The Loma Prieta, California, 

Earthquake of October 17,1 

Landslides

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The Loma Prieta, California, Earthquake of 

October 17, 1989—Landslides

DAVID K. KEEPER, Editor

STRONG GROUND MOTION AND GROUND FAILURE 

THOMAS L. HOLZER, Coordinator

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1551-C

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1998
DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

MARK SCHAFER, Acting Director

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February 28,1997

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CONTENTS

Page

Synopsis —————————————————————————————— C1 

By David K. Keefer

Regional distribution and characteristics of landslides generated

by the earthquake ——————————————————————— 7 

By David K. Keefer and Michael W. Manson

Coastal-bluff failures in northern Monterey Bay induced by the

earthquake —————————————————————————— 33 

By Gary B. Griggs and Nathaniel Plant

Landslide dams in Santa Cruz County, California, resulting

from the earthquake ————————————————————— 51 

By Robert L. Schuster, Gerald F. Wieczorek, and 

David G. Hope II

Large landslides near the San Andreas fault in the Summit

Ridge area, Santa Cruz Mountains, California ———————— 71 

By David K. Keefer, Gary B. Griggs, and Edwin L. Harp

Origin of fractures triggered by the earthquake in the Summit 

Ridge and Skyland Ridge areas and their relation to 

landslides ———————————————————— 129

By Edwin L. Harp

Evaluation of coseismic ground cracking accompanying the

earthquake: Trenching studies and case histories —————— 145 

By Jeffrey M. Nolan and Gerald E. Weber

Analysis of earthquake-reactivated landslides in the epicentral

region, central Santa Cruz Mountains, California—————— 165 

By William F. Cole, Dale R. Marcum, Patrick O. Shires, and 

Bruce R. Clark

III

THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17, 1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

SYNOPSIS

By David K. Keefer, 

U.S. Geological Survey

CONTENTS

Introduction————————————————————

Overall distribution and characteristics of landslides ———• 

Coastal landslides ——————————————————————- 

Landslide dams ———————————————————

Large landslides near the fault rupture in the Summit Ridge 

area ———————————————————————————

Characterization—————————————————

Relation of landslide cracks to other, earthquakegenerated cracks ———————————————————• 

Trenching investigations ————————————————• 

Seismic slope-stability analysis —————————————• 

Concluding remarks——————————————————————• 

References cited ——————————————————

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INTRODUCTION

Central California, in the vicinity of San Francisco and 

Monterey Bays, has a history of fatal and damaging landslides, triggered by heavy rainfall, coastal and stream erosion, construction activity, and earthquakes. The great 1906 

San Francisco earthquake (M^S.2-8.3) generated more 

than 10,000 landslides throughout an area of 32,000 km2 

(Keefer, 1984); these landslides killed at least 11 people 

and caused substantial damage to buildings, roads, railroads, and other civil works. Smaller numbers of landslides, which caused more localized damage, have also 

been reported from at least 20 other earthquakes that have 

occurred in the San Francisco Bay-Monterey Bay region 

since 1838 (Youd and Hoose, 1978; Keefer, 1984; 

Wieczorek and Keefer, 1987; D.K. Keefer, unpub. data, 

1992). Conditions that make this region particularly susceptible to landslides include steep and rugged topography, weak rock and soil materials, seasonally heavy 

rainfall, and active seismicity.

Given these conditions and history, it was no surprise 

that the 1989 Loma Prieta earthquake generated thousands 

of landslides throughout the region. Landslides caused one 

fatality and damaged at least 200 residences, numerous 

roads, and many other structures. Direct damage from landslides probably exceeded $30 million; additional, indirect

economic losses were caused by long-term landslide blockage of two major highways and by delays in rebuilding 

brought about by concern over the potential long-term 

instability of some earthquake-damaged slopes.

The 1989 Loma Prieta earthquake provided the first 

relatively complete data set on landslides generated by an 

earthquake of this size (M=7.0, M5=7.1) in central California. Previous landslide-producing earthquakes in the 

region, except for the 1906 event, were either much smaller 

or too poorly documented to accurately assess landslide 

effects. The papers in this chapter discuss many aspects 

of earthquake-induced landsliding, ranging from determinations of the overall distribution of landslides to detailed 

analyses of individual slope failures. Many of these aspects are consistent with the results of previous analyses 

based largely on data from other regions. Therefore, many 

of the slopes that failed during the earthquake could have 

been identified as susceptible to landsliding by using existing criteria and techniques (for example, Keefer, 1984; 

Wieczorek and others, 1985; Wilson and Keefer, 1985; 

Keefer and Wilson, 1989).

Several other aspects of the landsliding, however, had 

few, if any, historical analogs and thus lead to new conclusions concerning the generation of landslides by seismic events. Among these new conclusions are that (1) 

many ground cracks throughout the epicentral region were 

caused by incipient landslide movement, (2) seismically 

generated landslides may be important contributors to longterm coastal erosion, (3) areas of ground cracking adjacent to some fault ruptures may produce landslides 

substantially larger and more complex than those produced 

at greater distances from the fault by shaking alone, (4) 

many ground cracks associated with landslides near fault 

ruptures show evidence of previous movement and thus 

may be detectable by subsurface investigations, and (5) 

slope-stability analyses based on material strengths measured on small-diameter samples may underestimate actual earthquake-induced landslide displacements.

The first three papers in this chapter discuss various 

facets of the regional distribution of landslides: overall 

distribution and characteristics, coastal landslides, and generation of landslide dams. The next four papers discuss 

large, complex landslides that were generated near the

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C2 LANDSLIDES

rupture zone, including their overall characterization, 

analysis of their relation to ground cracks produced by 

processes other than landsliding, subsurface investigations 

documenting previous movement histories, and use of these 

well-documented features to calibrate methods of dynamic 

slope-stability analysis. Other chapters in this volume also 

discuss topics closely related to landslides: soil liquefaction, which produces types of landslides not described 

here (see O'Rourke, 1992; Holzer, in press); and ground 

cracks near the rupture zone (Ponti, in press).

OVERALL DISTRIBUTION AND 

CHARACTERISTICS OF LANDSLIDES

The landslides produced by the 1989 Loma Prieta earthquake occurred throughout an area of 15,000 km2, including most of the heavily populated San Francisco Bay region 

(fig. 1). Landslides damaged hundreds of residences and 

other structures, some as far from the epicenter as San 

Francisco (fig. 1), and blocked major highways for periods as long as weeks to months, but they caused only one 

fatality. The overall distribution and characteristics of these 

earthquake-generated landslides (except for those caused 

by soil liquefaction) are discussed by Keefer and Manson 

(this chapter).

Landslides were most abundant in an area of about 2,000 

km2 in the southern Santa Cruz Mountains (fig. 1). Despite its rugged topography, this mountainous area is populated, and the landslides caused substantial damage. At 

least 1,050 and, possibly, as many as about 3,000 landslides occurred. The most common types of landslides 

were rock falls, rock slides, and disrupted soil slides, types 

that also are most common in earthquakes worldwide 

(Keefer, 1984). Deeper seated and more coherent slumps 

and block slides were moderately common.

Most landslides were in materials that previous studies 

had identified as highly susceptible to earthquake-induced 

failure, including weakly cemented rocks, artificial fills, 

uncemented alluvial materials, and preexisting landslide 

deposits (Keefer, 1984; Wieczorek and others, 1985; 

Wilson and Keefer, 1985; Keefer and Wilson, 1989). In 

fact, many of the same geologic units that produced abundant landslides in this earthquake had previously been 

identified as highly susceptible to seismic slope failure 

in a study of San Mateo County, immediately to the north 

(Wieczorek and others, 1985). More than 85 percent of 

the earthquake-induced landslides occurred southwest 

of the fault rupture, almost certainly because the fault 

separated groups of rocks with significantly different 

properties.

In addition to cracks associated with clearly defined 

landslides, the earthquake opened cracks in hundreds of 

ridgetops and hillsides throughout the southern Santa Cruz

Mountains. Analysis of the locations and characteristics 

of these cracks leads to the conclusion that many of the 

cracks were, in fact, produced by incipient landslides. 

Keefer and Manson (this chapter) conclude that if conditions had been wetter at the time of the earthquake or if 

shaking had been more severe, many of these sites probably would have produced fully developed landslides, and 

so the total landslide damage would have been much 

greater.

COASTAL LANDSLIDES

Outside the Santa Cruz Mountains, landslides occurred 

along a 240-km-long stretch of the central California coast 

(fig. 1), which is characterized in most places by steep, 

high bluffs. Griggs and Plant (this chapter) discuss these 

coastal landslides, with emphasis on the northern Monterey 

Bay area, where these landslides were most abundant. 

Coastal landslides damaged or imperiled residences and 

other structures both on bluff tops and in beach areas at 

the bases of bluffs. In addition to fully developed landslides, the earthquake generated cracks on bluff tops as 

much as 10 m inland from bluff faces, raising questions 

about the long-term stability of many sites.

Griggs and Plant (this chapter) note that seismic failure 

of coastal bluffs may be a hazard not widely recognized 

during planning for construction in coastal areas. This 

absence of recognition derives from the common practice 

of basing construction setbacks or other hazard-mitigation 

procedures on calculated long-term average rates of bluff 

erosion. Because the period of recorded data may be too 

short to include major, well-documented earthquakes, the 

contribution of earthquake-induced failure may not be 

taken into account. Our experience in the 1989 Loma Prieta 

earthquake thus can be used to improve recognition and 

mitigation of hazards in the coastal area.

LANDSLIDE DAMS

Another effect of the earthquake-triggered landslides 

was the formation of landslide dams in the Santa Cruz 

Mountains, as described by Schuster and others (this chapter). Such landslide dams can pose delayed or long-term 

hazards because these dams are prone to fail from buildup 

of water behind them. Historical failures of landslide dams 

and consequent flooding events have killed as many as 

100,000 people. The 1989 Loma Prieta earthquake produced five documented landslide dams, all within about 

10 km of the epicenter (fig. 1). These dams were small, 

ranging from 200 to 50,000 m3 in volume, and impounded 

lake volumes ranging from 150 to 6,000 m3 . Two of the 

dams were removed by human excavation, and three failed
SYNOPSIS C3

123°

Loma Prieta 

rupture zone

10 20 30 40 KILOMETERS

i i i i

122

Figure 1 .—San Francisco Bay-Monterey Bay region, Calif., showing approximate limit of area affected by earthquake-generated landslides 

and locations of study areas of papers in this chapter. Base map from Edwards and Batson (1990).
C4 LANDSLIDES

naturally, producing only minor flooding that did not exceed channel capacity. Schuster and others conclude that 

the generation of landslide dams by the earthquake was 

limited by the uncommonly dry conditions in the San Francisco-Monterey Bay region at the time of the event.

trolled by elements of the regional geologic structure, such 

as bedding planes and local faults. Harp shows how these 

structurally related cracks interacted with downslope movement to produce the unusually complex structure of the 

landslides in the Summit Ridge area.

LARGE LANDSLIDES NEAR THE

FAULT RUPTURE IN THE SUMMIT

RIDGE AREA

CHARACTERIZATION

The most unusual slope-failure effects generated by the 

earthquake were the large, complex, deep-seated landslides 

that occurred in and around Summit Ridge, near the rupture zone (fig. 1; see Keefer and others, this chapter). 

Initial recognition of these landslides was hindered by 

their occurrence in an area of pervasive ground cracks, 

many of which originated from mechanisms other than 

landsliding (Ponti and Wells, 1991; Ponti, in press). Detailed mapping (Spittler and Harp, 1990) and analysis of 

ground-surface features in the Summit Ridge area eventually delineated 20 landslides, each with surface areas of 

more than 10,000 m2. The largest individual landslide, 

which had a surface area of 680,000 m2 and an estimated 

volume of 27 million m3, encompassed about 165 homes, 

many of which were severely damaged. An unusual feature of the landslides in the Summit Ridge area, in addition to their size and complexity, was the discontinuity of 

the surface features defining their boundaries. Keefer and 

others conclude that these discontinuous boundary features probably resulted from the small landslide displacements, ranging from 0.25 to 2.44 m.

According to Keefer and others (this chapter), the close 

association of these landslides with other, earthquake-generated ground cracks, the historical record of landslides in 

the Summit Ridge area, and analytical slope-stability modeling all suggest that future reactivation of the landslides 

is unlikely except during earthquakes. Although two possible analogs of these landslides, generated by previous 

earthquakes, are discussed by Harp (this chapter), such 

landslides are extremely rare in the historical record. Thus, 

the landslides in the Summit Ridge area represent a newly 

recognized type of landslide hazard along fault traces.

RELATION OF LANDSLIDE CRACKS TO 

OTHER, EARTHQUAKE-GENERATED CRACKS

Differentiation of landslide-generated ground cracks 

from other cracks in the Summit Ridge area is detailed by 

Harp (this chapter), who concludes that the locations and 

orientations of most nonlandslide ground cracks were conTRENCHING INVESTIGATIONS

Trenching investigations designed to determine the subsurface characteristics and movement histories of ground 

cracks are described by Nolan and Weber (this chapter). 

Their study includes detailed analyses of the features observed in two postearthquake trenches and a synthesis of 

data from 24 previous (both preearthquake and 

postearthquake) trenching investigations in the Summit 

Ridge area. One of the trenches studied in detail was excavated across the well-defined main scarp of a large landslide; the other was excavated across a ground crack that 

could be related to landsliding, tectonic deformation, or 

both. Both trenches revealed features indicative of at least 

four displacement events, including the 1989 Loma Prieta 

earthquake. These events occurred within a time period of 

approximately 3,200 years in one place and of approximately 2,000 years in the other. One movement event on 

one crack was probably associated with the 1906 San Francisco earthquake.

From their analysis of these and the 24 other trenching 

investigations, Nolan and Weber (this chapter) conclude 

that most ground cracks showing more than 3 to 5 cm of 

extension and (or) more than 1 to 3 cm of vertical displacement attributable to the 1989 Loma Prieta earthquake 

also showed clear subsurface evidence of previous movement. They also found that although not all ground cracks 

with previous displacements were reactivated by the earthquake, most cracks with previous displacements were associated with recognizable surface topographic features. 

The results of their study thus indicate that careful subsurface investigation before construction can minimize the 

potential hazard from earthquake-induced ground cracking in such areas.

SEISMIC SLOPE-STABILITY ANALYSIS

The landslides in the Summit Ridge area presented a 

rare opportunity to calibrate existing techniques for analyzing potential slope displacements under seismic conditions. The study by Cole and others (this chapter) involved 

sampling, material testing, and seismic slope-stability 

analysis of two of these landslides. Cole and others conclude that slope-stability analyses conducted by measuring material strengths in the laboratory on small-diameter 

samples may underestimate actual landslide displacements. 

They also found that calculated landslide displacements
SYNOPSIS C5

are highly sensitive to predicted ground-water levels within 

the analyzed slopes and to the ground-motion record used 

as input to the analysis. Their study shows that seismic 

slope-stability analysis may be made more accurate if material strengths can be estimated from backanalysis of previous slope failures.

1989 Loma Prieta earthquake has provided both a warning about the need for these efforts and significant new 

information with which to improve them.

REFERENCES CITED

CONCLUDING REMARKS

The 1989 Loma Prieta earthquake was the most important seismic event in a major U.S. urban area since 1906. 

As such, it provided both a significant test of existing 

hazard-reduction techniques and a case study for improving those techniques. In the areas of landsliding, techniques for identifying slopes susceptible to failure, 

developed largely during the past 10 years, were largely 

proved correct: Most slopes that failed in the earthquake 

could have been accurately characterized with sufficiently 

detailed preearthquake mapping and analysis. 

Postearthquake studies, however, have also led to the recognition of types of landslide hazards not fully appreciated in the past, including the potential for particularly 

large and complex landslides along fault traces, pervasive 

ridgetop and hillside cracking due to incipient landslide 

movement, and the contribution of earthquake-induced failure to coastal erosion. In addition, the earthquake provided an opportunity for the calibration and improvement 

of site-specific numeric methods, used to estimate potential earthquake-induced slope displacements.

The landslide damage and loss of life associated with 

the earthquake were fortuitously light, given the nigh population density and geologic conditions in the Loma Prieta 

region. However, the damage that did occur, the longterm disruption of transportation by landslides along roads, 

and the delays in recovery due to possible continuing instability of earthquake-damaged slopes all illustrate the 

types of problems that landslides can create as a result of 

earthquakes, both in this region and in other regions of 

the United States and the rest of the world. Minimizing 

the potential damage and loss of life from landslides in 

future earthquakes will require continued application and 

enhancement of the techniques of hazard reduction. The

Edwards, Kathleen, and Batson, R.M., 1990, Experimental digital shadedrelief maps of California: U.S. Geological Survey Miscellaneous 

Investigations Series Map 1-1848, scale 1:1,000,000, 2 sheets.

Holzer, T.L., ed., in press, The Loma Prieta, California, earthquake of 

October 17, 1989—liquefaction: U.S. Geological Survey Professional Paper 1551-B.

Keefer, D.K., 1984, Landslides caused by earthquakes: Geological Society of America Bulletin, v. 95, no. 4, p. 406-421.

Keefer, D.K., and Wilson, R.C., 1989, Predicting earthquake-induced 

landslides, with emphasis on arid and semi-arid environments, in 

Sadler, P.M., and Morton, D.M., eds., Landslides in a semi-arid 

environment with emphasis on the Inland Valleys of Southern California: Riverside, Calif., Inland Geological Society of Southern California Publications, v. 2, no. 1, p. 118-149.

O'Rourke, T.D., ed., 1992, The Loma Prieta, California, earthquake of 

October 17, 1989—Marina District: U.S. Geological Survey Professional Paper 1551-F, p. F1-F215.

Ponti, D.J., ed., in press, The Loma Prieta, California, earthquake of 

October 17, 1989—ground ruptures: U.S. Geological Survey Professional Paper 1551-D.

Ponti, D.J., and Wells, R.E., 1991, Off-fault ground ruptures in the 

Santa Cruz Mountains, California; ridge-top spreading versus tectonic extension during the 1989 Loma Prieta earthquake: Seismological Society of America Bulletin, v. 81, no. 5, p. 1480-1510.

Spittler, T.E., and Harp, E.L., compilers, 1990, Preliminary map of 

landslide features and coseismic fissures in the Summit Road area 

of the Santa Cruz Mountains triggered by the Loma Prieta earthquake of October 17, 1989: U.S. Geological Survey Open-File Report 90-688, 53 p., scale 1:4,800, 3 sheets.

Wieczorek, G.F., and Keefer, D.K., 1987, Earthquake triggered landslide at La Honda, California, in Hoose, S.N., ed., The Morgan 

Hill, California, earthquake of April 24, 1984: U.S. Geological Survey Bulletin 1639, p. 73-79.

Wieczorek, G.F., Wilson, R.C., and Harp, E.L., 1985, Map showing 

slope stability during earthquakes in San Mateo County, California: 

U.S. Geological Survey Miscellaneous Investigations Series Map 

I-1257-E, scale 1:62,500.

Wilson, R.C., and Keefer, D.K., 1985, Predicting areal limits of earthquake-induced landsliding, in Ziony, J.I., ed., Evaluating earthquake 

hazards in the Los Angeles region—an earth-science perspective: 

U.S. Geological Survey Professional Paper 1360, p. 317-345.

Youd, T.L., and Hoose, S.N., 1978, Historic ground failures in northern 

California triggered by earthquakes: U.S. Geological Survey Professional Paper 993, 177 p.

THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

REGIONAL DISTRIBUTION AND CHARACTERISTICS OF 

LANDSLIDES GENERATED BY THE EARTHQUAKE

By David K. Keefer, 

U.S. Geological Survey;

and

Michael W. Manson, 

California Division of Mines and Geology

CONTENTS

Page 

Abstract ———————...——..——————————....... C7

Introduction ———————————————————————————— 7

Setting of the earthquake———————————————————— 8

Climate, vegetation, and population ——————————— 8

Geology —————————————————————————— 8

Seismicity —————————————————————————— 10

Previous landslide activity ————————————————— 10

Distribution and characteristics of landslides generated by

the earthquake ———————————————————————— 11 

General characteristics and classification of earthquakeinduced landslides ——————————————————— 11 

Identification and mapping of landslides after the

earthquake ——————————————————————— 11 

Southern Santa Cruz Mountains ———————————-—— 12 

Disrupted slides and falls (category 1 landslides)—— 12 

Coherent slides (category 11 landslides) —————— 15 

Ground cracks ————-——————————————— 18 

Estimated total number of landslides in the

southern Santa Cruz Mountains—————————— 19

Landslides in other areas —————————————————— 20

Coastal landslides —————————————————— 20

Peripheral area ———————————————————— 23

Discussion and conclusions——————————————————— 24

Acknowledgments ———————————————————————— 28

References cited ————————————————————————— 29

ABSTRACT

The 1989 Loma Prieta earthquake generated thousands 

of landslides, most of which occurred within an area of 

approximately 2,000 km2 in the rugged, heavily vegetated 

southern Santa Cruz Mountains, but also along a 240-km 

stretch of the central California coast and throughout another 13,000 km2 of the San Francisco Bay-Monterey Bay 

region. Landslides triggered by the earthquake damaged 

or destroyed more than 100 residences and other structures, blocked roads throughout the affected region, and 

caused one fatality.

Near the rupture zone itself, the earthquake generated 

several large, complex landslides with surface areas as 

great as 85 ha and depths possibly exceeding 100 m. 

Throughout the rest of the southern Santa Cruz Mountains, the most abundant landslides were shallow, internally disrupted rock falls, rock slides, and soil slides, 

although more coherent and deeper seated landslides also 

were moderately common. In addition to landslides, the 

earthquake opened ground cracks at hundreds of localities 

throughout the Santa Cruz Mountains. Many of these 

cracks almost certainly marked incipient landslides, which 

would have developed more fully had ground conditions 

been wetter or the earthquake shaking more severe.

The shallow, internally disrupted landslides were most 

common in weakly to moderately cemented sedimentary 

rocks, whereas the more coherent landslides were most 

abundant in artificial fill and preexisting landslide deposits. All types of landslides were more abundant southwest 

of the San Andreas fault than to the northeast, probably 

owing to the widespread distribution of relatively weakly 

consolidated rocks in the area southwest of the fault. Along 

the coastal cliffs bordering the Pacific Ocean, large landslides occurred out to relatively great distances from the 

rupture zone, indicating that these seacliffs are especially 

susceptible to seismically induced failure.

INTRODUCTION

The Af=7.0 Loma Prieta earthquake triggered thousands 

of landslides throughout approximately 15,000 km2 of 

central California, including most of the San Francisco 

Bay-Monterey Bay region (fig. 1). These landslides were

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C8 LANDSLIDES

of several different types, occurred in various geologic 

environments, and locally caused significant damage to 

public infrastructure and private property. As part of an 

effort to document the effects of this earthquake, the largest in the region since 1906, mapping of landslides and 

related ground-failure features was begun the day after 

the earthquake. The mapping was carried out by more 

than 50 individuals from Government agencies, universities, and private firms; landslide localities and detailed 

descriptions from the area most heavily affected by landslides were compiled by Manson and others (1992). This 

paper uses these and other data to summarize general characteristics of the landslides generated by the earthquake 

and to compare these landslides with those generated by 

other historical earthquakes.

SETTING OF THE EARTHQUAKE

The region affected by the 1989 Loma Prieta earthquake stretches inland from the Pacific coast, through the 

broad lowlands around San Francisco Bay and Monterey 

Bay, to parts of the Coast Ranges of central California 

(fig- 1).

CLIMATE, VEGETATION, AND POPULATION

The affected region has a Mediterranean climate, characterized by warm, dry summers and cool, rainy winters. 

Virtually all precipitation occurs as rain, 90 percent of 

which falls during the winter months of November through 

April. Mean annual precipitation ranges from about 300 

to about 2,000 mm throughout the region; coastal mountains generally receive the most precipitation, and inland 

valleys the least (Rantz, 1971). Precipitation varies widely 

from year to year. The 1989 Loma Prieta earthquake occurred during the fourth year of drought conditions, when 

rainfall was only about 50 to 70 percent of normal, and at 

the end of the dry summer season—a combination of circumstances that made ground conditions especially dry.

The San Francisco Bay-Monterey Bay region (fig. 1) is 

heavily urbanized, with a population of almost 6 million 

people. Vegetation varies throughout the nonurbanized 

parts of the region, with areas of marsh, grassland, coastal 

scrub, chaparral, oak, evergreen, redwood, and mixed forests (Thomas, 1961).

Most of the landslides produced by the earthquake occurred in the southern Santa Cruz Mountains, through 

which the rupture zone passes (fig. 1). Topography in this 

area ranges from gently rolling hills to steep, rugged ridges 

separated by narrow canyons; altitudes range from near 

sea level to 1,155 m. Much of the area is covered by 

dense redwood or oak forest or chaparral, and the dense

vegetation precluded systematic mapping of landslides 

from aerial photographs or airborne observations. Mean 

annual precipitation in the area ranges from about 500 to 

1,500 mm (Rantz, 1971).

GEOLOGY

The Coast Ranges and intervening lowlands (fig. 1) are 

underlain by a wide variety of sedimentary, igneous, and 

metamorphic rocks and unconsolidated sedimentary deposits, most of which range in age from Jurassic through 

Holocene. The rocks vary greatly in composition, degree 

of consolidation, amount of deformation, and depth of 

weathering. Shale, siltstone, sandstone, and volcanic rocks 

predominate. Colluvium of varying depth and composition mantles most hillslopes.

In the southern Santa Cruz Mountains, the geologic conditions and bedrock units southwest of the San Andreas 

fault differ significantly from those to the northeast. Southwest of the fault, the predominant bedrock units are Tertiary sedimentary rocks—primarily sandstone, siltstone, 

mudstone, and shale—and smaller bodies of intrusive, volcanic, and metamorphic rocks (Brabb, 1989). The most 

widespread sedimentary units are the Purisima Formation 

(siltstone with sandstone interbeds), the Butano Sandstone 

(sandstone, siltstone, and minor conglomerate), the Santa 

Cruz Mudstone (mudstone), the Monterey Formation (mudstone and siltstone), the San Lorenzo Formation (sandstone, mudstone, and shale), the Vaqueros Sandstone 

(sandstone with interbeds of mudstone and shale), and the 

Santa Margarita Sandstone (sandstone). These rocks, which 

typically strike northwest, are poorly to moderately consolidated, varyingly weathered, intensely folded, and locally sheared and faulted. They are commonly covered by 

colluvial and residual soils, as much as several meters 

thick. Near the coast and in the southern part of the area, 

near Pajaro Valley (see pi. 1), these rocks are overlain by 

poorly consolidated to unconsolidated Quaternary alluvial 

and terrace deposits.

Northeast of the San Andreas fault, large parts of the 

area are underlain by rocks of the Central Belt of the 

Franciscan Complex (Upper Cretaceous to Lower 

Eocene?). Although these rocks are more indurated than 

those southwest of the fault, they are intensely and pervasively sheared. The predominant unit is melange, composed of resistant blocks of different sizes and rock types 

enclosed in a less resistant matrix of penetratively sheared 

argillite, tuff, and sandstone (McLaughlin and others, 

1991). Other types of Franciscan rocks include limestone, 

chert, basalt, and metasandstone (McLaughlin and others, 

1988, 1991). Also present in the area northeast of the 

fault are rocks of the Coast Range ophiolite (Middle? to 

Late Jurassic), made up of serpentinite, ultramafic rocks, 

gabbro, diabase, igneous dikes and sills, and volcanic
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C9

38°-

37°-

36°-

Summit 

Ridge

Southern 

Santa Cruz 

Mountains

Pajaro Rive

Loma Prieta 

rupture zone

0 510 20 30 40 KILOMETERS Big

ii i i i

Approximate

123 121 C

Figure 1.—San Francisco Bay-Monterey Bay region, showing geographic limit of landslides generated by 1989 Loma Prieta earthquake and 

locations of landslide zones discussed in text in relation to epicenter and rupture zone. Base map from Edwards and Batson (1990).
CIO LANDSLIDES

rocks (McLaughlin and others, 1988, 1991; Clark and 

others, 1989).

The areas immediately northeast of the San Andreas 

fault is underlain by sedimentary rocks that range in age 

from Upper Jurassic through Tertiary. These rocks vary 

widely in degree of induration, but on average they are 

more indurated than the sedimentary rocks in the area 

southwest of the fault. Predominant rock types are 

conglomerate, sandstone, mudstone, and shale. These 

rocks also are intensely folded and locally sheared and 

faulted.

SEISMICITY

The region affected by the 1989 Loma Prieta earthquake lies along the boundary between the North American and Pacific plates. This region contains many active 

and potentially active faults, including the plate-bounding 

San Andreas fault. In addition to myriad small and moderate historical earthquakes, 23 earthquakes of A/>6.0 have 

occurred in the region since 1836 (Wesnousky, 1986; U.S. 

Geological Survey, 1990), two of which were as large as 

the 1989 Loma Prieta earthquake (A/=7.0 in 1838 and 

A/=7.0 in 1868) and one substantially larger (A/=8.2-8.3 

in 1906) (U.S. Geological Survey, 1990).

The earthquake occurred at 5:04 p.m. P.d.t. on October 

17, 1989, and had a Richter surface-wave magnitude 

(A/5) of 7.1 (Plafker and Galloway, 1989) and a moment 

magnitude (M) of 7.0 (Hanks and Krawinkler, 1991). 

The hypocenter was at approximately 18-km depth and 

was located at lat 37°02/ N., long 121°53' W., in the 

Santa Cruz Mountains (fig. 1; Plafker and Galloway, 1989). 

The earthquake is inferred to have ruptured a 40-km-long 

segment of either the San Andreas or a nearby fault 

(Plafker and Galloway, 1989; Working Group on 

California Earthquake Probabilities, 1990; Hanks and 

Krawinkler, 1991); the rupture zone extended from near 

the Pajaro Gap, east of Watsonville, northwestward to 

near California Highway 17 (fig. 1; Plafker and Galloway, 1989; Working Group on California Earthquake 

Probabilities, 1990).

No throughgoing surface fault rupture was found. Instead, areas of the ground surface adjacent to the trace of 

the San Andreas fault exhibited complex patterns of 

coseismic fissures (U.S. Geological Survey staff, 1989; 

Spittler and Harp, 1990; Ponti and Wells, 1991; Aydin 

and others, in press). Geodetic surveys showed that the 

fault slip caused as much as 173 mm of subsidence in a 

zone mostly northeast of the mapped trace of the San 

Andreas fault, and as much as 592 mm of uplift in a zone 

mostly southwest of the mapped fault trace (Marshall and 

others, 1991).

PREVIOUS LANDSLIDE ACTIVITY

Prehistoric landslide deposits are widespread in the 

mountains of the San Francisco Bay-Monterey Bay region (for example, Brabb and others, 1972 and CooperClark and Associates, 1975), and abundant historical 

landslides have occurred there in association with earthquakes (Lawson, 1908; Youd and Hoose, 1978; Keefer, 

1984; Marshall, 1990), storms (Keefer and others, 1987; 

Ellen and Wieczorek, 1988), and other events, such as 

construction activity and coastal erosion. These landslides 

have caused several deaths and significant property damage, and landsliding is a common and recurring hazard in 

the region.

Landslides are known to have occurred during the October 8, 1865, earthquake (A/=6.5) on the San Andreas 

fault and the October 21, 1868, earthquake (A/=7.0) on 

the Hayward fault. These earthquakes, however, were 

poorly documented, and the available historical information is fragmentary (Youd and Hoose, 1978; Marshall, 

1990).

Documentation of landslides caused by the much larger 

April 18, 1906, San Francisco earthquake (M=8.2-8.3), 

though incomplete, was substantially more extensive 

(Lawson, 1908; Youd and Hoose, 1978; Keefer, 1984; 

Marshall, 1990). This earthquake probably generated more 

than 10,000 landslides throughout an area of 32,000 km2 

(Keefer, 1984; Keefer and Wilson, 1989). In addition to 

causing many shallow, highly disrupted landslides of the 

types most common in earthquakes (Keefer, 1984), the 

1906 earthquake triggered or reactivated many deep-seated 

rotational slumps in the Coast Ranges (Lawson, 1908). In 

addition to being larger than the 1989 earthquake, the 

1906 earthquake occurred when the region was relatively 

wet as a result of recent precipitation (Lawson, 1908; Youd 

and Hoose, 1978; Schuster and others, this chapter).

More recent seismic events in the region for which landslides have been documented were smaller than the 1989 

Loma Prieta earthquake. They include the 1957 Daly City 

earthquake (A/L=5.3), the 1979 Coyote Lake earthquake 

(ML=5A), the 1980 Greenville-Mount Diablo earthquake 

sequence (max A/^=5.8), and the 1984 Morgan Hill earthquake (A/^6.1), each of which triggered a few dozen to a 

few hundred landslides (predominantly shallow rock falls, 

soil falls, and disrupted soil slides) within areas of about 

10 to 500 km2 around the epicenters (Keefer, 1984; O.K. 

Keefer, unpub. data, 1992). In addition, the 1984 Morgan 

Hill earthquake triggered a large, deep-seated landslide at 

an anomalously large distance of 53 km from the epicenter (Wieczorek and Keefer, 1987).

Substantial landslide activity due to precipitation in the 

region has been documented during the rainfall seasons of 

1949-50, 1955-56, 1961-62, 1962-63, 1964-65, 1966- 

67, 1968-69, 1969-70, 1972-73, 1974-75, 1977-78, 1982-
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE Cll

83, 1983-84, and 1986-87 (Keefer and others, 1987; 

Brown, 1988; Ellen and Wieczorek, 1988). The greatest 

documented landslide activity was associated with the 

storm of January 3-5, 1982, which produced as much as 

610 mm of rain within 34 hours and caused more than 

18,000 landslides and more than $66 million in landsliderelated damage (Ellen and others, 1988).

subaqueous landslides, all of which move primarily by 

fluidlike flow. Most category III landslides are associated 

with soil liquefaction. Soil liquefaction caused by the 

1989 Loma Prieta earthquake is the subject of other 

chapters of this volume (O'Rourke, 1992; Holzer, in 

press), and so category III landslides are not discussed 

further here.

DISTRIBUTION AND 

CHARACTERISTICS OF LANDSLIDES 

GENERATED BY THE EARTHQUAKE

GENERAL CHARACTERISTICS AND

CLASSIFICATION OF EARTHQUAKE-INDUCED

LANDSLIDES

Earthquake-induced landslides were classified into 

three major categories and 14 individual types by Keefer 

(1984), on the basis of the landslide terminology 

defined by Varnes (1978). Landslides in the first major 

category—called "disrupted slides and falls" by Keefer 

(1984) and "category I landslides" by Keefer and Wilson 

(1989)—include landslides with a high degree of 

internal disruption that typically originate on steep slopes 

and travel at high velocities of several meters per hour to 

100 km/h or more. The six individual types of landslides 

in this category include falls, slides, and avalanches in 

both soil and rock. Rock falls and soil falls move by 

falling, bouncing, and rolling; rock slides and soil slides 

move by translational sliding on discrete basal shear 

surfaces; and rock avalanches and soil avalanches move 

by complex mechanisms involving both sliding and 

fluidlike flow. Except for rock avalanches, most category 

I landslides are less than 3 m thick; rock avalanches, which 

have volumes of more than O.SxlO6 m3 , are typically 

thicker.

Category II landslides, or "coherent slides," include five 

individual types that move primarily by sliding on discrete basal shear surfaces. These landslides are more coherent than those in category I and typically consist of 

one to several moving blocks; they also are relatively deep 

seated, typically more than 3 m thick. Included in this 

category are rock slumps and soil slumps, which move on 

concave-upward basal shear surfaces with significant 

headward rotation, and rock block slides, soil block slides, 

and slow earth flows, all of which move primarily by 

translational sliding on planar or gently curved basal shear 

surfaces. Typical velocities of landslides in this category 

range from a few millimeters per hour to several meters 

per hour.

Category III landslides, or "lateral spreads and flows," 

include soil lateral spreads, rapid soil flows, and

IDENTIFICATION AND MAPPING OF 

LANDSLIDES AFTER THE EARTHQUAKE

Landslide identification and mapping began the day after the earthquake. Initial observations from fixed-wing 

aircraft and preliminary examination of postearthquake 

aerial photographs showed that because of the dense vegetation cover in much of the epicentral region, mapping 

of landslide distribution from the air was not feasible. 

Therefore, subsequent mapping was conducted primarily 

on the ground, consisting of traverses in vehicles along 

all primary and many secondary roads throughout this 

region, and on foot through selected areas. Additional data 

on landslides in Santa Cruz County was provided from 

reports by building inspectors, who noted instances of 

building damage after the earthquake. Landslide localities 

were compiled on U.S. Geological Survey l:24,000-scale 

7.5-minute topographic quadrangles, and data on landslide types and other characteristics were compiled from 

field notes (Manson and others, 1992).

In the southern Santa Cruz Mountains, where most of 

the landslides occurred, landslide localities were digitized, 

and maps and accompanying field-locality descriptions 

were published by Manson and others (1992); these 

l:48,000-scale maps cover 15 7.5-minute quadrangles 

(Manson and others, 1992). In this paper, those data were 

used to determine the numbers and characteristics of landslides in categories I and II. The data include some localities where multiple landslides were reported, at a few of 

which the number of landslides was estimated from the 

field descriptions because it was not reported directly. For 

each landslide locality, the geologic unit was determined 

from either the field description or a geologic map. In 

addition, each landslide source was classified according 

to geomorphic environment (ridgecrest, midslope, 

streambank, artificial cut or artificial fill), using either the 

field description or a topographic map.

In addition to this regional mapping, more detailed mapping, involving closely spaced traverses on foot, was carried out in the Summit Ridge area (fig. 1; Spittler and 

Harp, 1990) and in Forest of Nisene Marks State Park 

(see pi. 1), immediately around the epicenter (Weber and 

Nolan, 1989). In these two areas, most earthquake-induced 

landslides were probably identified. In other areas, 

however, the number of landslides that occurred was
C12 LANDSLIDES

probably higher than that reported, because of heavy vegetation, remoteness of some areas, and lack of access to 

some private property. As discussed below, the detailed 

mapping in Forest of Nisene Marks State Park was used 

to estimate the total number of landslides that occurred 

throughout the affected region.

The next subsection discusses the characteristics, abundance, and geologic environments of landslides that occurred in the southern Santa Cruz Mountains, followed by 

data on coastal landslides (discussed in more detail by 

Griggs and Plant, this chapter) and on landslides that occurred at widely scattered localities throughout the rest of 

the San Francisco Bay-Monterey Bay region. Finally, we 

discuss these results and compare the landslides generated by the 1989 Loma Prieta earthquake with those generated by other historical earthquakes.

SOUTHERN SANTA CRUZ MOUNTAINS

After the earthquake, a total of about 1,280 landslides 

from 1,046 different sources were mapped within in an 

area of approximately 2,000 km2 in the southern Santa 

Cruz Mountains (see pis. 1, 2; fig. 1): about 1,050 landslides were mapped during the general reconnaissance; 

210 in Forest of Nisene Marks State Park, immediately 

around the epicenter (Weber and Nolan, 1989); and 20, 

particularly large, in the Summit Ridge area (see Keefer 

and others, this chapter). Altogether, these landslides damaged more than 160 homes (Seed and others, 1990; Spittler 

and Harp, 1990; Manson and others, 1992); damaged other 

structures, such as retaining walls and water-supply pipes 

and tanks (Manson and others, 1992); damaged and 

blocked numerous roads (Manson and others, 1992); and 

dammed creeks at five localities (see Schuster and others, 

this chapter). In addition to landslides, ground cracks were 

reported in another 300 localities (see pis. 1, 2; Weber 

and Nolan, 1989; Manson and others, 1992), as well as in 

the Summit Ridge area, where coseismic ground cracking 

was pervasive throughout an area of about 30 km2 (Spittler 

and Harp, 1990; Ponti and Wells, 1991). As discussed 

below, many of the ground cracks in the southern Santa 

Cruz Mountains were probably caused by incipient landslide movement.

DISRUPTED SLIDES AND FALLS 

(CATEGORY I LANDSLIDES)

Of all the landslides identified in the southern Santa 

Cruz Mountains, 950 (74 percent) were disrupted slides 

and falls (category I landslides), primarily rock falls, rock 

slides, and disrupted soil slides. Deposits of those landslides typically consisted of shallow, highly disrupted 

masses of soil and rock (figs. 2, 3), some containing boulders more than 1 m in diameter (fig. 4). These landslides

occurred on steep roadcuts, streambanks, and ridge flanks. 

A third (33 percent) of the landslides were adjacent to 

roads, and most of these landslides involved artificial cuts. 

Of the remaining category I landslides, 31 percent originated low on slopes, along streambanks; 6 percent near 

ridgecrests; and 62 percent in midslope localities.

Most of the rock falls, rock slides, and soil slides had 

volumes of less than 100 m3 , but about 90 were larger, 

ranging in volume from 100 m3 to several thousand cubic 

meters. These larger landslides were restricted to localities within about 10 km of the rupture zone (fig. 1), mostly 

in the drainages of Bear Creek, Zayante Creek, Lyndon 

Canyon, Corralitos Creek, Rider Creek, Browns Creek, 

Hinckley Creek, or Soquel Creek (see pis. 1, 2). Four 

rock falls and rock slides, ranging in volume from 200 to 

3,500 m3 , dammed creeks in this area (see Schuster and 

others, this chapter).

One of the largest rock falls (fig. 5), with an estimated 

volume of 5,000 to 8,000 m3 , blocked both northbound 

lanes of California Highway 17, the main road between 

Santa Cruz and the San Francisco Bay region, for 33 days 

after the earthquake, necessitating $1.8 million in repairs 

(Spittler and others, 1990). This rock fall originated from 

two source areas in moderately cemented sandstone of the 

Purisima Formation on a steep roadcut, 15 m high. One 

source was in highly fractured rock with three conspicuous, orthogonal joint sets; the other source was in rock 

that was both highly fractured and intensely weathered. 

Between these two sources, the rock was relatively unweathered massive sandstone, with an average joint spacing of about 1 m.

Another complex of rock falls and rock slides, which 

resulted from failures of cut slopes on both sides of California Highway 17, temporarily blocked that highway at 

the Santa Cruz-Santa Clara County line (fig. 2). The landslide source was a sequence of steeply dipping beds of 

sandstone and shale of the Vaqueros Sandstone, which 

were massive but locally closely fractured. Rock falls were 

generated only within the shale beds, and open fissures on 

the ridgecrest above the source west of the highway indicated that these rock falls were related to an incipient 

slope failure which was much larger and deeper. Together, 

the rock falls and rock slides at this locality contained an 

estimated several hundred cubic meters of material.

The bedrock units in which rock falls, rock slides, and 

soil slides occurred in the southern Santa Cruz Mountains 

are listed in table 1 in decreasing order of landslide abundance. Whereas 32 different geologic units produced category I landslides, only 7 units produced more than 24 

landslides each (table 1). Five of these seven units were 

Tertiary sedimentary rocks southwest of the San Andreas 

fault, one was a sedimentary unit northeast of the fault, 

and another was composed of preexisting landslide deposits (table 1). The Purisima Formation produced far more 

landslides than any other unit (more than 25 percent of
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C13

Figure 2.—Part of deposit from complex of rock falls and 

rock slides along California Highway 17 at Santa Cruz-Santa 

Clara County line (see pi. I for location). Landslide source 

t*l was *n mtensely fractured rock.

Figure 3.—Rock fall from nearvertical cut along Old San Jose 

Road, about 0.9 km northwest 

of Sugarloaf Mountain (see pi. 2 

for location). Source rock is weakly 

to moderately cemented sandstone, 

broken by conspicuous, throughgoing joints. Estimated volume of 

deposit is 500 to 1,000 m3.
C14 LANDSLIDES

the total), consistent with the generally poor consolidation of its rocks (Clark and others, 1989) and their widespread distribution near the rupture zone (fig. 1; Brabb, 

1989). Where observed in outcrop, the sources of category I landslides in all units were typically in materials 

that were weakly cemented, closely fractured, intensely 

weathered, and (or) broken by conspicuous, throughgoing 

joints.

Figure 4.—Boulder, approximately 1.5 m in maximum diameter, from earthquake-induced rock fall near conference 

ground about 2.5 km north of Sequel (see pi. 2 for location). Several boulders of nearly comparable size bounced 

and rolled into gently sloping field from steep slope above.

Figure 5.—Part of rock-fall deposit 

from cut slope above California 

Highway 17 in Glenwood area (see 

pi. 2 for location).
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C15

More indurated igneous rocks, southwest of the San 

Andreas fault, and Franciscan Complex rocks, northeast 

of the fault, produced few category I landslides (table 1); 

in all, 85 percent of the category I landslides occurred 

southwest of the San Andreas fault.

To compare the landslide susceptibility of the various 

units, we calculated the number of landslides per unit area 

(table 1). Data on the areas of occurrence of the various 

units were derived from digitized U.S. Geological Survey 

geologic maps (Brabb, 1989; Clark and others, 1989; 

McLaughlin and others, 1988, 1991). The results of this 

comparison showed that sedimentary rocks, primarily 

southwest of the fault and primarily of Tertiary age, were 

most susceptible to generating category I landslides. (Some 

Quaternary units, particularly alluvium, artificial fill, and 

preexisting landslide deposits, were incompletely represented on these maps and so were omitted from the calculations.)

Many of the units that produced more than 10 reported 

category I landslides had been identified as highly susceptible to earthquake-induced slope failure in San Mateo 

County, just north of the southern Santa Cruz Mountains, 

by Wieczorek and others (1985). Those high-susceptibility units included the Purisima Formation, the San Lorenzo 

Formation, preexisting landslide deposits, the Monterey 

Formation, and the Lambert Shale.

COHERENT SLIDES 

(CATEGORY II LANDSLIDES)

In the southern Santa Cruz Mountains, 330 (26 percent) 

of the mapped landslides were slumps or block slides (category II landslides; fig. 6). These landslides typically were 

deeper and more coherent than the category I landslides 

and consisted of one or a few blocks of displaced material. Commonly, the category II landslides were bounded 

at their heads by distinct scarps or fissures, and many 

contained internal fissures and compressional features as 

well.

The slumps and block slides triggered by the earthquake ranged in size from small features, a few meters 

long and a few meters wide, to the large, complex landslides in the Summit Ridge area, as much as 980 m long 

by 1,300 m wide (see Keefer and others, this chapter). 

Measured displacements on category II landslides ranged 

from less than 1 to more than 300 cm, and most displacements were from 10 to 100 cm. Of these landslides, 30 

percent involved roadcuts, fills, or embankments; 15 percent originated on ridgecrests; 33 percent occurred in 

midslope localities; and 22 percent were along ridge flanks. 

Average slopes where these landslides occurred ranged 

from about 12° to near vertical.

Probably the most common category II landslides were 

small slumps in roadfill (fig. 7), which were typically 

characterized by arcuate, concave-downslope main scarps

and several subsidiary fissures that disrupted the road surface and continued into underlying material. Such slumps 

were common on both paved and unpaved roads throughout the southern Santa Cruz Mountains.

Many larger, more complex slumps also occurred, both 

in the Summit Ridge area and elsewhere in the southern 

Santa Cruz Mountains. One of the largest of these slumps 

outside the Summit Ridge area, with an estimated volume 

of 50,000 m3 , dammed the west branch of Sequel Creek 

(see Schuster and others, this chapter). Another large slump 

occurred in the Rebecca Drive area of the community of 

Boulder Creek, where five residences were heavily damaged by a combination of earthquake shaking and landslide movement (fig. 8).

The Rebecca Drive landslide originated near the crest 

of a long, broad ridge. The main scarp (fig. 9), which 

passed under two houses, was 150 m long, and discontinuous small scarps and cracks continued along its strike 

for an additional 90 m. Maximum measured displacement 

across the main scarp was 57 cm. Subsidiary cracks were 

present both upslope from the main scarp and within the 

body of the landslide. Material exposed at the surface in 

the head of the landslide was a slightly cohesive, finesand fill, but the landslide almost certainly extended downward into the underlying soil and Santa Margarita 

Sandstone bedrock.

No compressional feature marking the landslide toe was 

found, but because the ridge flank was heavily vegetated, 

small compressional features may have been present but 

not observed. No indications of landslide displacement, 

however, were found along a roadcut into the ridge about 

150 m downslope from the main scarp, and so the slump 

was probably less than 150 m long from crown to toe.

Several large block slides in bedrock occurred from the 

ends of prominent ridges in the southern Santa Cruz Mountains. These landslides had distinctive shapes and were 

recognized primarily by displacements in road surfaces 

and adjacent roadcuts. Their features typically consisted 

of (1) zones of linear or arcuate, concave-downslope scarps 

and cracks above the roads, which marked the main scarps 

and heads of the block slides (figs. 10, 11); (2) flank 

cracks trending perpendicular to the long axes of the roads 

and exhibiting displacements consistent with downslope 

movement (figs. 10, 12); and (3) small rock falls or soil 

slides dislodged where flank cracks intersected steep cuts 

above the roads (figs. 10, 12). The block slides typically 

were several tens to a few hundreds of meters wide and 

moved from a few to a few tens of centimeters. Because 

these landslides did not develop visible compressional features marking their toes, their lengths could not be determined.

Most category II landslides were in unconsolidated Quaternary deposits (especially artificial fill, preexisting landslide deposits, or alluvium) or in poorly to moderately 

indurated Tertiary sedimentary rocks (especially the
C16 LANDSLIDES

Table 1.—Geologic units that produced category I landslides

[Predominant lithologies from Dibblee and Brabb (1978), Brabb and Dibblee (1979), Brabb and Pampeyan (1983), McLaughlin and others (1988, 1991), Brabb (1989), Clark 

and others (1989), and E.E. Brabb (oral commun., 1992). Ages: K, Cretaceous; J, Jurassic; Mz, Mesozoic; Pz, Paleozoic; Q, Quaternary; T, Tertiary, n.d., not determined]

Average

Direction number of 

from San landslides 

Andreas per square 

Unit___________________Predominant lithology_______________________Age Fault___kilometer

More than 200 landslides reported

Purisima Formation- Weakly consolidated sandstone and siltstone- SW 1.62

50 to 100 landslides reported

Preexisting landslide deposits--

Sandstone and shale).

... ... T

T

------ Q

sw 

sw

Both

NE

1.67 

1.68

n.d. 

2.18

25 to 49 landslides reported

Butano Sandstone—• 

Monterey FormationSandstone and siltstone—————————- 

Organic mudstone and sandy siltstoneSW 

SW

.44 

J9

10 to 24 landslides reported

Highland Way unit———— 

Lambert Shale—————•-—• 

Santa Cruz Mudstone— 

Aromas Sand-———————- 

Franciscan Complex—--— 

(sandstone and argillite). 

Quartz diorite————

Carbonaceous shale and minor sandstone—————— 

Organic mudstone——————————————————— 

Siliceous organic mudstone—————————————- 

Semiconsolidated fluvial clay, silt, sand, and gravelIntensely sheared sandstone and argillite———————

T 

T 

T

Q 

T,K

Quartz diorite, grading to granodiorite—————————————— K

NE 

SW 

SW

sw

NE 

SW

2.52

1.26

.19

.43

.60

.11

5 to 9 landslides reported

Alluvium—————-————— 

Lompico Sandstone————— 

Mount Chual unit—————— 

Santa Margarita SandstoneGranite and adamellite———• 

Older alluvium————----—•

Unconsolidated fluvial silt and sand with local clay and gravel— Q Both 

Calcareous arkosic sandstone——————————————————— T SW 

Mudstone and minor sandstone and conglomerate————————— T NE 

Friable arkosic sandstone————————————————————— T SW 

Granite and adamellite——————————————————————— K SW 

Unconsolidated gravel, sand, and silt——————————————— Q____Both

n.d. 

.42 

.83 

.15 

.88

n.d.

1 to 5 landslides reported

Nonmarine deposits-------—

Marine sandstone and shale— 

Sierra Azul unit————-----—

(conglomerate). 

Zayante Sandstone—————-- 

Franciscan Complex—-———

(basalt and tuff). 

Franciscan Complex—————

(metasandstone). 

Franciscan Complex—-———

(sheared rocks). 

Metasedimentary rocks—--—• 

Ultramaflc rocks—----———— 

Great Valley Sequence,

lower part———--————— 

Santa Clara Formation———— 

Locatclli Formation—————-- 

Basalt———————————

Fluvial sand and silt———————————————- 

Sandstone, silty sandstone, and silty mudstonePebbly to bouldery conglomerate————————

Arkosic sandstone with interbedded siltstone and conglomerate-- 

Basalt flow breccia with minor flows and tuff———————————

Complexly folded metasandstoneSheared and faulted rocks————

Q,T

T

K

T 

T,K

T,K 

T,K

SW

NE 

NE

SW

NE

NE 

NE

Pelitic schist and quartzite——————————————————————— 

Partially to completely serpentinized, sheared ultramafic rocks— 

Cherty shale, commonly sheared, and minor sandstone—————

Coarse fluvial gravel and fanglomerateMicaceous siltstone——————————— 

Basalt———————--———-————

Mz,Pz SW

J NE

K,J NE

Q,T NE

T SW

T SW

1.89 

.15

.25

26

.18

.10 

.05

.13 

.41

.71

.15 

J4 

.60
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C17

B

Figure 6.—Block diagrams showing idealized coherent (category 

II) landslides. A, Slump, consisting of single coherent block, 

showing movement with significant headward rotation on curved 

basal shear surface. B, Block slide, consisting of single coherent 

block, showing translational movement on planar basal shear 

surface. Adapted from Varnes (1978, fig. 2.1); copyright © U.S. 

National Academy of Sciences, used with permission.

Purisima Formation, Vaqueros Sandstone, or San Lorenzo 

Formation) (table 2). All of these units except the Vaqueros Sandstone were identified by Wieczorek and others 

(1985) as highly susceptible to slope failure in earthquakes. 

The more indurated Franciscan Complex rocks northeast 

of the San Andreas fault produced only one reported landslide of this type, whereas the intrusive rocks southwest 

of the fault produced none. Approximately 95 percent of 

these landslides occurred in the area southwest of the San 

Andreas fault.

At least 20 particularly large and complex category II 

landslides and landslide complexes with distinctive characteristics occurred in an area of about 30 km2, on and 

around Summit Ridge, immediately southwest of the rupture zone (fig. 1). These landslides are described by Keefer 

and others (this chapter), and various aspects of them are 

discussed by Cole and others (this chapter), Harp (this 

chapter), and Nolan and Weber (this chapter). These 

landslides occurred in an area that also contained the greatest abundance of earthquake-induced ground cracks 

(Spittler and Harp, 1990; Ponti and Wells, 1991), and the 

landslides and ground cracks in this area destroyed or 

damaged more than 100 residences (Spittler and Harp, 

1990).

The landslides and landslide complexes ranged in 

surface area from 1 to 85 ha. Drilling and other evidence 

indicated that the landslides were at least 27, and 

possibly more than 100 m, deep (William Cotton and Associates, Inc., 1990; Cole and others, 1991, this chapter; 

Keefer, 1991; Keefer and others, this chapter). The maximum estimated volume of an individual landslide was 

27xl06 m3 (see Keefer and others, this chapter). In addition to their large sizes, the landslides were characterized by

Figure 7.—Small slump in road fill 

near Bean Hill (see pi. 2 for location). Note arcuate, concavedownslope main scarp nearly 

parallel to long axis of road. 

Slumps in fill were most common 

type of category II landslides in the 

southern Santa Cruz Mountains. 

Photograph by David M. Peterson, 

U.S. Geological Survey.
CIS LANDSLIDES

highly irregular shapes and boundaries marked by discontinuous sets of ground cracks and other surficial features. 

These landslides occurred on moderate slopes, typically 

ranging from 12° to 30°, in an area underlain by poorly to 

moderately cemented Tertiary sandstone, siltstone, mudstone, and shale, varyingly weathered and sheared and 

covered by colluvial and residual soils, more than 20 m 

thick. The rocks are tightly folded and locally faulted, and 

most of the largest landslides were along the southwest 

flank of Summit Ridge, where the folding is tightest and 

faults and coseismic ground cracks most numerous (See 

Keefer and others, this chapter). Virtually all of the large 

landslides and landslide complexes also contained material from preexisting landslide deposits that had been identified before the 1989 Loma Prieta earthquake (CooperClark and Associates, 1975).

GROUND CRACKS

In addition to landslides, the earthquake generated 

numerous ground cracks throughout the southern Santa 

Cruz Mountains. During the general reconnaissance, 

such fissures were identified at about 220 localities 

(see pi. 1), and the intensive mapping in Forest of Nisene

Marks State Park identified about 80 more localities (Weber and Nolan, 1989). In addition, ground cracks were 

pervasive throughout the Summit Ridge area, as noted 

above.

Earthquake-induced ground cracks throughout the 

southern Santa Cruz Mountains evidently were caused 

by several different processes, including differential 

settlement, particularly in roadfills and embankments; local adjustments to the tectonic uplift that occurred southwest of the rupture zone (fig. 1); and incipient landsliding. 

Ground cracks interpreted as marking incipient landslides 

commonly occurred along or near ridgecrests and (or) 

upslope from either preexisting landslide deposits or 

earthquake-generated landslides. These ground cracks 

typically were also nearly parallel to slope contours, were 

linear or concave downslope in plan view, and exhibited 

displacements consistent with downslope movement. An 

example of a ground crack interpreted as an incipient landslide feature is shown in figure 13. Reported ground cracks 

were most common in artificial fill, preexisting landslide 

deposits, and the Tertiary sedimentary rocks southwest 

of the San Andreas fault, especially in the Purisima 

Formation, Vaqueros Sandstone, Lambert Shale, Butano 

Sandstone, and San Lorenzo Formation.

Figure 8.—Severely damaged 

house on Rebecca Drive landslide 

(see pi. 1 for location). Main scarp 

of landslide passes under house. 

Note ground cracks in foreground.
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C19

ESTIMATED TOTAL NUMBER OF LANDSLIDES IN 

THE SOUTHERN SANTA CRUZ MOUNTAINS

An extrapolation to the total number of landslides in 

the southern Santa Cruz Mountains may be obtained by 

comparing the landslide concentration in Forest of Nisene 

Marks State Park with that in adjacent areas, as determined by regional reconnaissance mapping. That study 

was designed to map all earthquake-induced landslides in 

the rugged, heavily vegetated terrain of the park through 

extensive traverses on foot (Weber and Nolan, 1989). The 

bedrock units in Forest of Nisene Marks State Park are 

the same as those that are extensive throughout adjacent 

areas and consist largely of folded Tertiary sedimentary 

rocks, including the Purisima Formation, Vaqueros Sandstone, San Lorenzo Formation, Butano Sandstone, Lambert Shale, and Zayante Sandstone.

Within the study area of 38.85 km2, intensive mapping 

identified 210 earthquake-induced landslides, and so the 

average landslide concentration was 5.4 landslides per 

square kilometer. Throughout the rest of the Laurel and 

Loma Prieta 7.5-minute topographic quadrangles, in which 

Forest of Nisene Marks is located, 559 landslides were

mapped by the general reconnaissance (see pis. 1, 2). The 

total area of these two quadrangles outside the park is 

269.3 km2 , and so the landslide concentration in this 

area is 2.1 landslides per square kilometer. Thus, the average, mapped landslide concentration within the park is 

2.6 times the average concentration throughout the rest of 

the two quadrangles. If (1) geologic conditions in the park 

were the same as throughout the rest of the two quadrangles and (2) the difference in landslide concentration 

is assumed to be due entirely to the more intensive mapping effort within the park, then the total number of landslides in the southern Santa Cruz Mountains outside the 

park may be about 2.6 times greater than the 1,050 actually mapped, or a total of 2,730. In addition to the 210 

landslides mapped inside the park, then, the total number 

of earthquake-generated landslides in the southern Santa 

Cruz Mountains may be as many as 3,000. Because Forest of Nisene Marks State Park surrounds the epicenter 

(see pi. 2) and rocks that predominate there are among 

the most susceptible to landsliding (table 1), this estimate 

probably represents an upper limit to the actual number of 

landslides that occurred in the southern Santa Cruz 

Mountains.

Figure 9.—Part of main scarp of 

Rebecca Drive landslide (see pi. 1 

for location). Notebook (circled) is 

20 by 27 cm. Approximate 

downslope displacement at this locality was 40 cm; maximum measured displacement across scarp 

was 57 cm.
C20 LANDSLIDES

LANDSLIDES IN OTHER AREAS

COASTAL LANDSLIDES

The earthquake also generated many landslides on the 

steep seacliffs, dunes, and terraces that border the Pacific 

Ocean. The San Francisco Bay-Monterey Bay region has 

an emergent, actively eroding coastline; typically, behind 

the beach, the coastal area contains a steep, rugged seacliff 

that in many places is topped with a gently sloping terrace 

surface. In some places, such as along the Big Sur coast, 

south of Monterey (fig. 1), the seacliff directly borders steep 

mountain slopes, hundreds of meters high. In a few other 

areas, such as along Sunset State Beach near the mouth of 

the Pajaro River (fig. 1), the beach is bordered by dunes.

Earthquake-induced landslides occurred along the coast 

from Marin County, north of San Francisco, to the Big 

Sur area south of Monterey, a distance of about 240 km

(fig. 14). Coastal landslides caused several million dollars 

in damage and killed one person (see Griggs and Plant, 

this chapter). The seacliff along much of this stretch of 

coast is composed largely of Tertiary sedimentary rocks, 

capped in many places by Quaternary terrace deposits, 

but also locally includes granitic or metamorphic rocks, 

Franciscan Complex materials, and semiconsolidated Quaternary sedimentary deposits.

The greatest concentration of coastal landslides was between Seabright State Beach in Santa Cruz and Sunset 

State Beach near the Santa Cruz-Monterey County line 

(fig. 14; Plant and Griggs, 1990; Sydnor and others, 1990; 

Griggs and Plant, this chapter). The predominant seacliff 

materials along this stretch of coast are (1) siltstone, mudstone, and fine-grained sandstone of the Purisima Formation, capped by unconsolidated terrace deposits; (2) weakly 

cemented, eolian Aromas Sand; and (3) Pleistocene dune 

deposits. This stretch of coast produced 80 mapped landV

5

_L

10 METERS

Figure 10.—Sketch map showing rock fall and block slide from end of ridge in Big Basin Redwoods State Park (see pi. 1 for location). Road curves 

around end of ridge in midslope position. Prominent scarps on ridgecrest above road (hachured on downdropped side, dashed where approximately 

located) showed as much as 6 cm of downslope displacement. Small arrows denote relative senses of motion on cracks in road, which typically 

trend perpendicular to long axis of roadway. Maximum local lateral displacement was 1 .5 cm. Large arrow indicates approximate, inferred direction 

of movement of slide. No compressional features were found in heavily vegetated area downslope from road. Rock and soil falls (shaded areas) 

occurred on cut slope above road.
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C21

Figure 11.—Part of main scarp of 

rock block slide on end of ridge 

near Laurel (see pi. 2 for location). 

Notebook is 20 by 27 cm.

Figure 12.—Lateral margin (left flank) of block slide near Laurel (see 

pi. 2 for location) defined by crack across road, showing left-lateral 

movement. Note small rock fall where crack intersects steep cut slope 

across road.

slides: 69 disrupted slides and falls and 11 coherent slides 

(Plant and Griggs, 1990; Manson and others, 1992). At 

least 13 of the disrupted slides and falls had volumes 

greater than 1,000 m3 , and another 22 had volumes greater 

than 100 m3 (see pi. 1; Manson and others, 1992).

North of Seabright State Beach landslides along the 

seacliffs were moderately common as far north as Lake 

Merced in San Francisco (fig. 14); many of these landslides also were relatively large (fig. 15), with reported 

volumes of as much as 40,000 m3 . The landslides along 

this stretch of coast were most common in Santa Cruz 

Mudstone, Purisima Formation, Pigeon Point Formation, 

and the terrace deposits that locally cap these rocks. In 

the Lake Merced area, landslides occurred in weakly 

cemented sand, silt, and clay of the Merced Formation— 

the same materials that produced several landslides in the 

1906 San Francisco earthquake (M^S.2-8.3) (Lawson, 

1908) and the 1957 Daly City earthquake (ML=5.3) 

(Bonilla, 1960).

North of the Lake Merced area, small landslides triggered by the 1989 Loma Prieta earthquake were observed 

as far north as Bolinas, in Marin County (fig. 14), and one 

large landslide reactivation was reported between Muir 

Beach and Stinson Beach (Seed and others, 1990). This 

reactivated landslide, approximately 300 m wide by 30 m 

deep, involved highly weathered and sheared sandstone 

and shale of the Franciscan Complex. Since previously 

activated as a result of severe storms in winter 1982, this 

landslide had been moving relatively continuously at a 

rate of less than 30 cm/yr. This rate increased significantly immediately after the earthquake, and between 2 

and 3 weeks after the earthquake, the coastal highway
C22 LANDSLIDES

Table 2.—Geologic units that produced category II landslides

[Predominant lithologies from Dibblee and Brabb (1978), Brabb and Dibblee (1979), Brabb and Pampeyan (1983), McLaughhn and others (1988, 1991), Brabb (1989), 

Clark and others (1989), and E.E. Brabb (oral commun.. 1992). Ages: K, Cretaceous; Q, Quaternary; T, Tertiary, n.d., not determined]

Unit Predominant lithology Age

Direction 

from San 

Andreas 

Fault

Average 

number of 

landslides 

per square 

kilometer

More than 50 landslides

Artificial fill Artificial fill, composed of various mixtures Q 

of clay, silt, sand, gravel, and coarser material.

Both n.d.

25 to 50 landslides

Preexisting landslide depositsPurisima Formation—————• 

Vaqueros Sandstone—————•

Colluvium and various rocks, displaced downslope Q Both

Weakly consolidated sandstone and siltstone——— T SW

Sandstone, shale, and mudstone————————— T SW

n.d. 

.25 

.60

10 to 24 landslides

San Lorenzo FormationAlluvium————————•Mudstone, sandstone, and shale————————— T SW 

Unconsolidated fluvial silt and sand with local clay Q Both 

and gravel——————————————————_________

.62 

n.d.

5 to 9 landslides

Santa Margarita SandstoneAromas Sand————————•

Butano Sandstone————- 

Sierra Azul unit————— 

__(sandstone and shale).

Friable arkosic sandstone————————————— T SW 

Semiconsolidated fluvial clay, silt, sand, gravel and Q SW

eolian sand.——————————————————— 

Sandstone and siltstone—————————————— T SW 

Sandstone and argillite——————————————— K NE

.56 

.64

.05 

.21

1 to 4 landslides

Highland Way unit—————

(sandstone and shale). 

Lambert Shale-———————• 

Monterey Formation————- 

Sierra Azul unit——————--

(conglomerate). 

Older alluvium———————— 

Nonmarine deposits————— 

Marine sandstone and shale— 

Lompico Sandstone————— 

Franciscan Complex————

(sheared rocks). 

Dune sands—————————— 

Santa Cruz Mudstone———• 

Zayante Sandstone—————

Carbonaceous shale and minor sandstone————— T NE

Organic mudstone——————————————————— T SW

Organic mudstone and sandy siltstone——————— T S W

Pebbly to bouldery conglomerate————————— K NE

Unconsolidated gravel, sand, and silt——————— Q Both

Fluvial sand and silt————————————— Q, T SW

Sandstone, silty sandstone, and silty mudstone—— T NE

Calcareous, arkosic sandstone——————————— T SW

Sheared and faulted rocks————————————— T, K NE

Dune sand————————————————— Q SW

Siliceous organic mudstone————————————— T SW

Arkosic sandstone with interbedded siltstone and T SW 

conglomerate—————————————————_________

.38

.20 

.05 

.17

n.d. 

.76 

.06 

.09 

.05

1.96 

.01 

.01

(California Highway 1) was closed at this locality pending repairs, which were estimated by the California Department of Transportation to involve the excavation of 

460,000 m3 of material (Seed and others, 1990).

South of Sunset State Beach (see pi. 1), along the 

Monterey County coast, small landslides were observed 

along the seacliffs as far south as the Big Sur area (figs. 

1, 14). The southernmost landslide, a small rock fall, was
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C23

reported from a locality known locally as Raining Rocks 

Point (Edwin L. Harp, oral commun., 1989), in an apparent reference to the common occurrence of rock falls there.

PERIPHERAL AREA

Outside the southern Santa Cruz Mountains and the 

coastal area, landslides occurred at widely separated localities throughout an additional 13,000 km2 of the San 

Francisco Bay-Monterey Bay region. Several dozen landslides, mostly small rock falls, rock slides, or soil slides 

less than 100 m3 in volume, were mapped. However, the 

mapping in this area was not as thorough as in the Santa 

Cruz Mountains, and extrapolating from available observations, several hundred landslides may actually have occurred in this area. The east limit of the landslide-affected 

area was determined as a line through reported landslide

localities near San Ardo, at Pacheco Pass, near Livermore, 

and through Mount Diablo (fig. 1). A few landslides in 

this peripheral area were rotational slumps.

One large, particularly destructive landslide was reported 

in central San Francisco, along Eighth Avenue between 

Ortega and Moraga Streets (Seed and others, 1990; J.J. 

Lienkaemper, unpub. data, 1990). The slope on which the 

landslide occurred was approximately 22°-38° and was 

composed of dry, loose to weakly cemented dune sand. 

The landslide formed a zone of cracks about 350 m long, 

with estimated maximum displacements of 10 to 20 cm. 

These cracks were near the top of the slope, which was 

about 20 to 35 m high. At least 30 residences were destroyed or heavily damaged by the landslide displacement 

(Seed and others, 1990). Indications of slope instability 

had been reported since the first houses were built there, 

approximately 50 years before the earthquake (Seed and 

others, 1990).

Figure 13.—Ridgecrest fissure and associated landslide along Robinwood Way (see 

pi. 2 for location). A, Ground crack, also 

breaking pavement and foundation slab. 

Crack is nearly linear, trends parallel to 

long axis of steep, high, narrow ridge, and 

is immediately upslope from slump shown 

in figure 135. Downslope direction is toward right. B, Rotational slump originating on edge of ridgecrest, immediately 

below ground crack shown in figure 13A. 

Position and orientation of crack and relation to slump indicate that crack is an incipient landslide feature.
C24 LANDSLIDES

DISCUSSION AND CONCLUSIONS

The 1989 Loma Prieta earthquake generated landslides 

throughout an area of about 15,000 km2 . Category I landslides occurred as far as 133 km from the epicenter and 

100 km from the rupture zone (fig. 1); category II landslides occurred as far as 111 km from the epicenter and 

99 km from the rupture zone. These area and maximum 

distance values are within the ranges of similar data compiled from other historical earthquakes (figs. 16-18). However, most of the landslides triggered by the 1989 Loma 

Prieta earthquake (75-85 percent of estimated total; see 

below) were within a comparatively small area of about 

2,000 km2 in the southern Santa Cruz Mountains and 

within 25 km of the fault zone. Similar concentrations of 

landslides relatively close to a fault rupture were noted in 

the 1980 Mammoth Lakes, Calif., earthquake (A/^6.1) 

and the 1980 Mount Diablo, Calif., earthquake (M^S.8),

two of the only three other earthquakes for which such 

data have been reported (Keefer and Wilson, 1989).

Estimates of the total number of landslides generated 

by the earthquake (exclusive of those caused by soil liquefaction) range from the approximately 1,500 actually 

mapped (1,280 in the southern Santa Cruz Mountains, 80 

along the coast, and several dozen elsewhere) to approximately 4,000 (3,000 in the southern Santa Cruz Mountains, as extrapolated from mapping in Forest of Nisene 

Marks State Park, and a few hundred elsewhere).

Studies of other historical earthquakes have shown that 

the total number of landslides generated by an earthquake 

of a given magnitude varies widely, depending on the 

geologic conditions in the affected region and the characteristics of the ground motion (Keefer, 1984; Keefer and 

Wilson, 1989). Keefer and Wilson, who presented data 

from eight previous earthquakes of M^=7.0-1.2, estimated 

that the numbers of landslides generated by those earthB

->,

^-,.,1? 1 Figure 13.—Continued.
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C25

37

Bolinas 

Muir 

Beach

Many

landslides Lake 

Merce

Several 

rockfalls

Several 

landslides

1989 

epicenter

Seabri 

State Beach

Loma Prieta

rupture zone

0 10 20 30 40 KILOMETERS

i i i i i

Raining

Rocks

Point

— 36

123 

I

Figure 14.—San Francisco Bay-Monterey Bay region, showing limits of coastal landslides and areas of abundant coastal landslides. Base map 

from Edwards and Batson (1990).
C26 LANDSLIDES

quakes ranged from less than 100 to 20,000. The number 

of landslides generated by the 1989 Loma Prieta earthquake falls in about the middle of this range. The high 

proportion of category I landslides generated by the earthquake (74 percent of those in the southern Santa Cruz 

Mountains and most of those elsewhere) is also consistent 

with data from other historical earthquakes (Keefer, 1984; 

Keefer and Wilson, 1989).

Most category I landslides generated by the 1989 Loma 

Prieta earthquake involved weakly to moderately cemented 

rocks that locally were deeply weathered, closely fractured, and (or) broken by conspicuous, throughgoing joints. 

A significant number also occurred in preexisting landslide deposits. Preexisting landslide deposits and rocks 

with such characteristics have been identified as highly to 

extremely highly susceptible to slope failure, on the basis

Figure 15.—Coastal landslide about 0.8 km 

south of mouth of San Gregorio Creek (see 

fig. 14 for location). Toe of deposit is approximately 60 m wide. Photograph by 

Mary M. Donate, U.S. Geological Survey.
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C27

of studies of other earthquakes (Keefer, 1984, 1993); many 

of the specific geologic units that produced abundant landslides had also been previously identified as particularly 

susceptible to failure (Wieczorek and others, 1985). Similarly, category II landslides were mostly in materials previously identified as highly susceptible to failure, especially 

uncemented to poorly cemented artificial fills, alluvium, 

and many of the same rock types that produced category I 

landslides (Keefer, 1984).

The distribution of landslides generated by the earthquake was significantly asymmetric with respect to the 

rupture zone (fig. 1): In the southern Santa Cruz Mountains, 85 percent of the category I landslides and 95 percent of the category II landslides were southwest of the 

San Andreas fault. This asymmetry probably is due mostly 

to differences in geologic conditions on the two sides 

of the fault: Relatively weakly consolidated rocks (primarily Tertiary sedimentary rocks) are significantly more 

abundant southwest of the fault than to the northeast. The 

southwest side of the fault, however, was also the hanging-wall block above a high-angle reverse fault and underwent tectonic ground-surface deformation as well;

thus, the asymmetry could also be partly due to this 

condition.

Outside the southern Santa Cruz Mountains, the slopes 

most susceptible to failure, as indicated by the number 

and size of landslides, were the seacliffs along the Pacific 

coast. This high susceptibility also conforms to findings 

in other historical earthquakes, including the 1906 San 

Francisco earthquake (Lawson, 1908).

Three aspects of the landsliding that were anomalous 

relative to other earthquakes were (1) the occurrence of 

large, complex landslides in the Summit Ridge area (fig. 

1); (2) the generation of large, deep-seated block slides 

from the ends of prominent ridges; and (3) the occurrence 

of ground cracks at hundreds of localities in the southern 

Santa Cruz Mountains. The large, complex landslides in 

the Summit Ridge area are described in more detail by 

Keefer and others (this chapter). These landslides were 

restricted to an area of pervasive ground cracking adjacent to the San Andreas fault, and their generation probably is directly related to that ground cracking. 

Descriptions of similar landslides are rare in reports on 

previous earthquakes (see Harp, this chapter), and so the

<^ 100,000

CO

LJ 

O

ZJ

CO

o

CO

uj 1 0,000

< o

>

CD LJ

o

LJ CO

LJ

a:

1000

100

10

1989

Loma Prieta 

earthquake

log A - M -3.46

56789 

EARTHQUAKE MAGNITUDE

10

Figure 16.—Area of landslides generated by 1989 Loma Prieta earthquake, 

A, as a function of earthquake magnitude, M, in comparison with other historical earthquakes with epicenters 

onshore (dots) and offshore (x's). Most 

data points and upper-bound curve 

(solid line) from Keefer (1984); additional data points and log-linear mean 

(dashed line) from Keefer and Wilson 

(1989).
C28 LANDSLIDES

Summit Ridge landslides are manifestations of a newly 

recognized, probably recurrent hazard in areas adjacent to 

some fault traces.

The generation of large, deep-seated block slides from 

the ends of prominent ridges was also an occurrence not 

recognized in reports on previous earthquakes, although 

previous studies had identified the ends of ridges as susceptible to shallow rock falls (for example, Harp and others, 1981). The block slides in the Santa Cruz Mountains 

were marked by the occurrence of small rock falls along 

their flanks, and other earthquake-induced rock falls in 

similar environments might also indicate the presence of 

larger, deeper seated landslides. The ends of prominent 

ridges in the Santa Cruz Mountains thus are also environments subject to a newly recognized, probably recurrent 

hazard.

Finally, the earthquake generated ground cracks at hundreds of localities in the southern Santa Cruz Mountains. 

Many of these ground cracks showed characteristics 

consistent with the formation of incipient landslides. In 

similar environments, the 1906 San Francisco earthquake,

which occurred when ground conditions were comparatively wet, generated abundant deep-seated landslides 

(Lawson, 1908). Under wetter conditions and (or) more 

severe ground shaking during the Loma Prieta earthquake, 

many of these zones of ground cracks probably would 

have developed into large, deep-seated landslides, and so 

the magnitude of landslide damage could have been much 

greater.

ACKNOWLEDGMENTS

We gratefully acknowledge the following people who 

mapped the landslides generated by the earthquake: A.G. 

Barrows, R.L. Baum, T.L. Bedrossian, K. Boyle, R. 

Brumbaugh, W.A. Bryant, J. Bussman, R.H. Campbell, 

M.C. Carey, S. Carson, K.H. Custis, M.M. Donato, G. 

Dunfield, R. Gibson, C. Giovannoni, T. Goddard, G.B. 

Griggs, S. Guiney, K.M. Haller, J. Hayes, R. Haltenhoff, 

E.L. Harp, D. Hope, J.K. Howard, P. Irvine, R.W. Jibson, 

D. Johnston, M. Jordan, P. Levine, H.H. Majmundar, M.A. 

McKittrick, D. Murray, J.M. Nolan, S.F. Personius, D.M.

1 000

o

Q_

O

ct:

o

in

Q

ID

<

1 00

10

1 f I

1 989

Loma Prieta 

ea rthq uake

0.1

5678 

EARTHQUAKE MAGNITUDE

Figure 17.—Maximum distance of landslides from 

epicenter of 1989 Loma Prieta earthquake as a function of earthquake magnitude, in comparison with 

other historical earthquakes. A, Disrupted slides and 

falls (category I landslides). B, Coherent slides (category II landslides). Most data points and upperbound lines from Keefer (1984); additional data 

points from Wieczorek and Keefer (1987) and Harp 

and Keefer (1990).
REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C29

Peterson, N.G. Plant, R. Powers, J. Rigby, J.P. Schlosser, 

K.M. Schmidt, R.L. Schuster, I.E. Slosson, R. SmithEvernden, J.A. Sowma, I.E. Spittler, J.G. Staude, L.R. 

Stevens, R.H. Sydnor, S. Tan, J. Thornberg, J.C. Tinsley, 

J. Treiman, J. Van Velsor, B.V. Vassil, D.L. Wagner, 

G.E. Weber, G.F. Wieczorek, CJ. Wills, and R.C. Wilson. Sarah Christian, Monique Jaasma, Donna Knifong, 

and Carl Wentworth helped with the analysis of digital 

data. The manuscript was improved by the thoughtful reviews of Gerald Wieczorek and Raymond Wilson.

REFERENCES CITED

Alger, C.S., and Brabb, E.E., 1985, Bibliography of United States landslide maps and reports: U.S. Geological Survey Open-File Report 

85-585, 119 p.

Aydin, Atilla, Johnson, A.M., and Fleming, R.W., in press, Coseismic 

right-lateral and left-lateral surface rupture and landsliding along

the San Andreas and Sargent fault zones during the earthquake, in 

Ponti, D.J., ed., The Loma Prieta, California, earthquake of October 

17, 1989—ground ruptures: U.S. Geological Survey Professional 

Paper 1551-D.

Bonilla, M.G., 1960, Landslides in the San Francisco south quadrangle, 

California: U.S. Geological Survey Open-File Report, 44 p.

Brabb, E.E., 1989, Geologic Map of Santa Cruz County, California: 

U.S. Geological Survey Miscellaneous Investigations Series Map 

1-1905, scale 1:62,500.

Brabb, E.E., and Dibblee, T.W., Jr., 1979, Preliminary geologic map of 

the Castle Rock Ridge quadrangle, Santa Cruz and Santa Clara 

Counties, California: U.S. Geological Survey Open-File Report 79- 

659, scale 1:24,000.

Brabb, E.E., and Pampeyan, E.H., compilers, 1983, Geological map of 

San Mateo County, California: U.S. Geological Survey Miscellaneous Investigations Series Map I-1257-A, scale 1:62,500.

Brabb, E.E., Pampeyan, E.H., and Bonilla, M.G., 1972, Landslide susceptibility in San Mateo County, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-360, scale 1:62,500.

Brown, W.M., III, 1988, Historical setting of the storm; perspectives on 

population, development, and damaging rainstorms in the San Francisco Bay region, chap. 1 of Ellen, S.D., and Wieczorek, G.F., eds.,

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REGIONAL DISTRIBUTION AND CHARACTERISTICS OF LANDSLIDES GENERATED BY THE EARTHQUAKE C31

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evaluation of analysis methods: final technical report to U.S. Geological Survey under contract 14-08,001-G1860, 33 p.

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1:24,000, 3 sheets.

Edwards, Kathleen, and Batson, R.M., 1990, Experimental digital 

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sheets.

Ellen, S.D., and Wieczorek, G.F., eds., 1988, Landslides, floods, and 

marine effects of the storm of January 3-5, 1982, in the San Francisco Bay region, California: U.S. Geological Survey Professional 

Paper 1434, 310 p.

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Introduction, in Ellen, S.D., and Wieczorek, G.F., eds., 1988, Landslides, floods, and marine effects of the storm of January 3-5, 1982, 

in the San Francisco Bay region, California: U.S. Geological Survey Professional Paper 1434, p. 1-5.

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California, earthquake and its effects; introduction to the special 

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California, earthquake sequence of May 2, 1983: U.S. Geological 

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the February 4, 1976, Guatemala earthquake: U.S. Geological Survey Professional Paper 1204-A, 35 p.

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C32 LANDSLIDES

California Division of Mines and Geology Special Publication 104, 

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THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17, 1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY 

INDUCED BY THE EARTHQUAKE

By Gary B. Griggs, 

University of California, Santa Cruz;

and

Nathaniel Plant, 

Oregon State University

CONTENTS

Abstract————————————————————————————- 

Introduction ——————————————————————————- 

The 1989 Loma Prieta earthquake ———————————

Coastal-bluff failures in northern Monterey Bay ——————

Summary and implications for land-use planning—————

Recommendations ——————————————————————- 

References cited ———————————————————————-

C33 

33 

37 

37 

41 

43 

50

ABSTRACT

The 1989 Loma Prieta earthquake caused bluff failures 

along 240 km of coastline between Marin County and the 

Big Sur coast of Monterey County, in various rock types 

and under varying slope conditions. Peak accelerations 

of 0.54 g horizontal and 0.60 g vertical were measured 10 

km from the epicenter on the coast at Capitola. Coastalbluff failures were most common nearest the epicenter, 

where extensive blufftop cracking and collapse led ultimately to the demolition of three homes and six apartment units. One death and several million dollars in 

damage were attributed to coastal landslides. Three 

types of coastal bluff failures were documented: (1) block 

falls in well-consolidated sedimentary rocks, (2) translational slides in more friable sandstone, and (3) sandflows 

in Quaternary dune deposits. Long-term stability of the 

central-coast bluffs, which underwent tensional cracking 

as far as 10 m inland from the bluff edge, is uncertain. 

The potential for earthquake-induced bluff failures exists 

along virtually the entire coastline of California, owing 

to the presence of active faults throughout the region. To 

date, however, this hazard has not been widely recognized.

INTRODUCTION

At least 86 percent of California's 1,750 km of shoreline is actively eroding, while 80 percent of the State's 

population now live within 50 km of the coast. The conflict between the population growth and the inherent instability of much of the shoreline has become increasing 

apparent in recent years, with $150 million in coastal storm 

damage during the decade 1978-88 alone.

Coastal erosion along the California shoreline is dominated by episodic events, typically the simultaneous occurrence of large storm waves and high tides. Although 

coastal-bluff erosion is commonly perceived as primarily 

a wave-induced phenomenon, seacliff failure is also a subaerial process. Landslides, slumps, and debris flows occur 

in response to elevated pore pressures resulting from either intense rainfall or elevated ground-water conditions, 

or overloading or oversteepening of a coastal bluff. Another significant process affecting coastal-cliff retreat is 

seismic shaking during large earthquakes. Although 

coastal-protection structures have slowed erosion of the 

coastline by wave action in some areas, failures continue 

to take place along both protected and unprotected bluffs, 

owing to terrestrial processes (fig. 1).

The potential for earthquakes that can affect coastal 

bluffs is significant along the entire length of the State's 

coastline (fig. 2). Of principal concern in northern California is the active San Andreas fault system, which follows the shoreline from Cape Mendocino to the San 

Francisco peninsula. Along the central California coast, 

the active San Gregorio-Hosgri fault extends from north 

of San Francisco southwesterly along the shoreline of San 

Mateo and northern Santa Cruz counties, across Monterey 

Bay, and along the coast of Monterey, San Luis Obispo, 

and northern Santa Barbara Counties. The Santa Ynez,

C33
C34 LANDSLIDES

More Ranch, Mesa, Arroyo Parida, and Oakridge faults 

extend along or intersect the shoreline between Point Conception and Ventura. The Malibu Coast, Palos Verdes, 

and Newport-Inglewood faults traverse or intersect the 

shoreline of Los Angeles County and the northern part of 

Orange County. Finally, the Rose Canyon Fault parallels 

the coast of much of San Diego County. No part of the 

coastline of California is more than 20 or 30 km from an 

active fault (Jennings, 1975), and most areas are considerably closer.

The effects of large earthquakes on coastal-bluff stability in central and northern California have generally been 

poorly documented, partly because of the infrequency of 

these events, the relatively short historical record in the 

region, the relatively low population densities along this 

part of the California coast at the time of previous large 

earthquakes, and, therefore, the absence of observations.

During the October 8, 1865, Santa Cruz Mountains earthquake (A/-6.5), the Santa Cruz Sentinel reported that "below Soquel the high cliffs crumbled into the sea" and "a 

continuous cloud of dust rose along the cliffs between 

Castro's Landing [now called Rio Del Mar] and Santa 

Cruz." One of us (N.P.) was on the beach at Rio Del Mar 

during the 1989 Loma Prieta earthquake and noted the 

same phenomenon: A continuous cloud of dust rose along 

the coastal bluffs as loose surficial material broke loose 

and cascaded downslope. During the great 1906 San Francisco earthquake (M=8.3), "much earth fell from bluffs 

near the town [of Capitola]" (Lawson, 1908). Lawson 

documented bluff failures in 1906 extending from Capitola 

on the south to Eureka on the north, a distance of about 

500 km. These few accounts constitute the extent of the 

readily available, published information describing historical earthquake-induced bluff failures along the

Figure 1.—Coastal-bluff failure in Rio Del Mar (see 

fig. 4 for location) during a 100-yr 24-hour rainstorm in January 1982
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C35

Monterey Bay shoreline and lead to the following conclu- 2

sions:

1. Large earthquakes apparently can cause instantaneous cliff 

retreat and weaken seacliffs through seismic shaking and 

the formation of cracks and fissures, which increase the 

seacliffs' susceptibility to failure. In addition to the hazards 

to structures or utilities at the top of a cliff, a risk also exists 

at the base of the seacliff, owing to the downslope movement, impact, and deposition of rock or soil from the 

adjacent cliff.

The intensity of development along the outer edge of the 

present seacliff along most of the California coast (fig. 3), 

relative to the location of coastal fault zones (fig. 2), 

indicates that the potential for cliff failures from seismic 

shaking was probably not considered when geologic or 

geotechnical evaluations were carried out, permits were 

issued, and construction took place. 

A recent statewide evaluation of the coastal-bluff setbacks required by local governments for new construction 

indicates that a wide variety of approaches to this hazard

COAST RANGE THRUST

CALIFORNIA

RODGERS CREEK-HEALDSBURG

SAN GREGORIO-HOSGRI 

NACIMIENTO SAN ANDREAS

50 100 OAKRIDGE 

MALIBU COAST

PALOS VERGES

NEWPORT-INGLEWOOD PACIFIC OCEAN

ROSE CANYON

Figure 2.—California, showing locations of active faults (irregular thin lines, dashed where approximately located, dotted 

where concealed) and thrust faults (sawteeth on upper plate) with potential to affect coastal bluffs.
C36 LANDSLIDES

exist (Griggs and others, 1991). Some municipalities require a complete geologic report, written according to specific guidelines and subject to peer review, and a minimum 

setback of 16 to 48 m (50-150 ft) but also use a 50- to 75- 

year structural lifetime to calculate a precise setback. At 

the other end of the spectrum, one coastal city has a minimum 6-m (20-ft) setback and does not require a geologic 

report.

Several approaches have been used to analyze the potential for bluff failures during the lifetime of a proposed 

new development. A standard approach is to use aerial 

photography and maps to determine an average long-term 

cliff-recession rate, typically based on 25 to 50 years of 

data. Using an average erosional rate in combination with 

a structural lifetime, a safe setback distance can be established. The assumption with this approach is that the 

timeframe covered by available maps and photographs is 

representative of long-term conditions (that is, rainfall, 

storm frequency, sea level, and so on). The longer the 

data base, the more representative the calculated erosional 

rates should be.

Along the central California coast, all of the recent (past 

150 years) historical earthquakes large enough to have 

potentially induced bluff failures took place before aerial 

photography. Evaluations of bluff stability under seismic

conditions, therefore, have had to rely on a combination 

of historical accounts, interpretation of more recent aerial 

photographs, existing bluff topography, and slope-stability analyses. These analyses have typically incorporated 

seacliff topography, strength parameters from blufftop or 

bluff-face borings, and peak accelerations based on a maximum probable earthquake. Slope-stability analysis is a 

common geotechnical tool; disruption of soil fabric during sampling, lateral and vertical variations in the strength 

of soil or rock, uncertainties in the peak horizontal accelerations at the site, and the role of ground water are important variables that affect the analysis. Whereas the 

strength values of the bedrock and overlying soils are 

used in the analysis, failure typically occurs along joint 

surfaces or other planes of weakness, such that the strength 

of the materials themselves may not be the critical factor 

affecting failure.

A key question in the evaluation of the potential for 

seismically induced bluff failures along this stretch of coast 

is whether historical failures are recognizable on older 

aerial photographs. For example, if deep-seated bluff failure took place during the 1906 San Francisco earthquake, 

should geomorphic evidence still be present 22 years later 

(1928) when the first aerial photographs were taken? The 

Monterey Bay region also sustained two M=6 earthquakes

Figure 3.—Intense blufftop development along coast of San Diego County, showing use of seawalls and gunnite to reduce bluff failure.
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C37

in October 1926 (presumably located on the offshore San 

Gregorio-Hosgri fault zone), just 2 years before these first 

aerial photographs. Site-specific analysis of several coastalbluff sites around the northern margin of Monterey Bay 

through interpretation of aerial photographs indicates no 

evidence of large-scale or deep-seated failures. Although 

shallow or surficial failures have taken place repeatedly, 

primarily during winter months of prolonged or intense 

rainfall (fig. 1), deep-seated or large-scale landsliding or 

slumping does not appear to have been common, at least 

during earthquakes in the 20th century.

THE 1989 LOMA PRIETA EARTHQUAKE

The 1989 Loma Prieta earthquake (M=7.1) ruptured a 

40-km-long segment of the San Andreas fault zone in the 

Santa Cruz Mountains about 85 km south of San Francisco (fig. 4). This earthquake, which was the largest to 

strike the central California coast since 1906, was felt 

over an area of 1 million km2, from Los Angeles in the 

south to the California-Oregon State line in the north. 

Within 15 to 20 s, the earthquake resulted in 67 known 

deaths, 3,757 injuries, more than 18,000 homes and 2,500 

businesses damaged, and more than 12,000 people homeless, as well as more than $6 billion in damage (U.S. 

Geological Survey, 1989). The earthquake ruptured a segment of the San Andreas fault beneath the Santa Cruz 

Mountains that had been recognized as having the greatest probability (30 percent for the next 30 years) for producing an M=6.5-7 earthquake of any fault segment north 

of the Mojave Desert in southern California (Plafker and 

Galloway, 1989).

The epicenter was about 8 km from the nearest segment of coastline. The most populous, densely developed 

section of coastal bluffs in Santa Cruz County sustained 

the most intense shaking. Four strong-motion-recording 

stations operated by the California Division of Mines and 

Geology provided quantitative records of shaking (Sydnor 

and others, 1990). At Capitola (fig. 4), about 13.5 km 

from the epicenter, the peak horizontal ground acceleration was 0.60 g, and the peak vertical acceleration was 

0.54 g. These peak accelerations offer a unique opportunity to compare actual peak ground-motion data with seismic coefficients typically used in slope-stability analyses 

by geotechnical engineers.

Coastal-bluff failures resulting from the earthquake occurred along approximately 240 km of coastline between 

the town of Bolinas in the north to the Big Sur coast in the 

south, in various rock types and under varying slope conditions (Plant and Griggs, 1989). Failures were most prevalent closest to the epicenter between Seabright Beach to the 

northwest and Sunset Beach to the southeast (figs. 4, 5).

The bluff failures at Daly City, south of San Francisco 

(fig. 6), are significant because of their size, the proximity of many homes precariously close to the cliff edge, 

and the epicentral distance of about 80 km at this site. 

The materials exposed in the 100- to 150-m-high bluffs 

consist of weakly cemented sandstone and siltstone. On 

the basis of the strong-motion data recorded within 5 km 

of the site, peak accelerations were about 0.10 to 0.14 g. 

Considering the height of the bluff, the failure was relatively thin (4.5-6 m). The 1957 Daly City earthquake 

(M=5.7), which generated similar accelerations, also produced significant landsliding along these bluffs, as well 

as apparent tension cracks landward of the crest of the 

bluffs (Sitar, 1990). However, the blufftop was not developed at that time.

The peninsular section of the San Andreas fault lies 

only a few hundred meters inland from these steep landslide-prone but intensively developed coastal cliffs. The 

most recent assessment of earthquake probabilities in the 

San Francisco Bay region (Working Group on California 

Earthquake Probabilities, 1990) concluded that there is a 

67-percent probability for at least one earthquake of M>7 

between 1990 and 2020. One of the fault segments most 

likely to rupture is the peninsular section of the San Andreas fault (fig. 4), which has been assessed as having a 

23-percent probability of rupturing within the next 30 

years. Depending on which of the San Francisco Bay region faults rupture, the intensity of future ground shaking 

within the area of figure 5 is estimated at 2 to 7 times 

greater than that sustained during the 1989 Loma Prieta 

earthquake (U.S. Geological Survey, 1990). The coastal 

bluffs along the Daly City shoreline probably will be severely disrupted during such an event.

COASTAL-BLUFF FAILURES IN 

NORTHERN MONTEREY BAY

The type and extent of failures that took place along 

the seacliffs and bluffs of northern Monterey Bay were 

closely tied to the lithologies of the cliff-forming materials. Three general types of seismically induced landslides 

occurred in the coastal bluffs: rockfalls, translational slides, 

and sandflows (figs. 7-9). Sedimentary bedrock cliffs are 

particularly susceptible to earthquake-induced landslides 

where they are closely jointed or where the bedrock is 

unconsolidated or weakly cemented, and where the base 

of the cliff is unprotected from wave attack.

Between Seabright Beach and Capitola (fig. 4), seacliffs 

are cut into a marine terrace that is as much as 25 m high. 

The lower part of the cliff consists of thick-bedded to 

massive siltstone, mudstone, and very fine grained sandstone of the Purisima Formation. Even though this bedrock is well indurated, it is extensively jointed and 

susceptible to failure and toppling along joint surfaces. 

The coastline in this area parallels the most pronounced
C38 LANDSLIDES

set of joints, which typically dip steeply seaward. Overlying the sedimentary bedrock is as much as 6 m of unconsolidated terrace deposits consisting of marine cobbles, 

sand, and, commonly, a more cohesive, clay-rich soil horizon. The near-vertical seacliffs in this area are actively 

eroding where they are not protected by seawalls or riprap.

Seismic shaking during the earthquake initiated numerous rockfalls and block slides along this section of the 

coast (fig. 10). Undercut bedrock and fractured promontories toppled downslope, blufftop terrace deposits collapsed, 

and weakened bedrock failed. Horizontal and downslope 

separation along joint surfaces allowed large blocks to 

separate from more intact rock (fig. 7). The size of these 

failures was a function of joint spacing and orientation, 

frictional or cohesive strength along the joint surface, cliff 

height, and toe support. High cliffs with widely spaced, 

subvertical joints and poor toe support sustained the largest instantaneous and incipient failures in weakened or 

undercut sedimentary rocks.

Aftershock Zone

EPICENTER

Capita la 

Santa Cruz

\ Seabright Beach

Rio Del Mar 

Sunset Beach x/

Monterey Bay

PACIFIC OCEAN

Figure 4.—San Francisco Bay-Monterey Bay region, Calif., showing locations of epicenter of 1989 Loma Prieta earthquake, aftershock zone, major faults (heavy lines, dashed where approximate, dotted where concealed), and coastal cities.
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C39

Damage to private property and public infrastructure 

was minimal along the cliffs in this area, except for those 

immediately east of Capitola (fig. 4). An apartment complex that was already overhanging the seacliff was additionally undermined as both bedrock and terrace deposits 

toppled to the beach below (fig. 11). Earthquake-induced 

failure led to partial loss of concrete caisson support and 

cracking of the foundation slab and perimeter wall of six 

apartments. Tension cracks were visible as far as 10 m 

inland of the cliff edge.

The apartment shown in figure 11 was originally built 

within a few meters of the bluff edge in 1967. Long-term 

(60 year) cliff-erosion rates at this site average 30 cm/yr 

(Griggs and Johnson, 1979). A cantilevered foundation 

was initially used to provide additional support in the 

event of the presumably anticipated foundation undermining due to the ongoing seacliff erosion. Continued bluff 

failure led to emplacement of a concrete caisson support 

system during the early 1980's. Plans to resupport the 

foundation and protect the base of the eroding bluff were 

in the design-review process at the time of the earthquake 

but were subsequently reconsidered by the project engineers and geologist. Owing to the extensive blufftop cracking, the loss of foundation support, and the cracking of 

walls and foundation, as well as the threat to beach users 

at the base of the bluff, the protection and foundation 

plans were no longer deemed adequate, and six threatened 

apartment units were demolished by the Federal Emergency Management Agency at a public cost of $85,000.

The longest continuous area of coastal landsliding was 

from east of Capitola to Rio Del Mar (figs. 4, 12, 13), 

where sandstone of the Purisima Formation supports 30- 

m-high cliffs. About 5 m of terrace deposits consisting of 

poorly consolidated sand and gravel and a soil horizon 

caps the Purisima in this area. Most of the base of the 

cliff has been isolated from wave erosion by seawalls, 

which protect the beachfront development. The steep upper parts of the cliffs fail periodically during periods of 

sustained and (or) intense rainfall, for example, during 

the severe winter of 1982 (fig. 1; Griggs, 1982).

Translational failure along joints or weathering surfaces 

produced many large (30-60 m wide) concave upward 

scarps in the upper 12 to 15 m of the cliff in this area 

(figs. 8, 12). These scarps tended to cut vertically through 

the terrace deposits and then flatten to parallel the slope 

as they approached the Purisima Formation. Deep tension 

cracks cut through the upper terrace deposits and soils as 

far as 6 m landward of many of the scarps; this distance 

tended to increase with the size of the scarp and the height 

of the cliff. Coherent blocks overlying loose soil disintegrated as they cascaded down the cliff face. Talus piles, 

as much as 12 m high, blocked access to homes (fig. 14) 

and partly buried automobiles. Two blufftop homes ultimately were demolished because of such failures, which 

produced deep tension cracks through the building sites

(fig. 15). Both homesites were on narrow, steep-sided peninsulas along the crest of the coastal bluff, locations where 

seismic shaking appears to have been particularly severe.

In the lower 10 m of the cliff, fracturing along intersecting joints undercut the sandstone. Blocks 1 m thick 

and as much as 3 m high broke up easily as they moved 

downslope and formed talus cones at the base of the bluffs. 

Though fractured and partly detached from the cliff, other 

blocks did not fall; these blocks remain as unstable areas.

From Rio Del Mar southward (figs. 4, 16), semiconsolidated eolian and fluvial sand of the Pleistocene Aromas Sand forms 25- to 40-m-high steep cliffs. The top 2 

to 5 m is more cohesive than the underlying, partly consolidated, weakly cemented sand. Seawalls protect much 

of the base of the bluff and the backbeach development 

from wave erosion. Interpretation of aerial photographs, 

however, indicates periodic shallow failure or sloughing 

of the upper parts of the bluffs in response to periods of 

intense or prolonged rainfall. These failures commonly 

initiated in the blufftop terrace deposits but typically extended to the base of the bluff.

Several types of earthquake-induced failures were typical along this stretch of coastal bluffs. The first type consisted of shallow (1-3 m deep), large (max 100 m wide) 

translational slides (figs. 8, 16, 17). At one site, such a 

failure undermined 5 to 10 m of the clifftop, leading to 

collapse and toppling of large cohesive blocks of soil. 

These detached masses rolled downslope and blocked access to condominiums built on the talus slope at the base 

of the bluff (fig. 18). This same failure also undermined a 

blufftop home that was subsequently demolished.

Another type of failure occurred in weakly consolidated 

Pleistocene dune sand near Sunset Beach (fig. 4) that has 

been partially stabilized by vegetation. Homes have been 

built on the edge of this ancient, partially vegetated dune 

complex. Slope failure took the form of shallow (max 1 m 

thick), dry sandflows that initiated well below the blufftop 

(figs. 9, 19). A vertical scarp cut through a thin, cohesive 

soil layer to a failure surface that paralleled the bluff face. 

Loose sand or small blocks of more cohesive material, 

commonly held together by roots, slid downslope along 

this surface and then ramped over the less steep talus 

deposits at the base of the bluff. The foundations of several homes were threatened, and one home was relocated.

Despite these recent failures, building permits are still 

being sought for new homes that would be sited on the 

talus slopes at the base of bluffs in the Rio Del Mar area 

(figs. 4, 20). The geologic hazards consistently identified 

at these sites in consulting reports are slope failure induced by excess water and by seismic shaking. The 1989 

Loma Prieta earthquake provided an opportunity to check 

the distribution and characteristics of seismically induced 

failures. Eyewitness accounts of the bluffs during the earthquake by one of us (N.P.) noted virtually instantaneous, 

shallow slope failures along the bluffs in this area during
C40 LANDSLIDES

/ Aptos Creek 

Ssl':-'.' . / Rio Del Mar

New Brighton State Beach

Seacliff State Beach

Figure 5.—North shoreline of Monterey Bay, Calif., showing locations of coastal-bluff failures (dots) induced by 1989 Loma Prieta earthquake.

the main shock. The mitigation measures proposed for 

downslope homes to reduce the risk of collapse involve 

upgrading standards for rear walls and roofs so as to absorb or deflect the loading associated with failure.

It is somewhat puzzling that the cliffs fronting northern 

Monterey Bay in the Rio Del Mar area (fig. 4), though 

consisting of only weakly cemented sand, are able to stand 

at such steep slopes. Past bluff failures, whether from 

intense rainfall or seismic shaking, have consistently produced relatively shallow slides. Large, deep-seated or rotational slides do not appear to have taken place in this 

geologic setting, despite its rainfall and seismic history.

Recent laboratory study and fieldwork indicate that cemented sand and gravel derive their strength from a combination of their interlocking grain structure and varying 

amount of cementation (Sitar, 1990). Seismic failures of 

steep slopes in natural sand and gravel deposits apparently initiate by tensile splitting in the upper parts of the 

slopes (Sitar, 1990), followed by toppling of the upper 

blocks or by shear failure of the lower slopes. Both types 

of failure lead to relatively shallow slides, rarely involving more than 2 to 5 m of material. Both natural and 

manmade slopes in cemented sand and gravel generally 

are quite steep and tend to perform surprisingly well when 

subjected to seismic loading. A problem does exist in

Figure 6.—Large coastal landslide near Daly City (fig. 4) resulting from 

1989 Loma Prieta earthquake. Note proximity of clifftop houses to 

headscarp.
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C41

evaluating the inplace strength of these deposits because 

conventional sampling techniques tend to break down the 

structure of cemented sand and are generally unsuitable 

for gravel. Sitar (1990) suggested alternative approaches, 

such as inplace freezing, block sampling, or onsite testing.

Although the lithology of the seacliff materials controlled the mode of seismic failure in most places, steepsided promontories and narrow peninsulas or ridges along 

the coast consistently failed most extensively throughout 

the study area. Scientists also noted this effect in the Santa 

Cruz Mountains as a result of the 1989 Loma Prieta earthquake (Plafker and Galloway, 1989) and in other areas 

where large earthquakes have occurred in the past (Harp 

and others, 1981). In these other, noncoastal areas, the 

topography reflected seismic waves; constructive interference amplified the earthquake motion and caused rock or 

soil failures due to the intensified dynamic stresses. However, the size of a structure that can reflect incident waves 

must be approximately one wavelength (Harp and others, 

1981). The abundance of failures along the coastal promontories and narrow peninsulas is probably attributable to 

the absence of lateral support at these bluff faces during 

the intense shaking.

SUMMARY AND IMPLICATIONS FOR 

LAND-USE PLANNING

Seismically induced coastal-bluff failures were common as far as 80 km from the epicenter of the 1989 Loma 

Prieta earthquake. The failures in developed areas of the 

coast posed risks to development on the blufftop, as well 

as to private and public development on the beach below. 

One death occurred on a beach north of Santa Cruz when 

a section of weak bedrock collapsed onto a sunbather.

Quat,

Block fall from earthquake

Figure 7.—Geologic structure and style of typical rockfall in seacliffs underlain by well-jointed siltstone of the Purisima 

Formation. Note undercut cliff and tension cracks in Pleistocene terrace deposits.
C42 LANDSLIDES

The top, slope, and base of seacliffs along northern 

Monterey Bay all proved to be unstable during the earthquake. At the clifftops, several structures were immediately damaged when cliff failure undermined their 

foundations. At several sites, cracks formed 3 to 10 m 

inland from the cliff edge and, in some places, cut through 

foundations. Cracking and bluff failure were sufficiently 

damaging at four blufftop sites that three homes and six 

apartment units had to be demolished. The blufftop cracks 

exposed at these sites appear to represent incipient slides 

or detachment surfaces that may fail under renewed seismic shaking or elevated pore pressures during prolonged 

rainfall. We believe that the extent of this cracking can be 

used as a reasonable indicator of expected failures during 

seismic shaking in other California coastal bluffs consisting of similar materials.

Hazards to houses at beach level resulted from failure 

of the cliffs above them. Several houses in the Rio Del

Mar area (fig. 4) were either damaged or posted as unsafe 

and evacuated when the cliffs behind them failed. The 

houses most susceptible to damage were those built closest to the base of the bluffs. In addition, the bluff areas 

where the most extensive failures occurred during the 

earthquake, and where homes were most heavily damaged, were those that had failed most frequently during 

past episodes of intense rainfall, as evidenced in aerial 

photographs. Along the north shoreline of Monterey Bay, 

the areas of most frequent failure were the steep bluffs of 

Purisima Formation from Capitola to Rio Del Mar and the 

bluffs consisting of weakly cemented Aromas Sand that 

extend down the coast to Sunset Beach. Owing to the 

high value of ocean-front or ocean-view property, new 

homes are still being proposed for construction on the 

talus slopes at the base of the in the Rio Del Mar area 

(fig. 20), with rear walls and roofs engineered to absorb 

the impact of expected future slope failures.

cracking from earthquake

Figure 8.—Geologic structure and typical debris slide in seacliffs underlain by sandstone of the Purisima Formation.
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C43

The 1989 Loma Prieta earthquake, which was the largest earthquake to strike northern California in 83 years, 

initiated slope failure along approximately 240 km of coastline. The slope failures induced by seismic shaking provided the geological, geotechnical, and land-use-regulatory 

community with a unique opportunity to observe the effects of a large earthquake on coastal-bluff stability. The 

potential hazards of seismic shaking to ocean-front and 

seacliff construction have been well documented. The close 

proximity of virtually the entire 1,750 km of the California coastline to active faults ensures that seismic shaking 

can reasonably be expected at any ocean-front site during 

the lifetime of any structure built today.

RECOMMENDATIONS

We need to both anticipate and plan for the inevitable 

earthquake-induced landslides that will take place along 

the State's coastal bluffs during future large earthquakes. 

Failure during the 1989 Loma Prieta earthquake occurred

not only along bluffs that were actively eroding but also 

along bluffs that were protected from marine erosion.

• Encroachment of public or private structures and utilities 

in the coastal zone should be permitted only after a complete assessment of geologic hazards and risks.

• We need a regional understanding of long-term bluff or 

shoreline erosional rates and their relation to both coastal 

processes and regional seismicity.

• Although the main shock of the earthquake caused widespread instantaneous failure, erosional and failure rates in 

earthquake-weakened bedrock and soils may increase as 

future winter storms and rain batter the coastline.

• Current and future development proposals along the coast 

should consider the evidence and experience gained 

from this earthquake to evaluate both building setbacks 

and the feasibility of hazard-mitigation measures. Reconstruction on earthquake-damaged sites should proceed 

only after extensive geologic and geotechnical investigations conclude that appropriate mitigation measures 

have been taken to reduce the future risks of damage to an 

acceptable level.

' Tension cracks

Figure 9.—Dry sandflow in uncemented eolian deposits. Note tension cracks in overlying, more cohesive surficial soils.
C44 LANDSLIDES

scarp (headscarp marked by hachures)-

debris path ~

debris -

0.1 0.2 Q3 0.4 0.5

Figure 10.—Plan view and longitudinal section showing locations and sizes of earthquake-induced seacliff failures and geology between Seabright 

Beach and Soquel Point (A) and between Soquel Point and New Brighton State Beach (B) (see fig. 5 for locations). Units: Qm, marine-terrace deposits 

(Quaternary); Tp, Purisima Formation (late Miocene and Pliocene).

Figure 11 .—Collapse of terrace deposits and underlying jointed bedrock 

of the Purisima Formation at Capitola (fig. 5) as a result of the 1989 Loma 

Prieta earthquake. These six apartment units were subsequently demolished, owing to blufftop cracking and loss of foundation support
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C45

Figure 12.—Earthquake-induced failure of terrace deposits and sandstone of the Purisima Formation between Capitola 

and Rio Del Mar (see fig. 5 for location).
C46 LANDSLIDES

scarp (headscarp marked by hachures) 

debris path

Figure 13.—Plan view and longitudinal section showing locations and sizes of seacliff failures and geology between New Brighton State Beach and 

Aptos Creek (A), and between Aptos Creek and Rio Del Mar (fi) (see fig. 5 for locations). Units: Qa, Aromas Sand (Pleistocene); Qd, dune sand 

(Quaternary); Qm, marine terrace deposits (Quaternary); Tp, Purisima Formation (late Miocene and Pliocene).

Figure 14.—Base of seacliff in area of figure 12 where material from bluff failure has blocked access to beach-front 

homes.
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C47

Figure 15.—Deep tension cracks (arrows) in terrace deposits in Rio Del Mar (fig. 5). Cracks cut through house 

foundation and indicate potential extent of future failure. At this site, house was demolished, and unstable material 

was removed to stabilize site.

scarp (headscarp marked by hachures)

100

Sunset State Beach

failure surface

Figure 16.—Plan view and longitudinal section showing locations and sizes of seacliff failures and geology between Rio Del Mar and Sunset State 

Beach (see fig. 5 for locations). Units: Qa, Aromas Sand (Pleistocene); Qd, dune sand (Quaternary).
C48 LANDSLIDES

Figure 17.—Large translational slide above condominium development south of Rio Del Mar (fig. 5). One house at 

top of bluff had to be demolished; homes at base of bluff were threatened, and access partly blocked (see fig. 18).

Figure 18.—Weakly consolidated sand of the Aromas Sand disintegrated and flowed downhill to block access and 

threaten condominiums at base of bluff (see fig. 16). Tires had been placed on slope in an attempt to control 

erosion.
COASTAL-BLUFF FAILURES IN NORTHERN MONTEREY BAY INDUCED BY THE EARTHQUAKE C49

Figure 19.—Large sandflows above Sunset State Beach (fig. 5). One blufftop house was to be relocated.

Figure 20.—Earthquake-induced bluff failures in Beach Drive area of Rio Del Mar (fig. 5). New homes are being proposed at base of this 

bluff behind existing beach-front homes.
C50 LANDSLIDES

REFERENCES CITED

Griggs, G.B., 1982. Flooding and slope failure during the January 1982 

storm, Santa Cruz County, California: California Geology, v. 35, 

no. 7, p. 158-163.

Griggs, G.B., and Johnson, R.E., 1979. Coastline erosion, Santa Cruz 

County: California Geology, v. 32, no. 4, p. 67-76.

Griggs, G.B., Pepper, J.E., and Jordan, M.E., 1991. California's coastal 

hazards policies; a critique, in Domurat, G.W. and Wakeman, T.H., 

eds., The California coastal zone experience: New York, American 

Society of Civil Engineers, p. 89-107.

Harp, E.L., Wilson, R.C., and Wieczorek, G.F., 1981. Landslides from 

the February 4, 1976, Guatemala earthquake: U.S. Geological Survey Professional Paper 1204-A, 35p.

Jennings, C.W., 1975, compiler, Fault map of California with locations 

of volcanoes, thermal springs, and thermal wells: California Division of Mines and Geology Geologic Data Map 1, scale 1:750,000.

Lawson, A.C., chairman, 1908, The California earthquake of April 18, 

1906; report of the State Earthquake Investigation Commission: 

Carnegie Institution of Washington Publication 87, 2 v.

Plafker, George, and Galloway, J.P., 1989, Lessons learned from the

Loma Prieta, California, earthquake of October 17, 1989: U.S. Geological Survey Circular 1045, 48 p.

Plant, Nathaniel, and Griggs, G.B., 1989, Coastal landslides caused by 

the October 17, 1989 earthquake, Santa Cruz County, California: 

California Geology, v. 43, no. 4, p. 75-84.

Sitar, Nicholas, 1990, Seismic response of steep slopes in weakly cemented sands and gravels: H.B. Seed Memorial Symposium, Berkeley, Calif., 1990, Proceedings, p. 67-82.

Sydnor, R.H., Griggs, G.B., Weber, G.E., McCarthy, R.J., and Plant, 

Nathaniel, 1990, Coastal bluff landslides in Santa Cruz County 

resulting from the Loma Prieta earthquake of 17 October 1989, 

in McNutt, S.R., and Sydnor, R.H., eds., The Loma Prieta (Santa 

Cruz Mountains) California earthquake of October 17, 1989: 

California Division of Mines and Geology Special Publication 104, 

p. 67-82.

U.S. Geological Survey, 1989. The Loma Prieta earthquake of October 

17, 1989: Menlo Park, Calif., 16 p.

————1990, The next big earthquake in the Bay Area may come sooner 

than you think: Menlo Park, Calif., 23 p.

Working Group on California Earthquake Probabilities, 1990, Probabilities of large earthquakes in the San Francisco Bay region, California: U.S. Geological Survey Circular 1053, 51 p.
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, 

RESULTING FROM THE EARTHQUAKE

By Robert L. Schuster and Gerald F. Wieczorek, 

U.S. Geological Survey;

and

David G. Hope II, 

Santa Cruz County Planning Department

CONTENTS the smaller size of the 1989 earthquake and because of 

________ drought conditions, the 1989 landslides and their resulting

dams also were smaller (max 7 m high), had smaller im-

., r*,8,e poundments (max 6,000 m3 volume), and were distrib- Abstract ————————————————————————— C51 r v /

Introduction————————————————————— 51 ute<^ over a smaller area than those that formed under the

Historical landslide dams in California ————————— 52 wetter soil conditions in 1906.

Landslide dams caused by the 1906 San Francisco earthquake 52 Two landslide dams on Corralitos Creek were removed

Landslide dams caused by the 1989 Lotna Prieta earthquake - 55 by «stage'> (that is, incremental) excavation to eliminate

Locations and characteristics——————————————— 55 _, -L-i-^r- j ^ nj- i*

„.„,_._, „ the possibility of minor downstream flooding as a result West Branch Sequel Creek——————--———— 55 v J „ r &t

Hinckley Creek —————————————— 59 °f breaching. One dam on West Branch Sequel Creek and

Corralitos Creek, upper dam —————————— 59 another on Hinckley Creek failed during subsequent heavy

Corralitos Creek, lower dam —————————— 60 rainfall, but downstream channel capacity was not exBrowns creek———————————————— 60 ceeded by the resulting small floods; thus, no flood damec s ———————————————————— occurred to homes or property. However, sediment

Upstream flooding —————————————— 60 6 ? r J > Downstream flooding -——----—---- 60 deposited immediately downstream from these two

Deterioration of water quality ————————— 62 breached dams locally damaged spawning beds for anadroDeterioration of fish habitats ———————— 62 mous (that is, sea-run) steelhead trout.

Factors affecting the distribution of landslide dams ————— 63 

Seismic intensity ————————————————————— 63 

Slope steepness/topography ——————-——---- 63 INTUOnTTPTfON

Lithology/weathering properties ——————————— 63 ir^ L J*vJU U ^ 1 1VJ11

Soil moisture/ground water ———————————————— 64 

Influence of climate and hydrology ————————— 65 Landslide dams form most frequently where narrow,

Mitigation of stream damming-------------- 68 ^ aTQ bordered by high, rugged mountains

Conclusions————————————————————— 68 r J , ««,«••• References cited —————————————————— 69 (Costa and Schuster, 1988). This setting is common m

geologically active areas where earthquakes, volcanoes,

________ or glacially oversteepened slopes occur. The most common types of landslides that form landslide dams are rock/ 

debris avalanches, earth slumps and slides, and debris

In mountainous areas, landslide blockages of streams flows. The most common initiation mechanisms for damare an occasional, but hazardous, result of landslides trig- forming landslides are excessive precipitation (rainfall and 

gered by seismic activity. These natural dams may have snowmelt) and earthquakes.

the following negative effects: (1) upstream (backwater) A landslide dam differs from a constructed embankflooding, (2) downstream flooding as a result of dam fail- ment dam because it consists of a heterogeneous mass of 

ure, (3) deterioration of water quality, and (4) deteriora- unconsolidated or poorly consolidated material and betion of fish habitats. Eight reported landslide dams occurred cause it includes no engineered water barrier (impervious 

over a widespread area in northern and central California core), no filter zone to prevent piping (that is, internal 

as a result of the 1906 San Francisco earthquake, and five erosion), and no drain zones to control pore pressures 

dams were formed in Santa Cruz County by landslides within the mass. It also has no spillway or other protected 

triggered by the 1989 Loma Prieta earthquake. Because of outlet; thus, landslide dams commonly fail. Failure mostly

C51
C52 LANDSLIDES

occurs by overtopping, followed by erosion by the 

overflowing water, although a few landslide dams have 

breached as a result of piping. Some dams fail soon after 

they form, some last for months or years, and some never 

fail. Evidence of long-term landslide dams can be noted 

in mountain ranges worldwide, where landslide-dammed 

lakes are common features.

In addition to catastrophic downstream flooding caused 

by failure of a landslide dam, upstream (that is, backwater) flooding behind dams can inundate communities and 

valuable agricultural land. Some of the most catastrophic 

failures have been of large landslide dams formed by earthquake-induced landslides. The most disastrous recorded 

case of downstream flooding occurred in central China in 

1786, when an estimated M=7.5 earthquake caused a major landslide that dammed the Dadu River for 10 days (Li 

Tianchi, 1989). When the landslide dam overtopped and 

breached, a flood that extended 1,400 km downstream 

drowned as many as 100,000 people, whereas the earthquake had killed only 400 to 500 people directly. A graphic 

account of the 1841 failure of an earthquake-induced landslide dam on the Indus River, India, was provided by 

Mason (1929). When this natural dam, which was more 

than 200 m high and impounded a lake 65 km long, 

breached, the entire lake drained within 24 hours, causing 

a tremendous flood. Hundreds of towns and villages were 

swept away, with great numbers of casualties. In 1933, at 

least 2,400 people died in the flood caused by failure of 

the 240-m-high earthquake-induced (M=7.5) Deixi landslide dam on the Min River in central China (Li Tianchi 

and others, 1986). In 1986, the sudden failure of an earthquake-induced (Af=7.1) debris-avalanche dam on the 

Bairaman River on the island of New Britain, Papua New 

Guinea, resulted in a 100-m-deep flood in the canyon 

downstream (King and others, 1989).

HISTORICAL LANDSLIDE DAMS IN 

CALIFORNIA

The locations of 29 historical landslide dams in California are shown in figure 1. All of these natural dams 

occurred in the north half of the State. Of the blockages, 

16 were formed by landslides triggered by precipitation 

(rainfall or snowmelt), and 13 were earthquake triggered 

(8 by the 1906 San Francisco earthquake and 5 by the 

1989 Loma Prieta earthquake). A literature search for possible leads to the historical occurrence of landslide dams 

in southern California found only the following two examples, both of which are either too controversial or lack 

enough detail to be included in figure 1. 

1. As reported by Townley and Alien (1939, p. 153), the San 

Francisco Chronicle of April 25, 1909, stated that an 

earthquake had caused a great slide in Santa Paula Canyon, 

in the Coast Ranges of Ventura County, blocking the

drainage and creating a lake. Townley and Alien noted, 

however, that no seismic activity had been reported in the 

vicinity; instead, the slide seemed to have been caused by 

extremely heavy spring rains.

2. As a result of the 1952 Arvin-Tehachapi earthquake (Af=7.6) 

in Kern County, Buwalda and St. Amand (1955) noted, 

"Canyons were dammed with rock debris, and some small 

lakes were formed."

Most of the recorded landslide dams in California failed 

without serious downstream flooding or were intentionally removed to prevent flooding; however, two large dams 

failed catastrophically, resulting in serious flooding. The 

earliest recorded landslide dam in California formed on 

December 20, 1867, on the South Fork of the Kaweah 

River (fig. 1) in what is now Sequoia National Park (Fry, 

1933). A period of 41 days of rain had triggered a large 

debris avalanche/debris flow that dammed the river at its 

confluence with Garfield Creek. The 20-m-high blockage 

soon failed; the flood swept down the Kaweah River, carrying with it trees and debris and inundating part of the 

San Joaquin Valley. The town of Visalia (fig. 1), 60 km 

downstream from the dam, was flooded to a depth of 

nearly 2 m on December 23. Several other rainfall-induced landslides have blocked streams in northern California since then, but none has produced a comparable 

flood.

The 1906 San Francisco earthquake (M-8.2) resulted in 

eight recorded landslide dams (Costa and Schuster, 1991), 

and the 1989 Loma Prieta earthquake (Af=7.1) triggered 

landslides that formed five stream blockages, all of which 

were small. This paper describes the landslide dams caused 

by the 1989 earthquake, in comparison with those caused 

by the 1906 earthquake.

LANDSLIDE DAMS CAUSED BY THE 

1906 SAN FRANCISCO EARTHQUAKE

The 1906 San Francisco earthquake triggered thousands 

of landslides throughout a 600-km-long stretch of the California Coast Ranges from southern Monterey County in 

the south to Eureka in the north (Lawson, 1908; Youd and 

Hoose, 1978; Keefer, 1984). As shown in figure 1, streams 

were dammed in eight places by landslides triggered by 

the earthquake. In addition, there were sketchy reports of 

other small landslide dams in Santa Cruz County, but available information on these dams is so vague that we have 

not included them in this paper. The locations of the recorded dams ranged from the Eel River near Island Mountain in Trinity County (approx 260 km north of San 

Francisco) to Hinckley Creek in Santa Cruz County 

(approx 90 km southeast of San Francisco). Most of these 

natural dams were small (max 10 m high, table 1). A 

typical example was the dam that formed on Los Gatos 

Creek in Santa Clara County (fig.2). However, the Olive
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C53

Springs landslide (also called the Sawmill slide because it 

destroyed the Loma Prieta Mill, fig. 3) in Santa Cruz 

County dammed Hinckley Creek to a depth of 20 m (Santa 

Cruz Morning Sentinel, 1906), and a large debris slide in

Lake County formed a 30-m-high blockage of Cache Creek 

(Scott, 1970; Manson, 1990).

Within 13 days after the earthquake, the 2.4x106-m3- 

volume Cache Creek debris slide in Lake County (approx

124° 122C 120°

42C

1965, 1970

South Fork Smith River

••

1964 Hurdygurdy Creek

__'OREGON I

m

EUREKA

197 

• •

979-80 Mosquito Creek

1890 Trinity River 

1955-64 Grouse Creek

40C

1964 Van • 

iDuzen River

•1970 Eel River 

19839 

Mattole River —

REDDING

EXPLANATION

Rainfall- induced landslide dams 

Earthquake-induced landslide dams: 

T 1906 San Francisco earthquake 

A 1989 Loma Prieta earthquake

\

1906 Eel River

1986 South Fork 

Eel River

1 1964 Long 

Valley Creek

CHICO

1906 Gualala River

1906 Cache Creek 

1906 Maacama Creek

T .

38C

PACIFIC OCEAN

1983 South ForkN 

American River

SACRAMENTO

^ < m

JSAN FRANCISCO

V 19Q6 LQS Qatos Creek (see fj 2) 

/ 19Q6 Hjnckley Creek (see fjg 3)

1982 Love Creek^ir /J&1986 Hinckley Creek 

1989 West Branch Soquel C'reek^A^4l989 Browns Creek

1989 Hinckley Creek\ \1989 Corralitos Creek 

(upper and lower dams)

1906 Purisima Creek 

1906 San Gregorio Creek^T

50 100 Kilometers

H

36C

FRESNO

MONTEREY

1867 South Fortk 

Kaweah River.

VISALIA *• 

• 1867' 

Garfield Creek

42°

40°

38C

36°

124° 122° 120° 

Figure 1.—Northern California, showing distribution of 29 historical landslide dams. Data primarily from Costa and Schuster (1991).
C54 LANDSLIDES

Table 1.—Landslide dams caused by the 1906 San Francisco earthquake

River/stream Dam height 

(m)

Comments References

Eel River ———————————————— ?

Cache Creek

Gualala River

Maacama Creek

Purisima Creek ——————————— 8-10

San Gregorio Creek

Los Gatos Creek 

(fig. 2).

Hinckley Creek - 

(fig. 3).

Reported by Northwestern 

Pacific Railroad.

30 Dam failure caused

downstream flooding.

? "*****river was temporarily dammed up by 

slides from both slopes."

? "*****avalanche cut its

way through a fir forest 

and dammed Maacama Creek. 1

•«*****slide*****dammed the 

creek to a depth of 25 or 

30 feet."

"*****creek was dammed up 

to a depth of 6 feet."

"Large pond" formed.

20 Nine killed by "Sawmill 

slide. "

Dwyer and others 

(1971).

Scott (1970), 

Manson (1990) .

Lawson (1908).

Do.

Do.

Do.

Do.

Santa Cruz Morning 

Sentinel (1906).

Figure 2.—Landslide that dammed Los Gatos Creek, Santa Clara County, Calif, (fig. 1), as a result of 1906 San 

Francisco earthquake. Photograph courtesy of J.C. Branner Papers, Stanford University Archives.
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C55

120 km north of San Francisco) formed; the slide dammed 

Cache Creek (fig. 1; Scott, 1970). In including this slide 

with others triggered by the earthquake, Lawson (1908, p. 

390) stated, "This earth-avalanche cannot be so directly 

referred to the earthquake of April 18 [1906] as the others 

heretofore described, but it was probably indirectly caused 

by the shock." In the absence of proof, this slide most 

probably was caused by a change in the ground-water 

regimen as a result of the earthquake. In this regard, Scott 

(1970) noted:

According to the Carnegie report [Lawson, 1908], 

Cache Canyon was in a seismic intensity zone of about 

V to VI. This ground motion could have opened incipient failure planes in the slide mass along which groundwater percolation and increased hydrostatic pressures 

developed until, 12 [sic] days later, the slope failed. 

Then, 5 days later, the natural dam failed by overtopping and (or) piping, releasing approximately 15xl06 m3 

of impounded water as a flood that entered Capay Valley 

to the southeast and devastated the small town of Rumsey 

on Cache Creek, 30 km downstream from the landslide. 

The flood destroyed several buildings, carried away the 

Rumsey post office and store, and damaged Southern Pacific Railroad facilities and the county bridge over Cache 

Creek (Manson, 1990). No one was killed or injured.

LANDSLIDE DAMS CAUSED BY THE 

1989 LOMA PRIETA EARTHQUAKE

LOCATIONS AND CHARACTERISTICS

Five landslides triggered by the 1989 Loma Prieta earthquake formed small dams on streams in Santa Cruz County 

(figs. 4, 5; table 2). All five of these landslides occurred 

in rocks of the upper Miocene and Pliocene Purisima Formation (McLaughlin and others, 1988; Brabb, 1989; Clark 

and others, 1989; Weber and Nolan, 1992). The general 

characteristics of these landslides and the resulting dams 

and impoundments are described below.

WEST BRANCH SOQUEL CREEK

The rockslide on West Branch Soquel Creek (fig. 6), 

which occurred on a rather remote stretch of the creek 

(fig. 4), is the largest of the five 1989 landslides discussed in this paper. This slide was approximately 250 m 

long, averaged 50 m in width, and had an estimated maximum thickness of 6 m and a volume of 50,000 m3 . The 

slide began in Purisima Formation sandstone, siltstone,

Figure 3.—Workers removing toe of earthquake-induced landslide that dammed Hinckley Creek at Olive Springs 

sawmill in 1906 (see fig. 4 for location). Landslide killed nine workers at Olive Springs and formed a 20-m-deep 

impoundment where it dammed the creek. Photograph courtesy of California Department of Parks and Recreation.
O

Ui

121° 55' 121° 52' 30"

37° 5'lr

Areas of 

figures 

4 and 5 - 0 0.5 1.0 Mile

121° 55' 121° 52' 30"

Figure 4.—Area of Santa Cruz Mountains, Calif., showing locations of 1989 landslide dams on West Branch Sequel Creek and Hinckley Creek and of epicenter of 1989 Loma Prieta earthquake 

(star). Base from U.S. Geological Survey 1 V2-niinute Laurel quadrangle, Calif.; contour interval, 40 ft.
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C57
C58 LANDSLIDES

Table 2.—Summary of characteristics of landslides and landslide dams in Santa Cruz County resulting from the 1989 Loma Prieta earthquake

Location/ 

stream

West Branch

Soquel

Creek

Hinckley

Creek

(Olive

Springs)

Corral itos

Creek ,

upper dam.

Corral itos

Creek,

lower dam.

Browns

Creek

Site Failed 

geology slope

Purisima 26°

Formation

(sandstone,

siltstone,

mudstone) .

Purisima 60°

Formation

( sandstone) .

—— do ——— 75°

Purisima 30°

Formation

(sandstone,

siltstone,

mudstone) .

Purisima 70°

Formation

(sandstone) .

Landslide Landslide Dam Lake Comments 

type volume height volume 

(m3 ) (m) (m3 )

Rock slide 50,000 4 1,800 Dam failed 61 days

after earthquake;

minor stream

flooding and

sedimentation .

Rock fall 1,600 4 3,500 Soil matrix of

dam failed in

February 1992.

—— do —— 3,500 7 6,000 Spillway excavated; then dam

removed .

Rock slide 3,500 3 1,200 Do.

Rock fall 200 2 150 Dam failed by

overtopping.

Figure 6.—Toe of this 50,000-m3-volume earthquake-induced slide 

dammed West Branch Soquel Creek (see fig. 4 for location). Dam 

failed in December 1989, 61 days after 1989 Loma Prieta earthquake. Photograph taken in May 1992.
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C59

and mudstone at the site of a spring on the valley wall. 

Initial failure probably occurred in a weak layer of silt or 

clay in the Purisima Formation. The slide crosses the 

Zayante fault (Clark and others, 1989) on a slope averaging 26°, and its toe lies on rocks of the Eocene Butano 

sandstone in the valley bottom. The existing, nearly vertical headscarp in Purisima Formation rocks is 12 m high 

and 30 m long and has a spring issuing from its base. As 

indicated by Clark and others, the strike of the Purisima 

Formation beds at this site approximately parallels West 

Branch Sequel Creek, and the dip is back into the valley 

wall. Thus, this slide was not a dip-slope failure; instead, 

failure probably was related to the low shear strength of 

mudstone and siltstone that had been saturated locally by 

ground water from the spring.

Although this slide is much larger than the others discussed here, it barely crossed the creek, forming only a 

low dam and a resulting small impoundment. The minimum crest height of the blockage formed by the toe of the 

slide was 4 m. The original landslide dam was 10 m long 

(perpendicular to the stream) by 55 m wide (parallel to 

the stream); it impounded an 1,800-m3-volume lake. The 

dam was composed of a heterogeneous, fairly impervious 

mass of clay, silt, sand, and small blocks of sandstone. On 

December 17, 1989, 61 days after the earthquake, the landslide dam failed, causing minor downstream flooding and 

deposition of 5,000 m3 of sediment in the streambed.

HINCKLEY CREEK

The 1989 Hinckley Creek rock fall occurred near Olive 

Springs (fig. 4) on a 60° slope in weathered sandstone of 

the Purisima Formation, 60 m high, about 800 m downstream from the 1906 Hinckley Creek "Sawmill slide" 

(table 1). As shown by Clark and others (1989), the 

Purisima Formation strikes nearly normal to Hinckley 

Creek at this site and dips about 20° upstream; thus, the 

attitude of the formation had little influence on the 1989 

slope failure. Weathering and joints at the surface of the 

sandstone probably were the important elements in failure 

of the slope. The 1989 rock fall had an estimated volume 

of 1,600 m3 , consisting mainly of broken blocks of soft 

sandstone that disintegrated to silt and fine sand in the 

valley bottom. This broken-up sandstone, combined with 

timber debris, dammed Hinckley Creek to a depth of 4 m, 

impounding a lake 160 m long with an estimated volume 

of 3,500 m3 . The blockage, which was 20 m long (normal 

to the creek) by 15 m wide (parallel to the creek), breached 

in February 1992 after heavy rain.

CORRALITOS CREEK, UPPER DAM

The upper dam on Corralitos Creek in Eureka Canyon 

(fig. 5) was formed by a rock fall (fig. 7) from the weathered surface of a 35-m-high, 75° face of Purisima Formation sandstone. In the vicinity of the rock fall, the Purisima 

Formation strikes nearly normal to the creek and dips 35° 

upstream (McLaughlin and others, 1988); thus, the failure 

was related to surficial weathering and jointing, not to the 

attitude of the sandstone forming the face. The volume of 

the rock fall was only 3,500 m3 , but because it fell into a 

very narrow (10 m wide) canyon bottom, it formed a blockage that was 7 m high at the low point of the crest. This

Figure 7.—Rock fall on upper Corralitos Creek (see fig. 5 for location) 

triggered by 1989 Loma Prieta earthquake. Note logs and brush intermingled with earth material to form landslide dam at base of slope. 

Impoundment is outside photograph to immediate left. Photograph taken 

October 25, 1989, 1 week after 1989 Loma Prieta earthquake.
C60 LANDSLIDES

mass of silt and fine sand from disintegrated rock-fall 

blocks impounded an estimated 6,000 m3 of water. Just 1 

day after the earthquake, a spillway was excavated across 

the 25-m-wide landslide dam by a Santa Cruz County 

Flood and Water Conservation District crew, and within 2 

weeks the dam was removed. No downstream flooding 

occurred.

CORRALITOS CREEK, LOWER DAM

The lower dam on Corralitos Creek in Eureka Canyon 

(100 m downstream from the upper dam, fig. 5) was 

formed by a shallow rockslide from a 30° valley wall 

consisting of weathered sandstone, siltstone, and mudstone 

of the Purisima Formation. A spring at the site of the 

slide was a prime factor in the failure. As shown by 

McLaughlin and others (1988), the Purisima Formation in 

the vicinity of the site strikes nearly normal to the creek, 

and the rocks dip 37° upstream; thus, the attitude of the 

formation probably was not a factor in the failure. The 

slide formed a dam of clay, silt, and fine sand that was 3 

m high, 20 m long (normal to the creek), and 20 m wide 

(parallel to the creek), and it impounded a pond with an 

estimated volume of 1,300 m3 . Just 1 day after the earthquake, a spillway was constructed across the landslide 

dam by a Santa Cruz County Flood and Water Conservation District crew, and within 2 weeks the dam was removed. No downstream flooding occurred.

BROWNS CREEK

The small landslide dam on Browns Creek (fig. 5) was 

formed by a 200-m3 rock fall on a 70° canyon wall of 

weathered and jointed Purisima Formation sandstone. In 

the vicinity of the dam, the Purisima Formation strikes 

nearly normal to the creek and dips about 20° downstream 

(McLaughlin and others, 1988); thus, the attitude of the 

bedding had little influence on the stability of the steep 

face. As in the rock falls described above, the failure was 

related primarily to jointing in the soft, weathered sandstone composing the steep face above the creek. The dam 

was only 2 m high at the low point in the crest, 8 m long 

(normal to the stream), and 8 m wide (parallel to the 

stream) at its narrowest part. The impoundment was only 

40 m long. The blockage failed by overtopping, but the 

volume of the pond (about 150 m3) was so small that no 

downstream flooding occurred.

In summary, all five of the landslides that formed dams 

occurred in Purisima Formation weathered sandstone, siltstone, or mudstone on slopes ranging from 26° for the 

slide on West Branch Sequel Creek to 75° for the rock 

fall that formed the upper dam on Corralitos Creek. Landslide volumes ranged from only 200 m3 for the rock fall

on Browns Creek to about 50,000 m3 for the slide on 

West Branch Sequel Creek. The effective heights of the 

landslide dams were all small, ranging from 2 m for the 

dam on Browns Creek to 7 m for the upper dam on 

Corralitos Creek. In addition, these small landslide dams 

occurred in narrow canyons with steep gradients; thus, 

the impoundments were also small, ranging from 150 m3 

behind the lower dam on Corralitos Creek to 6,000 m3 

behind the upper Corralitos Creek dam. None of the impounded lakes was large enough to cause damage from 

upstream flooding or to pose a serious downstream flooding hazard if its dam had failed.

EFFECTS

Damming of streams in Santa Cruz County by landslides triggered by the 1989 Loma Prieta earthquake caused 

no permanent change in the gradients of the streams because the dams were small and either were soon removed 

by the Santa Cruz County Flood and Water Conservation 

District or breached owing to natural causes. However, 

they did affect the streams in regard to (1) upstream flooding due to impoundment, (2) downstream flooding resulting from dam failure, (3) deterioration of water quality 

downstream, and (4) deterioration of fish habitats.

UPSTREAM FLOODING

As noted earlier, upstream or backwater flooding was 

minimal from all five landslide dams because of low dam 

heights, narrow canyons, and steep stream gradients, all 

of which resulted in small impoundments. The longest 

lake, behind the Hinckley Creek landslide dam, was only 

160 m long. In addition, no buildings or other structures 

were located immediately upstream of the dams. Thus, 

the impoundments caused no significant upstream damage or economic loss.

DOWNSTREAM FLOODING

Although the impoundments were small, there still was 

some danger of downstream flooding if the dams were to 

breach rapidly. For this reason, the Santa Cruz County 

Flood and Water Conservation District and its contractors 

excavated spillways across both of the Corralitos Creek 

landslide dams within 72 hours after the earthquake. Excavation was difficult because large amounts of tree and 

brush remnants (logs, limbs, and so on) were incorporated 

into the dams (fig. 8). This same wood material, however, 

protected the constructed spillways against erosion. Within 

2 weeks after the earthquake, both Corralitos Creek landslide dams were completely removed by Santa Cruz County
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C61

Flood and Water Conservation District crews because of 

the potential danger of dam failure and flooding during 

future heavy rains. Nothing was done to the Browns Creek 

landslide dam because its impoundment was too small to 

be considered a threat downstream; it was allowed to fail 

naturally.

On the day after the earthquake, the landslide dam on 

West Branch Soquel Creek (fig. 6) was noted from the air 

(Capt. Dennis Smith, Santa Cruz County Sheriff's Office, 

oral commun., 1989). However, because this landslide dam 

was in an unpopulated, untraveled area and its impoundment was small, no mitigative measures were undertaken.

Figure 8.—Rock-fall and timber debris forming dam on Hinckley Creek (see fig. 4 for location). Arrows denote 

downstream end of impoundment. Photograph taken on October 25, 1989, 1 week after 1989 Loma Prieta earthquake.

Figure 9.—Downstream view of remnants of 1989 Hinckley Creek landslide dam after it had failed in February 

1992. Arrows denote previous shoreline of now-drained 4-m-deep impoundment. Photograph taken May 1992.
C62 LANDSLIDES

Then, 61 days after the earthquake, the dam failed, causing minor downstream flooding in the channels of West 

Branch Sequel Creek and Sequel Creek. Channel capacity 

was not exceeded; thus, no damage occurred as a result of 

flooding.

The 4-m-high Hinckley Creek landslide dam was naturally reinforced by timber debris (logs and limbs) within 

the landslide, and so, because it was thought to be stable, 

it was not removed, as both the Corralitos Creek landslide 

dams were. During heavy rain in February 1992, however, the soil matrix of the dam was removed by erosion, 

resulting in rapid drainage of the impoundment (fig. 9) 

and minor downstream flooding that did not exceed channel capacity. Most of the interwoven logs, branches, and 

brush that had held the dam together remained in place 

(fig. 10).

DETERIORATION OF WATER QUALITY

No formal studies were conducted on the effects of the 

five 1989 landslide dams on the water quality of Soquel, 

Hinckley, Corralitos, and Brown Creeks, although it was 

noted that Corralitos Creek received only small amounts 

of sediment over a short period of time because its two 

landslide dams were effectively removed by Santa Cruz 

County Flood and Water Conservation District crews. Although the Browns Creek landslide dam failed, it was so 

small that only about 200 m3 of earth material washed 

down the stream.

Figure 10.—Closeup of log/brush remnant of 1989 Hinckley Creek landslide dam after soil matrix of dam had eroded away in February 1992. 

Photograph taken May 1992.

The effects of the West Branch Soquel Creek and 

Hinckley Creek landslide dams on downstream water quality were more serious because both of these dams failed 

and both were made up of moderate amounts of earth 

material that was flushed downstream. As noted above, 

the landslide dam on West Branch Soquel Creek failed in 

December 1989, 61 days after the earthquake. During this 

61-day period, approximately 2,500 m3 of soil eroded from 

the upstream part of the landslide and was deposited in 

the impoundment. When the dam failed, this material, plus 

an equal amount from the dam itself, was carried downstream by the floodwaters. Gravel and coarse sand were 

deposited within the first few hundred meters, but silt was 

carried as far as 2 km downstream on West Branch Soquel 

Creek. Clay-size particles were carried in suspension even 

farther.

When the soil matrix of the rest of the Hinckley Creek 

landslide dam eroded away in February 1992, the effect 

on downstream water quality was similar to that from the 

failure of the landslide dam on West Branch Soquel Creek, 

except that only about 1,000 m3 of sediment was washed 

downstream. The coarse materials were deposited within 

200 m of the dam, but a significant amount of silt and 

fine sand was carried as far as the confluence of Hinckley 

and Soquel Creeks, 1 km downstream (fig. 4).

The clay in sediment from the failures of the landslide 

dams on both West Branch Soquel Creek and Hinckley 

Creek evidently was carried farther downstream in suspension, as were small amounts of suspended sediment 

from both Corralitos Creek landslide dams, but these small 

amounts of suspended material had no evident long-term 

effect on water quality in the streams.

DETERIORATION OF FISH HABITATS

Soquel, Hinckley, Corralitos, and Browns Creeks serve 

as spawning grounds for steelhead trout and as permanent 

habitats for a few other species of nonsport fish. The 

anadromous steelhead require stable gravelly stream bottoms as spawning beds. Spawning is unsuccessful when 

these gravel beds are buried by silt and fine sand, as occurred immediately downstream of the failed landslide 

dams on West Branch Soquel Creek and Hinckley Creek. 

However, because the stretches of these streams that were 

subjected to fine-grained sedimentation constituted only a 

small percentage of the total spawning areas on the headwaters of these streams, and because very little sedimentation occurred in Corralitos and Browns Creeks, the 

overall effects of the 1989 landslide dams on steelhead 

reproduction were insignificant. Because spawning occurs 

in the fall and winter, the sediment was deposited at a 

critical time; however, other factors, such as below-normal streamflow due to the prolonged drought, may have 

had a greater effect on spawning in 1989-90 than did 

sediment from the failed landslide dams.
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C63

FACTORS AFFECTING

THE DISTRIBUTION

OF LANDSLIDE DAMS

SEISMIC INTENSITY

The distribution of landslide dams caused by the 1989 

Loma Prieta earthquake was geographically restricted close 

to the epicentral area (figs. 4, 5). All five landslide dams 

occurred within about 10 km or less northwest, west-northwest, or southeast of the epicenter (table 3). Bilateral fault 

rupture to the northwest and southeast of the epicenter 

(Beroza, 1991; Wald and others, 1991) may have influenced the distribution of strong ground shaking and of 

landslide-dam sites to the northwest and southeast in this 

vicinity.

The Corralitos strong-motion station on Eureka Canyon 

Road was located only 7 km west of the earthquake epicenter (figs. 4, 5); two other stations at Capitola and Santa 

Cruz were also near (<16 km from) the epicenter. All 

three stations recorded comparable peak accelerations of 

0.4 to 0.6 g (table 4) for both horizontal and vertical 

components. The fact that these three stations are at distances comparable to or slightly greater than the distances 

of all five landslide dams from the epicenter suggests that 

the peak accelerations recorded there were at least as great 

at these landslide-dam sites.

The Corralitos strong-motion station is located on a 

landslide deposit derived from the same bedrock geologic 

unit, the Purisima Formation (Brabb, 1989), as that of all 

five 1989 landslide dams. The Santa Cruz station, however, is founded on limestone, and the Capitola station on 

alluvium (Shakal and others, 1989). Therefore, the site 

response at the Corralitos station most nearly represents 

the strong ground motion at the landslide-dam sites because of a similarity of geologic materials and a distance 

from the epicenter that is comparable to those of all the 

landslide dams.

In addition to peak accelerations, the duration of strong 

shaking is important for evaluating seismic intensity and 

the ability to trigger landslides. Strong shaking from the 

earthquake lasted at least 15 to 20 s, as recorded at these 

three stations closest to the epicenter (Shakal and others, 

1989). The Arias intensity, Ia, is defined as:

where a is the acceleration, t is the time, and g is the 

acceleration due to gravity (Arias, 1970). The Arias intensity is expressed in units of velocity, usually meters per 

second. Because calculation of the Arias intensity involves 

the entire record, its value is proportional to the duration 

as well as the amplitude of the shaking record.

The high Ia values, ranging from 0.9 to 4.3 m/s (table 

4), for Corralitos and Capitola, the two stations closest to 

the epicenter, are significant because they greatly exceed 

7fl=0.11 m/s as a shaking threshold for falls, disrupted 

slides, and avalanches (category I landslides; Keefer and 

Wilson, 1989) and 7fl=0.32 m/sec as a shaking threshold 

for slumps, block slides, and slow earth flows (category II 

landslides; Keefer and Wilson, 1989). The large (50,000- 

m3 volume) rock slide on West Branch Soquel Creek is a 

category II landslide, whereas the other smaller (<3,500 

m3 volume) rock falls and rock slides at the other sites 

were category I landslides—generally shallow and highly 

to very highly disrupted internally.

With the combination of strong horizontal and vertical 

components and long duration of strong shaking, the high 

Ia values from these three strong-motion records nearest 

the epicenter are among the highest ever measured. This 

intensity of seismic shaking in the area of the landslide 

dams generally corresponds to the heavy structural damage and firsthand observations of extreme shaking in the 

near-epicentral area.

SLOPE STEEPNESS/TOPOGRAPHY

Slope steepness strongly affected the distribution of 

landslide dams caused by the earthquake. Steep slopes are 

common throughout the near-epicentral area. Regional 

tectonism has uplifted the southern Santa Cruz Mountains 

to maximum elevations of about 1,150 m at Loma Prieta. 

In this earthquake, the maximum surface uplift on the 

west side of the San Andreas fault was 592 mm (Marshall 

and others, 1991). Active tectonics, in combination with 

downcutting by streams in this area of moderately high 

average annual rainfall (860-1,120 mm; Rantz, 1971), has 

produced steeply sloped canyons that are prone to 

landsliding due to both seismic shaking and precipitation.

The sites of the landslide dams caused by the earthquake were all on steep slopes. The steepest slopes (>60°) 

sustained rock falls (Hinckley Creek; Corralitos Creek, 

upper dam; and Browns Creek). The other two sites 

(West Branch Soquel Creek and Corralitos Creek, lower 

dam) had lesser slopes (approx 30°) but also had springs 

near their crowns, suggesting that ground water played a 

significant part in slide initiation. No common or predominant slope aspect was observed.

LITHOLOGYAVEATHERING PROPERTIES

The Purisima Formation forms most of the surface area 

of the southern Santa Cruz Mountains in the vicinity of 

the five landslide-dam sites (figs. 4, 5; McLaughlin and 

others, 1988; Brabb, 1989, Clark and others, 1989). The
C64 LANDSLIDES

Table 3.—Distance and direction of landslide-dam sites from the epicenter of the 1989 Loma Prieta earthquake

Location/ 

stream

Slope Distance from 

aspect epicenter 

(km)

Direction from 

epicenter

West Branch SW 

Soquel Creek.

Hinckley Creek SE 

(Olive Springs).

Corralitos Creek, W-SW 

upper dam.

Corralitos Creek, W-SW 

lower dam.

Browns Creek. NW

7.0

2.8

6.8

7.0

10.2

NW

W-NW

SE

SE

SE

Table 4.—Seismic intensity of the 1989 Loma Prieta earthquake near landslide dams 

[From Shakal and others (1989); Arias intensities from E.L. Harp (written commun., 1992)]

Strongmotion

station

name

Corralitos

Capitola -•

Santa Cruz

Epicentral

distance

(km)

—— 7

- —— 9

—— 16

Component

90

Up

360

90

Up

360

90

Up

360

Peak

acceleration,

9

.50

.47

.64

.47

.60

.54

.44

.40

.47

Arias

Intensity,

Ia (m/s)

2.572

0.917

3.258

2.373

4.279

4.382

2.043

1.072

2.666

Site

Geology

Landslide

deposits.

Alluvium.

Limestone.

Purisima Formation consists of upper Miocene and 

Pliocene, very thick bedded, tuffaceous, diatomaceous siltstone with thick interbeds of semifriable, fine-grained 

andesitic sandstone. As mapped in some areas, the Purisima 

Formation includes Santa Cruz Mudstone, a medium- to 

thick-bedded, blocky-weathering, siliceous organic mudstone. The area is thickly vegetated by redwood and other 

evergreen forest. The relatively few outcrops expose deeply 

weathered bedrock. A generally deep weathering profile 

in the Purisima Formation bedrock resulted in mainly shallow (<3 m deep) landslides representative of most of those 

that dammed streams in this earthquake.

SOIL MOISTURE/GROUND WATER

Soil-moisture contents and ground-water levels at the 

time of the earthquake were probably at or near record 

historical lows. The third consecutive season (1986-87, 

1987-88, 1988-89) of abnormally low rainfall or drought 

conditions, combined with the end of the summer dry 

season before the onset of fall rains, set the hydrogeologic stage for the region at the time of the earthquake. In 

the Santa Cruz Mountains, hillside ground-water levels 

typically fluctuate about 5 m during years of near-average 

rainfall (Rojstaczer and Wolf, 1991). This semiannual cycle
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C65

ranges from high hillside ground-water levels at or slightly 

after the height of the January-March rainy season to low 

ground-water levels at the end of the summer dry season 

before the onset of fall rains in October or November (fig. 

11). The seasonal ground-water trends from 1976 to 1990 

and the low ground-water levels at the time of the earthquake in the Santa Cruz Mountains were well documented 

in one well that was monitored weekly (fig. 12), and confirmed by numerous other well and stream-gage records 

in basins in the area affected by the earthquake (Rojstaczer 

and Wolf, 1991).

The exceptionally low ground-water levels at the time 

of the earthquake probably minimized the number and 

volumes of landslides triggered by the earthquake, particularly limiting deeper-seated coherent landslides 

(category II landslides; Keefer and Wilson, 1989). Some 

deep hillside soils and rocks that normally would have 

been below ground-water levels and thus would have been 

saturated were probably dry and thus more resistant to 

sliding at the time of the earthquake. However, springs at 

the head of the West Branch Soquel Creek and lower 

Corralitos Creek sites indicated that in isolated places 

ground water remained moderately high and may have 

assisted in triggering landslides.

Ground-water flow in fracture networks immediately 

after the earthquake was suggested to be important. The 

earthquake apparently caused discharge of water from 

fractures and resulting short-term increases in streamflow. Water-well levels on ridgecrests generally fell 

within several weeks after the earthquake (Rojstaczer and 

Wolf, 1991), and streamflows in the basins increased by 

a factor of 2 to 5 within hours of the main shock (Briggs, 

1991; Curry and others, 1994). This sudden increase in 

fracture flow and streamflow probably had a minimal 

effect on the triggering of landslides at the time of the 

earthquake because of the delay times needed for water 

to flow through the fractures. However, the increased flows 

probably significantly affected landslide dams because 

their impoundments were able to fill at faster rates 

than would have been expected for preearthquake 

streamflow.

INFLUENCE OF CLIMATE AND 

HYDROGEOLOGY

Landslide dams are a potential hazard in areas that are 

subject to large earthquakes and vulnerable to landslides

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and Atmospheric Administration (1988, 1989, 1990).
C66 LANDSLIDES

on steep slopes. The seasonal climatic and hydrologic 

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Figure 11.—Continued.
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C67

streamflow are generally lower than in central and northern California.

On the basis of a comparison of the ground-failure effects of the 1906 and 1989 earthquakes, apparently more 

numerous deep-seated landslides (category II) and landslide dams occur during wet periods, although differences 

in earthquake magnitude and duration, topography, and 

geologic site conditions could account for some of the 

observed variations. The difference in size between the 

1906 San Francisco earthquake (M-8.2) and the 1989 

Loma Prieta earthquake (Af=7.1) can certainly account for 

the major difference in the number of landslides and landslide dams caused by these two earthquakes, but hydrogeologic conditions, as influenced by antecedent rainfall, 

probably also were a significant factor. The influence of 

antecedent rainfall on the seismic triggering of landslides 

that form dams can be examined by comparing the 1906 

San Francisco earthquake with the 1989 Loma Prieta earthquake. The 1906 earthquake triggered many large, deepseated landslides throughout the coastal area of central 

and northern California (Youd and Hoose, 1978), and created at least eight landslide dams, some of which were 

large (fig. 1; table 1). In contrast, the 1989 earthquake 

triggered mostly shallow landslides, commonly less than 

about 100 m3 in volume. Of the five landslides that 

dammed streams in Santa Cruz County in 1989, four were 

small (<3,500 m3 volume). The resulting small landslide 

dams might have gone unnoticed in remote areas or been 

underreported during postearthquake investigations, and 

so the number of landslide dams resulting from the 1906 

earthquake may have been even higher than reported. Thus, 

the largest (category II) landslide dam (on West Branch 

Soquel Creek) that occurred in the 1989 earthquake can 

be compared to each of six that occurred in the 1906

690

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640

1976 1981 1991

Figure 12.—Water-table elevation (from well on east edge of Pescadero 

Basin, 25-35 km northwest of landslide dams in this paper) as a function of precipitation for period 1976-90 (Rojstaczer and Wolf, 1991).

earthquake. Because category II landslides are commonly 

larger and more deep seated than category I landslides, 

they are more likely to fill or constrict narrow canyons 

and to form large landslide dams.

Although it had not rained for 17 days before the April 

18, 1906, earthquake (Youd and Hoose, 1978), the month 

of March 1906 was exceptionally wet (fig. 11). Cumulative rainfall for San Francisco, Santa Cruz, and Watsonville 

for 1-, 3-, and 6-month periods before the earthquake was 

about 1 V2 to 2 times normal (table 5). Thus, at the time 

of the 1906 earthquake, although surface soils may have 

dried, hillside ground-water levels probably were relatively 

high throughout central and northern California as a result 

of the high antecedent rainfall in the preceding several 

months (Youd and Hoose, 1978). The exact period of antecedent rainfall that is most effective in recharging and 

raising ground-water levels at a particular site depends on 

site-specific characteristics of the soil and rock—thickness, permeability, and stratigraphy. The fact that much 

higher than normal rainfall occurred consistently during 

the 1- to 6-month period before the 1906 earthquake suggests that ground-water levels were significantly above 

normal, if recent ground-water measurements after heavy 

seasonal rainfall in northern California are any indication 

(Wieczorek, 1981; Wilson and Dietrich, 1987; Wilson, 

1989; Rojstaczer and Wolf, 1991).

In comparison, rainfall recorded at San Francisco, Santa 

Cruz, and Watsonville before the 1989 earthquake was 

significantly below normal for the preceding 1-, 3-, 6-, 

and 12-month periods or for seasonal totals (table 5). In 

addition, significant gradual decreases in ground-water levels for the 1989-90 season before the earthquake were 

measured in the Santa Cruz Mountains (Rojstaczer and 

Wolf, 1991). These generally low ground-water levels were 

probably at least partly responsible for the relatively few 

large, deep-seated landslides that occurred or that moved 

only short distances in the earthquake (Plafker and Galloway, 1989; Keefer, 1991; Keefer and Manson, this volume; Keefer and others, this volume).

The time required for a landslide-dam impoundment to 

fill depends (1) on the rate of stream discharge, which in 

California is controlled by seasonal patterns of rainfall, 

infiltration, and runoff; and (2) on the basic characteristics of the dam, including height, potential storage capacity, and permeability. The 61-day period after the 1989 

Loma Prieta earthquake before the failure of the landslide 

dam on West Branch Soquel Creek attests at least in part 

to the relatively low runoff during the fall rains after a 3- 

year drought.

There appears to be inadequate accurate historical information available for a statistically significant characterization of the expected size of landslide dams in 

California. Given the specific size and location of a postulated earthquake, then the areal limits of category I and 

II landslides can be estimated by the methods of Keefer
C68 LANDSLIDES

Table 5.—Antecedent, seasonal, and average rainfall for 1-, 3-, 6, and 12-month periods before the 1906 San 

Francisco and 1989 Loma Prieta earthquakes measured in San Francisco (1849-1990), Santa Cruz (1879-1990), 

and Watsonville (1879-1990)

[Data from U.S. Department of Agriculture (1907) and U.S. National Oceanic and Atmospheric Administration (1988, 1989, 1990)]

Antecedent San 

time period

1 month:

Mar. 1906

(avg)

Sept. 1989

(avg)

3 months:

Jan. -Mar. 1906

(avg)

July-Sept. 1989

(avg)

6 months:

Oct. 1905-Mar. 1906

(avg)

April-Sept. 1989

(avg.)

12 months:

Apr. 1905-Mar. 1906

(avg)

Oct. 1988-Sept. 1989

(avg.)

Francisco 

rainfall 

(mm)

127.5

(65.5)

24.9

(6.1)

260.35

(251.2)

26.2

(9.1)

411.2

(431.5)

47.2

(59.4)

497.1

(544.3)

413.3

(544.3)

Santa Cruz 

rainfall 

(mm)

220.1

(96.3)

22.1

(9.9)

541.5

(383.8)

23.9

(17.3)

658.1

(638.8)

45.2

(97.3)

806.5

(736.1)

614.7

(736.1)

Watsonville 

rainfall 

(mm)

167.9

(74.2)

21.8

(8.9)

468.1

(278.6)

22.9

(13.5)

549.9

(471.4)

41.9

(72.4)

698.5

(543.8)

444.0

(543.8)

Seasonal totals: 

1905-6 (July 1905- 

June 1906)

1988-9 (July 1988- 

June 1989)

1989-90 (July 1989- 

June 1990)

Yearly average

518.7

387.9

363.7

544.3

819.4

590.8

444.2

736.1

645.4

421.1

377.2

543.8

and Wilson (1989) and used to predict the extent of landslide-dam occurrence.

MITIGATION OF STREAM DAMMING

The 1989 Loma Prieta earthquake occurred at 5:04 p.m. 

P.d.t. October 17. By 10:00 a.m. the next morning, reconnaissance by helicopter and fixed-wing aircraft, coupled 

with coordinated ground reconnaissance, had located all 

of the landslide dams caused by the earthquake. This rapid 

reconnaissance was a critical factor in the success of the 

dam-failure-control program.

After the reconnaissance, a priority assessment was conducted to review dam size, impoundment volume, potential for failure, and potential flood discharge. Dams close 

to homes and public infrastructure were given the highest

priority for mitigative efforts, although water quality and 

fish habitats were also considered. Access by earth-moving equipment to the two landslide dams on Corralitos 

Creek was facilitated by use of Santa Cruz County roads. 

The two landslide dams on Corralitos Creek were given 

the highest priority, and Santa Cruz County Flood and 

Water Conservation District crews immediately began to 

excavate spillways across both dams. The spillway inverts were positioned significantly below the crests of the 

dams, thus reducing the volume of impounded water and 

preventing erosion failures by overtopping of the unprotected crests of the original dams. Each spillway was constructed across the low side of the dam crest opposite the 

landslide source. In general, this part of the landslide darn 

contained the most woody debris. Wherever possible, this 

debris was left in place along the bottom and sides of the 

spillway for stabilization.
LANDSLIDE DAMS IN SANTA CRUZ COUNTY, CALIFORNIA, RESULTING FROM THE EARTHQUAKE C69

There was concern that impending winter storms would 

greatly increase streamflows, possibly mobilizing upstream 

logs and debris that could block the newly excavated spillways and cause dam failure. In addition, there also was 

concern that the dams would block upstream migration of 

anadromous steelhead trout. Thus, Santa Cruz County 

Flood Control and Water Conservation District crews used 

earth-moving equipment to remove the dams. The operation was completed by "stage" (that is, incremental) excavation. Soils and debris either were trucked to offsite 

locations or were spread in thin layers across existing 

flood plains.

Removal of landslide-dam materials was followed by 

seeding and mulching of the newly exposed surfaces and 

of spoil areas to which the materials had been hauled. The 

complete removal of the dams and seeding/mulching of 

fresh soil surfaces left the sites with the same stream gradients and crossprofiles that they had before the earthquake. The long-term effect on fish habitats was minimal. 

Water quality was temporarily affected by increased turbidity during operations, but monitoring of water conditions and aquatic-life responses indicated little or no loss 

of stream life or long-term damage to fish habitats.

CONCLUSIONS

Precipitation totals in Santa Cruz County in the months 

before the 1989 Loma Prieta earthquake were considerably lower than those before the 1906 San Francisco earthquake. As a result of a smaller earthquake and less 

antecedent rainfall, fewer landslides and landslide dams 

occurred in 1989 than in 1906; furthermore, the 1989 

blockages were smaller and impounded only small lakes. 

Because of their small size, the effects of these dams and 

their impoundments were minor. However, if future earthquakes in California are larger and (or) occur during or 

soon after periods of heavy and prolonged rainfall that 

result in saturated soil conditions, landslide dams can be 

expected to be more numerous and larger than those caused 

by the 1989 earthquake. Thus, the hazards due to potential landslide dams and their impoundments should be considered in planning for future earthquake hazard mitigation 

in California.

Downstream and upstream flooding are probably the 

most serious hazards to be expected from future landslide 

dams. Downstream flooding from breaching of a dam commonly is more serious, especially if the flood is unexpected; the severity of downstream flooding depends on 

the volume of the impoundment, the quickness of failure 

of the dam, and the velocity of flow. Upstream flooding is 

generally more gradual and leaves adequate time for evacuation. Other potential hazards from landslide dams include sedimentation, deterioration of water quality, and 

erosion of the channel or flood plain. Disruption or blockage of streamflow below the dam, as well as sedimentation, may pose a hazard to fish habitats.

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THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17, 1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT

IN THE SUMMIT RIDGE AREA, 

SANTA CRUZ MOUNTAINS, CALIFORNIA

By David K. Keefer, 

U.S. Geological Survey;

Gary B. Griggs, 

University of California, Santa Cruz;

and

Edwin L. Harp, 

U.S. Geological Survey

CONTENTS Renewed movement on the Upper Schultheis Road

__________ landslide —————————————————— 112

	Renewed movement on the Villa Del Monte

	Page landslide ——————————————————————— 114

Abstract ———————-————————————————————— C71 Renewed movement on the Hester Creek North

Introduction ————————————————————— 72 landslide—————————————————— 115

Physical, cultural, and seismic setting——————————— 72 Subsurface monitoring————————~————-— 116

Location, topography, vegetation, and population ———— 72 Inclinometers————————————————————— 116

Climate and rainfall ———————————————— 74 Piezometers ————————————————— 117

Geologic setting——————————————————————— 74 Discussion and conclusions——————————————————— 118

Seismicity and characteristics of the 1989 Loma Prieta Acknowledgments ———————————————-——- 127

earthquake —————————————————————-—— 75 References cited ———————————-————————————— 127

Previous landslides and landslide-hazard mapping in __________

the Summit Ridge area ————————————————— 75

Landslides caused by previous earthquakes ———— 75 ABSTRACT

Landslides caused by precipitation ————————— 75

Landslide damage to roads ———————— 81 In the Summit Ridge area, the 1989 Loma Prieta earthPrevious landslide-hazard mapping——————— 81 quake generated many anomalously large, complex landIdentification and characteristics of earthquake-induced slides that differed from other landslides triggered by this 

landslides ———————————————— 81 of Q^J comparable historical earthquakes. The area in

Data and criteria for identifying landslides ————-—— 81 , . , , , ,11-, , • ,- i

Summary of landslide characteristics--------- 83 which these large landslides occurred was immediately

Old Santa Cruz Highway landslide complex —————— 85 southwest of the San Andreas fault and contained abunUpper Schultheis Road landslide —————————— 89 dant other coseismic ground-deformation features. The

Rails Drive landslide———————————-———— 92 landslides were recognized and differentiated from other

Villa Del Monte landslide ———————————— 94 coseismic effects by detailed mapping and analysis of the

Sunset Drive area——————————————— 95 , , _/, , T _,_,.. Upper Skyview Terrace-Bel Air Court area ----- 95 geomorphology of the study area. In addition to mapping

Deerfield Road area ————————————— 96 and analysis, investigations included postearthquake moniLower Skyview Terrace area ————————— 99 toring of movement and ground-water levels; determinaOverall landslide dimensions, movement, and setting 99 tjon of subsurface conditions through drilling, trenching,

Subsurface conditions and materials ———————— 100 ^ ft § Qf earthquake.related damage to water wells;

Upper Morrell Road landslide——————————— 103 , , . , , , -,- it- Burrell landslide —-————————————- 104 and analytical slope-stability modeling.

Upper Redwood Lodge Road landslide —————— 104 Distinctive characteristics of the landslides in the SumAmaya Ridge landslide—————————————- 108 mit Ridge area included their large sizes: The landslides

Lower Schultheis Road West landslide-——-—--— 108 were as mucfr as 930 m long and as much as 1,220 m

Other large landslides--------------- 110 wid ^ ha(J surfac£ afeas a§ t a§ g5 ha Maximum

Postearthquake monitoring———————————-—————-— 110 ,,,.,,, , r r?~ .,-. ,

Surface monitoring---------------- no landslide depths ranged from 27 to possibly more than

Methods ——————————————————— no 100 m, and estimated landslide volumes were as great as

Results———————————————————— 111 27 million m3 .

	C71
C72 LANDSLIDES

Other distinctive features of the landslides included irregular shapes and boundaries, defined by discontinuous 

sets of surface cracks, scarps, and compressional features. 

Commonly, main scarps and heads were well developed 

and marked by conspicuous, relatively continuous scarps 

and cracks; flanks were moderately developed and delineated by discontinuous, locally echelon cracks; and toes 

were poorly developed, with local compressional features 

present on some landslides but absent on others. Measured landslide displacements, which ranged from 25 to 

244 cm, were small relative to landslide lengths, and the 

incomplete development of landslide boundaries probably 

resulted from these comparatively small displacements.

Material in the landslides was heterogeneous colluvium 

and bedrock consisting of poorly to moderately cemented 

sandstone, mudstone, siltstone, and shale, which were locally sheared, faulted, and folded. The largest landslides 

occurred where folding was most intense and faults were 

most numerous. Virtually all the earthquake-induced landslides exhibited geomorphic indications of previous landslide movement.

Postearthquake movement and ground-water levels were 

monitored through spring 1991. Rainfall during that 2- 

year period was below normal, except for a short time in 

February and March 1991, when renewed displacement 

and associated ground cracking occurred along and adjacent to the main scarps of three landslides. This activity, 

which was the only significant landslide movement recorded during the 2 years of monitoring, was probably 

due to local upslope migration (retrogressive failure) of 

the landslide heads. Measured ground-water levels during 

the 1991 period of high rainfall rose as much as 6.1 m.

These large, complex landslides occurred only where 

ground cracks produced by other coseismic processes were 

also abundant. This correlation indicates that the two types 

of features are mechanistically related. Many of the 

coseismic ground cracks were almost certainly produced 

either directly by tectonic deformation or by the effects of 

seismic shaking on preexisting planes of weakness. Once 

these ground cracks formed, they provided loci for 

downslope movement and landslide formation as shaking 

continued. This mechanism implies that landslides of this 

type are specifically generated by seismic shaking, a conclusion supported both by slope-stability modeling and by 

the historical record of landslides in the study area.

INTRODUCTION

The 1989 Loma Prieta earthquake (My=7.1, M=7.0) generated thousands of landslides throughout an area of approximately 15,000 km2 (see Keefer and Manson, this 

chapter). Most of these landslides were similar to those 

triggered by other worldwide historical earthquakes of 

approximately the same size (Keefer, 1984; Keefer and 

Manson, this chapter). In the Summit Ridge area, however, immediately southwest of the rupture zone (fig. 1), 

the earthquake produced many particularly large and complex landslides with uncommon characteristics. These landslides were restricted to a small area of approximately 40 

km2 that also contained the highest known concentration 

of coseismic ground cracks (see pi. 3; Spittler and Harp, 

1990; additional data from Ponti and Wells, 1991 and in 

press, and Harp, this chapter). Outside this area, landslides were predominantly shallow, small to moderatesize rock falls, rock slides, and soil slides and small to 

moderate-size slumps and block slides that typically had 

relatively simple morphologies (Manson and others, 1991; 

Keefer and Manson, this chapter). Inside this area, landslides were exceptionally large and deep, complex, irregular in shape, and bounded by intricate networks of 

discontinuous ground cracks and other features.

The landslides in the Summit Ridge area were first recognized and differentiated from other coseismic grounddeformation effects as a result of detailed mapping initiated 

immediately after the earthquake (Spittler and Harp, 1990). 

The landslides had caused considerable damage to residences, roads, and other structures, mostly in Santa Cruz 

County; and because of the potential continuing hazard, a 

comprehensive investigation was carried out as part of the 

U.S. Government disaster-relief effort to Santa Cruz 

County (Keefer, 1991). Components of this investigation 

included (1) mapping ground cracks; (2) surveying selected topographic profiles; (3) reviewing historical and 

scientific literature on pre-1989 landslides in the area; (4) 

compiling data on earthquake-induced damage to water 

wells in the area; (5) trenching across selected scarps; (6) 

drilling, sampling, and testing landslide materials; (7) installing and monitoring arrays of survey stakes and strain 

gages to measure any surficial, postearthquake landslide 

movements; (8) installing and monitoring borehole inclinometers and piezometers to measure subsurface movements and ground-water levels; and (9) performing 

analytical slope-stability modeling.

This paper describes the unusual features of these landslides, summarizes the results of investigations, and suggests a mechanism for landslide generation. Some aspects 

of the investigations are also discussed in other publications (Griggs and others, 1990; Spittler and Harp, 1990; 

William Cotton and Associates, 1990; Keefer, 1991; 

Marshall and Griggs, 1991; Harp, this chapter; Nolan and 

Weber, this chapter).

PHYSICAL, CULTURAL, AND SEISMIC 

SETTING

LOCATION, TOPOGRAPHY, VEGETATION, AND 

POPULATION

The Summit Ridge area is adjacent to the northeast 

edge of Santa Cruz County, Calif., approximately 80 km
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C73

southeast of San Francisco, 16 km south of San Jose, and 

16 km north of Santa Cruz (fig. 1). The boundaries of the 

area studied in most detail (see pi. 3), which contained 

most of the large, complex landslides, were chosen on the 

basis of criteria established for U.S. Government disaster 

assistance (Keefer, 1991).

Much of the north and northeast boundary of the study 

area coincides with the crest of the Santa Cruz Mountains, which separate the San Francisco Bay region to the 

north and east from the Monterey Bay region to the south 

and west. The highest point in the Santa Cruz Mountains 

is the summit of Loma Prieta, approximately 6 km east of 

the study area, which has an elevation of 1,155 m above 

mean sea level. Within the study area itself, elevation

ranges from approximately 160 to 661 m. The study area 

includes parts of Skyland Ridge, the features locally known 

as Summit Ridge and Amaya Ridge, and several adjacent 

ridges and valleys (see pi. 3). Surveyed profiles show that 

average slopes in the area typically range from 10° to 25°. 

Many slopes also exhibit irregular, benched topographic 

profiles, with alternating steep and gentle stretches. Much 

of the study area is covered with dense forest, dominated 

in some zones by coast redwood (Sequoia sempervirens) 

and in others by oak (Quercus sp.). Nonforested areas are 

typically covered by chaparral or grassland vegetation.

Despite the proximity of the study area to an urban 

metropolis of 5.9 million people in the San Francisco Bay 

region, the study area is primarily rural to locally subur122°15' 122°00' 121°45'

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SUMMIT Rinrr OIUUT RIDGE AREA

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Loma Prieta 

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epicenter

MONTEREY 

BAY

Figure 1.—Loma Prieta region, Calif., showing locations of study area, epicenter of 1989 earthquake, and rupture zone.
C74 LANDSLIDES

ban, with a total population of approximately 4,000. The 

largest communities are the Villa Del Monte neighborhood, which contains 195 residential lots and approximately 

165 single-family homes, and the Redwood Lodge-Summit Woods neighborhood, where approximately 100 lots 

are located. In addition to the several hundred private 

residences, one public school and several churches are 

located in the study area. More than 100 residences in the 

study area were heavily damaged by earthquake-induced 

landslides and ground cracks (Spittler and Harp, 1990).

CLIMATE AND RAINFALL

The area has a Mediterranean climate, characterized by 

warm, dry summers and cool, rainy winters. Temperatures rarely exceed 40°C or fall below 0°C. Virtually all 

precipitation occurs as rain, about 90 percent of which 

falls during the winter months of November through April, 

inclusive; precipitation varies substantially from year to 

year (fig. 2).

Mean annual precipitation in the Summit Ridge area 

ranges from 1,140 to 1,270 mm (Rantz, 1971). However, 

the 1989 Loma Prieta earthquake occurred in the midst of 

a 5-year drought: Annual precipitation during the 3 years 

before and the 2 years after the earthquake was, respectively, 71, 56, 64, 67, and 78 percent of normal (fig. 2). 

The earthquake also occurred near the end of the dry summer season: The only precipitation in the area between 

June 1 and October 17, 1989, was 30 mm of rain that fell 

between September 16 and 29. Thus, the area was unusually dry at the time of the earthquake.

GEOLOGIC SETTING

Bedrock in and around the Summit Ridge area consists 

primarily of Tertiary marine sedimentary rocks—mostly 

sandstone, mudstone, siltstone, and shale (see pi. 3). The 

rocks, which generally strike northwest, have been intensely folded and locally faulted, so that they typically 

dip steeply, are vertical, or are overturned.

The rocks in the area are assigned to the Butano Sandstone, San Lorenzo Formation, Vaqueros Sandstone, Lambert Shale, and Purisima Formation (see pi. 3: Clark and 

others, 1989, with landslide boundaries revised by R.J. 

McLaughlin and J.C. Clark, unpub. data, 1990; 

McLaughlin and others, 1991). These rocks are generally 

poorly to moderately cemented, contain numerous shear 

surfaces, and locally are deeply weathered, intensely fractured, or both. Exposures of bedrock within the area are 

few because the rocks are typically mantled by several 

meters or more of colluvium.

The geologic structure of the Summit Ridge area is 

dominated by northwest-striking faults and folds (see pi.

3). The San Andreas fault passes along the northeast 

boundary of the area. The Butano and Zayante faults, as 

well as many minor, unnamed faults, also pass through 

the area (see pi. 3). Major folds include the Laurel anticline, the Summit syncline, and the Glenwood syncline 

(see pi. 3).

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LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C75

SEISMICITY AND CHARACTERISTICS OF THE 

1989 LOMA PRIETA EARTHQUAKE

Previous historical earthquakes in the San Francisco 

Bay-Monterey Bay region have included 23 events of 

M>6.0, of which 2 were approximately as large as the 

1989 Loma Prieta earthquake (M=7.0 in 1838 and 

M=7.0 in 1868) and 1 was substantially larger (M=8.2-8.3 

in 1906; Wesnousky, 1986; U.S. Geological Survey, 

1990).

The hypocenter of the 1989 Loma Prieta earthquake 

(M^=7.1; M=7.0) was at a depth of approximately 18 km, 

located at lat 37°02' N., long 121°53' W., 4.1 km southeast of the study area (fig. 1). The earthquake is inferred 

to have ruptured a 40-km-long fault segment extending 

from near Pajaro Gap, east of Watsonville, northwestward 

through the Summit Ridge area, to just north of California 

Highway 17 (Plafker and Galloway, 1989; Working Group 

on California Earthquake Probabilities, 1990).

The aftershock distribution indicates that the fault plane 

dips approximately 70° SW. under the Summit Ridge area 

(Plafker and Galloway, 1989). The coseismic fault slip at 

depth had both right-lateral strike-slip and vertical, compressional components. The inferred fault slip at depth 

was approximately 1.9 m right lateral, 1.3 m reverse, and 

2.3 m total (Plafker and Galloway, 1989). No throughgoing 

surface fault rupture was found.

Preearthquake and postearthquake leveling surveys 

showed that the fault slip produced a zone of groundsurface subsidence northeast of the San Andreas fault and 

a zone of uplift southwest of the fault, in and around the 

Summit Ridge area. Maximum measured uplift was 592 

mm, and maximum measured subsidence was 173 mm 

(Marshall and others, 1991).

During the earthquake, ground shaking in the Summit 

Ridge area was violent, as indicated by eyewitness accounts and such effects as the snapping of large redwood 

trees, movement of heavy vehicles, and destruction of 

homes and other structures. The free-field strong-motion 

station closest to the study area was in Corralitos (fig. 1), 

maintained by the California Division of Mines and Geology. That station, located on landslide deposits 200 m 

from the San Andreas fault and 12 km southeast of the 

study area, recorded a peak horizontal ground acceleration of 0.64 g and peak vertical acceleration of 0.47 g 

(Shakal and others, 1989).

PREVIOUS LANDSLIDES AND LANDSLIDEHAZARD MAPPING IN THE SUMMIT RIDGE 

AREA

Preexisting landslide deposits are widespread in the 

Summit Ridge area, as well as throughout much of the 

rest of the Santa Cruz Mountains, and abundant historical

landslides have occurred in the Santa Cruz Mountains in 

association with earthquakes (Lawson, 1908; Youd and 

Hoose, 1978; Marshall, 1990), storms (Keefer and others, 

1987; Ellen and Wieczorek, 1988), and other events.

LANDSLIDES CAUSED BY PREVIOUS 

EARTHQUAKES

Landslides are known to have occurred in the Santa 

Cruz Mountains during both the October 8, 1865, earthquake (M=6.5) on the San Andreas fault and the October 

21, 1868, earthquake (M=7.0) on the Hayward fault. The 

landslides in those events, however, were poorly documented, and the available historical information is fragmentary (Youd and Hoose, 1978; Marshall, 1990).

Documentation of landslides caused by the much larger 

April 18, 1906, San Francisco earthquake (M=8.2-8.3), 

though incomplete, was substantially more extensive, owing to the report by Lawson (1908) and many other reports, books, and newspaper articles. The 1906 earthquake 

triggered or reactivated thousands of landslides throughout an area of approximately 32,000 km2 (Keefer, 1984), 

including all of the Santa Cruz Mountains.

The severity of landsliding in the Summit Ridge 

vicinity during the 1906 earthquake was described in general terms by an article in the Santa Cruz Morning Sentinel of May 1, 1906 (in Griggs and others, 1990), as 

follows:

From all reports, the higher altitudes of the Santa Cruz 

Mountains all the way from beyond Saratoga to Loma 

Prieta, on both slopes, appear to have been more seriously disturbed than many localities in the valleys and 

foothills. In places the roads are or were impassable, not 

only on account of great avalanches of stones and earth, 

but of wide deep cracks in the earth where the ground 

was rent asunder.

More specific, detailed descriptions of the ground cracks 

and landslides caused by the 1906 earthquake in and 

around the Summit Ridge area are listed in table 1. Even 

though the reports from the 1906 earthquake are incomplete and locations are, for the most part, imprecise, the 

available information indicates that landslides occurred in 

many parts of the Summit Ridge area, including Summit 

Ridge, Skyland Ridge, the Morrell, Burrell School, and 

Laurel areas, and along Old San Jose and Redwood Lodge 

Roads (see pi. 3). These reports indicate that several of 

the landslides were large and that, at several localities, 

many landslides occurred.

LANDSLIDES CAUSED BY PRECIPITATION

As with the historical record of landslides caused by 

earthquakes, the record concerning landslides caused by
C76 LANDSLIDES

Table 1.—Landslides and ground cracks in and around the Summit Ridge area produced by the San Francisco earthquake 

of April 18, 1906 [Modified from Youd and Hoose (1978, table 6) and Marshall (1990)]

Location 

(pi. 4)

Description Original Reference

Eva A 10-acre slide dammed the creek at Eva station until the water crossed the 

railroad tracks following a new raised channel.

A "huge earth slide dammed the creek at Eva station, creating a natural lake 

that blocked all [railroad] travel...for months." It took until December to 

remove the slide and lake.

Alma to Wrights The railroad between Alma [6.4 km northwest of Wrights] and Wrights was 

impassable, owing to several landslides and boulders on the tracks.

A landslide dammed Los Gatos Creek at the News Letter Ranch, forming a 

lake ranging in depth from 50 to 100 ft.

Santa Cruz Morning Sentinel. 

April 26, 1906, p. 8.

Young (1979, p. 39)

Santa Cruz Morning Sentinel. 

April 26, 1906, p. 8.

Santa Cruz Morning Sentinel. 

May 1, 1906, p. 2.

Patchin to 

Wrights

Wrights

"On the ridge road, about 5 miles northwest of Wright Station, the fault 

again shows slightly in a few 2-inch cracks.... Going down the slope from 

here to Wright, the cracks rapidly become larger. ...At Patchin, 3 miles west 

of Wright Station, there are fissures over a foot wide trending mainly in the

direct line of the fault (S. 33° E.). Several stretches of numerous small 

cracks alternating with a few long continuous fissures, mark the course from 

Patchin to Wright Station."

"Just north of Wright's Station, on the west bank of Los Gatos Creek, there 

was a landslide 0.5 mile wide which had slid into the creek and dammed it. 

The top of this slide was near the Summit school-house and was close to the 

main fault-line."

"The main fault fracture is about 500 feet northeast of the [Summit] hotel, 

and a secondary crack close to it had a downthrow of from 5 to 7 feet on the 

north or downhill side. The crack was about 4 feet wide here, and the line of 

fracture was parallel with the direction of the ridge. The Summit schoolhouse was dropt [sic] 4 feet downhill from its original position toward the 

northeast." Just below the Summit schoolhouse was the headscarp of the 

landslide that dammed Los Gatos Creek near Wright Station.

"At Freely's place, 4 or 5 miles north of Morrell's, some 15 acres of 

woodland have slid into Los Gatos Creek, making a large pond. There are 

many other slides in the neighborhood and many broken trees."

"Into this [Los Gatos] creek, from the Freely ranch, some ten acres of land 

was thrown in a great landslide. At the head of the creek is the long tunnel 

which cuts under the saddle, from Wright's to Laurel."

"Landslides were abundant, especially in the Santa Cruz Mountains, where 

the topography is more rugged. One slide, a few miles from Wright's 

Station, involved eight to ten acres of ground."

"Large fissures and ridges" formed in the ground at Wrights.

Lawson (1908, p. 109-110).

Lawson (1908, p. 276).

Lawson (1908, p. 275-276).

Lawson (1908, p. 278).

Jordan (1907, p. 27).

Carey (1906, p. 297).

Santa Cruz Evening Sentinel. 

April 21, 1906, p. 2.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C77

Table 1.—Continued.

Location 

(pl. 4)

Description Original Reference

Wrights to Laurel The Wrights-to-Laurel railroad tunnel collapsed in the earthquake just hours 

before the planned inaugural run of the first standard-gage train along this 

previously narrow-gage line. Where the tunnel crosses the summit, it was 

offset laterally 5 ft. Almost all other railroad tunnels in the Santa Cruz 

Mountains partly collapsed or were blocked by slides at their entrances.

The Wrights-to-Laurel railroad tunnel cracked in the middle and settled 

several inches out of line.

"The tunnel floors have raised as much as three to four feet in places...."

Payne (1978, p. 49).

Young (1979, p. 38-39).

Santa Cruz Morning Sentinel. 

April 26, 1906, p. 8.

Laurel to 

Glenwood

Morrell Ranch

Minor slides blocked the Laurel-to-Glenwood tunnel.

About 400 ft of tunnel No. 3 between Laurel and Glenwood caved in.

Young (1979, p. 39).

Santa Cruz Evening Sentinel. 

April 19, 1906, p. 5.

"The Morrell ranch is located 1 mile south of Wright's Station.... the house Lawson (1908, p. 276-277).

itself was built exactly upon a fissure, which opened up under the house at the

time of the earthquake. The house was completely wrecked, being torn in

two pieces and thrown from its foundation....There was an apparent

downthrow upon the northeast side of the fault, as seen in the orchard; but

under the house the vertical movement was not so apparent. ...The fence and

road near the house were crost [sic] by the fault and showed an offset which

indicated a relative movement of the southwest side toward the southeast.

...The "splintering" of the main fracture raised a long, low ridge across

which a creek had been forced to cut its way thru a vertical distance of 1.5 feet

to get down to its original level."

"The earthquake crack past thru [sic] [the Morrell] ranch, a branch of it going 

under the house. The main body of the house was thrown to the east, away 

from the crack, the ground there slumping several feet and the house being 

almost totally wrecked. All thru [sic] the orchard the rows of trees are shifted 

about 6 feet, those on the east side being farther north, and the east side, 

which is downhill, seems to have fallen. The crack is largely open and in 

one place is filled with water. This should be attributed to slumping. A little 

farther on, the crack passes thru [sic] a grassy hill on which there is no 

slumping. The Morrells say that this hill has been raised. What appears to 

be the fact is that the east side of the hill overrides the other. The whole top 

of the hill is more or less cracked for a width of about 10 feet. The east side 

is a little higher than the west side, and it looks as though the hill had been 

shoved together and raised, the east overriding. About 1 mile beyond 

Morrell's house, at the end of the ranch, there is a blacksmith shop, and the 

road is crost [sic] by the crack. Here there is a break of 3 or 4 feet like a 

waterfall, the east side being the lower; but this is part, I take it, of the 

general slumping of the east side of the crack where it stands near the ravine 

above Wright. Morrell's place is right over the Wright tunnel, the tunnel 

and the rocks near by being finely broken rock and very much subject to 

slides and other breaks."

Lawson (1908, p. 277-278).
C78 LANDSLIDES

Table 1.—Continued.

Location 

(pi. 4)

Description Original Reference

Burrell

Burrell [Laurel] 

Creek

Highland

Skyland

"In the Burrell district there is one fissure in the hillside fully 3 feet wide. 

This crossed the road and tumbled Ingraham's store building into the gulch."

"Near the Burrell school-house, 1.5 miles southeast of Wright Station, a 

crack extends across the road by a blacksmith shop and shows a downthrow 

of four feet on the northeast."

"Gulches appear to have been contracted as the bridges crossing them show 

that they were squeezed. The banks of Burrell Creek appear to have 

approached each other, so that the creek has become very much narrowed. 

Water pipes were broken and twisted, and filled with dirt."

"Half a mile to the northwest of the [Beecher] house [on Loma Prieta 

Avenue], a fissure 2 feet wide appeared.... The fissure runs from north to 

south, and the earth was piled up on the west side from 2 to 4 feet high across 

the road. On Highland, a mile to the west, a fissure 5 feet was opened at an 

elevation of 2,500 feet."

"Large landslides occurred in the neighborhood."

'The road between the King and Crane places has slid into the orchard 

below."

'There seems to have been a narrow strip, about two miles wide, east of 

Skyland, with Skyland as the center, where hardly a building remains 

standing or unbroken. ..."One section of road of about 3 miles long is 

hardly without a crack.... At one place in the road it has been lifted fully 5 

feet." The road was still impassable after 3 days of heavy work by a crew of 6

Santa Cruz Morning Sentinel. 

April 24, 1906, p. 7).

Lawson (1908, p. 276).

Lawson (1908, p. 276).

Lawson (1908, p. 276).

Lawson (1908, p. 278).

S anta Cruz Evening Sentinel. 

April 21, 1906, p. 4.

Santa Cruz Morning Sentinel. 

April 24, 1906, p. 7.

"...the cracks run up over the ridge just west of Skyland. Large fissures show 

in the orchards and fields on the eastern side of the ridge, but are not so 

evident on the western slope. Here instead, great landslides occurred, and 

redwoods were snapt [sic] off or uprooted."

"The slides which obliterated Fern Gulch at Skyland...lie to the west of the 

crack [fault]."

On the western slopes of the ridge just west of Skyland, several earthavalanches were caused by the shock; and great slides of a similar character 

occurred on both sides of Aptos Creek for 0.75 mile. Besides these, there 

were many smaller earth-avalanches in many parts of the Santa Cruz 

Mountains which can not be enumerated."

Lawson (1908, p. 110).

Lawson (1908, p. 278).

Lawson (1908, p. 389-390).
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C79

Table I.—Continued.

Location 

(pl. 4)

Description Original Reference

"About Four 

Miles South of 

Wright Station" 

[probably near 

Laurel township]

San Jose-Soquel 

Road

Redwood Lodge 

Road

Upper Sequel 

Creek

Hinckley Creek 

(Olive Springs) 

[2.6 km SSE of 

study area]

'The ridge...was full of cracks, ranging up to 2 and 3 feet in width, and in 

length from a few rods to 0.25 mile, all trending west of north to northwest. 

...The canyon south of us was filled with landslides. In this canyon the 

stratification of the rocks is plainly shown. The strike is northwestsoutheast and the dip is almost vertical. The cracks coincide in direction 

with the strike of the strata. Cold water was flowing from some of the 

cracks."

San Jose-Soquel Road was extensively damaged in the earthquake but was 

reopened by July 4, 1906.

The earthquake "severely damaged Redwood Lodge Road, and workmen took 

until June 1906 to complete repairs."

A newspaper article of May 7, 1906, reported an eyewitness story that the 

headwaters of Sequel Creek were dammed by two landslides, forming a pond 

100 ft deep. The upper Sequel Creek basin was reportedly ravaged by fallen 

trees and boulders, as well as "great fissures and landslides. ...The roads were 

gone, and in their stead were chaotic masses of debris from the hillsides." 

An article of May 9 corrected the account after the site was visited by another 

eyewitness. This second account claimed that the damming of the creek 

resulted not from landslides but from an upward vertical displacement of the 

creekbed of from 5 to 30 ft in places. The pond averaged 15 ft in depth but 

was no more than 20 ft deep. Many fissures in the ground near Sequel Creek 

were "now largely filled in."

"The mountains are said to have come together and 17 lives...lost." [Nine 

people were actually killed.]

With the first severe shock of the earthquake, a landslide 500 ft wide, 

extending up to the ridgetop, descended with "extraordinary speed," burying 

the Loma Prieta lumber mill under a mass of rock and trees of "about 100 feet 

in depth at the worst places and gradually diminishing at the edges to 25 

feet." Nine men were buried instantly, while others, only several hundred 

feet away, were spared. "The mountainside where the land fell was swept bare 

of vegetation. Massive redwoods and pines were jammed on top of the mill 

in the gulch below. ...The landslide filled the water course. The stream was 

dammed, and the water rose to a depth of sixty feet in the gulch. A pump was 

set to working, and the water is now being used to wash away the earth from 

the machinery." Hundreds of people were involved in a massive digging 

effort in the following week, but only three bodies had been discovered by 5 

days later. [More than a year passed before the last body was finally removed 

from the debris.]

Lawson (1908, p. 278).

Payne (1978, p. 17).

Payne (1978, p. 17).

Santa Cruz Evening Sentinel. May 

7, 1906, p. 1, and May 9, 1906, 

p. 1)

Santa Cruz Evening Sentinel. 

April 18, 1906, p. 1, and April 

19, 1906, p. 7.

Santa Cruz Morning Sentinel. 

April 26, 1906, p. 1)
C80 LANDSLIDES

Table I .—Continued.

Location 

(pi. 4)

Description Original Reference

The mill was buried under 60 ft of earth and trees, whereas the nearby 

bunkhouse, where nine men were sleeping, was buried under 10 to 15 ft of 

debris.

A second slide occurred during an aftershock at 11 p.m. April 19, interfering 

with rescue efforts.

"Near Olive Springs, 12 miles north of Santa Cruz, an earth-avalanche 

demolished Loma Prieta Mill and killed several men."

"At Santa Cruz the inhabitants reported that near Olive Springs, 12 miles 

north of Santa Cruz, a landslide demolished Loma Prieta Mill and killed 9 

men."

"...the [fault] crack goes into Hinckley's Gulch, in which the Loma Prieta 

Mills are situated, and which are buried under the slides."

"On the northern side of Bridge Creek Canyon there are typical cracks from 1 

to 8 inches wide, and here also occurred a great landslide which buried the 

Loma Prieta Mill."

"Wreck of Loma Prieta Sawmill, Hinckley's Gulch, Santa Cruz County." 

[Picture caption]

"Site of Loma Prieta Sawmill, covered to a depth of 125 feet." [Picture 

caption]

"Loma Prieta Lumber Company's Mill. The mill, boarding house and other 

buildings of the plant were situated in a gulch, and were overwhelmed by a 

portion of the mountain, 1500 feet long, 400 feet wide, and 100 feet deep, 

which slid down upon them. The mill and everything in the gulch were 

forced up the opposite slope of the mountain and there buried to a depth of 

one hundred feet. Pine and redwood trees 100 feet high came down with the 

slide and are now standing over the mill site as though they had grown there. 

Nine men were killed."

"LOMA PRIETA GO'S LOSS. When the earthquake occurred yesterday 

morning it caused a large mountain of earth to slide into the canyon and 

completely covering the new mill. Continuing its course up the mountain 

on the other side it covered what is known as the bunk house and buried ten 

men, who were asleep at the time."____ ____________________

Patten (1969, p. 79).

Santa Cruz Evening Sentinel. 

April 21, 1906, p. 2.

Lawson (1908, p. 389).

Lawson (1908, p. 271).

Lawson (1908, p. 278).

Lawson (1908, p. 110).

Jordan (1907, p. 30).

Jordan (1907, p. 31).

Salinas Daily Index. April 25, 

1906, p. 3.

Salinas Daily Index. April 19, 

1906, p. 3.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C81

rainfall in and around the Summit Ridge area is incomplete. Sources of available data include the following.

1. Marshall (1990), who compiled information on landslides 

in the central Santa Cruz Mountains before and after the 

1865 and 1906 earthquakes, reported that landslides were 

abundant there in rainstorms that occurred during the 

winters of 1866-67 and 1906-7. For example, on December 29,1866, the Santa Cruz Evening Sentinel reported that 

the road between Santa Cruz and Santa Clara was impassable after heavy rains caused landslides and flooding 

(Marshall, 1990). Landslides along roads and railroads in 

the central Santa Cruz Mountains were reported from at 

least five storms in the winter of 1906-7, although the 

accounts do not mention the Summit Ridge area specifically (Marshall, 1990).

2. Wieczorek and others (1988) mapped landslides in Santa 

Cruz County caused by the storm of January 3-5, 1982. 

This storm, which produced as much as 610 mm of rain 

within 34 hours, caused more than 18,000 landslides and 

more than $66 million in landslide-related damage throughout the San Francisco Bay-Monterey Bay region (Ellen and 

others, 1988). This is the only storm for which detailed 

and comprehensive data on landslide distribution are available. Within the Summit Ridge area, approximately 20 

shallow, fast-moving debris flows were mapped (Wieczorek 

and others, 1988); this number was probably a minimum 

because the dense vegetation cover in many areas made 

detection of debris flows difficult. In addition to mapped 

debris flows, Santa Cruz County road-repair records 

show that lower Schultheis and Redwood Lodge Roads 

required repairs as a result of landslide damage in 1982 

(fig. 3; table 2).

3. Keefer and others (1987) included the Summit Ridge area 

in a zone of high landslide concentration associated with 

the storm sequence of February 12-21, 1986, but did not 

report the locations of individual landslides within that 

zone.

4. In addition to the landslide activity discussed above, 

Brown (1988) reported that rainstorms caused significant landslide activity in parts or all of the San Francisco Bay-Monterey Bay region during the winters of 

1949-50, 1955-56, 1961-62, 1962-63, 1964-65, 

1966-67,1968-69,1969-70,1972-73,1974-75, and 1977- 

78. Documentation of landslides from these seasons, however, is fragmentary, and the number of landslides that may 

have occurred in the Summit Ridge area, if any, is unknown.

LANDSLIDE DAMAGE TO ROADS

Griggs and others (1990) described landslide damage 

to roads in the Summit Ridge area, obtained from a review of Santa Cruz County Public Works Department files

and a field trip through the area with Ray Geyon, one of 

the department's senior staff members. Landslide damage 

reportedly occurred at seven localities in 1975, 1976, 1980, 

1982, 1983, and 1986 (circled numbers, fig. 3; table 2). 

Although all seven of these localities were also sites of 

landslides and (or) ground cracks generated by the 1989 

Loma Prieta earthquake, the pre-1989 landslides were all 

smaller than those triggered by the earthquake.

PREVIOUS LANDSLIDE-HAZARDS MAPPING

Preexisting landslide deposits in the Summit Ridge area 

are shown on the map of landslide deposits in Santa Cruz 

County by Cooper-Clark and Associates (1975), which 

was prepared from interpretation of vertical, black-andwhite aerial photographs (fig. 4). Limitations of that map 

include a minimum size of landslide deposits that could 

be mapped (length or width of 15 m), possible incomplete 

identification of landslide deposits in areas of dense vegetation, and difficulties in differentiating landslide deposits from such similar-appearing features as alluvial terraces 

(Cooper-Clark and Associates, 1975). To reflect the various degrees of certainty in identification, landslide deposits were classified as "definite" (D), "probable" (P), or 

"questionable" (?) (fig. 4). Many landslide deposits are 

also shown on U.S. Geological Survey geologic-quadrangle 

maps of the study area (see pi. 3; Clark and others, 1989, 

with landslide boundaries revised by RJ. McLaughlin and 

J.C. Clark, unpub. data, 1990; McLaughlin and others, 

1991).

IDENTIFICATION AND 

CHARACTERISTICS OF EARTHQUAKEINDUCED LANDSLIDES

DATA AND CRITERIA FOR IDENTIFYING 

LANDSLIDES

Coseismic ground cracks in the Summit Ridge area were 

probably the result of several different processes, including landsliding. Other crack-forming processes are inferred 

to include discontinuous surface fault rupture (Aydin and 

others, in press), slip along bedding planes (Cotton, 1990; 

Ponti and Wells, 1991), adjustments to the local tectonic 

uplift that accompanied the earthquake (Ponti and others, 

1990; Ponti and Wells, 1991), and extension over active 

bedrock folds (Ponti and Wells, 1991). Many of the ground 

cracks produced by the various coseismic processes, including landsliding, showed evidence of control by structures in the underlying bedrock, such as bedding planes 

and faults (Ponti and others, 1990; Keefer, 1991; Ponti 

and Wells, 1991 and in press; Harp, this chapter).
C82 LANDSLIDES

Table 2.—Landslide-related damage to roads before the 1989 Loma Prieta earthquake, as reported by the Santa 

Cruz County Public Works Department

[Modified from Griggs and others (1990)]

Locality Description and date of damage 

(fig. 3)

Lower Schultheis Road-1982: The slope below Lower Schultheis Road failed in 

1982 approximately 0.5 km east of the intersection with Laurel Road, destroying 

the roadway and requiring evacuation of local residents. A private company was 

retained to engineer the slope stabilization. Since repair work was completed in 

1983(7), the road has settled at least an additional 60 cm. In 1989, earthquakeinduced ground cracks were found across the road, around the margins of the 

previous failure. This failure was on the northeast flank of the larger Lower 

Schultheis Road East landslide.

Redwood Lodge Road—repeated failures: Over the years, Redwood Lodge Road 

has had many maintenance problems due to slope failure associated with heavy 

rainfall, in particular during 1983 and 1986. Numerous landslides, both large and 

small, have plagued the road along its entire length. The only landslide with 

significant movement as a result of the 1989 Loma Prieta earthquake was the 

Upper Redwood Lodge Road landslide, which displaced the road west of its 

intersection with Old San Jose Road. This section of road, in particular, has a long 

history of repeated landslide damage and repair.

Redwood Lodge Road--1981-82 and 1983:

During the winter of 1981-82, a large landslide covered the road near the 

confluence of Laurel Creek and Burns Creek. Residents living below the damaged 

road were trapped and had to be evacuated. The road was reconstructed, sustained 

additional failure during 1983, and was again reconstructed. Individual slide 

blocks still remain on the steep slope above the road. Slope instability in this area 

is typically associated with heavy rainfall and has been a concern for at least 30 to 

40 years. The 1989 Loma Prieta earthquake reactivated the landslide; ground 

cracks were opened along approximately 240 m of steep hillside above the road 

and extended headward from the preexisting main scarp.

Morrell Road-1982-83: A landslide, approximately 60 m long by 60 m wide, 

buried Morrell Road immediately south of its crossing of Laurel Creek. The main 

scarp of the landslide was approximately 10 m upslope from the road. The County 

of Santa Cruz had to evacuate local residents and rebuild the road, which was 

covered with more than 6 m of debris. The slope was stabilized in 1984. Ground 

cracks from the Lower Morrell landslide complex, which moved during the 1989 

Loma Prieta earthquake, crossed the road on both the right and left flanks of the 

1982-83 landslide.

Morrell Road-1980: Northwest of where Morrell Road crosses Laurel Creek, a 

stretch of the road moved downhill approximately 2 to 3 m during a 2-year period. 

A house was moved to protect it from landslide damage. The slope movement was 

relatively rapid, forming large cracks and a 1.2-m-high scarp across the road; 

however, the landslide did not bulge below the road to compensate for the 

extensional movement associated with this scarp. County of Santa Cruz road 

crews graded and filled the roadway until the movement stopped. This area was on 

the eastern margin of the Upper Morrell Road landslide, which moved during the 

1989 Loma Prieta earthquake.

Morrell Road-repeated damage: Although the area downslope from the 1989 

Upper Morrell Road landslide was never reported as a problem area, several 

stretches of the road have been patched repeatedly. Road-repair records did not 

identify this patching as landslide related, but the repeated, slow offset of the road 

may be related to a landslide. Ground cracks caused by the 1989 Loma Prieta 

earthquake crossed the road in several places. Cutbanks both upslope and 

downslope from the road have required cribbing and maintenance in the past, 

evidently because of local roadcut failures. Several of these roadcuts failed during 

the 1989 Loma Prieta earthquake.

Villa Del Monte-1975-76 and repeated damage: Within the Villa Del Monte 

landslide are several areas that have required road maintenance, most of which is 

evidently related to local cribbing or fill failures. In 1975-76, the hillside upslope 

from Skyview Terrace moved slowly downslope during a period of 2 years. County 

of Santa Cruz maintenance crews cleared the road approximately once a week 

during that time.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C83

Because of the complexity of the coseismic ground 

cracking, the extent of large landslides was recognized 

only by interpreting detailed postearthquake maps of the 

area (Spittler and Harp, 1990, with landslide boundaries 

defined by Keefer, 1991). These maps were initially compiled on a planimetric base at a scale of 1:4,800 (Spittler 

and Harp, 1990), and later on a topographic base at a 

scale of 1:6,000 (Keefer, 1991).

Supplemental data for delineating some landslide features were obtained from a postearthquake survey of water wells (Brumbaugh, 1990). However, these water-well 

data were used only on a limited basis because (1) most 

reports of damage (or absence of damage) were secondhand, (2) many well locations were determined only approximately, (3) the causes of well damage were typically

unknown, (4) the extent to which reportedly undamaged 

wells were actually surveyed is unknown, and (5) the 

ground displacements that the wells could tolerate without damage are also unknown.

SUMMARY OF LANDSLIDE CHARACTERISTICS

Within the Summit Ridge area, 20 large landslides and 

landslide complexes, with surface areas larger than 1 ha, 

were identified as having moved during the earthquake 

(see pi. 3; table 3). (The term "landslide complex" here 

denotes a feature made up of several to many coalescing, 

juxtaposed, and (or) superimposed landslides.) The largest landslide complex had an area of about 85 ha. Of

121°57'30"

37°07'30" —

EXPLANATION

Preearthquake landslide damage 

to road - see table 2

Secondary

Unpaved

Stream

Base from Griggs and others, 1990 

Contour interval 200 feet

0.5 

I

1 KILOMETER 

j

Figure 3.—Summit Road area, showing localities of preearthquake landslide damage to roads (circled numbers) as reported by Santa Cruz County 

Public Works Department (modified from Griggs and others, 1990). See table 2 for detailed descriptions of localities.
C84 LANDSLIDES

these landslides and landslide complexes, 18 were within 

the area delineated for detailed study (see pi. 3). Most of 

the largest landslides and landslide complexes were on 

the southwest flank of Summit Ridge, in an area extending eastward from California Highway 17 to near Morrell 

Road. In that area, about 60 percent of the ridge flank was 

involved in large earthquake-induced landslides (see pi. 

3). The characteristics of the landslides are summarized 

in table 3, and the terms used to describe various landslide features are defined in figure 5.

In general, such surficial features as scarps, ground 

cracks, or pressure ridges were not continuous around the

boundaries of the landslides in the Summit Ridge area 

(fig. 6). The best developed, most continuous sets of cracks 

and scarps typically occurred in and around the landslide 

crowns and heads (figs. 5, 6). Those cracks and scarps 

commonly (1) had trends approximately across the slope 

(perpendicular to the gradient); (2) were linear or, more 

commonly, arcuate and concave downslope in plan view; 

and (3) exhibited extensional components of displacement 

and (or) downslope-facing scarp segments, features consistent with generation by downslope movement. Such 

cracks and scarps formed the main scarps of the landslides, as defined in figure 5.

12V57'30" 12V55'

37'07'30'

0 0.5 1 MILE 

i i i

i i

0 0.5 1 KILOMETER

Limit of 

study area

37*05' —

Figure 4.—Summit Ridge study area, showing generalized boundaries of large earthquake-generated landslides (diagonal-lined areas) in 

relation to generalized landslide deposits and landslide scarps (dotted areas) identified on preearthquake landslide-hazard map of Cooperdark and Associates (1975): D, definite; P, probable; ?, questionable.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C85

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Downslope from the main scarps, landslide 

flanks were delineated either by cracks trending 

nearly parallel to gradient or by complex sets of 

echelon cracks (fig. 6). In some places, flank 

cracks were continuous with the main scarps farther upslope. These flanks cracks exhibited relative displacements consistent with downslope 

movement, with components of right-lateral shear 

along right flanks and left-lateral shear along left 

flanks.

Downslope from some landslide flanks were 

local to relatively continuous pressure ridges or 

areas of bulging ground, convex upward in profile, that were interpreted as marking landslide 

toes (fig. 5). In several places where such features were absent, the positions of other ground 

cracks suggested that the landslides extended all 

the way to the stream channels at the bases of the 

slopes. In addition to main scarps, flank cracks, 

and toe features, the landslides also contained 

abundant internal scarps and cracks, as well as a 

few internal ridges (fig. 5).

In the following sections, we describe several 

of these landslides and landslide complexes individually to illustrate their characteristics and to 

provide the evidence used to delineate them and 

to differentiate them from other coseismic features. The landslides and landslide complexes described here include all of the largest, those where 

the most detailed investigations were performed, 

and those with features judged to be particularly 

significant. Following the individual descriptions 

is a summary of the characteristics of the other 

landslides.

OLD SANTA CRUZ HIGHWAY 

LANDSLIDE COMPLEX

The Old Santa Cruz Highway area (area 2, pi. 

3), on the southwest flank of Summit Ridge, encompasses a complex of earthquake-induced landslides, about 670 m long, 1,300 m wide, and 85 

ha in area (fig. 7).

The longest ground-cracking feature within this 

area is a nearly continuous, southeast-trending line 

of cracks and scarps (fig. 8), approximately 1,200 

m long (leg A-A', fig. 7A). Displacements across 

this feature vary along its length; maximum displacement components are about 60 cm vertical 

(down to the southwest), 75 cm left lateral, and 

15 cm extensional.

A second, virtually continuous, 420-m-long 

scarp strikes parallel to this line of cracks and 

scarps and crosses Summit Road 500 m farther
C86 LANDSLIDES

ORIGINAL GROUND 

SURFACE

MAIN SCARP - A steep surface on the undisturbed ground around the periphery of the slide,

caused by the movement of slide material away from undisturbed ground. The projection

of the scarp surface under the displaced material becomes the basal shear surface. 

INTERNAL SCARP - A steep surface on the displaced material produced by differential

movements within the sliding mass. 

HEAD - The upper parts of the slide material along the contact between the displaced

material and the main scarp.

TOP - The highest point of contact between the displaced material and the main scarp. 

TOE OF BASAL SHEAR SURFACE - The intersection (sometimes buried) between the lower

part of the basal shear surface and the original ground surface. 

TOE - The margin of displaced material most distant from the main scarp. 

TIP - The point on the toe most distant from the top of the slide. 

FOOT - That part of the displaced material that lies downslope from the toe of the basal

shear surface. 

MAIN BODY - That part of the displaced material that overlies the basal shear surface

between the main scarp and the toe of the basal shear surface. 

CROWN CRACKS - Cracks in the ground surface upslope from the crown. 

CROWN - The material that is still in place, practically undisplaced, and adjacent to the

highest parts of the main scarp. 

FLANK - The side of the landslide. 

ORIGINAL GROUND SURFACE - The slope that existed before the movement that is being

considered took place. 

LEFT AND RIGHT - Compass directions are preferable in describing a slide, but if these

terms are used, they refer to the slide as viewed downslope from the crown. 

SURFACE OF SEPARATION - The surface separating displaced material from stable material

but not known to have been a surface on which failure occurred. 

DISPLACED MATERIAL - The material that has moved away form its original position on the

slope. It may be in a deformed or undeformed state. 

ZONE OF DEPLETION - The area within which the displaced material lies below the original

ground surface. 

ZONE OF ACCUMULATION - The area within which the displaced material lies above the

original ground surface. 

LENGTH (L) - The distance from top to tip. 

WIDTH (W) - The maximum distance from the left flank to the right flank, measured

perpendicular to length. 

DEPTH (D) - The maximum depth to the basal shear surface, measured perpendicular to the

ground surface. 

BASAL SHEAR (or RUPTURE) SURFACE - The surface at the base of the main body on which

landslide movement took place.

Figure 5.—Idealized 

deep-seated, complex 

landslide, illustrating 

nomenclature for constituent parts (modified 

from Varnes, 1978).
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C87

east (leg B-B', fig. 7A). Measurements across this scarp 

showed 20 to 30 cm of movement downslope (to the southwest). Stratigraphic and structural relations exposed in a 

trench excavated across the scarp revealed at least three 

and, possibly, many more, pre-1989 movements, including one that probably took place during the 1906 San 

Francisco earthquake (Keefer, 1991; Nolan, in press; Nolan 

and Weber, this chapter).

These two long ground-crack features have characteristics that are consistent with formation by landslide movement and significant control by bedrock structure. 

Characteristics consistent with a landslide origin are strikes 

approximately perpendicular to the slope inclination and 

measured displacements that in nearly all places are 

downslope (to the south and southwest; U.S. Geological 

Survey staff, 1989; Spittler and Harp, 1990). Characteristics that indicate structural control and, possibly, a tectonic rather than landslide origin are (1) strikes parallel to

other structurally controlled cracks, (2) linear trends and 

great lengths, (3) a few uphill-facing scarp segments, and 

(4) location of the 420-m-long scarp approximately over a 

fault mapped by McLaughlin and others (1991), as shown 

in plate 5.

If these two features are of landslide origin, the landslide movement was relatively deep and involved part of 

the crest of Summit Ridge, north of Summit Road near 

California Highway 17. Even if the cracks and scarps were 

produced originally by coseismic tectonic processes, these 

features could also have served as surfaces of weakness 

along which landslide movement took place later in the 

earthquake. The seven reportedly undamaged water wells 

between these two features (wells 173-175, 177-180, fig. 

IB) and the linearity of the line of cracks and scarps (leg 

A-A', fig. 7A) where it crosses a deep valley north of 

Summit Road (loc. C, fig. 7) are consistent either with 

landslide movement on a basal shear surface more than

Tension cracks and scarps

Left-stepping, echelon 

tension cracks

Right-stepping, echelon 

tension cracks

Warped and bulged ground 

(may or may not be present)

Figure 6.—Idealized landslide bounded by discontinuous surface features, as was typical for large landslides in Summit Ridge area (modified from 

William Cotton and Associates, Inc., 1990).
C88 LANDSLIDES

0 100 200 300 METERS

I If;
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C89

100 m deep or with movement produced by some other 

deep-seated process. Damage to wells immediately east 

of California Highway 17 and south of Old Summit Road 

(wells 171, 220, 222, fig. IB), all reportedly disturbed at 

depths of 21 to 23 m (Brumbaugh, 1990), indicates that a 

21- to 23-m-deep landslide shear surface underlies that 

area.

Downslope and southwest of the line of cracks and 

scarps (leg A-A', fig. 7A) are shorter, arcuate scarps and

EXPLANATION

[Composite explanation for part A of figures 7,9, 12, 17, and 20 and figures 

19, 21, 22, and 24-28. Surface features modified from Spittler and Harp 

(1990). Symbols may be combined]

Fissure, fracture, or shear generated by the 1989 Loma Prieta earthquake—Dashed where approximately located; 

dotted where concealed; queried where uncertain. Arrow 

indicates direction of separation where present

Fracture generated by the 1989 Loma Prieta earthquake 

along which there has been horizontal displacement— 

Half arrows indicate direction of relative horizontal movement

Scarp generated by the 1989 Loma Prieta earthquake— 

Formed by relative vertical displacement along fissure. 

Dashed where approximately located; hachures on downthrown side

Compressional feature generated by the 1989 Loma Prieta 

earthquake—Queried where uncertain

Small (<1 ha) landslide generated by the 1989 Loma Prieta 

earthquake—Dashed where approximately located; queried where uncertain; opened where only scarp was mapped. 

Half arrows indicate direction of downslope movement

• — — Approximate boundary of large (>1 ha) landslide or landslide complex—From Keefer (1991)

8-37 Geologic notes locality—See Spittler and Harp (1990)

•^ Strike and dip of bedding

(§) Locality mentioned in text—May include shaded area

•—@ Quadrilateral array—Numbers are array identifiers, as discussed in text and by Griggs and others (1990) and Marshall 

and Griggs( 1991)

o—(?) Quadrilateral array removed before December 1990—Numbers are array identifiers, as discussed in text and by Griggs 

and others (1990) 

• Strain gage

Base note:

Planimetric base from Santa Cruz County, 1989

Figure 7.—Old Santa Cruz Highway landslide complex, showing 

boundary of landslide and locations of (A) surficial features, surfacemonitoring stations, and localities discussed in text and of (B) boreholes 

(from Cole and others, 1991) and water wells (from Brumbaugh, 1990). 

Ground cracks and compressional features from Spittler and Harp (1990); 

landslide boundaries from Keefer (1991). Base from Towill, Inc., using 

topography from U.S. Geological Survey Los Gates, Calif., 7.5-minute 

quadrangle and culture from County of Santa Cruz planimetric base and 

interpretation of aerial photographs taken July 24, 1990. Contour interval, 40 ft.

cracks and other features that indicate the presence of 

several smaller, shallower landslides. These scarps, cracks, 

and other features, which include localized areas of compression or bulging ground, indicating landslide toes, are 

so numerous as to form a virtually continuous landslide 

complex between the longest ground-cracking feature and 

Burns Creek (fig. 7).

Slope inclinations in the Old Santa Cruz Highway area 

average 15°-20°, and much of the area also displays the 

uneven, hummocky topography and benched topographic 

profiles characteristic of landslide terrain. The southeastern part of this area was mapped as a landslide deposit by 

Cooper-Clark and Associates (1975) (fig. 4) and by 

McLaughlin and others (1991) (see pi. 3).

The Butano fault is projected under the southeastern 

part of this area (see pi. 3), but the local strike of this 

fault is oblique to the main zones of ground cracking. 

Northeast of the projected fault trace, the landslide complex is underlain by the Vaqueros Sandstone; southwest 

of the trace, bedrock is the Butano Sandstone. The axis of 

the Summit syncline passes through the northeastern margin of the area, and the axis of the Laurel anticline passes 

through the southern part of the area and is truncated 

against the Butano fault (see pi. 3). Because of these complex structural relations, the dip of the bedrock beneath 

the landslide complex varies but under most of the complex is probably northeastward (into the slope).

Two 61-cm-diameter boreholes were drilled into the 

southeastern part of this landslide complex (Cole and others, 1991). The boreholes, logged to depths of 11 and 17 

m, evidently did not penetrate through landslide material 

into undisturbed bedrock. They revealed fractured, deeply 

weathered siltstone and sandstone overlain by as much as 

0.3 m of topsoil (silty sand and silty clay) and 5 m of 

colluvium (clayey sand, clayey silt, sandy silt, and silty 

sand). Free water was penetrated at depths of 10 and 12.5 

m (Cole and others, 1991).

UPPER SCHULTHEIS ROAD LANDSLIDE

The Upper Schultheis Road landslide is immediately 

east of the Old Santa Cruz Highway landslide complex, 

separated from it by less than 100 m of ground with no 

observed cracks (area 3, pi. 3). The crown, head, and 

upper west flank of the landslide are delineated by a continuous main scarp that is arcuate and concave downslope 

in plan view (figs. 9, 10). The east flank of the landslide 

is marked by a set of discontinuous pressure ridges and 

cracks that extend about 275 m downslope from the main 

scarp. Together, these features encompass a landslide about 

300 m wide. A few small pressure ridges are present low 

on the slope above Burns Creek; these features are interpreted to be internal ridges on the basis of their small size 

and discontinuity. The landslide probably extends
C90 LANDSLIDES

200 300 METERS 11 i n c\ MI M

B

Figure 7.—Continued.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C91

downslope through the steep, heavily vegetated terrain 

below Schultheis Road to Burns Creek itself (fig. 9). The 

landslide thus delineated is approximately 460 m long 

and 13 ha in area.

The Upper Schultheis Road landslide exhibits a benched 

topographic profile (fig. 11), indicative of previous landslide movement. The slope inclination along this profile 

averages 15°. The arcuate main scarp of the landslide also 

partly coincides with the base of a steep slope, inferred to 

be the scarp of a preexisting landslide. Both Cooper-Clark 

and Associates (1975) and McLaughlin and others (1991) 

mapped almost all of the area of the 1989 landslide as 

landslide material (see pi. 3; fig. 4).

Displacements measured along a transect across the main 

scarp indicate that the head of the landslide moved 1.16 

m downslope. An additional 1.28 m of downslope displacement was measured across a series of internal fissures and scarps that extended for 62 m downslope from 

the main scarp. The total measured displacement of the 

landslide along that transect was thus 2.44 m.

Bedrock beneath the landslide is sandstone, siltstone, 

and shale of the Butano Sandstone and Vaqueros Sandstone, which are separated by the Butano fault (see pi. 3). 

Upslope from this fault, dips are steeply northeastward 

(into the slope). Downslope from the fault, bedding is 

folded around the axis of the Laurel anticline, but except 

for local southwestward dips under the inferred toe of the 

landslide, dips downslope from the fault also are predominantly steeply northeastward (see pi. 3; fig. 11).

Subsurface exploration of the landslide included five 

small-diameter (12.4-16.5 cm) boreholes drilled to depths

EXPLANATION

[Composite explanation for part B of figures 7, 9, 12, and 20. See figure 1'A 

for explanation of surface features and localities discussed in text. Waterwell data from Brumbaugh (1990); borehole data from William Cotton 

and Associates (1990) and (or) Cole and others (1991)]

179

A

90

140

27

SR-4 

•

Water well reported undamaged by 1989 Loma Prieta earthquake — Upper number, well number; lower number, total 

depth (in meters)

Water well reported damaged by 1989 Loma Prieta earthquake — Upper number, well number; upper lower number, 

total depth (in meters); lower lower number, depth of damage (in meters); queried where no depth of damage given

Borehole — Number identifies borehole, as discussed in text 

and by William Cotton and Associates ( 1 990) and Cole and 

others (1991)

Figure 7. — Continued.

of 25 to 92 m (William Cotton and Associates, Inc., 1990), 

and three 61-cm-diameter boreholes drilled to depths of 

16 to 22 m (Cole and others, 1991). Materials consisted 

of (1) a layer of colluvium (locally mantled by topsoil and 

manmade fill), ranging in thickness from less than 0.6 to 

about llm, consisting mainly of silty clay, clayey silt, 

silt, sandy silt, and silty sand; (2) a layer, 3 to 18 m thick, 

made up of varyingly weathered shale, siltstone, sandstone, clay, sandy silt, and sand; and (3) bedrock consisting of approximately 58 percent sandstone, 9 percent 

siltstone, and 33 percent shale and claystone. (The percentages were virtually the same for both bedrock units 

represented.) The rock was locally weathered and (or) 

intensely fractured and contained localized zones of 

crushed, very soft, or sheared materials and zones where 

drilling circulation was lost, indicating open fractures at 

depths as great as 55 m.

Particularly conspicuous shear zones were penetrated 

in borehole SR-2 at a depth of 27.1 to 27.4 m, and in the 

larger-diameter boreholes SR-5, SR-6, and SR-7 at depths 

of 6.7, 5.3 to 6.7, and 10.4 m, respectively (figs. 9B, 11). 

The deeper shear zone contained a sheared mixture of 

highly plastic clay, silty clay, sandstone, and fractured 

shale overlying 0.5 to 1.3 cm of sheared, mixed silty clay 

and clayey silt of varying plasticity (William Cotton and 

Associates, Inc., 1990); the shallower shear zone separated intensely weathered and oxidized materials from 

unoxidized rocks (Cole and others, 1991). In borehole 

SR-5, this shallower shear zone consisted of 3 to 13 cm 

of sheared siltstone and clay with polished surfaces; in 

borehole SR-6, the zone contained three separate faulted 

and sheared contacts; and in borehole SR-7, the zone was 

a 5- to 10-cm-thick layer of swelling clayey siltstone, 

which separated an overlying brecciated and fractured siltstone from an underlying sandstone (Cole and others, 

1991).

Two of three known water wells within the landslide 

were reportedly damaged during the earthquake. In well 

141 (fig. 95), located less than 30 m from borehole SR-2, 

the well-pump motor was reportedly broken off, and the 

motor shaft bent to a 45° angle at 30-m depth, corresponding closely to the 27-m depth of the shear zone in 

borehole SR-2. In well 218, about 3 m from borehole 

SR-6 (fig. 95), an offset of 20 to 30 cm was reported to 

have occurred at about 4.5-m depth in a 76-cm-diameter 

casing. This offset corresponds closely in depth to the 

shallower shear zone. These correspondences indicate that 

landslide displacements during the earthquake probably 

occurred in both the shallower and deeper shear zones. A 

cross section showing the inferred landslide basal shear 

surfaces is shown in figure 11. If the average landslide 

thickness is assumed to be half of the maximum thickness 

shown on this cross section, the total volume of the landslide is 4.3xl06 m3 .
C92 LANDSLIDES

RALLS DRIVE LANDSLIDE

The Rails Drive landslide is immediately east of the 

Upper Schultheis Road landslide, on the southwest flank 

of Summit Ridge (area 4, pi. 3). The main scarp is a set of 

ground cracks and scarps, some arcuate and concave 

downslope and others linear, intersecting to form an angular or zigzag pattern in plan view (leg B-B', fig. 9A). 

Two faults with northwestward strikes, parallel to segments of the main scarp, also pass through the head of the 

landslide (see pi. 3). This association, as well as the angular pattern of the main scarp between localities B and B' 

(fig. 9A), indicates that bedrock structure controlled scarp 

formation there.

Part of the east flank of the Rails Drive landslide is 

delineated by a set of northwest-trending echelon scarps 

and cracks (fig. 9); the west flank abuts the Upper 

Schultheis Road landslide. A major internal ground crack, 

striking approximately downslope, divides the Rails Drive 

landslide into two main blocks (leg B-C, fig. 9A). The 

smaller, western block formed on the spur ridge adjacent 

to the Upper Schultheis Road landslide, and the larger, 

eastern block formed around and under a short valley, 

tributary to Burns Creek. Both landslide blocks were moderately disrupted by other internal scarps and cracks; the 

area of greatest disruption was adjacent to the east flank 

(fig. 9). Local indications of compression were present 

near the downslope termination of the crack separating 

the two blocks, particularly where this crack curved to the 

east (leg C-C', fig. 9A).

The Rails Drive landslide is about 730 m wide and at 

least 480 m long. No visible surface feature marks the 

toe, but small internal scarps low on the slope suggest 

that the landslide extends an additional 150 to 430 m 

downslope from the flank cracks to Burns Creek. The 

area of the landslide thus is approximately 50 ha.

Displacements measured across the main scarp and internal scarps are downslope, to the south, southeast, or 

southwest. These measurements indicate that the head of 

the western block moved downslope 0.55 to 1.02 m, that 

the head of the eastern block moved downslope 0.28 to 

0.97 m, and that the most disrupted parts of the eastern 

block moved downslope as much as 1.80 m.

Within the Rails Drive landslide, one well (125, fig. 

95) was reportedly damaged at 17-m depth, whereas five 

wells (19, 20, 119, 122, 126, fig. 95), 46 to 61 m deep, 

were reportedly undamaged by the earthquake. Three of 

the reportedly undisturbed wells are near the main scarp, 

and the other three are within the eastern block. The reported absence of damage to these wells could indicate 

either that local displacements were not large enough to 

damage the wells or that the basal shear surface is deeper 

than 46 to 61 m. The damage to well 125 may thus indicate a shallower, subsidiary shear surface, or it may be 

unrelated to landslide movement.

The slope through the Rails Drive landslide averages 

15°, but it is also irregular and contains benches and steeper 

segments, indicating previous landslide activity. Most of 

the landslide was mapped as landslide material by Cooper-Clark and Associates (1975) and McLaughlin and

Figure 8.—Segment of 

Tranbarger fracture (leg AA', fig. 7A), a 1,200-m-long 

zone of scarps and cracks in 

Old Santa Cruz Highway 

landslide complex. View 

northwestward from north 

edge of Summit Road (loc. 

D, fig. 7A).
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C93

others (1991) (see pi. 3; fig 4), and part of the main scarp 

was at the base of an older scarp.

The trace of the Butano fault passes through the Rails 

Drive landslide about 140 m downslope from the crown 

(see pi. 3). Upslope from the fault, bedrock is the northeast-dipping Vaqueros Sandstone; downslope from the

fault, bedrock is mostly the Butano Sandstone, with a 

small area underlain by the San Lorenzo Formation (see 

pi. 3). The axis of the Laurel anticline is projected to pass 

near the base of the slope; thus, most rocks downslope 

from the Butano fault dip northeast (into the slope), 

whereas rocks under the landslide toe dip southwest.

RALLS DRIVE LANDSLIDE

\V-"

300 METERS

\ \ \ \

Figure 9. — Upper Schultheis Road and Rails Drive landslides, showing 

boundary of landslides and locations of (A) surficial features, surfacemonitoring stations, and localities discussed in text and of (B) boreholes 

(from William Cotton and Associates, 1990, and Cole and others, 1991),

cross section in figure 11, and water wells (from Brumbaugh, 1990). 

Ground cracks and compressional features from Spittler and Harp (1990); 

landslide boundaries from Keefer (1991). See figure 7 for explanation 

and base-map information.
C94 LANDSLIDES

VILLA DEL MONTE LANDSLIDE

The Villa Del Monte landslide is also on the southwest 

flank of Summit Ridge, on the broad spur ridge between 

the Rails Drive landslide and Taylor Gulch (area 5, pi. 3; 

fig. 12). Surveyed topographic profiles show average slope 

inclinations of 12°-15°. Many gently sloping benches alternate with steeper stretches of slope, especially in the

central and eastern parts of the area; slopes near the base 

of the ridge, in particular, are relatively steep. Numerous 

roads and building sites have been located in this area to 

take advantage of these gently sloping benches, and the 

Villa Del Monte neighborhood, which contains approximately 165 homes, is the most densely populated part of 

the Summit Ridge area. Earthquake-induced ground movement caused substantial damage in this neighborhood.

B

RAILS DRIVE 

LANDSLIDE

Figure 9.—Continued.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C95

Several distinct zones of ground cracks are present 

within the Villa Del Monte landslide. The major zones of 

ground cracks are along Sunset Drive, in the area of upper Skyview (or "Sky View") Terrace and Bel Air (or 

"Belair," "Bel-Air," or "Bel Aire") Court, along and east 

of Deerfield Road, and along lower Skyview Terrace (fig. 

12). Additional, smaller zones of ground cracks along the 

westernmost extension of Deerfield Road and around the 

Troy Road-Skyview Terrace intersection (fig. 12) are probably associated with separate, smaller landslides.

formation was controlled by bedrock structures. Displacements measured across individual cracks and scarps were 

from 8 to 53 cm, downslope toward the southwest and 

southeast.

Six wells in this area (wells 71, 74, 75, 82, 83, 88, fig. 

121?) were reportedly damaged. The damage to wells 74 

and 75, which involved collapsed casings at 5- and 23-m 

depth (Brumbaugh, 1990), suggests that the ground crack 

immediately upslope from them marks the upslope boundary of the landslide.

SUNSET DRIVE AREA

The ground cracks and scarps in the Sunset Drive area 

(figs. 12, 13) extend approximately 420 m across the slope 

(leg A-A', fig. 12A) and 210 m down the slope (leg B-B', 

fig. 12A). For parts of their lengths, these cracks and scarps 

cross a gentle slope downslope from a 3- to 5-m-high 

bedrock scarp composed of intensely fractured sandstone. 

A few northwest-striking ground cracks in this area are 

subparallel to local faults (see pi. 3), indicating that their

UPPER SKYVIEW TERRACE-BEL AIR COURT 

AREA

The upper Skyview Terrace-Bel Air Court area, in the 

eastern part of the Villa Del Monte neighborhood, is the 

second zone of concentrated and pervasive ground cracking (figs. 12, 14). The cracks and scarps in this area extend approximately 300 m across the slope and 670 m 

downslope, from immediately above upper Skyview Terrace (leg C-C', fig. 12A) to Laurel Creek (leg D-D', fig.

Figure 10.—Part of main scarp of Upper 

Schultheis Road landslide. Measurements 

near this locality showed 1.07 m of extensional displacement and 46 cm of vertical displacement across main scarp. 

View eastward from near landslide crown 

(loc. A, fig. 9A).
C96 LANDSLIDES

12A). At the upslope margin of this zone is a scarp (leg 

C-C', fig. 12A), 240 m long, that is arcuate and concave 

downs lope, toward the south. The ground cracks between 

upper Skyview Terrace and Bel Air Court (leg E-E', 

fig. 12A), along Bel Air Court and middle Skyview 

Terrace (leg F-F', fig. 12A), and along Tree View Trail 

and lower Skyview Terrace (leg D-D', fig. 12A) also form 

arcuate-downslope patterns subparallel to the upper scarp 

zone. In addition, the area contains several small pressure 

ridges.

With one exception, displacement measurements in the 

upper Skyview Terrace-Bel Air Court area showed movement to the southeast, south, and southwest, toward Laurel Creek. The sole exception, which showed 15 cm of 

extension and 33 cm of vertical displacement on an 

upslope-facing scarp, could be due to graben formation or 

differential movement within the landslide. Measured local downslope displacements elsewhere in the area were 

10 to 107 cm.

Several wells along the eastern margin of the area of 

ground cracks (wells 26, 91-94, fig. 12B) were reportedly 

undamaged by the earthquake, indicating that the landslide did not extend eastward of that margin to Taylor 

Gulch. This margin defines the east flank of the landslide.

DEERFIELD ROAD AREA

In the western part of the Villa Del Monte area, another 

major zone of ground cracks, scarps, and pressure ridges 

occurs along Deerfield Road and between Deerfield Road 

and Evergreen Lane (area G-G'-H-H', fig. 12A). This 

zone extends approximately 240 m from west to east and 

370 m downslope (fig. 12). The cracks and scarps (fig. 

15) in the zone strike predominantly northeast, oblique to 

the main structural elements in the bedrock (see pi. 3). 

Displacement measurements show predominantly southeastward to locally southwestward movement of 5 to 41 

cm. Both the strike of the main ground cracks and the 

predominantly southeastward direction of displacements 

indicate that this zone forms the west (or right) flank of 

the landslide.

All five wells in this zone, along Deerfield Road (wells 

85, 88, 89, 233, 234, fig. 125), were reportedly damaged

100 200

DISTANCE, IN METERS 

300 400 500 600 700

500

450

400

EXPLANATION

Borehole —Data from William Cotton and Associates, Inc. (1990) 

and Cole and others (1991)

\///\ Colluvium

[i*V>j Sandstone

[T>] Siltstone

fooc^ Shale

— Distressed materials 

Rock units (Tertiary)

TV Voqueros Sandstone

Tb Butano Sandstone

—•—^— Contact—Approximately located

————^ Fault—Approximately located

Inferred landslide basal shear surface, Half—arrows 

indicate direction of downslope movement

Crocks generated by 1989 Loma Prieta earthquake

Borehole SR-4 

PROJECTED 20 M WEST

Borehole SR-7 

PROJECTED 30 M EAST

Borehole SR-2 

PROJECTED 143 M WEST

Borehole SR—6

PROJECTED 42 M EAST

I I Borehole SR-1

' ' PROJECTED 11 M WEST

Borehole SR-5

NORTHEAST

SUMMIT 

ROAD

250

Figure 11.—Geologic cross section through Upper Schultheis Road landslide (fig. 9B). Locations of landslide basal shear surfaces inferred from 

borehole data of William Cotton and Associates, Inc. (1990), and Cole and others (1991).
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C97

VILLA DEL MONTE r 

LANDSLIDE

200 300 METERS /

i i

Figure 12.—Villa Del Monte landslide, showing boundary of landslide fied from Keefer (1991). Base from Towill, Inc., using topography from 

and locations of (A) surficial features, surface-monitoring stations, and U.S. Geological Survey Los Gates and Laurel, Calif., 7.5-minute quadlocalities discussed in text and of (B) boreholes (from William Cotton and rangles and culture from County of Santa Cruz planimetric base and interAssociates, 1990, and Cole and others, 1991), cross section in figure 16, pretation of aerial photographs taken July 24, 1990. See figure 7 for 

and water wells (from Brumbaugh, 1990). Ground cracks and compres- explanation. Contour interval, 40 ft. 

sional features from Spittler and Harp (1990); landslide boundaries modi-
C98 LANDSLIDES

by the earthquake. However, seven of the eight wells along reportedly undamaged. This zone of undamaged wells sugwesternmost Deerfield Road and westernmost Skyview gests that the smaller zones of ground cracks farther west

Terrace (wells 105-108, 235a, 235b, 237, fig. 125) were (fig. 12) were not part of the larger landslide. These

VILLA DEL MONTE 75

Figure 12.—Continued.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C99

westernmost ground cracks were almost certainly associated with smaller landslides. Although the boundary of 

the large landslide is inferred to lie generally in the area 

between the damaged and undamaged wells (approx between Iocs. H and I, fig. 12A), the landslide limits in this 

area were poorly defined by the available data.

LOWER SKYVIEW TERRACE AREA

Along lower Skyview Terrace, for a distance of approximately 400 m east of Cove Lane Road, is a zone of 

mostly arcuate, concave-southward scarps and cracks (leg 

H'-J, fig. 12A). The zone extends approximately 200 m 

from north to south, on both sides of lower Skyview Terrace. The water well within the area of most concentrated

cracking (well 104, fig. 12B) was reportedly damaged, 

whereas three nearby wells (wells 101-103, fig. 12B), in 

less disturbed areas, were reportedly undamaged. One 

measurement across a ground crack in this area showed 

33 cm of displacement downslope, toward the southeast.

OVERALL LANDSLIDE DIMENSIONS, 

MOVEMENT, AND SETTING

The Villa Del Monte landslide extends downslope from 

the Sunset Drive area to Laurel Creek and thus is 980 m 

in overall length. The right flank of the landslide is inferred to be the western margin of the Deerfield Road 

area, and the better defined left flank is the eastern margin of ground cracking in the upper Skyview Terrace-Bel

Figure 13.—Segment of main scarp of 

Villa Del Monte landslide in Sunset Drive 

area. View northwestward from near quadrilateral-array station 14 (fig. 12A). Photograph by Kevin M. Schmidt, U.S. 

Geological Survey.
C100 LANDSLIDES

Air Court area (leg C'-D, fig. 12A). Thus, the overall 

width of the landslide is 870 m, and its surface area is 

about 68 ha. Displacements measured across individual 

ground cracks and scarps throughout the Villa Del Monte 

landslide ranged from 5 to 107 cm; most local displacements ranged from 33 to 53 cm.

Cooper-Clark and Associates (1975) mapped the east 

two-thirds of the Villa Del Monte landslide as a "probable" landslide (fig. 4). Clark and others (1989, with landslide boundaries revised by R. J. McLaughlin and J.C. 

Clark, unpub. data, 1990) and McLaughlin and others 

(1991) show all but the westernmost part of the landslide 

as underlain by a particularly large preexisting landslide 

deposit that stretches along Summit Ridge for about 1.6 

km; most of the northern, eastern, and southern margins 

of the Villa Del Monte landslide correspond closely in

location to the mapped boundaries of that deposit (see pi. 

3). In addition, the benched topography in the Villa Del 

Monte area indicates previous landslide activity, and Santa 

Cruz County road-repair records show that previous landslide movement occurred in the Skyview Terrace area in 

1975-76 (fig. 3; table 2).

SUBSURFACE CONDITIONS AND MATERIALS

Bedrock beneath the Villa Del Monte landslide is the 

Butano Sandstone and the Twobar Shale and Rices Mudstone Members of the San Lorenzo Formation (see pi. 3). 

The axis of the Laurel anticline is projected to pass through 

the southwestern part of the landslide. Rocks northeast of 

this axis dip obliquely into the slope, to the north and

Figure 14.—Internal scarp of Villa Del 

Monte landslide in Upper Skyview Terrace-Bel Air Court area. View northwestward from near quadrilateral-array station 

23 (fig. 12A). Photograph from Griggs and 

others (1990).
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C101

Figure 15.—Internal scarps and compressional features of Villa Del Monte landslide in Deerfield Road area. A, Internal scarps, which are part of a zone 

about 200 m long. View northeastward from just east of Deerfield Road (loc. K, fig. 12A). B, Internal ridge, consisting of compressed ground, marked 

by broken and buckled pavement. View northwestward from near east end of Deerfield Road (loc. B' on fig. 12A).
C102 LANDSLIDES

northeast. Southwest of the axis, dip may be steeply southwestward, locally overturned to the northeast, or both (see 

pi. 3). Several northwest-striking faults are mapped through 

the crest and upper flanks of Summit Ridge, into and just 

north of the landslide (see pi. 3).

A total of 13 small-diameter (14.9-16.5 cm) boreholes 

(fig. 12B), drilled to depths as great as 92 m, showed 

materials consisting of as much as 4 m of fill and colluvium and 3 to 17 m of weathered rock overlying bedrock 

(William Cotton and Associates, Inc., 1990). The fill and 

colluvium were composed of clayey silt, silty clay, silty 

sand, sandy clay, and clayey sand. The weathered rock 

contained similar materials, as well as weathered sandstone, siltstone, and shale that were locally brecciated, 

crushed, or intensely fractured. Bedrock to the depths penetrated by the boreholes consisted of sandstone, siltstone, 

and shale. Among the three geologic units, the Twobar 

Shale Member contained the most sandstone (85 percent 

of drilled interval) and the least siltstone (12 percent) and 

shale (3 percent). The drilled interval of the Butano Sandstone consisted of 32 percent sandstone, 44 percent siltstone, and 25 percent shale, and that of the Rices Mudstone

Member consisted of 39 percent sandstone, 41 percent 

siltstone, and 20 percent shale.

The soil, weathered rock, and bedrock contained approximately 45 intervals in the boreholes with indications 

of distressed materials—sheared, intensely fractured, or 

faulted rocks; very soft zones; crushed materials; intervals of collapsing or caving of the boreholes; or zones of 

lost circulation (fig. 16; William Cotton and Associates, 

Inc., 1990). These zones occurred in all rock types at 

depths from near the surface to near the maximum depth 

of drilling. However, the small diameters, discontinuous 

sampling, and absence of geophysical logging of the boreholes made identification of landslide shear surfaces within 

the boreholes uncertain. The numerous zones with indications of disturbance and reports of water well damage at 

various depths (fig. 12B) indicate that localized internal 

shearing occurred at several depths during landslide movement. An inferred deep basal shear surface, which is compatible with the drilling and surface data, is shown in 

figure 16. If the average landslide thickness is assumed to 

be half of the maximum thickness shown on this cross 

section, the landslide volume is 27x106 m3 .

100 200 300 400

DISTANCE, IN METERS 

500 600 700 800 900 1000 1100 1200

500

400

300

SOUTHWEST 

Laurel 

Creek

200

EXPLANATION 

Borehole — Data from William Cotton end Associates, Inc. (1990)

\//A Colluvium 

m'lVJ Sandstone 

{-:-] Siltstone 

F::>:^ Shale

— Distressed materials 

Rock units (Tertiary) 

TV Vaqueros Sandstone

Rices Mudstone Member of the Son Lorenzo Formation

Twobor Shale Member of the San Lorenzo Formation

Butano Sandstone

Contact — Approximately located

Fault — Approximately located

Inferred landslide basal shear surface. Half-arrows 

indicate direction of downslope movement

Tsr 

Tst 

Tb

Borehole DMPROJECTED 76 M 

Borehole DM-2 

PROJECTED 119 M WEST

1 

WEST

LPEQ fT crocks gene "TV/

Borehole ED1 

PROJECTED 60 M EAST 

Borehole DM-3 

PROJECTED 73 M WEST

SKYVIEW

EVERGREEN

1

NORTHEAST 

LPEQ

rated by 1989 Loma Prieta earthquake

Borehole DM-4 

PROJECTED 157 M WEST

LPEQ

Figure 16.—Geologic cross section through Villa Del Monte landslide (see fig. 12B for location). Basal shear surface inferred from borehole data of 

William Cotton and Associates, Inc. (1990). See figure 11 for explanation.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C103

UPPER MORRELL ROAD LANDSLIDE

The Upper Morrell Road landslide is on the south flank 

of Summit Ridge east of Taylor Gulch (area 7, pi. 3). The 

main scarp of the landslide is made up of two adjoining 

zones of ground cracks and scarps, each of which is arcuate, concave downslope in plan view (leg A-A', fig. 17). 

Together, these zones extend approximately 550 m across 

the slope. The orientation of ground cracks in these zones

and the relation of these cracks to other northwest-striking cracks nearby (leg B-B', fig. 17) suggest significant 

control by bedrock structures. Displacement measurements 

across the main zones of cracks (fig. 18) indicate 

downslope movement of as much as 2.01 m. Southeast of 

the main scarp, a shorter scarp is inferred to mark part of 

the east flank of the landslide (loc. C, fig. 17).

The toe of the landslide is a gentle, east-west-striking 

bulge in the ground surface, 430 m long and 180 to 270 m

IP '»

TAYLOR GULCH 

LANDSLIDE

200 300 METERS

Figure 17.—Taylor Gulch and Upper Morrell Road landslides. Ground cracks and compressional features from Spittler and Harp 

(1990); landslide boundaries from Keefer (1991). See figure 7 for explanation and figure 12 for base-map information.
C104 LANDSLIDES

downslope from the main scarp. A zone of fractures and 

grabens along the crest of this bulge is evidence of local 

extension of the ground that accompanied upward bulging 

of the landslide toe. Between the crown cracks and the 

toe, the landslide encompasses an area of approximately 

14 ha.

Two topographic profiles surveyed through the landslide exhibit the benched form characteristic of landslide 

terrain and show that the average slope is about 15°. In 

agreement with the topographic indications of previous 

landslide activity, Cooper-Clark and Associates (1975) and 

Clark and others (1989, with landslide boundaries revised 

by R.J. McLaughlin and J.C. Clark, 1990) mapped most 

or all of the Upper Morrell Road landslide as landslide 

material (see pi. 3; fig. 4). Santa Cruz County road-repair 

records additionally indicate several episodes of previous 

landslide damage to Morrell Road in this area (fig. 3; 

table 2). Bedrock underlying the landslide is mostly mudstone and sandstone of the Rices Mudstone Member of 

the San Lorenzo Formation, and small areas are underlain 

by igneous diabase and gabbro, the Vaqueros Sandstone, 

and the Butano Sandstone (see pi. 3). The beds dip generally northeast, into the slope.

BURRELL LANDSLIDE

The Burrell landslide is on the south flank of Summit 

Ridge, less than 100 m from the trace of the San Andreas 

fault (area 9, pi. 3). Features interpreted as the main scarps

of two landslide blocks (legs A-A', B-B', fig. 19) were 

differentiated from other ground cracks in the area on the 

basis of their arcuate forms, their location at the upslope 

margin of a gently sloping topographic bench, and the 

occurrence of a pressure ridge downslope from one of 

them, which partly delineates the landslide toe (loc. C, 

fig. 19). These two main scarps were separated by a zone 

of discontinuous cracks. The landslide is approximately 

230 m long by 460 m wide and encompasses an area of 

about 9 ha. Downslope displacements measured across 

the main scarps were from 28 to 71 cm. The slope averages 15°-20°, and bedrock is the Butano Sandstone (see 

pi. 3), which dips east to northeast, oblique to the slope 

direction.

Trenching studies in the main scarp area show that the 

Burrell landslide is a reactivated preexisting landslide that 

first moved about 3 ka ago and later moved at least twice 

before the 1989 Loma Prieta earthquake (Keefer, 1991; 

Nolan and Weber, this chapter). The western part of the 

landslide was mapped as landslide material by CooperClark and Associates (1975) (fig. 4).

UPPER REDWOOD LODGE ROAD LANDSLIDE

The Upper Redwood Lodge Road landslide is in the 

Redwood Lodge area, about 2 km south of Summit Road 

(area 10, pi. 3). The landslide was delineated primarily by 

three sets of discontinuous ground cracks and scarps, which 

indicate the presence of three large landslide blocks. The

Figure 18.—Part of main scarp 

of Upper Morrell Road landslide 

where scarp crosses gravel road. 

View northeastward from just 

east of Morrell Road (loc. E, 

fig. 17).
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C105

main scarp of the landslide is a series of discontinuous, 

straight to slightly arcuate cracks and scarps approximately 

580 m long, that strikes about N. 30° W. (leg A-A', fig. 

20A). The linearity and strike direction suggest that crack 

formation was controlled by bedrock structures (Keefer, 

1991; Harp, this chapter). The most conspicuous ground 

cracks and scarps are in a nearly flat meadow, at the base 

of a steep stretch of slope; for part of their length, they 

are associated with a preexisting graben, about 30 m long, 

a few meters wide, and about 1 m deep. These relations 

indicate that the area is the head both of a large landslide 

that moved during the 1989 Loma Prieta earthquake and 

of a large preexisting landslide. Earthquake-induced displacements across the ground cracks and scarps were as 

great as 25 cm.

A second conspicuous zone of ground cracks and scarps 

crosses Soquel-San Jose Road ("Old San Jose Road") north 

of its intersection with Redwood Lodge Road (leg B-B', 

fig. 20A). This zone is arcuate and concave downslope in

plan view. The southern part of the zone is at the base of 

an older bedrock scarp, and the slope between this zone 

and the creek channel to the west has a benched topographic profile. Both the older scarp and the benched profile are additional indicators of previous landslide 

movement. Santa Cruz County road-repair records also 

describe repeated landslide damage to Redwood Lodge 

Road there (fig. 3; table 2). Earthquake-related displacements of 3 to 30 cm were measured across individual 

ground cracks in this zone but were inconsistent in direction.

A third conspicuous zone of cracks and scarps, farther 

downslope, trends west-northwest along and downslope 

from Redwood Lodge Road, between Old San Jose Road 

and Long Branch Gulch (leg C-C', fig. 22A). This zone, 

which is 580 m long, is the most continuous of the three.

The location of the northeast boundary of the landslide 

is poorly determined but is inferred to be at least as far 

northeast as the limits of the three conspicuous zones of

BURREU. 

LANDSLIDE

s* ''0

L

r^v.

100i

/ \

200 3(

i i

RC

Figure 19.—Burrell landslide. Ground cracks and compressional features from Spittler and Harp (1990); landslide boundaries from 

Keefer (1991). Base from Towill, Inc., prepared using topography from U.S. Geological Survey Laurel, Calif., 7.5-minute quadrangle and 

culture from County of Santa Cruz planimetric base and interpretation of aerial photographs flown July 24, 1990. See figure 7 for 

explanation. Contour interval, 40 ft.
UPPER REDWOOD 

LODGE ROAD 

LANDSLIDE

Figure 20.—Upper Redwood Lodge Road and Long Branch landslides, showing locations of (A) surficial features, surface-monitoring stations, and local

(B) water wells (from Brumbaugh, 1990). Ground cracks and compressional features from Spittler and Harp (1990); landslide boundaries from Kee

explanation and figure 19 for base-map information.
UPPER REDWOOD 

LODGE ROAD 

LANDSLIDE LONG BRANCH 

LANDSLIDE

In , r^ ^ 

Figure 20.—Continued.
C108 LANDSLIDES

cracks and scarps (fig. 20); the landslide is thus at least 

640 m wide. The southwest (downslope) boundary is likewise not marked by any surface features, but the position 

and pattern of the westernmost cracks and scarps suggest 

that the landslide extends to the creek at the base of the 

slope (fig. 20). The landslide is thus about 820 m long 

and at least 37 ha in area. The slope through the landslide 

averages 15°-20°.

Two wells within the landslide were reportedly damaged by the earthquake—well 159, at a depth of 27 m and 

well 38, at undetermined depth—and four wells (35- 

37, 40, fig. 205), with depths of 45 to 113 m, were reportedly undamaged (Brumbaugh, 1990). The two damaged 

wells are both located near the margins of the landslide. 

The four reportedly undamaged wells are farther from 

these margins, and the absence of reported damage could 

indicate either a basal shear surface locally deeper than 

the wells or local displacements too small to cause damage.

Cooper-Clark and Associates (1975) mapped the area 

of the Upper Redwood Lodge landslide as containing three 

preexisting landslide deposits (fig. 4), and two landslides 

were mapped in this area by Clark and others (1989, with 

landslide boundaries revised by R.J. McLaughlin and J.C. 

Clark, 1990; see pi. 3). Bedrock consists of mudstone and 

sandstone of the Rices Mudstone Member of the San 

Lorenzo Formation and the Vaqueros Sandstone (see pi. 

3). Dips range from steeply southwestward (oblique to 

slope direction) to vertical.

AMAYA RIDGE LANDSLIDE

The Amaya Ridge landslide is on the southwest flank 

of Amaya Ridge, in the southern part of the study area 

(area 13, pi. 3). Two sets of ground cracks and associated 

features are present (fig. 21). The northwestern set consists of a relatively continuous, arcuate zone of ground 

cracks that are concave downslope in plan view (leg AA', fig. 21), and a nearly straight compression feature 

farther downslope (leg B-B', fig. 21). These features are 

interpreted to be, respectively, the main scarp and toe of a 

landslide, approximately 120 m long, 240 m wide, and 

1.6 ha in area. Measurements across the main scarp indicated local downslope displacements of 8 to 100 cm or 

more.

Southeast of the landslide is a 300-m-long zone of 

ground cracks and scarps (line C-C', fig. 21) that is linear 

to slightly arcuate, parallel, and echelon to the main scarp 

to the northwest. This southeastern set of ground cracks 

and scarps may represent the main scarp of a separate 

landslide, an easterly and upslope extension of the other 

landslide, or cracks produced by some other process. One 

displacement measurement across a ground crack in this 

set indicated movement of 15 to 20 cm downslope, to the

southwest. The area between this set of ground cracks and 

scarps and the base of the slope is 4.5 ha.

Both sets of ground cracks and scarps (legs A-A', CC', fig. 21) are near breaks in slope; the slope above them 

is relatively steep, and the slope below them is a broad, 

gently sloping bench, 90 m wide. A topographic profile 

surveyed through the northwestern set of ground cracks 

and scarps shows an average slope inclination of 22° and 

an inclination of the steep segment above the scarps of 

24°. Marshy areas just below the base of the steep stretch 

of slope indicate a local water table near the surface.

The northwestern landslide (fig. 4) is in preexisting landslide material, as mapped by Cooper-Clark and Associates (1975), and the southeastern set of ground cracks and 

scarps closely follows the trace of a landslide scarp shown 

on their map. Bedrock consists of sandstone and siltstone 

of the Purisima Formation that dip southwest, generally 

parallel to the slope direction (see pi. 3).

LOWER SCHULTHEIS ROAD WEST LANDSLIDE

The Lower Schultheis Road West landslide is near Laurel, on a short ridge immediately south of Summit Ridge 

(area 18, pi. 3). The main scarp of the landslide is a 90-mlong series of slightly arcuate cracks and a graben (leg AA', fig. 22). For part of its length, this main scarp is south 

of the ridge crest (fig. 23), and so part of the ridge crest is 

involved in the landslide, having been displaced to the 

north. Measurements along the scarp showed 30 to 65 cm 

of northward displacement.

A set of discontinuous ground cracks (leg A'-B, fig. 

22) west of the scarp indicates that the landslide may 

extend an additional 90 m in that direction. These ground 

cracks also nearly parallel the bedrock strike (see pi. 3), 

suggesting that they and, by extension, the better developed main scarp to the east probably are structurally controlled. An elongate bulge in the ground about 110 m 

downslope from the main scarp marks part of the landslide toe (loc. C, fig. 22). The landslide defined by this 

bulge and the main scarp is at least 110 m long, 110 m 

wide, and 1.2 ha in area; if the landslide also encompasses the zone of ground cracking to the north and also 

extends there as far downslope as the bulge, its area may 

be as large as 2 ha. The slope through the landslide averages 15°-20°.

The area within and adjacent to the Lower Schultheis 

Road West landslide contains several surface features almost certainly associated with shallow preexisting landslides, and the topography of the ridge and the meadow 

immediately to the north suggests that deep-seated landslide movements have also occurred in the past. CooperClark and Associates (1975) mapped this area as being 

within a landslide deposit (fig. 4). Bedrock consists of 

mudstone and sandstone of the Rices Mudstone Member
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C109

of the San Lorenzo Formation, which are overturned 

steeply northeast or, locally, vertical (see pi. 3).

Subsurface exploration, consisting of direct observations in two 61-cm-diameter boreholes, 4 and llm 

downslope from the main scarp, revealed two shear surfaces (Cole and others, 1991 and this chapter). The shallower shear surface, at 4.9-m depth, consisted of a layer 

of soft, slickensided clay, 1.2 to 5 cm thick, which separated overlying, oxidized material from underlying, 

unoxidized but intensely fractured rock. The oxidized 

material consisted of clayey silt containing sandstone 

and siltstone clasts; the underlying rocks consisted of moderately hard sandstone and siltstone. The deeper shear surface, which was exposed only in the upslope borehole, 

at 7.3-m depth, consisted of a sheared siltstone interbed 

within a massive sandstone unit. This shear surface

AMAYA 

RIDGE 

/-LANDSLIDE

POSSIBLE 

LANDSLIDE

200 300 METERS

Figure 21.—Amaya Ridge landslide. Ground cracks and.compressional features from Spittler and Harp (1990); landslide boundaries from 

Keefer (1991). See figure 7 for explanation and figure 19 for base-map information.
C110 LANDSLIDES

separated overlying, fractured rock from underlying, 

unfractured material (Cole and others, 1991 and this 

chapter).

maximum depth of 29 m, was also reported in the Redwood Estates area in Santa Clara County, about 1.6 km 

north of the study area (Seed and others, 1990).

OTHER LARGE LANDSLIDES

The earthquake also caused displacements on nine other 

large landslides and landslide complexes within the study 

area (areas 1, 6, 8, 11, 12, 14-17, pi. 3) and on two 

additional landslides in the Summit Ridge area but outside the study area (areas A, B, pi. 3). Though typically 

smaller than the landslides described individually above, 

these 11 landslides had characteristics generally similar to 

those of the landslides discussed in detail, including (1) 

boundaries marked by discontinuous cracks and scarps, 

(2) well developed main scarps and head areas, (3) moderately well-developed flanks, (4) poorly developed toes, 

(5) involvement of preexisting landslide material, (6) occurrence on slopes of 15°-30°, (7) displacements of 25 to 

79 cm, and (8) association with ground cracks that show 

evidence of control by bedrock structures. The characteristics of these landslides are summarized in table 3, and 

detailed maps are shown in figs. 17, 20, 22, and 24 through 

28. One additional landslide with similar characteristics, 

which encompassed an area of about 10 ha and had a

POSTEARTHQUAKE MONITORING 

SURFACE MONITORING

A total of 11 landslides and landslide complexes (areas 

1-5, 7, 10, 13-15, 17, pi. 3) were monitored for 

postearthquake displacements from December 1989 

through June 1990 and again from December 1990 through 

July 1991, using surveys of arrays of stakes; three of these 

landslides (areas 3, 5, 7) were also monitored continuously, using recording strain gages, as described in detail 

by Griggs and others (1990) and Marshall and Griggs 

(1991).

METHODS

Survey stakes were initially placed in 51 quadrilateral 

arrays of four stakes each, according to the method of 

Baum and others (1988). These arrays were placed to span 

selected cracks and scarps along the heads and flanks of

\\\\ \ \ \ \

LOWER SCHULTHEIS 

ROAD WEST LANDSLIDE

\ \ < 

POSSIBLE 

LANDSLIDE

\

LOWER 

REDWOOD 

LODGE ROAD 

LANDSLIDE

Figure 22.—Lower Redwood Lodge Road, Lower Schultheis Road East, and Lower Schultheis Road West landslides. Ground cracks 

and compressional features from Spittler and Harp (1990); landslide boundaries from Keefer and others (1991). See figure 7 for 

explanation and figure 19 for base-map information.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA cm

the landslides. The locations of these arrays are shown in 

figures 7A, 9A, 12A, 17, 20A, 21, 22, 24, and 27. Stakes 

were set at least 0.5 m away from crack edges; depending 

on crack size and geometry, distances between stakes were 

typically from 1 to 3 m, and in some places as much as 7 

m. A fiberglass tape was used to measure the distances 

between stakes, and repeated measurements at control arrays showed a precision of ±1.0 cm.

During the first winter after the 1989 Loma Prieta earthquake, arrays were surveyed every 15 to 20 days and after 

significant rainfall. Of the 51 original quadrilateral arrays, 13 were either damaged, removed, or abandoned 

before the second winter of monitoring. During this second winter, most of the remaining 38 arrays were checked 

at least once a month, and as often as once a week during 

a period of heavy rainfall in March 1991. Two new quadrilateral arrays were installed during the March 1991 storms 

at sites of renewed cracking on the Upper Schultheis Road 

and Villa Del Monte landslides.

Four pairs of potentiometer-based strain gages, linked 

to continuously recording data loggers, were installed 

across scarps in the Upper Schultheis Road, Villa Del 

Monte, and Upper Morrell Road landslides (figs. 9A, 12A, 

17). Details of strain-gage operation and installation were 

given by Griggs and others (1990) and Keefer (1991). 

The data loggers were programmed to record at 30-minute 

intervals, and displacements were measured with a resolution of ±0.5 mm. All four strain-gage systems operated 

during the first monitoring period, and three (all but the 

Deerfield Road instrument, fig. 12A) were reinstalled for 

the second period.

RESULTS

During the first 7-month monitoring period, from December 1989 through June 1990, no significant displacements that could be related to renewed landslide movement

Figure 23.—Part of main scarp of Lower 

Schultheis Road West landslide. View eastward from just west of Schultheis Road 

(loc. A', fig. 22). Ridgecrest to left (north) 

is involved in landslide.
C112 LANDSLIDES

were recorded at any of the quadrilateral arrays or straingage sites. During this period, rainfall was exceptionally 

low—total rainfall in the Summit Ridge area during the 

winter of 1989-90 was only about 737 mm, or 67 percent 

of normal (fig. 2). Thus, the absence of recorded displacements provided no indication about the potential for renewed landslide movement during wetter periods. 

Quadrilateral arrays and strain gages did record contraction across some cracks and scarps during rainy periods 

in February and March 1990; this contraction was evidently due to swelling of the near-surface soil as it absorbed moisture.

Rainfall in the area during most of the second winter 

after the earthquake also was substantially below normal; 

total rainfall from July 1, 1990, through February 25, 1991, 

was only 171 mm (table 4). However, the period between

\ \ '

Figure 24.—Majestic Drive landslide. Ground cracks and compressional 

features from Spittler and Harp (1990); landslide boundaries from Keefer 

(1991). See figure 7 for explanation and base-map information.

February 26 and March 26, 1991, was exceptionally wet: 

Approximately 668 mm of rain fell during this 29-day 

period (table 4). Monitoring of quadrilateral arrays was 

intensified during this period of heavy rainfall, and displacements were measured on March 5, 15, 19, 25, and 

27, after major storms. During this period, the quadrilateral-array data showed cross-crack contractions of 2 to 4 

cm across cracks at 12 monitored sites on the Majestic 

Drive, Upper Schultheis Road, Rails, Villa Del Monte, 

and Upper Redwood Lodge landslides and the Old Santa 

Cruz Highway landslide complex (arrays 3, 5, 10, 11, 14, 

21, 30, 37, 46, 52-54, figs. 7A, 9A, 12A, 20A, 24A). Field 

observations indicated that these contractions were probably caused by swelling of surficial soil as water contents 

increased (Marshall and Griggs, 1991).

During this period, greater cross-crack contractions of 

9 to 14 cm were recorded at array 55 on the Upper Morrell 

Road landslide (fig. 17) and 15 to 36 cm at array 16 on 

the Villa Del Monte landslide (fig. 12A). Also during this 

period, cross-crack extensions of 3 to 7 cm were recorded 

at array 36 on the Upper Morrell Road landslide (fig. 17), 

and 3 to 4 cm at array 49 on the Hester Creek North 

landslide (fig. 27). The cross-crack extension at array 36 

and the contraction at array 55 were caused by local slumping of material bordering the cracks (Marshall and Griggs, 

1991). The displacements at arrays 16 and 49, as well as 

other observed displacements that did not disturb survey 

stakes, were associated with deeper-seated, renewed cracking along and near major scarps formed during the earthquake (Marshall and Griggs, 1991). This renewed cracking 

along major scarps occurred on the Upper Schultheis Road, 

Villa Del Monte, and Hester Creek North landslides (areas 3, 5, 14, pi. 3) as discussed below.

RENEWED MOVEMENT ON THE UPPER 

SCHULTHEIS ROAD LANDSLIDE

The first evidence of postearthquake ground cracking 

related to landslide movement was discovered on March 

5, 1991, when new echelon cracks were observed on repaired pavement where Schultheis Road crosses the main 

scarp of the Upper Schultheis Road landslide (loc. A, fig. 

29). The cracks were 1.5 to 3 m long and 1 to 2 cm wide, 

and showed as much as 2 cm of downslope vertical displacement. Observations on March 15 and 19, after additional rain, revealed increases of 1 to 2 cm in crack widths 

and vertical displacements, and increases of 0.5 to 1 m in 

crack lengths (Marshall and Griggs, 1991).

On March 25, a new set of cracks was observed upslope 

from the main scarp that formed during the earthquake 

(leg B-B', fig. 29). Because these new cracks did not 

occur along the traces of the preexisting, earthquake-induced cracks, associated displacements were not detected 

at nearby quadrilateral arrays 51 or 52 or by the adjacent
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C113

Schultheis strain gage (fig. 29). The new set of cracks 

extended eastward and southeastward from a point (B, 

fig. 29) on the earthquake-generated main scarp. From 

there eastward, a new crack was virtually continuous for 

about 50 m. This crack was arcuate and concave downslope 

in plan view, and along part of the crack was a graben, 15 

m long, 2 to 3 m wide, and 0.5 m deep, that showed 20 to 

30 cm of net vertical displacement between its upslope 

and downslope margins. Elsewhere, the crack was 5 to 10 

cm wide and showed 5 to 10 cm of vertical displacement. 

Southeast of the continuous crack, shorter echelon cracks 

continued for an additional 50 m (to loc. B', fig. 29). 

These shorter cracks were 10 to 20 cm wide and showed

as much as 10 cm of vertical displacement. At several 

localities upslope from the main scarp, storm runoff was 

observed flowing into the newly formed cracks on March 

25 (Marshall and Griggs, 1991).

A new quadrilateral array (56, figs. 9A, 29) was installed on March 27, 1991, across the newly formed graben. Measurement of this array 2 weeks later, on April 

11, showed 3 cm of additional downslope displacement. 

Subsequent measurements on April 18, 23, and 30, May 

8, and July 17 indicated no additional displacements, but 

field observations during those surveys showed that minor ground cracking had occurred in several places along 

the main scarp (between Iocs. A and B, fig. 29).

LOWER MORRELL 

ROAD LANDSLIDE 

COMPLEX

Figure 25.—Lower Morrell Road landslide complex. Ground cracks and compressional features from Spittler and Harp (1990); landslide 

boundaries from Keefer (1991). See figure 7 for explanation and figure 19 for base-map information.
C114 LANDSLIDES

RENEWED MOVEMENT ON THE VILLA DEL 

MONTE LANDSLIDE

Renewed ground cracking and displacement along the 

major scarp near upper Skyview Terrace was detected on 

March 19, 1991, when a survey of quadrilateral array 16 

(figs. 12, 30), showed 1 to 2 cm of contraction. Field 

inspection revealed that the contraction was associated 

with extension across a new crack, less than 1 m upslope 

from the site (inset, fig. 30). This crack, which formed 

sometime between March 15 and 19, was 2 to 5 cm wide 

and showed no vertical displacement. The crack was traceable 10 m eastward from the array, until lost beneath 

thick vegetation. Small cracks, less than 1 cm wide, also 

broke the pavement of upper Skyview Terrace near quadrilateral array 16 (point A, fig. 30).

Measurements at quadrilateral array 16 on March 25 

revealed additional contractions of 11 to 19 cm. The new 

crack noted on March 19 appeared to be unchanged, but 

a larger crack, about 1 m farther upslope, had formed 

between March 19 and 25. This new crack, 5 to 15 cm 

wide and showing 5 to 15 cm of downslope vertical 

displacement, was subparallel to the preexisting scarp

and traceable continuously approximately 40 m eastward. From that point east, a series of shorter cracks were 

traceable an additional 70 to 80 m. These shorter 

cracks were 5 to 15 cm wide and showed vertical 

displacements of as much as 15 cm. The cracks in 

Skyview Terrace (loc. A, fig. 30) had also become larger 

by March 25.

On March 27, another new crack was observed, trending obliquely upslope, northeast from the earthquake-generated scarp (inset, fig. 30). This crack, which averaged 5 

cm in width and showed as much as 5 cm of vertical 

displacement, was traceable for 4 m and passed between 

two of the stakes in quadrilateral array 16 (stakes A, B, 

inset, fig. 30). The March 27 survey showed 11 cm of 

cross-crack contraction on the two legs involving stake B 

(that is, legs B-C, B-D) and no significant change on the 

two legs involving stake A (that is, legs A-C, A-D). On 

March 27, a new quadrilateral array (57, figs. -12A, 30) 

was installed across the new crack observed on March 25 

(fig. 30). Subsequent measurements made at both quadrilateral arrays 16 and 57 on April 11, 18, and 23 and May 

8, 1991, showed that no additional displacements had occurred.

DOWNSLOPE

BOUNDARY

UNCERTAIN

Figure 26.—Stetson Road landslide. Ground cracks and compressional features from Spittler and Harp (1990); landslide boundaries 

from Keefer (1991). See figure 7 for explanation and figure 19 for base-map information.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C115

RENEWED MOVEMENT ON THE HESTER CREEK 

NORTH LANDSLIDE

Renewed displacement was first detected along the main 

scarp of the Hester Creek North landslide on March 19, 

1991. Measurements at quadrilateral array 49 (fig. 27) 

indicated 2 cm of extension between March 15 and 19,

and on March 25, measurements at the site revealed an 

additional 2 cm of extension. During the March 25 survey, renewed minor cracking was also observed across 

the unpaved access road, downslope from quadrilateral 

array 49 (fig. 27), at both localities where the road crossed 

the main scarp. Those cracks were 1 to 2 cm wide and 

showed slight vertical displacements.

SOQUEL-SAN JOSE 

ROAD LANDSLIDE

POSSIBLE 

LANDSLIDE i HESTER CREEK 

NORTH LANDSLIDE

HESTER CREEK 

J-lSpUTH LANDSLIDE

Figure 27.—Hester Creek North, Hester Creek South, and Soquel-San Jose Road landslides. Ground cracks 

and compressional features from Spittler and Harp (1990); boundaries of Hester Creek North and South 

landslides from Keefer (1991). See figure 7 for explanation and figure 19 for base-map information.
C116 LANDSLIDES

SUBSURFACE MONITORING

Subsurface monitoring of postearthquake conditions 

in the Upper Schultheis Road and Villa Del Monte 

landslides was carried out by using borehole inclinometers, for measuring subsurface movement, and borehole 

piezometers, for measuring ground-water levels and porewater pressures. Piezometers and inclinometers were installed by William Cotton and Associates, Inc.; the 

instruments and their installation were described in detail 

by William Cotton and Associates, Inc. (1990), and Keefer 

(1991).

INCLINOMETERS

Inclinometer casings were installed in all the smalldiameter boreholes in the Upper Schultheis Road and Villa 

Del Monte landslides except SR-1A, SB-1A, and SB-4A 

(figs. 9B, 125; table 5). The inclinometer probe used to 

monitor vertical deflections in the casings was a SINCO 

Digitilt Sensor, with a sensitivity of 1 part in 10,000 and a 

rated system error of less than 6.4 mm of deflection per 

30 m of casing.

Initial readings on the inclinometers were taken during 

the period January-March 1990 as installation was com-

\ \

COMSTOCK 

MILL ROAD 

LANDSLIDE

Figure 28.—Comstock Mill Road landslide. Ground cracks and compressional features from Spittler and Harp (1990). See figure 7 for 

explanation and figure 19 for base-map information.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C117

Table 4 —Rainfall in the area of the Burrell landslide, July 1, 1990- 

June 30, 1991

Daily Cumulative 

Date rainfall rainfall 

(1990-91) (mm) (mm)

July 1-Nov. 18

Nov. 19

Nov. 20

Nov. 25

Dec. 10

Dec. 14

Dec. 15

Dec. 18

Dec. 19

Jan. 6

Jan. 7

Jan. 8

Jan. 9

Feb. 1

Feb. 2

Feb. 4

Feb. 26

Feb. 27

Feb. 28

Mar. 1

Mar. 2

Mar. 3

Mar. 4

Mar. 10

Mar. 12

Mar. 13

Mar. 14

Mar. 16

Mar. 17

Mar. 19

Mar. 20

Mar. 21

Mar. 23

Mar. 24

Mar. 25

Mar. 26

Apr. 1

Apr. 19

Apr. 20

Apr. 24

Apr. 30

May 13

June 26

June 27

June 28

.3

.5

10.2

14.5

14.0

9.7

4.8

3.8

5.3

2.3

8.1

.3

25.1

25.9

22.4

1.3

54.6

42.7

16.8

55.1

134.6

28.7

22.9

27.4

1.5

2.8

25.4

46.2

25.4

10.4

3.8

27.2

96.5

34.3

9.9

4.3

11.4

2.5

.5

.8

1.3

.3

5.1

16.3

23.6

23.9

24.4

34.5

49.0

63.0

72.6

77.5

81.3

86.6

88.9

97.0

97.3

122.4

148.3

170.7

172.0

226.6

269.2

286.0

341.1

475.7

504.4

527.3

554.7

556.3

559.1

584.5

630.7

656.1

666.5

670.3

697.5

794.0

828.3

838.2

842.5

853.9

856.5

857.0

857.8

859.0

859.3

864.4

881.4

pleted; subsequent readings were taken in May 1990, November 1990, and March 1991. Between early 1990 and 

March 1991, these inclinometer measurements recorded 

only one displacement that exceeded the rated instrument 

system error; this displacement of 1.3 to 1.9 cm occurred 

about 15 to 30 m below the surface in borehole DM-4 

(figs. 12B, 31). Although this displacement indicated localized postearthquake movement, probably due to smallscale slumping toward the steep creekbank downslope from 

the instrument, the absence of any other significant displacements indicated that large-scale reactivation of the 

monitored landslides did not occur. Full records of the 

inclinometer measurements were presented by Keefer 

(1991).

PIEZOMETERS

The piezometers installed in most boreholes were continuously recording electronic-strain-gage Thor model 

DPXE instruments with a pressure range of 690 kPa and 

an overrange capacity of 1.5x. The piezometers were installed in canvas bags, which were filled with sand, secured onto the outside of the inclinometer casings, and 

sealed with the relatively impermeable grout used to 

emplace the inclinometer casings. Three such piezometers 

were installed in all boreholes except SR-1A, SB-1A, and 

SB^A at depths ranging from 10.7 to 88.1 m (tables 5, 6).

To monitor the performance of the strain-gage piezometers, Casagrande open-standpipe piezometers were installed at two depths each in boreholes SR-1A and SB-1 A 

and at depths near the intermediate strain-gage piezometer tips in boreholes DM-1, DM-3, SB-2, SB-3, SR-3, 

and SR-4 (table 5). Data from these Casagrande piezometers generally agreed well with those from the adjacent 

strain-gage piezometers (Keefer, 1991).

In all the boreholes, ground-water levels were progressively deeper, as measured by the deeper piezometer tips 

(table 6); differences in ground-water levels among the 

various boreholes also indicated a complex hydraulic regime. Within the Upper Schultheis Road landslide, the 

highest ground-water levels as measured by shallow piezometers were at 8.5- to 20-m depth; comparable measurements were 3.4- to 30-m depth as measured by 

intermediate-depth piezometers and 5.1- to 49-m depth as 

measured by deep piezometers. In the Villa Del Monte 

landslide, the highest ground-water levels were at 0.3- to 

20-m depth as measured by shallow piezometers, 4.5- to 

30-m depth as measured by intermediate-depth 

piezometers, and 13- to 42-m depth as measured by deep 

piezometers (table 6).

Ground-water levels were relatively constant throughout most of the monitoring period but rose significantly 

in response to the period of heavy rainfall in spring
C118 LANDSLIDES

1991 (figs. 32, 33; table 6). The dates and times of rise 

in ground-water levels due to the spring 1991 rains 

are listed in table 7. These times reflect a relatively 

rapid ground-water response to the rainfall, particularly 

to that in February and early March. Complete plots 

of the piezometric records were presented by Keefer 

(1991).

DISCUSSION AND CONCLUSIONS

The landslides in the Summit Ridge area were larger 

and more complex than virtually all other landslides triggered by the 1989 Loma Prieta earthquake (Keefer and 

Manson, this chapter) or by other historical earthquakes 

of comparable magnitude (Keefer, 1984). The occurrence

Schultheis 

strain gage

56 Graben

EXPLANATION

Earthquake —generated ground 

crack, October 17, 1989- 

hachures on downdropped side

56.

Renewed ground cracking, 

March 1991— hachures on 

downdropped side

Quadrilateral array

Strain gage

Dirt road or path

Locality mentioned in text

30 METERS 100 FEET 

_I

Figure 29.—Area around main scarp of Upper Schultheis Road landslide, showing ground cracks and scarps that resulted 

from earthquake; ground cracks, scarps, and grabens formed during period of heavy rainfall in March 1991; and locations 

of quadrilateral arrays and strain gage.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C119

of these landslides only in the Summit Ridge area indicates that their formation was related to specific seismic 

and (or) geologic conditions there. Among those conditions is the proximity of the landslides to the trace of the 

San Andreas fault. All of these landslides are within 4.1 

km of the fault trace, and all of those larger than 10 ha in 

area are within 1.8 km of the fault trace (fig. 34). In 

addition, although the data plotted in figure 34 show considerable scatter, a generally inverse relation between landslide area and distance from the fault trace is evident.

The occurrence of these large landslides only near the 

San Andreas fault could correlate with exceptionally strong 

near-field shaking that attenuated rapidly with distance 

away from the fault. Eyewitness accounts and other evidence of such effects as the snapping of redwood trees, 

movement of vehicles and other heavy objects, and major 

shaking damage to residences imply locally severe ground 

shaking. The Corralitos strong-motion station, 12 km

southeast of the study area and 200 m from the fault (fig. 

1), recorded a moderately high peak acceleration of 0.64 

g (Shakal and others, 1989) and a moderately high Arias 

intensity of 4.0. Shaking could have been even more severe in the Summit Ridge area. The absence of strongmotion recordings in that area preclude direct verification 

of this hypothesis; however, from a study of 20 strongmotion records, Beroza (1991) concluded that an area of 

concentrated fault slip was centered 7 km northwest of 

the hypocenter, nearly under the Summit Ridge area. Such 

concentrated slip could presumably cause anomalously 

strong shaking.

The Summit Ridge area was also in the region of maximum tectonic ground deformation, as determined by geodetic analysis of surface uplift and mapping of off-fault 

ground cracking. The maximum measured coseismic uplift of 592 mm was near the south boundary of the study 

area, and uplift within the study area itself ranged from

Renewed ground 

cracking, March

1991 3/25/9-

EXPLANATION

Earthquake-generated ground 

crack, October 17, 1989- 

hachures on downdropped side

Renewed ground cracking, 

March 1991— hachures on 

downdropped side

Quadrilateral array 

Locality mentioned in text

30 METERS 0 

III I

Figure 30.—Area around part of upper Skyview Terrace on Villa Del Monte landslide, showing ground cracks and scarps that resulted from 

earthquake; ground cracks and scarps formed during period of heavy rainfall in March 1991; and locations of quadrilateral arrays.
C120 LANDSLIDES

Table 5.—General information on boreholes drilled in the Upper Schultheis Road and Villa Del Monte landslides 

[Do., ditto]

Date (1989-90)

Borehole Total 

depth 

(m)

SR-1

SR-1A

SR-2

SR-3

SR-4

ED-1

ED-2

SD-1

DM-1

DM-2

DM-3

DM-4

SB-1

SB-1A

SB-2

SB-3

CR 4 \JL> *T

SB-4A

62.0

25.0

46.8

75.6

76.2

63.4

61.6

61.9

43.0

62.5

76.2

77.1

62.2

30.5

89.0

92.4

47.2

6.1

Start

12/18

12/29

01/03

01/08

01/29

01/17

01/23

01/04

01/29

01/15

01/30

01/18

12/21

01/02

01/02

01/10

01/11

01/15

Finish

12/22

01/02

01/05

01/02

03/13

01/23

01/30

01/10

01/31

01/23

03/09

01/25

12/30

01/03

01/10

01/18

01/12

01/15

Piezometer depths (m)

Drilled by Depth of Date of initial Shallow Interinclinometer inclinometer mediate 

casing (m) reading

Pifph^r

—do-— — — — -

...do..............

—do— — — - —

—do— — — — -

—do— — — — -

—do— — — — -

—do— ----- — —

All Terrain——

Pitcher-———

—do— — — — -

All Terrain ——

Pitrhpr

— do— — — -----

All Terrain ——

...do—————-

Pitcher ____

—do—————

61

--

46

76

74

61

61

61

43

61

76

76

61

-

88

91

46

1/8/90

-

1/9/90

3/19/90

2/1/90

3/8/90

1/12/90

1/12/90

3/14/90

2/27/90

3/14/90

2/14/90

1/9/90

--

3/13/90

3/13/90

3/1/90

12

--

21.2

18.2

20.1

15.1

16.8

11.8

12

21.2

18.3

21.3

11.6

-

21.2

21.2

10.6

24.2

-

30.3

51.7

35.4

28.8

33.4

29

18.6

36.4

36.6

39.6

27.4

-

45.6

42.5

21.2

Deep Casagrande

60.4

-

45.6

73.2

53.6

60.7

60.8

57.5

42.5

60.8

76.2

76.2

39.6

-

88.1

66.9

45.6

11.6,23.5

-

55.3

35.4

-

-

-

18.6

-

36.6

-

--

11.2,27.3

45.7

42.1

--

about 100 to slightly less than 592 mm (fig. 35; Marshall 

and others, 1991). The Summit Ridge area also had the 

highest concentration of ground cracks produced by the 

earthquake (see pi. 3; Spittler and Harp, 1990; Ponti and 

Wells, 1991). Within the area, ground cracks were most 

abundant along the south flank of Summit Ridge, where 

most of the largest landslides occurred.

The coseismic ground cracks evidently formed parallel 

to elements of the bedrock structure, such as bedding 

planes, joint surfaces, and faults; their orientations are 

generally consistent with tectonic extension across the crest 

of the uplifted hanging-wall block, but most of them also 

exhibit displacements indicating extension due to gravitational, downslope movement (Ponti and Wells, 1991). The 

relation of ground cracks to the large landslides is demonstrated in detail by the many specific examples of structurally controlled ground cracks forming landslide 

boundaries and other landslide elements described above, 

by Keefer (1991), and by Harp (this chapter). This evidence indicates that although many ground cracks were 

produced directly by tectonic deformation, by the initial 

effects of seismic shaking on preexisting planes of weakness, or both, once these cracks formed, they then served 

as loci around which continued shaking caused downslope 

movement and landsliding. This mechanism of landslide 

formation is consistent with the highly irregular shapes 

and typically discontinuous boundaries of the landslides 

and with the restriction of these landslides to the Summit 

Ridge area; other areas that had no significant numbers of 

ground cracks also produced no large landslides.

Within the Summit Ridge area, all but one of the largest landslides occurred along the southwest flank of Summit Ridge itself (see pi. 3). That part of the area, which is 

generally nearest the San Andreas fault, also has the most 

complex geologic structure, as evidenced by overturned, 

tightly folded bedding and numerous subsidiary faults. 

Because of this complex structure, subsurface shear zones 

and other planes of weakness probably are most common 

under that part of the area, which, as noted above, also 

had the highest concentration of ground cracks.

Among the bedrock units in the study area, the Vaqueros Sandstone was most involved in landsliding: earthquake-induced landslides covered about 51 percent of the 

area underlain by this unit (table 8), in comparison with 

26 percent for the Rices Mudstone Member of the San 

Lorenzo Formation, 18 percent for the Butano Sandstone, 

10 percent for the Twobar Shale Member of the San 

Lorenzo Formation, 6 percent for the Purisima Formation, 

and 0 percent for the Lambert Shale. The Vaqueros Sandstone, Rices Mudstone Member, Butano Sandstone, and 

Twobar Shale Member were sampled during drilling in 

the upper Schultheis Road and Villa Del Monte areas. For 

those units, some correlation exists between the abundance of shale and the percentage of area involved in 

landsliding (table 8): The Vaqueros Sandstone contained 

the most shale (33 percent), the Twobar Shale Member 

the least (3 percent), and the Rices Mudstone Member 

and Butano Sandstone intermediate amounts (20 and 29 

percent, respectively). Neither the Purisima Formation nor 

the Lambert Shale was sampled. The Lambert Shale un-
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C121

derlies only a small part of the study area; however, the 

Purisima Formation is more extensive and is described as 

being relatively poorly consolidated (Clark and others, 

1989). The relatively small percentage of area involved in 

landsliding on this unit is probably due to its occurrence 

in the study area farther away from the San Andreas fault 

than the other units, to its lesser structural complexity, 

and to the relatively low concentration of coseismic ground 

cracks in that part of the study area.

A high correlation exists between the locations of large 

landslides triggered by the 1989 Loma Prieta earthquake 

and mapped, preexisting landslide deposits (fig. 4; table 

3). Of the 20 large landslides in and near the study area, 

18 were partly or completely within areas mapped as preexisting landslide deposits (Cooper-Clark and Associates, 

1975), and virtually all of the earthquake-generated landslides examined in the field also exhibited clear geomorphic evidence of previous movement. Additional evidence 

of recurrent movement of the landslides comes from 

trenching studies, discussed by Keefer (1991) and Nolan 

and Weber (this chapter). Thus, the earthquake largely 

reactivated preexisting landslide material in the Summit 

Ridge area.

INDICATED DEFLECTION (RESULTANT), 

IN CENTIMETERS

CO

o

CO

Q-z.

Z)

o 

a: 

o

o

_i

LJ 

00

m hQ_

LJ 

Q

0

10

20

30

40

50

60

70

80

0 1

\

November 

1990

March 

1991

From William Cotton 

and Associates, Inc., 

May 1990

Bottom of inclinometer DM —4 

I I I I_____

Figure 31.—Deflections measured by inclinometer in borehole DM-4 

on Villa Del Monte landslide (see fig. 125 for location).

Landslide occurrence and direction of movement did 

not generally correlate with the dip of bedding. Of the 20 

large landslides, 8 moved in directions opposite to the 

dip, 7 moved in directions oblique to the dip, and only 5 

possibly moved generally in the same direction as the dip 

(table 3). Along the southwest flank of Summit Ridge, in 

particular, in the area of the largest landslides, bedding 

most commonly dips northeast or is overturned to the 

northeast, into the slope (see pi. 3). Thus, the other conditions discussed above evidently were more significant than 

the dip of bedding in determining where large landslides 

formed.

The distinctive surface morphology of the large landslides in the Summit Ridge area is probably due in part to 

earthquake-induced displacements that were small relative to the landslide lengths. Typically, landslide heads 

and main scarps were relatively well developed, flanks 

were moderately well developed, and toes were poorly 

defined by surface features. Such morphology is consistent with small displacements and longitudinal strains (defined as the ratio of displacement to landslide length), 

whereby extensional displacements in the head of a landslide may be accommodated by distributed internal compression downslope, so that little or no actual displacement 

of the landslide toe may occur. This process is illustrated 

for a longitudinal strain of 0.2 percent in figure 36. Longitudinal strains calculated for the landslides in the Summit Ridge area ranged from 0.05 to 0.83 percent (table 3).

With increasing longitudinal strain, landslide features, 

particularly those in the landslide toes, should be more 

fully formed. The relation between longitudinal strain and 

the formation of surface features across toes for the landslides in the Summit Ridge area is plotted in figure 37. 

Half of the landslides exhibited no mapped pressure ridges 

or other features marking the toes; however, the toe features observed on other landslides were typically small, 

and so many toe features could have been hidden in heavily 

vegetated terrain. For those landslides where toe features 

were mapped, the data show a trend of increasing development of toe features with increasing longitudinal strain.

The landslide materials themselves were heterogeneous 

and composed of generally thick layers of colluvium and 

varyingly sheared and weathered sandstone, siltstone, and 

shale bedrock. Basal shear surfaces were identified in boreholes at depths ranging from about 5 to 27 m, and other 

data from boreholes and from undamaged water wells suggests that some basal shears may be more than 100 m 

deep. The identified basal shears consisted of (1) a zone 

of sheared material, 30 cm thick, containing highly plastic 

clay, silty clay, sandstone, and fractured shale and a basal 

layer, 0.5 to 1.3 cm thick, of sheared silty clay and clayey 

silt; (2) a 5- to 10-cm-thick layer of swelling, clayey siltstone separating oxidized from unoxidized materials; (3) 

a 2.5- to 5-cm-thick layer of soft, slickensided clay separating oxidized from unoxidized rock; and (4) a sheared
C122 LANDSLIDES

Table 6.—Piezometric data on the Upper Schultheis Road and Villa Del Monte landslides 

[n.d., no data; do., ditto]

Borehole Piezometer Surface Elevation of Depth of Depth of Fluctuation 

depth elevation highest piezometer highest in groundOn) ground- tip ground- water level 

water level (m) water level during 

(m) (m) Spring 1991 

(m)

DM-1

DM-2

DM-3

DM-4

ED-1

ED-2

SB-1

SB-2

SB-3

SB-4

SD-1

DM-1

DM-2

DM-3

DM-4

ED-1

ED-2

SB-1

SB-2

SB-3

SB-4

cr\ i

\JLJ- 1

DM-1

DM-2

DM-3

DM-4

ED-1

ED-2

SB-1

SB-2

SB-3

SB-4

SD-1

SR 1

SR-2

SR-3

SR-4

OK.-1 QT? 1

SR-2

SR-3

SR-4

OK-1 QT? 1 

SR-2

SR-3

SR-4

Shallow—————

... do— — — — —

... do— — — —— -

—do——————

— do— — — ————

— do— ——— —— —

— do— — — — —

— do— —— — ———

—do———————

—do— — — — —

—do—————

Intermediate ———

... do— ——— ——

...do— — — — —

...do— — — — —

—— do———— —— ———

— do————— — — -

—do—————

— do——— — — - -

...do— — — — —

...do— — — — —

Ho

Deep— — — — — -

... do—— — ———

... do— — — — —

...do——————

... do— —— — —

— do——— — ——

— do— — ——— — -

—do—————————

— do——— — ————

...do—————

—do— — — — —

Shallow————

...do— — — ——

..-do— — ——— — -

—do—————

Intermediate ———

do— — — — —

... do— — — ——

— do—— — -----

Dee

— do— ——— — ——

— do-——— ——— —

——do——————————

486.7

460.9

425.2

332.4

439.5

420.3

435.3

321.4

385.2

470.3

470.6

486.7

460.9

425.2

332.4

439.5

420.3

435.3

321.4

385.2

470.3

470.6

486.7

460.9

425.2

332.4

439.5

420.3

435.3

321.4

385.2

470.3

470.6

441.4

412.0

413.8

390.3

441.4

412.0

413.8

390.3

441.4

412.0

413.8

390.3

482.5

447.1

410.3

312.4

424.0

406.9

434.9

309.7

n.d.

469.1

470.3

476.7

431.3

406.3

303.7

415.1

406.9

420.9

293.2

375.8

465.7

465.6

468.5

419.1

385.6

n.d.

414.5

405.4

395.3

283.5

372.5

452.0

445.6

430.7

n.d.

405.4

370.3

429.8

408.7

396.2

360.3

425.5

406.9

390.8

341.4

12.0

21.2

18.3

21.3

15.1

16.8

11.6

21.2

21.2

10.7

11.8

1 Q C. lo.o

36.4

36.6

39.6

28.8

33.4

27.4

45.6

42.5

21.2

29.0

42.5

60.8

76.2

76.2

60.7

60.8

39.6

88.1

66.9

45.6

57.5

12.0

21.2

18.1

20.1

24.2

30.3

51.7

35.4

60.4

45.6

73.2

53.6

4.2

13.8

14.9

20.0

15.5

13.4

.3

11.8

n.d.

1.2

.3

10.0

29.7

18.9

28.7

24.3

13.4

14.4

28.2

9.4

4.5

5.0

18.2

41.8

39.6

n.d.

24.9

14.9

40.0

38.0

12.7

18.3

25.0

in s lU.o

n.d.

8.4

20.0

11.7

3.3

17.6

30.1

15.9

5.1

23.1

49.0

1.7

2.7

4.3

.0

.0

1.5

2.1

.9

n.d.

1.5

4.0

1.6

.3

2.7

-1.2

2.4

.0

.6

.6

2.1

2.4

3.5

.3

-3.0

.0

n.d.

n.d.

2.1

.9

.3

1.5

2.7

3.7

.9

n.d.

6.1

.0

3.0

1.8

1.5

.0

2.4

.6

.6

.6
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C123

siltstone interbedded with a massive sandstone (Cole and basal shears evidently separated weathered material from

others, 1991 and this chapter). No simple relation between bedrock, and two were within the bedrock itself,

geologic materials, stratigraphy, or local structure and the Two shear surfaces each were documented under the

formation of basal shear surfaces was evident. Two of the Upper Schultheis Road and Lower Schultheis Road West

420

410 -

. 400 -

390 -

Ground surface

Shallow

Intermediate

Deep

380

1/1/90 4/1/90 7/1/90 10/1/90 1/1/91 4/1/91 7/1/91 10/1/91

, J. I , , , . , , J. , I . , . . . ,l . . I , , . . ,l , . , I . . , ,l . , . , I , , .

100 200 300 400 

TIME, IN DAYS

500 600

Figure 32.—Elevation of piezometric surface versus time for piezometers in borehole SR-3 on Upper Schultheis Road 

landslide (see fig. 9B for location).

490 -

480 -

LLJ

ui 470

Ground surface

Intermediate

Deep

1/1/90 4/1/90 7/1/90 10/1/90 1/1/91 4/1/91 7/1/91 10/1/91

460 i i i i i i i i i i i i i i i i i i i i i i i i i i i i i >

100 200 300 400 

TIME, IN DAYS

500 600

Figure 33.—Elevation of piezometric surface versus time for piezometers in borehole DM-1 on Villa Del Monte 

landslide (see fig. 12B for location).
C124 LANDSLIDES

Table 7.—Rises in ground-water level in response to rainfall, February-March 1991

Piezometer 

Depth Date of Initial Rise (1991) Amount of Rise (m)

Range Average Range Average

Shallow————

Intermediate ——

— — Feb. 24 to Mar. 4 Mar. 1 0-6.1 1.7

— — Feb. 26 to Apr. 10 Mar. 7-8 -1.2 to 3.5 1.3

... „ Pf»h A tn Ar>r OH \Air- f, -^ H tn ^ 7 Q

landslides (William Cotton and Associates, Inc., 1990; 

Cole and others, 1991 and this chapter). Additionally, borehole data from the Villa Del Monte landslide and the 

distribution of well damage there and elsewhere suggests 

(1) that multiple shear surfaces could exist under other 

landslides as well and (2) that the degree of development 

of shear surfaces varied locally. In particular, the absence 

of reported well damage within some landslide areas suggests that local basal shear displacements were small, distributed over a broad range of depths, or both.

Direct comparisons between the large landslides generated by the 1989 Loma Prieta earthquake and the landslides triggered or reactivated by the 1906 San Francisco 

earthquake are difficult because of the incomplete descrip100

<I— 

o

LJ

n:

cc: 

<

10

-i—i i i i

'10

^9

15

11 .13

0.1 1 10

DISTANCE OF CROWN FROM FAULT TRACE,

IN KILOMETERS

Figure 34.—Area of landslide versus distance from landslide crowns to 

surface trace of San Andreas fault. Letters and numbers refer to landslides mapped on plate 3 and listed in table 3.

tions and imprecise locations in reports on the 1906 earthquake. However, those reports do suggest that landslide 

activity in and around the Summit Ridge area was more 

widespread and severe in 1906 than in 1989 (table 1). The 

greater landslide activity in 1906 is consistent with at 

least two major differences in conditions: (1) the 1906 

earthquake (M=8.2-8.3) was much larger than the 1989 

earthquake (My=7.1); and (2) rainfall was heavier before 

the 1906 earthquake, and so ground-water levels were presumably higher than in 1989 (Youd and Hoose, 1978; 

Schuster and others, this chapter).

Surface and subsurface monitoring of selected landslides 

in the Summit Ridge area showed no significant 

postearthquake displacements during the unusually dry 

period from December 1989 through late February 1991. 

Precipitation of 668 mm during late February and March 

1991 did trigger localized ground cracking and displacements near the main scarps of the Upper Schultheis Road, 

Hester Creek North, and Villa Del Monte landslides. The 

maximum movement recorded across ground cracks and 

(or) surface-monitoring arrays was 13 to 20 cm. The 

ground cracking and displacement were probably due to 

local upslope migration, or retrogressive failure, of the 

landslide heads, a process that could continue through 

subsequent periods of heavy rainfall.

The period of heavy rainfall and observed landslide 

cracking in February and March 1991 coincided with a 

period of rising ground-water levels, as monitored by piezometers in the upper Schultheis Road and Villa Del 

Monte areas. Ground-water levels measured by individual 

piezometers during this period rose as much as 6.1 m 

from February through April. The average rise in groundwater level was 1.7 m as measured by shallow piezometers, 1.3 m as measured by intermediate-depth 

piezometers, and 0.9 m as measured by deep piezometers. 

These rises in ground-water level were associated with a 

month-long period of heavy rainfall, which occurred after 

a much longer (5 year) period of exceptionally light rainfall. Although these data show that significant changes in 

ground-water levels are possible within these slopes as a
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C125

result of rainfall, extrapolation to other short- or longterm rainfall cycles requires additional data and analysis.

The dry ground conditions before and during the 1989 

Loma Prieta earthquake almost certainly limited the extent and severity of landsliding. If the earthquake had 

occurred under wetter conditions, when ground-water levels in slopes were higher, many additional areas of 

coseismic ground cracks that exhibited small downslope 

displacements would probably have developed additional 

landslides. A larger earthquake under wetter conditions 

would probably produce much more severe and extensive 

landsliding in the Summit Ridge area, as evidently occurred during the 1906 earthquake.

The inferred mechanism for landslide formation in the 

Summit Ridge area, with initiation associated with 

coseismic ground cracks, implies that such landslides are 

specifically generated by earthquakes. This inference is

supported both by the historical record and by slope-stability analyses of the Upper Schultheis Road and Villa 

Del Monte landslides. The historical record indicates that 

although landslides are commonly triggered by intense or 

long-duration rainfall in the area, such landslides are much 

smaller, shallower features, primarily debris flows and 

less complex slumps and block slides. Slope-stability 

analyses, as described in detail by Keefer (1991), indicate 

that (1) deep-seated basal shear surfaces of large areal 

extent are the most unstable under seismic conditions and 

(2) shallower shear surfaces of more limited areal extent 

are the most unstable under conditions of rising groundwater levels in the absence of seismic shaking. Thus, the 

large landslides in the Summit Ridge area probably are 

uniquely earthquake related, and, in this area, such earthquake-generated landslides are probably major recurring 

hazards and significant agents of landscape evolution.

37°20'

37°10'

37°00'

36°50'

122°20' 122° 10 s 122°00 121°50J 121 °40

°'

121 °30

°'

Figure 35.—Loma Prieta area, showing contours of coseismic uplift and subsidence caused by 1989 earthquake (from Marshall and 

others, 1991). Star, earthquake epicenter; triangle LP, geodetic station on Loma Prieta; dots, bench-mark locations; irregular thin 

lines, faults (dashed where approximately located, dotted where concealed). Contour interval, 50 mm; subsidence contours hachured.
C126 LANDSLIDES

Table 8.—Percentage of areas and shale in geologic units involved in landslides

[Percentage of area is based on ratio of area of landslides within unit to total mapped area of unit in study area. Percentage of shale is based on ratio 

of total length of shale to total length of unit in boreholes drilled in the Upper Schultheis Road and Villa Del Monte areas; drilling logs from William 

Cotton and Associates, Inc. (1990). n.d., not determined]

Geologic unit

Vaqueros Sandstone —————— 

Rices Mudstone Member of the San Lorenzo Formation ——— • Butano Sandstone ————————

Twobar Shale Member of the San Lorenzo Formation — —— —

Purisim a Formation-— — — — ——

Lambert Shale —— -- — — — — —— -

Percentage of area 

involved in landsliding

51 

i^

18 

10

6 

0

Percent 

shale 

(percent)

33 

20 

29 

3 

n.d. 

n.d.

Zone of distributed internal compression

Zone of 

extensional 

cracking in

landslide

head

in HLJ 2H

cr LJ

=3 0

< LJ

^ Q_ L_

_l Z

<

z

£~") 1 i J

c/5 2

LJ (/)

a: on

0- 0

2 LT

0 0

0 <

1 UU

80

60

40

20

W

f~^

i i i i i i i i i i i i i f*i i

— —

"•

* •

13-

— —

• 3 _

4

5^ • 9

10 B 15 176* 1114

I A I I I I .1. 1 — . 1 — . J. -4' U 1 1 ] 1

0.01 0.1 12

LONGITUDINAL STRAIN, IN PERCENT

Figure 36.—Idealized landslide cross section, illustrating accommodation of extensional displacement in head by distributed internal compression of rest of landslide, with no bulging of ground surface at 

landslide toe. Lengths of arrows along basal shear surface represent 

relative downslope displacement of points along basal shear surface; 

dots, no displacement. For example, within a landslide 1,000 m long, 2 

m of extensional displacement across main scarp could be accommodated by 0.2 percent of distributed longitudinal compressive strain within 

landslide material, with no movement at toe and no bulging of ground 

surface (assuming a constant thickness of landslide material).

Figure 37.—Percentage of landslide toes marked by compressional features (calculated from ratio of total length of compressional features in 

landslide toe to width of toe) versus longitudinal strain (calculated 

from ratio of displacement to landslide length) for landslides in Summit Ridge area. Letters and numbers refer to landslides mapped on 

plate 3 and listed in table 3.
LARGE LANDSLIDES NEAR THE SAN ANDREAS FAULT IN THE SUMMIT RIDGE AREA C127

ACKNOWLEDGMENTS

This research was supported by the Federal Emergency 

Management Agency as part of the U.S. Government's 

disaster-relief program, provided at the request of the 

County of Santa Cruz. We are grateful for the assistance 

of the following people, who aided in various aspects of 

our investigation: Thomas E. Spittler (California Division 

of Mines and Geology), Paia Levine and Robert 

Brumbaugh (County of Santa Cruz); John M. Andersen, 

Sonia Diaz, Kenneth Harrington, Mary E. Hynes, Colin 

C. McAneny, Arijs A. Rakstins, H.M. Taylor, Brian Walls, 

and Tak Yamashita (U.S. Army Corps of Engineers); 

William M. Brown III, Ethel P. Lopez-Cavender, Alan F. 

Chleborad, Raymond R. Eis, Randall W. Jibson, Mark E. 

Reid, Kevin M. Schmidt, and Robert L. Schuster (U.S. 

Geological Survey); Nicholas Sitar (University of California, Berkeley); I.M. Idriss and Martin Hudson (University of California, Davis); Jeffrey S. Marshall and Nan A. 

Rosenbloom (University of California, Santa Cruz and 

Gary B. Griggs and Associates); Gerald E. Weber (University of California, Santa Cruz); Jeffrey M. Nolan (Nolan 

Associates); and William F. Cole, William R. Cotton, 

Dale R. Marcum, and Patrick O. Shires (Cotton, Shires 

Associates, Inc.). We also thank the many residents of 

the study area, who cooperated in providing access to 

private property and in sharing their observations of earthquake effects.

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C128 LANDSLIDES

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landslide features and coseismic fissures, in the Summit Road area 

of the Santa Cruz Mountains, triggered by the Loma Prieta earthquake of October 17, 1989: U.S. Geological Survey Open-File 

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may come sooner than you think: Menlo Park, Calif., 23 p.

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Debris flows and other landslides in San Mateo, Santa Cruz, Contra 

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and factors influencing debris-flow distribution, in Ellen, S.D., and 

Wieczorek, G.F., eds., 1988, Landslides, floods, and marine effects 

of the storm of January 3-5, 1982, in the San Francisco Bay region, 

California: U.S. Geological Survey Professional Paper 1434, p. 133- 

161.

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THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

ORIGIN OF FRACTURES TRIGGERED BY THE EARTHQUAKE

IN THE SUMMIT RIDGE AND SKYLAND RIDGE AREAS

AND THEIR RELATION TO LANDSLIDES

By Edwin L. Harp, 

U.S. Geological Survey

CONTENTS

	Page 

Abstract ————————————————————... ci 29

Introduction ———————————————————————————— 129

Geologic setting —————————————————————— 130

Methods of fracture mapping and determination of origin —— 132

Criteria for discriminating fracture origins ——————————— 132

Fractures related to landslide movement ————————— 132

Fracture orientation —————————————————— 132

Variation in trend ——————————————————-- 132

Displacement magnitude——————————————— 133

Sense of relative displacement ———————————— 133

Fractures related to regional structure ————-———— 133

Fracture orientation —————————————————— 133 

Trends of displacement vectors and sense of relative

displacement ———————————————————-- 134

Displacement magnitude——————————————— 135 

Fractures related to both landslide movement and

regional structure ———————————————————— 135

Tranbarger fracture zone——————————————— 135

Church fracture zone ————————————————— 136

Interpretation of fracture patterns ——————————————— 136

Effect of regional structure on landslides ————————-- 136

Upper Morrell Road landslide ———————————— 136

Villa Del Monte landslide ———————————— 137

Other examples and summary ——————————————— 137 

Similarity of fracture patterns to those produced in other

earthquakes —————————————————————————— 138

The 1986 North Palm Springs, California, earthquake ----- 138

The 1980 El Asnam, Algeria, earthquake ——————— 138

Summary ———————————————————————————— 142

References cited ——————————————-————————— 143

ABSTRACT

The 1989 Loma Prieta earthquake generated a complex 

zone of distributed fractures related to both landslide movement and regional structure, such as bedding, joints, and 

faults, within a 3- by 10-km area primarily on the southwest flank of Summit Ridge in Santa Cruz County adjacent 

to the San Andreas fault zone. Instead of predominantly 

left stepping, right-lateral fractures confined to the mapped 

trace of the fault, these fractures were primarily extensional, and components of left-lateral slip were more common than those of right-lateral slip.

Fractures primarily related to landslide movement and 

those primarily related to regional structure were discriminated on the basis of orientation, variation in trends, displacement magnitude, sense of relative displacement, and 

spatial relation to existing geomorphic features. Fractures 

related primarily to landslide movement varied more 

widely in azimuthal trend and dip and had greater maximum displacements than those related primarily to regional structure. Interaction between landslide boundaries 

and structurally controlled fractures is evident in that the 

regional-structural trend (N. 30°-80° W.) is reflected in 

the shapes of some of the landslides reactivated by the 

earthquake. Several landslide headscarps that formed along 

structurally controlled discontinuities paralleled the regional-structural trend, and the landslide boundaries 

showed asymmetries imposed by regional structure.

The distributed off-fault fractures caused by the earthquake are similar to the fractures triggered by other recent 

earthquakes. For example, the 1986 North Palm Springs, 

Calif., and 1980 El Asnam, Algeria, earthquakes generated extensive off-fault fractures and landslides. Fracture 

patterns, extensional graben features, and landslide formation related to distributed horizontal shear, gravitational 

effects, and uplift-induced extension during the 1980 El 

Asnam earthquake suggest a useful qualitative model for 

similar features produced during the 1989 Loma Prieta 

earthquake.

INTRODUCTION

Initial efforts to locate a surface rupture from the 1989 

Loma Prieta earthquake were frustrated largely by the confusing pattern of landslide-related fractures and numerous 

other ground cracks that displayed features generally considered to be uncharacteristic of the San Andreas fault. 

Instead of predominantly left stepping, right-lateral fractures confined to the mapped trace of the fault, fractures

C129
C130 LANDSLIDES

were distributed throughout a wide zone southwest of the 

fault, along the crests of Summit and Skyline Ridges (fig. 

1), approximately 3 km wide transverse to the ridgecrest 

by 10 km long parallel to the ridge, approximately between Hutchinson Road and Skyland Ridge (see pi. 4; fig. 

1). Instead of the expected primarily right-lateral displacements, most fractures were primarily extensional, and components of left-lateral slip were more common than those 

of right-lateral slip.

This paper presents the fracture pattern produced by 

the earthquake in the Summit Ridge and Skyland Ridge 

areas (see pi. 4), discriminates fractures related to landslide movement and those related to regional structure 

(bedding, faults, joints), and discusses the relations between the existing structural fabric and the landslides generated by the earthquake.

GEOLOGIC SETTING

The geology of Summit Ridge itself is dominated by 

three units of Tertiary age: the Butano Sandstone, San

Lorenzo Formation, and Vaqueros Sandstone. All of these 

units consist of weakly cemented sandstone, siltstone, 

mudstone, and shale. According to the most recent geologic mapping in the study area (Clark and others, 1989, 

with landslide boundaries revised by RJ. McLaughlin and 

J.C. Clark, unpub. data, 1990; McLaughlin and others, 

1991), the overall trend of bedding and other regional 

structure is northwest-southeastward. Bedding in the study 

area typically strikes N. 30°-80° W. The rocks are intensely folded, and dips are typically steep. Locally, the 

rocks dip southwest or northeast, are vertical, or are overturned to the northeast (fig. 2).

Several major geologic structures pass through the study 

area (fig. 2). The Laurel anticline trends approximately N. 

45° W. through the south flank of Summit Ridge and is 

truncated by the Butano fault approximately 1.4 km southeast of the California Highway 17-Summit Road intersection. This fault, which diverges from the San Andreas 

fault 800 m northwest of the Old San Jose Road-Summit 

Road intersection, traverses the Summit Ridge area with 

local strikes of N. 60°-75° W. The Summit syncline trends 

N. 60°-65° W. and extends approximately 300 m west of

122°

Figure 1.—Loma Prieta area, Calif, showing locations of major faults (dashed where inferred, dotted where 

concealed) and of study area in the Santa Cruz Mountains where widespread landsliding and coseismic 

ground fractures were generated by 1989 Loma Prieta earthquake. Star, epicenter.
ORIGIN OF FRACTURES TRIGGERED BY THE EARTHQUAKE AND THEIR RELATION TO LANDSLIDES C131

f

I "47*

Morrell Road, where the synclinal axis is truncated 

by an unnamed fault trending N. 45° W. that is inferred to offset the Butano fault left laterally (fig. 2; 

McLaughlin and others, 1991). In the Villa Del Monte 

and Morrell Road areas, several faults striking approximately N. 45° W. extend across the Butano fault 

and the Summit syncline. Most of these faults are 

shorter than 1 km in mapped length and appear to 

offset structures little or not at all.

Several grabens are present near the crest of Summit Ridge (see pi. 4A; fig. 2); they are subparallel to 

the ridge and have a local relief of 1.5 to 6 m across 

their boundaries. Renewed displacement occurred on 

parts of graben boundaries during the earthquake, as 

evidenced by fractures with vertical displacements of 

from several centimeters to as much as 30 cm. Three 

of the most conspicuous grabens are near Summit 

Road (see pi. 4A; fig. 2); the westernmost of these is 

about 60 m southeast of Old Summit Road and about 

750 m east of California Highway 17. This graben 

(Tranbarger graben, pi. 4A; fig. 2) is part of the extended Tranbarger fracture zone in the Old Santa Cruz 

Highway area. There, earthquake-induced fractures 

formed on both margins near its northwest limit, where 

it is approximately 15m wide. To the southwest, the 

graben increases in width to approximately 60 m 

within a horizontal distance of 100 m. Measured vertical displacements during the earthquake ranged from 

2.5 to 15 cm (Spittler and Harp, 1990).

Another visible graben along Summit Road lies 

adjacent to and southwest of a 430-m-long fracture 

(Church ruptures of Ponti and Wells, 1990) that extends southeastward from the Melody Lane-Summit 

Road intersection (see pi. 4A; fig. 2), crossing Summit Road twice. The long fracture along the northeastern margin of this graben is the only significant 

earthquake-induced fracture associated with it. Its 

maximum displacement is about 0.3 m (Spittler and 

Harp, 1990). As discussed below in the subsection 

entitled "Church Fracture Zone," no single origin is 

obvious for this feature.

Beginning at the Summit Road-Old Santa Cruz 

Highway intersection, a 30- to 50-m-wide graben extends approximately 250 m southeastward along the 

southwest side of Summit Road; it trends N. 45° W., 

diverging from the trend of Summit Road by about 

5°. Both southwest- and northeast-facing scarps 

formed here on fractures (Corral graben, pi. 4A; fig. 

2) triggered by the earthquake, with vertical displacements ranging from 5 to 36 cm (Spittler and Harp, 

1990). Within this graben, earthquake-induced fractures did not wholly coincide with the graben margins but also formed southwest- and northeast-facing 

scarps within the graben itself. These grabens and the 

fractures created within them during the earthquake
C132 LANDSLIDES

parallel the prevailing structural trends in this area (N. 

40°-60° W.), as discussed below in the subsection entitled "Fractures Related to Regional Structure."

METHODS OF FRACTURE MAPPING 

AND DETERMINATION OF ORIGIN

The fractures or ground cracks that formed in the Summit Ridge area as a result of the earthquake were mapped 

by a team of geologists from Federal, State, and county 

agencies, universities, and private consulting firms. The 

map of landslide-related and other coseismic fractures (see 

pi. 4) was derived from the original map by Spittler and 

Harp (1990), using l:6,000-scale aerial photographs taken 

October 27, 1989. Outside of this coverage, mapping was 

done on enlargements of l:24,000-scale U.S. Geological 

Survey topographic maps, l:31,680-scale aerial photographs taken in June 1989, and l:4,800-scale County of 

Santa Cruz planimetric maps.

The locations of features mapped from the l:6,000-scale 

aerial photographs are accurate to within 10 m where vegetation is sparse and to within 10 to 50 m where vegetation is dense. Outside the coverage of the 1:6,000 aerial 

photographs, accuracy is diminished. Where distinctive 

and (or) topographic features are present, locations are 

accurate to within 20 to 50 m; where no such distinctive 

features exist and where tree cover is dense, accuracy is 

estimated at within 50 to 100 m.

Some fractures may have been missed during the mapping because of dense vegetation, poor access, and the 

obliteration of many features due to repair work before 

mapping could be completed. The purpose of plate 4 is to 

systematically document and interpret the landslide and 

other coseismic fractures and to display their distribution 

with a minimum of generalization. Because these features 

were plotted by hand and are not precisely surveyed at a 

large scale, such as 1:500 or larger, some generalization 

has necessarily been incorporated into the mapping. Therefore, plate 4 is useful as a reconnaissance tool and as a 

guide to subsequent geologic and geotechnical investigations but is insufficiently precise to serve as a map on 

which to base site-specific engineering or planning decisions.

The various distinguishing characteristics of fractures, 

such as orientation, continuity, and relation to geomorphic features, were used to discriminate their origins. The 

criteria described below were used to discriminate on plate 

4 between fractures primarily related to landslide movement and those primarily related to regional structure, such 

as bedding, joints, or faults. A few major fractures and 

fracture zones are designated as related to both types of 

processes because evidently both landslide movement and 

regional structure significantly influenced their geometry.

Relatively small ground cracks, whether within or outside 

landslide boundaries, were not individually categorized. 

Ground-crack patterns were analyzed primarily with the 

data of Spittler and Harp (1990, pi. 5) and Wells (in press).

CRITERIA FOR DISCRIMINATING 

FRACTURE ORIGINS

FRACTURES RELATED TO LANDSLIDE 

MOVEMENT

The typical association of fractures showing different 

relative displacements and orientations within the various 

parts of a landslide is well documented (Varnes, 1978) 

and forms a basis for relating fractures to landslides or 

other causes. The relation of fractures to slope morphology and to each other is also important in such a determination. The large landslides in the Summit Ridge area and 

the fractures associated with them are described by Keefer 

and others (this chapter), along with a discussion of the 

general criteria for identifying such landslides. To facilitate the discussion of fracture origins, however, in the 

following sections I discuss the criteria used to discriminate between fractures related to landslide movement and 

those related to regional structure.

FRACTURE ORIENTATION

The orientation and shape of a fracture are primarily 

important in evaluating its origin. As shown on plate 3 

and in figure 1 of Keefer and others (this chapter), if the 

main (head) scarp and both flanks of a rotational slump or 

block slide are completely defined by fractures, these fractures describe a horseshoe-shaped arc opening in the 

downslope direction, although, as discussed by Keefer and 

others, the main scarps of block slides may be only gently 

curved or relatively straight. Therefore, in general, fractures related to main scarps may vary in trends within 

approximately 180° on a single landslide. For example, 

on the southwest flank of Summit Ridge, most major 

groups of fractures associated with landslides had trends 

that spanned this full range although the full range of 

possible trends was not observed on all individual landslides.

VARIATION IN TREND

Fractures associated with the main scarps of landslides 

are typically arcuate or curved. If the main scarp or flanks 

are exceptionally long, however, these fractures may be 

relatively straight over much of their length. Most, but
ORIGIN OF FRACTURES TRIGGERED BY THE EARTHQUAKE AND THEIR RELATION TO LANDSLIDES €133

not all, fractures associated with landslides in the Summit 

Ridge area were in zones that remained straight for less 

than 100 m; the most notable exceptions to this limit were 

associated with the largest landslides.

DISPLACEMENT MAGNITUDE

Though not a distinguishing criterion, maximum displacements measured across fractures within landslides in 

the Summit Ridge area were commonly larger than across 

fractures unrelated to landslide movement. Fracture displacements outside of landslide boundaries were typically 

less than 50 cm, whereas those of more than 60 cm across 

individual cracks were not uncommon within landslides. 

Thus, both fractures related to landslide movement and 

those related to regional structure had displacements of 

less than 50 cm, whereas only fractures related to landslide movement commonly had displacements of more than 

60 cm, probably reflecting a gravity-driven component of 

landslide displacement in addition to a shaking-induced 

component. Cumulative displacements across fracture 

zones within landslides were as large as 2.4 m.

SENSE OF RELATIVE DISPLACEMENT

Within a landslide mass that has a completely developed main scarp and flanks, vertical, right-lateral, and 

left-lateral senses of displacements are all represented. In 

the Summit Ridge area, this was not the case for all landslides because within some landslides (specifically, the 

Upper Morrell Road and Bel Air Court landslides, pi. 4A, 

4B), fractures defining the flanks of the landslides were 

not equally developed. Throughout the study area, however, displacements across fractures along landslide margins were relatively consistent—dextral along the right 

flanks and sinistral along the left flanks.

FRACTURES RELATED TO REGIONAL 

STRUCTURE

The origins of the fractures in the Summit Ridge area 

unrelated to landslide movement is still in debate. Ponti 

and Wells (1991) attributed much of the displacement 

across such fractures to gravitational movement and suggested that displacement vectors are in the direction of 

regional or local slope. Cotton and others (in press) invoke bedding-plane slip to account for the largely extensional style of the displacements across fractures, and 

similar past deformation to account for the grabens, in the 

Summit Ridge area; they consider this deformation to be 

tectonic in origin. Aydin and others (in press) found rightlateral displacements with reverse slip-slip components in 

short discontinuous segments along both the Sargent and

the San Andreas fault zones. They consider the displacements within these segments to be kinematically compatible with the source mechanism of the earthquake, and the 

broad zone of distributed fractures on Summit Ridge to be 

tectonic but otherwise obscure in origin. Johnson and 

Fleming (1993) considered fractures in the Summit Ridge 

area unrelated to landslide movement to be extensional 

features formed within a zone of distributed right-lateral 

shear and the observed left-lateral displacements observed 

across such fractures to derive from rotation accompanying the distributed right-lateral deformation. They stated 

that this deformation is tectonic, consistent with the source 

mechanism of the earthquake, and that the displacements 

observed in the Summit Ridge area are probably typical 

for past earthquakes there. Regardless of their specific 

origin, most of these fractures are probably openings along 

bedding surfaces or along other structures trending parallel to bedding.

Extension was the predominant mode of displacement 

across the fractures related to regional structure throughout the study area. Even those fractures that displayed 

lateral and (or) vertical offset generally lacked slickensides or surficial ridges of disturbed ground ("mole 

tracks"), which are created primarily by shear. Although 

both left- and right-lateral displacements occurred, leftlateral displacements were more common.

FRACTURE ORIENTATION

In addition to their unrelatedness to any recognizable 

landslides, the principal criteria for identifying fractures 

related to regional structure were an absence of curvature 

and a limited variation in orientation parallel to regional 

structure, in contrast to the wide variation in orientation 

characteristic of the fractures related to landslide movement. Most fractures in the Summit Ridge area outside 

the boundaries of well-defined landslides parallel or nearly 

parallel bedding, as shown in figure 2. Most of these fractures and most bedding surfaces trend and strike in the 

range N. 30°-80° W. As previously stated, the trends of 

fractures related to landslide movement varied much more 

widely than those of fractures related to regional structure. The rose diagrams shown in figures 2, 3A, and 3B 

plot the cumulative trends of fractures related to regional 

structure in the northwest and southeast halves, respectively, of the study area (see pi. 4). Most fractures in the 

northwest half of the study area along Summit Ridge (figs. 

2, 3A) trend approximately N. 45° W., whereas those in 

the southeast half of the study area along Skyland Ridge 

(figs. 2, 3fi) show an additional clustering of trends around 

N. 15°-20° W., as well as a maximum trend of N. 45° W., 

reflecting a more northerly trend of regional structure and 

strike of bedding in parts of the Skyland Ridge area than 

in the Summit Ridge area.
C134 LANDSLIDES

TRENDS OF DISPLACEMENT VECTORS AND 

SENSE OF RELATIVE DISPLACEMENT

The trends of displacement vectors and their sense of 

relative displacement were inconclusive in discriminating 

between fractures related to landslide movement and those 

related to regional structure. Instead, these criteria were 

supplemental to more deterministic data on orientation, 

fracture straightness, and proximity to topographic features. The rose diagrams in figures 3C and 3D plot the

trends of displacement vectors across fractures outside 

landslide boundaries in the northwest and southwest halves, 

respectively, of the study area (see pi. 4; fig. 2). The 

trends of displacement vectors across fractures in the northwest half of the study area (fig. 3Q show maximums that 

are slightly counterclockwise from perpendicularity to the 

maximums in figure 3A. Comparison of figures 3A and 

3C reveals the mostly extensional style of relative displacement (normal to the maximum trend) with a small 

component of sinistral displacement across fractures in

E

D

E

Figure 3.—Rose diagrams showing cumulative trends of (A, B), and trends of displacement vectors across (C, D), fractures inferred to be related to 

regional structure in northwest (A, C) and southeast (B, D) halves of study area (see fig. 2). Values on ordinate axis are numbers of fractures and 

correspond to radial length. Azimuth interval, 5°. Data from Spittler and Harp (1990) and Wells (in press).
ORIGIN OF FRACTURES TRIGGERED BY THE EARTHQUAKE AND THEIR RELATION TO LANDSLIDES €135

the northwest half of the study area; comparison of figures 3B and 3D reveals similar relations in the trends of 

displacement vectors across fractures in the southeast half 

of the study area (see pi. 4; fig. 2).

As stated above, the sense of relative displacement 

across fractures was not a criterion in discriminating 

between fractures related to landslide movement and 

those related to regional structure. Both landslides and 

other structurally controlled processes affected fractures, 

producing relative vertical displacements that were down 

on the downslope side. Conversely, some fractures with 

upslope sides displaced relatively downward were found 

within landslide masses. Such fractures can be common 

within landslides just below the main-scarp area where an 

extensional domain exists or, in places where large 

displacements have occurred, within the toe region of a 

landslide, where compression commonly results in differential vertical deformation. Because displacements within 

landslide masses were relatively small during the earthquake, creating little evidence of compressional-toe formation, most long, straight fractures parallel to the 

regional-structural trend (especially those with upslopefacing scarps) were interpreted to be related to regional 

structure, even if they occurred within a recognizable landslide body.

DISPLACEMENT MAGNITUDE

The magnitudes of displacement across fractures outside landslide boundaries were generally lower than of 

those across fractures within landslides. As stated above, 

displacements across fractures related to regional structure were observed to be generally less than 50 cm, 

whereas those of more than 60 cm across individual fractures within landslide masses were not uncommon. Displacements across a few fractures related to regional 

structure were more than 1.0 m, but most displacements 

of more than 60 cm were across fractures related to landslide movement.

FRACTURES RELATED TO BOTH LANDSLIDE 

MOVEMENT AND REGIONAL STRUCTURE

The criteria described in the previous subsections allowed interpretation of the overall geometries of most fracture zones in the Summit Ridge area and enabled 

discrimination between fractures related to landslide movement and those related to regional structure. Several of 

the longest fracture zones within the Summit Ridge area, 

however, may be related to both landslide movement and 

regional structure, suggesting a possible interaction between the structural fabric of the study area and landslide 

processes during the earthquake.

TRANBARGER FRACTURE ZONE

The Tranbarger fracture zone, which received much attention during the first few days after the earthquake, 

crossed Summit Road 640 m east of California Highway 

17 (loc. A, pi. 4A). It extended approximately 670 m 

along an average trend of N. 55° W. that changed locally 

to slightly west of north where the fracture crossed Old 

Summit Road (see pi. 4A; U.S. Geological Survey staff, 

1989).

This fracture and several other approximately parallel 

fractures to the southwest formed a zone approximately 

200 m wide along Summit Road (loc. B, pi. 4A). To the 

southeast, this fracture zone was approximately 120 m 

wide at Old Summit Road and less than 30 m wide at a 

point 200 m farther southeast. The fracture zone widened 

southeastward from this point to 120 m and ended near 

Old Santa Cruz Highway (loc. C, pi. 4A). The average 

trend of this somewhat discontinuous fracture zone was 

N. 45° W. It extended for more than 1,200 m and was 

thus the longest such zone in the Summit Ridge area. The 

trends of individual fractures in the zone ranged from N. 

10° W. to N. 65° W. Near the northwest limit of this 

fracture zone, vertical displacements across fractures were 

generally less than 13 cm and were consistently southwest side down. Southeast of Old Summit Road, the zone 

was less complex in terms of magnitudes and senses of 

displacements. There, several grabenlike fractures formed. 

Farther southeast, upslope- and downslope-facing scarps 

faced away from each other, creating a horst. Still farther 

southeast, near the end of the zone, was another set of 

narrow grabenlike fractures. Vertical displacements on 

these fractures ranged from 5 to 60 cm and averaged 15 

cm. Extension across the fractures in this set was typically 15 cm, and horizontal shear displacements typically 

sinistral and ranged from 2.5 to 80 cm in magnitude.

The trend of the Tranbarger fracture zone paralleled the 

regional trend of bedding. This trend, and the left-lateral 

shear displacements along fractures not suitably oriented 

with respect to the slope to be lateral shears of a landslide, suggest deformation along regional structure. The 

grabenlike fractures, however, also suggest a zone of extension developing along the main scarp of a large landslide; the sinistral shear displacements were small in 

comparison with the vertical displacements in most places. 

Thus, the 1,200-m-long fracture zone could well be the 

main scarp of a landslide affecting much of the southwest 

slope of Summit Ridge. Because the fractures within this 

zone formed along the regional-structural trend, their origin is thus ambiguous: It is unclear whether they simply 

formed along bedding planes due to seismic shaking, 

whether they indicate graben formation due to extension 

of the ridgecrest, or whether they formed in response to 

landslide movement along a large main scarp parallel to 

regional structure.
C136 LANDSLIDES

CHURCH FRACTURE ZONE

Another long fracture approximately parallel to the regional-structural trend extended for approximately 430 m 

southeastward along Summit Road from near the Melody 

Lane-Summit Road intersection (loc. D, pi. 4A). This fracture intersects Summit Road approximately 200 m from 

the northwest end of the fracture. Southeastward from 

there, the fracture runs along the southwest edge of Summit Road for approximately 60 m and then recrosses Summit Road and parallels the road for an additional 140 m. 

The overall trend of this fracture is N. 40° W., and its 

trend ranges locally from N. 15° to 80° W. Throughout 

most of its length the fracture is a single discontinuity, 

with vertical displacements of as much as 30 cm, down to 

the southwest (downslope).

Like the Tranbarger fracture zone, the Church fracture 

zone has most of the characteristics of a ground crack 

opening along the regional-structural trend; however, it 

also has characteristics of the main scarp of a large landslide. Unlike the Tranbarger fracture zone, the sense of 

vertical displacement across the Church fracture zone is 

consistently downslope side down except in a few localities near its northwest and southeast ends, where small 

(max 2.5 cm) vertical displacements occurred that were 

upslope side down.

Ponti and Wells (1991) interpreted the displacements 

across this fracture as deformation along bedding on the 

northeast limb of the Summit syncline, the axis of which 

trends N. 60°-65° W. They suggested that this fracture, 

as well as others in the vicinity, may reflect slip along 

bedding surfaces because beds on both limbs of the syncline appear to move toward the center of the syncline. 

They furthermore suggested that movement of beds toward the syncline center may be due to ridge spreading 

caused by lateral movement of underlying mudstone units, 

allowing gravitational relaxation of the syncline.

Although the above model is consistent with vertical 

displacements of this fracture along its length, other fractures on the southwest limb of the syncline (loc. E, pi. 

4A) have southwest-facing scarps, indicating movement 

of material in the opposite direction, away from the synclinal center. Although some fractures in this vicinity have 

northeast-facing scarps, other nearby fractures not obviously related to landslide movement have southwest-facing scarps.

A trench dug across the Church fracture zone at a locality on the southwest side of Summit Road (see Nolan 

and Weber, this chapter) showed that the fracture dips 

45°-63° SW. Data from this trench, as well as surface 

data, suggest that this fracture could be the main scarp of 

a large deep-seated landslide involving much of the southwest slope of Summit Ridge. If both this fracture and 

those constituting the Tranbarger fracture zone are related 

to landslide movement, then this fracture along Summit

Road would be a main scarp along the ridgetop, and the 

Tranbarger fracture zone would be a zone of internal scarps 

downslope and within a large landslide that includes most 

of the southwest flank of Summit Ridge between California Highway 17 and Old Santa Cruz Highway. In fact, the 

Church fracture zone crosses Summit Ridge at its northwest end, occupying a position slightly to the north and 

downslope of the crest of Summit Ridge at this point. For 

a landslide scarp to occupy a position on the opposite side 

of a ridgecrest from the slope toward which it is inclined 

is not uncommon where bedding surfaces or other structure has a similar inclination. According to the geologic 

map (McLaughlin and others, 1991), bedding in this area 

dips southwest. For the Church fracture zone to be the 

scarp of a deep-seated landslide, however, its failure surface would have to depart from the geometry of the Summit Ridge syncline in its lower reaches.

In addition, the Church fracture zone may be related to 

a fault mapped in approximately the same place and with 

the same overall trend (fig. 2; McLaughlin and others, 

1991). The fault as mapped extends beyond the present 

fracture zone approximately 0.3 km farther northwestward 

and slightly more than 1 km farther southeastward. At 

present, however, the origin of the Church fracture zone 

is still uncertain, and any one or, possibly, all of these 

origins may have contributed to its formation.

INTERPRETATION OF FRACTURE 

PATTERNS

The fractures on Summit Ridge created by the earthquake revealed a complex interaction between landslide 

processes as triggered by the earthquake and preexisting 

regional structure, such as bedding surfaces, joints, and 

faults. The fractures related to landslide movement evidently are superimposed on and, therefore, postdate the 

network of regional structural features.

EFFECT OF REGIONAL STRUCTURE ON 

LANDSLIDES

The spacing and orientation of regional structure within 

the study area (see pi. 4) and generalized from McLaughlin 

and others (1991) evidently affected the shape and position of many of the margins of the larger landslides. Several examples of this effect are discussed below.

UPPER MORRELL ROAD LANDSLIDE

The Upper Morrell Road landslide, on the south flank 

of Summit Ridge approximately 370 m east of Taylor 

Gulch, shows a strong influence of regional structure on
ORIGIN OF FRACTURES TRIGGERED BY THE EARTHQUAKE AND THEIR RELATION TO LANDSLIDES C137

the position and shape of the fractures forming the landslide margins. The fractures forming the main scarp and 

the flanks of the landslide are asymmetric (loc. F, pi. 4B), 

and fractures forming the left flank of the landslide parallel the regional-structural trend (N. 55°-60° W.) for as 

much as 240 m.

The fractures forming the right flank (looking 

downslope) of this landslide curved to the southwest from 

the main-scarp area and trended approximately N. 20° E. 

where they crossed Morrell Road. At that point, they were 

predominantly right lateral, with approximately 60 cm of 

horizontal displacement and 15 to 25 cm of vertical displacement, down to the southeast. Approximately 23 m to 

the southwest along this fracture zone, its trend abruptly 

swung clockwise to N. 60° W. (loc. G, pi. 4B), parallel to 

regional structure. Immediately west of this change in 

trend, the fracture was predominantly extensional, and several upslope-facing scarp segments were present, with 

maximum displacements of 5 cm. At this change in trend, 

the fracture zone ceased to exhibit the characteristics of 

right-lateral shear along the flank of a landslide and took 

on the attributes (largely extensional displacements, fractures parallel to regional structure and bedding, and 

upslope-facing scarps well downslope from graben-forming areas of the Upper Morrell Road landslide) of fractures related to regional structure.

VILLA DEL MONTE LANDSLIDE

Several fracture zones in the Villa Del Monte area 

seemed to be related to both landslide movement and regional structure. For example, near the junction of Sky view 

Terrace and Bel Air Court, a fracture zone trending N. 

50° W. formed part of an internal landslide scarp (loc. H, 

pi. 4B). Similarly to the Upper Morrell Road landslide, 

this fracture zone, forming the left flank of the internal 

landslide, paralleled the regional-structural trend. The right 

flank of the fracture zone curved counterclockwise to the 

southwest, trending about N. 20° E.

Near the junction of Sunset Drive and Evergreen Lane, 

as well as immediately to the north, are several fractures 

that parallel the regional-structural trend but also resemble 

fractures forming the main scarp of a large, deep-seated 

landslide. This fracture zone trends approximately N. 65° 

W., but one fracture (near loc. I, pi. 4A) curves counterclockwise, trending N. 45° E., as displacements change 

from predominantly extensional to primarily dextral. This 

change in trend and the displacements along this fracture 

are common features of the transition from the main scarp 

to the right flank of a landslide.

One effect of the regional system of discontinuities on 

the shape of landslides along the south flank of Summit 

Ridge is a consistent asymmetry in their horizontal dimensions. Generally, the right flanks are more clearly defined by fractures than are the left margins. Examples are 

the Upper Morrell Road landslide (see pi. 4B), the Burrell 

landslide (loc. H, pi. 4B), and the Villa Del Monte landslide (loc. I, pi. 4A). Because the left flanks of the landslides on the south side of Summit Ridge more nearly 

parallel the regional-structural trend, the fractures forming these flanks generally occur along previously existing 

structural discontinuities rather than propagating through 

unbroken ground, and generally are extensional rather than 

dominantly shear, as is more typical in the flanks of a 

landslide.

OTHER EXAMPLES AND SUMMARY

Many fractures that approximately parallel the regionalstructural trend extend across landslide boundaries or 

through landslides, evidently affecting the shapes of the 

landslides. For example, a N. 50° W.-trending fracture 

related to regional structure cuts through one of the main 

scarps in the Burrell landslide (loc. H, pi. 4B) and continues into the landslide. Similarly, a N. 50°-67° W.-trending fracture cuts across the Taylor Gulch landslide (loc. J, 

pi. 4B). Finally, a set of N. 60° W.-striking fractures related to regional structure cut through the northwest part 

of the main scarp and the right flank of the Stetson Road 

landslide (loc. K, pi. 4D).

As these examples show, the pattern of fractures triggered by the earthquake on the southwest slopes of Summit and Skyland Ridges reveals an interaction between 

landslides triggered by the earthquake and a regional system of structural discontinuities, most likely bedding surfaces and faults subparallel to bedding. This interaction 

between landslide movement and regional structure takes 

two forms. One form of interaction involves the influence 

of the spacing and orientation of fractures related to regional structure on the position and shape of fractures 

forming landslide boundaries. This form of interaction also 

imparts an asymmetry, in that fractures outlining the left 

flanks of landslides commonly are poorly formed in comparison with those outlining the right flanks. In several 

places, the left flanks of landslides that formed along fractures which parallel the regional-structural trend were long 

and straight, whereas those forming the right flanks were 

arcuate. Another form of interaction involves the crosscutting of landslides by fractures related to regional structure, with little apparent effect on the landslide shape.

Thus, in the 1989 Loma Prieta earthquake, a wide zone 

of distributed coseismic fractures and landslides was triggered to the southwest and approximately parallel to the 

mapped trace of the San Andreas fault. The pattern of 

landslides and fractures resulted from a complex interaction between strong shaking, fault movement, geologic 

structure, and gravitational slope movement. The 1989 

Loma Prieta earthquake is not unique, however, in pro-
CBS LANDSLIDES

ducing such a complex pattern of fractures and slope failures within its epicentral area. Other recent earthquakes 

have also triggered complex patterns of surface deformation that are difficult to interpret.

SIMILARITY OF FRACTURE PATTERNS

TO THOSE PRODUCED IN OTHER

EARTHQUAKES

The pattern of fractures formed on the southwest slopes 

of Summit and Skyland ridges by the 1989 Loma Prieta 

earthquake is similar to those produced in other recent 

earthquakes: The 1986 Palm Springs, Calif., and the 1980 

El Asnam, Algeria, earthquakes both produced extensive 

zones of distributed fractures and reactivated landslides 

along and near the traces of the causative faults. We discuss these earthquakes here to provide a basis of comparison for evaluating the possible origins of the fractures in 

the Summit Ridge area and their relation to landslides 

triggered by the 1989 Loma Prieta earthquake.

THE 1986 NORTH PALM SPRINGS, CALIFORNIA, 

EARTHQUAKE

The July 8, 1986, North Palm Springs, Calif., earthquake (ML=5.9) produced an extensive network of fractures within a diffuse zone along both sides of the Banning 

fault, which dips about 50° N. in this area. The fracture 

zone extended 8.6 km along the fault and 90 to 300 m 

wide. Although discontinuous fractures and landslides were 

most abundant, and displacements greatest, where slopes 

were steepest within this fracture zone, gentle slopes and 

nearly level ground were also cracked. Extension and small 

(several millimeters) right-lateral offsets were the most 

common type of displacements. The fractures on nearly 

level ground were probably related to fault slip rather 

than to slope movement. Intensely fractured ground along 

ridgecrests was also a common feature within the fracture 

zone (Morton and others, 1989).

Many of the fractures within the diffuse zone along the 

Banning fault were interpreted to have been caused by 

shallow gravitational slope displacements (landslides) induced by the seismic shaking. The distribution of the fractures with respect to the fault trace, however, indicated 

that these fractures did not result from strong shaking 

alone. Morton and others (1989) concluded that this 

anomalous fracture zone formed from a combination of 

strong ground shaking, gravitational effects, and a localized surficial response to static shear strain induced near 

the fault trace by rupture at depth. Their model, based on 

elastic-dislocation theory, predicted that the surface fractures not caused by slope movements extended no deeper

than 100 m and that the fault rupture at depth extended 

only to within 4 km of the surface and was not connected 

to the surface fractures.

Similarities between the 1986 Palm Springs earthquake 

and the 1989 Loma Prieta earthquake are (1) the respective source mechanisms, which were both right lateral 

with a thrust component; and (2) the presence of discontinuous fracture zones away from the fault traces that 

showed dominantly extensional displacement. Also, many 

landslides were triggered within this diffuse zone of fractures, creating difficulties for geologists seeking to discriminate between fractures outlining the slope failures 

and other fractures due to shaking or faulting.

Differences between the fracture pattern created by the 

1986 North Palm Springs earthquake and that created by 

the 1989 Loma Prieta earthquake were (1) shear displacements across fractures that were dominantly right lateral 

in the 1986 earthquake as opposed to dominantly left lateral in the 1989 earthquake, and (2) the presence of fractures and slope failures on both sides of the Banning fault 

in the 1986 earthquake. In the Summit Ridge area, most 

observed ground cracks were confined to the southwest 

side of the ridge or, at least, to the hanging wall (with 

respect to the thrust component of the Loma Prieta source 

mechanism) of the San Andreas fault zone, whereas the 

diffuse zone of fractures originating from the 1986 earthquake extended across the Banning fault trace.

In summary, both the 1986 North Palm Springs earthquake and the 1989 Loma Prieta earthquake formed broad 

zones of complex fractures as a result of strong ground 

shaking, gravitational effects, and fault rupture at depth. 

Differences existed as to the dominant sense of shear across 

fractures and the distribution of fracture zones with respect to the fault traces.

THE 1980 EL ASNAM, ALGERIA, EARTHQUAKE

A recent earthquake showing possibly even more similarities to the 1989 Loma Prieta earthquake in producing 

off-fault ground cracks was the October 10, 1980, El 

Asnam, Algeria, earthquake (Afy=7.3). Cotton and others 

(1990 and in press) also pointed out the similarities between the landslides and off-fault fractures caused by these 

two earthquakes.

The 1980 El Asnam earthquake generated 40 km of 

primary surface rupture that ranged in trend from N. 45° 

to 80° E. (fig. 4). The primary rupture was somewhat 

discontinuous over this 40 km, but distances between individual fractures in the rupture zone generally were less 

than 100 m. The surface fault displacement was predominantly thrusting, with some left-lateral slip in the southern 

part of the fault trace (fig. 4; Philip and Meghraoui, 1983; 

Sorriso-Valvo, 1986). The source mechanisms indicated a 

dip of approximately 50° N. (Ouyed and others, 1981).
1°20' 1*30' 

36°15' -

36°10' -

1 2 KILOMETERS

Area of 

figure 6 /

Normal Fau

located. 

Strike-slip fa

direction 

Vertical crac

Natural dam 

Road

Figure 4.—El Asnam area, Algeria, showing ground cracks from the October 10, 1980, earthquake (after Philip and Meghraoui, 1983). 
C140 LANDSLIDES

Few fractures formed south of the fault trace, on the footwall; however, along the hanging wall near the main trace, 

various compressional and extensional fractures and landslides (rotational slumps) formed in a zone as much as 1 

km wide.

Several large rotational slumps were created where the 

surface of the hanging wall near the thrust fault was uplifted and folded. Landslide scarps formed immediately 

adjacent and parallel to the thrust fault and in the hanging 

wall as it was uplifted and extended by folding (fig. 5).

Compressional features were related to deformation at 

the leading edge of the thrust fault and to the regional 

stress field associated with it. Pressure ridges south of, or 

in front of, the thrust fault were observed where the fault 

became imbricate. Extension fractures perpendicular to 

the fault trace and parallel to the inferred regional direction of maximum principal stress were found primarily 

on the hanging-wall side. Extension fractures parallel to 

the fault trace occurred both as simple extension cracks

with little relative displacement and as large grabens (figs. 

5,6).

The patterns of fractures and landslides produced by 

the 1980 El Asnam and 1989 Loma Prieta earthquakes are 

similar in the spatial relations of landslides and extension 

fractures/grabens. Both earthquakes had thrusting source 

mechanisms and strike-slip displacement components. 

Also, most of the off-fault fractures and landslides generated by the earthquakes were confined to the hanging walls 

of the respective faults, and hundreds of discontinuous 

extension fractures were created, with little vertical or 

horizontal shear and orientations subparallel to the main 

fracture zones.

Unlike the 1989 Loma Prieta earthquake, the 1980 El 

Asnam earthquake produced an extensive echelon graben 

field at a high angle to the main fault (fig. 7). These 

grabens are oriented with their long dimensions at an angle 

of approximately 35° to the fault trace. The echelon pattern was right stepping, consistent with extensional disEXTENSIONAL 

CRACK

Landslide 

>carp\

Pressure ridge

0 10 METERS

Thrust fault -Sawteeth 

on upper plate

Figure 5.—Block diagram illustrating relation of pressure ridges, extension fractures, and slump scarps to 

main fault break in 1980 El Asnam earthquake (from Philip and Meghraoui, 1983).

500

Extensional 

crack Graben Thrust fault-Sawteeth 

ion upper plate

Figure 6.—Block diagram illustrating relation of graben features to main fault break in 1980 El Asnam 

earthquake (from Philip and Meghraoui, 1983).
ORIGIN OF FRACTURES TRIGGERED BY THE EARTHQUAKE AND THEIR RELATION TO LANDSLIDES C141

placement within a sinistral shear zone. Philip and 

Meghraoui (1983) observed that displacements across individual graben scarps had horizontal right-lateral components, and they interpreted this echelon zone as due to 

distributed left-lateral deformation within a 1-km-wide 

zone northwest of the main fault. They interpreted the 

right-lateral displacements as due to small counterclockwise rotations of structural elements within the graben 

zone. Vertical displacements forming the grabens occurred

in response to extension generated by uplift and arching 

at the surface within the hanging wall of the thrust fault. 

Although the 1989 Loma Prieta earthquake produced 

no such extensive echelon graben fields, horizontal displacements opposite in sense to those of the main fault 

within this zone in the 1980 El Asnam earthquake have 

been interpreted similarly to those horizontal displacements opposite in sense to that of the fault source in the 

1989 Loma Prieta earthquake (Johnson and Fleming, 1993).

r

y

y

Figure 7.—Features of the 1980 El Asnam, 

Algeria, earthquake. A, Part of main fault 

trace (see fig. 4 for location), showing echelon graben features aligned at an angle of 

approximately 35° to fault trace. Right-lateral displacements at graben margins are 

due to rotation of structural elements within 

this zone. 1, normal faults, dashed where 

uncertain, hachures on downthrown side; >2, 

extension cracks, dashed where uncertain; 

3, echelon cracks; 4, thrust faults and piessure ridges, sawteeth on upthrown side; 5, 

attitude of bedding planes; 6, horizontal 

stratification. Shaded areas denote 

downfaulted areas in graben. B, Block diagram illustrating relation of echelon graben 

features to fault and to sense of horizontal 

shear in this part of rupture zone (from 

Philip and Meghraoui, 1983).
C142 LANDSLIDES

The fault slip associated with the 1989 Loma Prieta 

earthquake had subequal components of strike slip and 

thrusting, whereas that of the 1980 El Asnam earthquake 

was primarily thrusting, and the senses of strike-slip motion on the two faults were opposite. Bedding in nearsurface rocks of the El Asnam area is subparallel to the 

ground surface and directly reflects the deformation of 

this and similar past earthquakes, whereas bedding in the 

Summit Ridge area is more highly deformed. Finally, the 

1980 El Asnam earthquake produced a conspicuous surface fault rupture, with surface offsets commonly of more 

than 2 m and regional uplift of greater than 6 m, whereas 

the 1989 Loma Prieta earthquake produced little, if any, 

primary fault rupture on a single trace or narrow zone at 

the ground surface and a maximum regional uplift of only 

about 56 cm (Anderson, 1990).

Despite the obvious differences between these two earthquakes, the patterns of surface fracturing and landslide 

generation have many similarities. Both earthquakes triggered landslides and extension fractures that were concentrated on the hanging walls of the respective fault zones, 

and both earthquakes produced zones of distributed shear 

and extension. Johnson and Fleming (1993) interpreted 

the dominantly left-lateral slip within the Summit Ridge 

and Skyland Ridge areas as due to rotation between structural blocks responding to right-lateral shear across a wide 

zone rather than a narrow zone or single fault trace. Their 

model portrays the San Andreas fault within these areas 

as a wide zone with horizontal right-lateral deformation 

distributed completely across it, forming extension fractures that subsequently rotate clockwise and produce predominantly left-lateral shear displacements along structural 

elements within this zone. They maintained that the fractures unrelated to landslide movement within the Summit 

Ridge area that formed during the 1989 Loma Prieta earthquake are tectonic and consistent with the source mechanism of the earthquake, and that this deformation is 

probably typical of movement on the San Andreas fault in 

this area.

Philip and Meghraoui (1983) similarly interpreted the 

horizontal right-lateral displacements along graben margins in the central part of the El Asnam rupture zone as 

due to rotation of structural elements within the echelon 

graben field, which was itself a result of left-lateral deformation over a wide zone.

In the 1980 El Asnam earthquake, much of the graben 

formation and many of the extension fractures were probably related to regional and local uplift, which was a maximum of 6 m. According to the model of Johnson and 

Fleming (1993), the fractures produced by the 1989 Loma 

Prieta earthquake were probably due to distributed horizontal deformation rather than to regional uplift, which 

was comparatively small.

Alternatively, Ponti and Wells (1990) attributed most 

of the extensional deformation across fractures in the Summit Ridge and Skyland Ridge areas to gravitational spreading. They contended that the displacement vectors across 

individual fractures generally parallel the regional or local slope direction and that the displacements, which are 

largely extensional, are governed by gravity-induced 

downslope movement.

Cotton and others (in press) build a case for beddingplane slip as the origin for many of the fractures in the 

Summit Ridge area. They argue that the fractures caused 

by the 1989 Loma Prieta earthquake and previous earthquakes in this area formed by slip along bedding planes 

which have undergone flexing due to compression and 

uplift from the reverse-slip component of the earthquake 

source mechanism. They also attribute much of the extension across fractures to stretching created by uplift and 

compression-generated folding.

The models of Ponti and Wells (1990), Johnson and 

Fleming (1993), and Cotton and others (in press) are all 

probably applicable in respective proportions. Distributed 

right-lateral shear across a wide zone, gravitational spreading, and bedding-plane slip may all have been active to a 

greater or lesser degree and have interacted while also 

being subjected to deformation by landslides generated by 

the earthquake. I expect that other such complex patterns 

will have to be studied from future earthquakes before the 

relative merits of such models can be properly evaluated.

SUMMARY

The mapping of fractures triggered by the 1989 Loma 

Prieta earthquake revealed a complex pattern that was 

inconsistent with the pattern previously expected from 

ground rupture within the San Andreas fault zone. The 

fractures were widely distributed on the southwest side of 

Summit Ridge within a broad zone, approximately 2.4 km 

wide by 8.0 km long, extending from near the California 

Highway 17-Summit Road intersection to Skyland Ridge.

The fracture pattern indicated that many landslides were 

triggered or reactivated by the earthquake and that fractures unrelated to landslide movement paralleled the regional trend of bedding or other structures. On the basis 

of the orientation, sense and magnitude of displacement, 

relation to adjacent landforms, and degree of curvature, 

most fractures were characterized as related to either landslide movement or regional structure.

The influence of regional structure was apparent in the 

trend and shape of many landslide margins. The main 

scarps of several large landslides formed along 

discontinuities that are part of the regional structure— 

probably mostly bedding surfaces. Many of the large landslides were distinctly asymmetric in that many right flanks 

were arcuate where they connected to their main scarps, 

whereas left flanks were typically straight, parallel to regional structure, and nonarcuate at points of intersection
ORIGIN OF FRACTURES TRIGGERED BY THE EARTHQUAKE AND THEIR RELATION TO LANDSLIDES C143

with main scarps. Some fractures related to regional structure extended across landslide margins and through landslides with little deviation.

Two of the longest fracture zones—the Tranbarger fracture zone and the Church fracture zone—have many of 

the characteristics of landslide main scarps and may be 

associated with a landslide that involves much of the southwest flank of Summit Ridge; yet these fracture zones also 

formed parallel to the regional-structural trend and may 

have originated along bedding surfaces.

The 1989 Loma Prieta earthquake was similar to other 

recent earthquakes producing off-fault fractures and triggering landslides near the causative faults. Both the 1986 

North Palm Springs, Calif., and 1980 El Asnam, Algeria, 

earthquakes produced extensive off-fault fractures and 

landslides. Extensional grabens and landslides were generated by the 1980 El Asnam earthquake, a thrusting event 

with accompanying left-lateral shear displacement; and 

the relations between distributed horizontal shear, fracture patterns, and landslide movements in that earthquake 

appear analogous to those in the Summit Ridge area associated with the 1989 Loma Prieta earthquake. Regional 

uplift in the 1989 Loma Prieta earthquake was small in 

comparison with that in the 1980 El Asnam earthquake 

and probably does not account for much of the extensional deformation in the Summit Ridge and Skyland Ridge 

areas.

REFERENCES CITED

Anderson, R.S., 1990, Evolution of the northern Santa Cruz Mountains 

by advection of crust past a San Andreas fault bend: Science, v. 

249, no. 4967, p. 397-401.

Aydin, Attila, Johnson, A.M., and Fleming, R.W., in press, Coseismic 

right- and left-lateral surface ruptures and landsliding along the San 

Andreas and Sargent fault zones during the earthquake, in Ponti, 

D.J., ed., The Loma Prieta, California, earthquake of October 17, 

1989—ground ruptures: U.S. Geological Survey Professional Paper 

1551-D.

Clark, J.C., Brabb, E.E., and McLaughlin, R.J., 1989, Geologic map and 

structure sections of the Laurel 7 l/i' Quadrangle, Santa Clara and 

Santa Cruz counties, California: U.S. Geological Survey Open-File 

Map 89-676, 31 p., scale 1:24,000, 2 sheets.

Cotton, W.R., Fowler, W.L., and Van Velsor, J.E., 1990, Coseismic 

bedding plane faults and ground fissures associated with the Loma 

Prieta earthquake of 17 October 1989, in McNutt, S.R., and Sydnor, 

R.H., eds., The Loma Prieta (Santa Cruz Mountains), California, 

earthquake of 17 October 1989: California Division of Mines and 

Geology Special Publication 104, p. 95-104.

Cotton, W.R., Hardin, B.C., and Smelser, M.G., in press, Origin and 

characteristics of coseismic ground fissures associated with the earthquake, in Ponti, D.J., ed., The Loma Prieta, California, earthquake 

of October 17, 1989—ground ruptures: U.S. Geological Survey Professional Paper 1551-D.

Johnson, A.M., and Fleming, R.W., 1993, Formation of left-leteral fractures within the Summit Ridge shear zone, 1989 Loma Prieta, California, earthquake: Journal of Geophysical Research, v. 98, no. 

B12, p. 21823-21837.

McLaughlin, R.J., Clark, J.C., Brabb, E. E., and Helley, E. J., 1991, 

Geologic map and structure sections of the Los Gatos 7 l/i' Quadrangle, Santa Clara and Santa Cruz counties, California: U.S. Geological Survey Open-File Report 91-593, 48 p., scale 1:24,000, 3 

sheets.

Morton, D.M., Campbell, R.H., Jibson, R.W., Wesson, R.L., and 

Nicholson, Craig, 1989, Ground fractures and landslides produced 

by the North Palm Springs, California, earthquake of July 8, 1986, 

in Sadler, P.M., and Morton, D.M., eds., Landslides in a semi-arid 

environment with emphasis on the inland valleys of southern California: Riverside, Calif., Inland Geological Society, p. 183-196.

Ouyed, Merzouk, Meghraoui, Mustapha, Cisternos, Armando, 

Deschamps, Anne, Dorel, Jacques, Frechet, Julien, Gaulon, Roland, 

Hatzfield, Denis, and Philip, Herve, 1981, Seismotectonics of the 

El Asnam earthquake: Nature, v. 292, no. 5818, p. 26-31.

Philip, Herve, and Meghraoui, Mustapha, 1983, Structural analysis and 

interpretation of the surface deformations of the El Asnam earthquake of October 10, 1980: Tectonics, v. 2, no. 1, p. 17-49.

Ponti, D.J., and Wells, R. E., 1990, Origin of surface ruptures that 

formed in the Santa Cruz Mountains, California, during the Loma 

Prieta earthquake [abs.]: Seismological Research Letters, v. 41, no. 

1, p. 17.

———1991, Off-fault ground ruptures in the Santa Cruz Mountains, 

California; ridge-top spreading versus tectonic extension during the 

1989 Loma Prieta earthquake: Seismological Society of America 

Bulletin, v. 81, no. 5, p. 1480-1510.

Sorriso-Valvo, Marino, 1986, Landslide activity in the area of the El 

Asnam 1980 earthquake (Algeria): Geologia Applicata e 

Idrogeologia, v. 21, no. 2, p. 291-304.

Spittler, T. E., and Harp, E. L., compilers, 1990, Preliminary map of 

landslide features and coseismic fissures in the Summit Road area 

of the Santa Cruz Mountains, triggered by the Loma Prieta earthquake of October 17, 1989: U.S. Geological Survey Open-File Report 90-688, 31 p., scale 1:4,800, 3 sheets.

Varnes, D.J., 1978, Slope movement types and processes, in Schuster, 

R.L., and Krizek, R.J., eds., Landslides—analysis and control: U.S. 

National Academy of Sciences, Transportation Research Board Special Report 176, p. 11-33.

Wells, R.E., in press, Surface ruptures produced by the earthquake; their 

characteristics, origin, and hazard implications, in Ponti, D.J., ed., 

The Loma Prieta, California, earthquake of October 17, 1989— 

ground ruptures: U.S. Geological Survey Professional Paper 1551- 

D.

U.S. Geological Survey staff, 1989, Preliminary map of fractures formed 

in the Summit Road-Skyland Ridge area during the Loma Prieta, 

California, earthquake of October 17, 1989: U.S. Geological Survey Open-File Report 89-686, scale 1:12,000.

THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

EVALUATION OF COSEISMIC GROUND CRACKING ACCOMPANYING

THE EARTHQUAKE: 

TRENCHING STUDIES AND CASE HISTORIES

By Jeffrey M. Nolan, Nolan Associates; and 

Gerald E. Weber, University of California, Santa Cruz

CONTENTS

	Page 

Abstract——————————~~———————————. C145

———————................. 145

...................................... 147

——.—............................ 147

—————......................... 150

———————————.————_ 152

Introduction —————————————————————————

Trenching investigations ——————————————————

Trench 1 ————————————————————

Trench 2———————————————————

Summary of case histories -———————————————— 

Case history 1: Preearthquake investigation for Loma 

Prieta Elementary School —————————————• 

Case history 2: Postearthquake investigation for a

family residence —————————————————— 

Case history 3: Postearthquake investigation at

Robinwood Ridge —————————————————— 

Case histories: Discussion and conclusions ————————• 

General conclusions —————————————————————• 

References cited ——————————————————————

153

156

156

161

162

162

ABSTRACT

Although the 1989 Loma Prieta earthquake, centered in 

Santa Cruz County, Calif., was associated with little observable fault-related surface rupture, it caused abundant 

coseismic ground cracking throughout the epicentral region. Many homes and other structures in the Santa Cruz 

Mountains were damaged by ground cracking through their 

foundations. The purpose of this study is to assess the 

level of risk associated with this phenomenon and to determine effective means for mitigating the hazard to structures. The study included geologic trenching investigations 

of two ground-crack systems, using paleoseismic techniques, and a case-history review of geologic-hazard reports for sites that sustained ground cracking.

Geologic trenching across two separate ground cracks 

revealed well-defined, southwest-dipping shear or slip surfaces associated with normal displacement during the earthquake. In trench 1, earliest Holocene 14C ages from the 

basal part of a colluvial wedge on the downdropped side 

of the shear indicate that slip on this shear surface has 

been ongoing throughout the Holocene. Evidence for four 

separate movement episodes are recorded in the stratigraphic section for approximately the past 2,000 years.

Trench 2, located across a ground crack clearly associated 

with a reactivated landslide, revealed a moderately dipping slip surface overlain by landslide debris, with a small 

colluvial wedge at the head of the landslide mass. A 14C 

age on soil from the basal part of the colluvial wedge 

indicates initial formation of this landslide about 3.2 ka. 

The wedge records three additional movements within 

about the past 2,000 years, including the 1989 Loma Prieta 

earthquake.

To supplement our trenching studies, we reviewed all 

geologic-hazard studies on file with the Santa Cruz County 

Planning Department for the Summit Ridge area of the 

Santa Cruz Mountains, the area of most severe damage. 

Trench logs in the 24 reports we selected for study indicated that most large ground cracks (those with more than 

about 5 cm of displacement) could have been identified in 

advance by appropriate geologic investigation; however, 

small ground cracks (those with less than 3-5 cm of displacement) were not so predictable.

We conclude that the geologic hazard due to ground 

cracking in the Summit Ridge area is moderate but may 

be reducible to a relatively low level by appropriate site 

investigations and project design. Comparison of the geomorphic features in the Summit Ridge area with those 

reported by researchers in other seismically active regions 

shows that ground cracking may represent a significant 

geologic hazard in many other localities.

INTRODUCTION

The 1989 Loma Prieta earthquake was associated with 

right-lateral, re verse-oblique slip on a steeply southwest 

dipping fault surface that is part of the San Andreas fault 

system (fig. 1; Dietz and Ellsworth, 1990). The earthquake was associated with little, if any, ground surface 

rupture attributable to slip on this fault surface (Aydin 

and others, 1990). Extensive coseismic ground cracking, 

however, occurred throughout the epicentral region, especially on ridgecrests and steep slopes in the Santa Cruz 

Mountains (Ponti and Wells, 1991). The geologic hazard

C145
C146 LANDSLIDES

associated with these ground cracks was clearly demonstrated by the damage caused to roads and structures. 

Where the ground cracks intercepted buildings, the structures commonly were severely damaged or destroyed.

Similar ground cracking has been observed in other large 

earthquakes, including more severe ground cracking in 

the same area of the Santa Cruz Mountains during the 

great 1906 San Francisco earthquake (Lawson, 1908) and 

ground deformation associated with the 1980 El Asnam, 

Algeria, earthquake (Philip and Meghraoui, 1983) or the 

1980 Irpinia, Italy, earthquake (Cotecchia, 1982). The purpose of this study is to determine whether the groundcracking hazard observed in the 1989 Loma Prieta 

earthquake can be mitigated by appropriate geologic site 

investigations and (or) project design. The study therefore 

focuses on the predictability of ground-crack locations, 

displacement magnitudes, and frequency of occurrence, 

without necessarily determining the mechanisms of the

ground cracking. Our research pursued two avenues of 

investigation: (1) a geologic trenching investigation of two 

ground cracks to study the structures responsible for the 

ground cracks and to evaluate their movement histories, 

and (2) a case-history study that included a review of all 

geologic-hazard reports on file with the County of Santa 

Cruz through November 1990. In all, 24 of these reports 

included trenching investigations and were therefore selected for detailed study.

In addition to assessing the geologic hazard associated 

with ground cracks, the trenching studies also provide 

insight into the seismic history of the study area. In contrast to standard paleoseismic studies, the movement histories derived from coseismic ground cracks record the 

recurrence of a certain intensity of seismic shaking, rather 

than movements on a specific fault, and therefore elucidate a different aspect of the seismic record from what is 

normally seen.

Figure 1.—Loma Prieta area, Calif., showing locations of study area (shaded) and major faults.
EVALUATION OF COSEISMIC GROUND CRACKING ACCOMPANYING THE EARTHQUAKE C147

Large ground cracks produced by the 1989 Loma Prieta 

earthquake were commonly associated with recognizable 

older scarps, an indication of previous movements on structures in the same localities. Such structures had, in fact, 

been identified before the earthquake, both on 

photointerpretative maps (Sarna-Wojcicki and others, 

1975) and in geologic trenching investigations of linear 

topographic features in the Summit Ridge area (Foxx, 

Nielsen, and Associates, 1988; Rogers E. Johnson and 

Associates, 1988). These structures were formerly interpreted as recently active faults. This section summarizes 

the results of a trenching investigation of two ground 

cracks formed by the earthquake; a more complete discussion of the trenching investigations summarized here 

was presented by Nolan (1992). The 14C ages of samples 

from trenches 1 and 2 cited in the following discussions 

are listed in table 1.

TRENCH 1

DESCRIPTION OF TRENCH SITE

Trench 1 was excavated across a 1989 ground crack 

that occurred along a 10-m-high, 0.5-km-long, northwesttrending scarp along Summit Road, approximately 1.4 km 

southeast of California Highway 17 (see pi. 5). The trench 

extends across a topographic basin formed on the 

downdropped side of the scarp. Surface topography at the 

trench has been altered slightly by grading for an old 

alignment of Summit Road.

Measurements in two places along the ground crack 

yield a consistent slip direction of S. 32° W. and a net slip 

magnitude of 21 to 24 cm. The 1989 ground crack showed 

approximately 18 cm of vertical separation and 12 cm of 

horizontal separation where it crossed the trench. Bedrock 

in the vicinity is the Vaqueros Sandstone, a sequence of 

thick-bedded, fine- to medium-grained arkosic sandstone 

with shale interbeds (McLaughlin and others, 1991). The 

trench site lies on the southwest-dipping limb of a synclinal fold aligned with Summit Ridge.

DESCRIPTION OF TRENCH EXPOSURE

The trench revealed Vaqueros Sandstone and shale offset in a normal sense by a southwest-dipping shear surface, possibly aligned with bedding (fig. 2). A sequence 

of colluvial deposits, 3.6 m thick, overlies the 

downdropped block. The shear surface is a sharply defined parting lined with a 3- to 4-mm-thick layer of gray 

clay and a band of iron oxide staining. Slickensides observed on the shear surface have rakes of 76°-77° SE. 

Movement therefore is dominantly normal, dip slip, with 

a slight left-lateral component.

The sedimentary deposits overlying the Vaqueros Sandstone bedrock on the downdropped block are entirely colluvial, except for some artificial fill at the surface. The 

colluvium is deposited in a closed depression that evolved 

because of multiple offsets on the shear surface in the 

trench.

SUMMARY OF MOVEMENT HISTORY

The long-term displacement history on the shear surface in trench 1 is recorded by the colluvial deposits overlying the downdropped block (fig. 2). Deposition of 

colluvium began approximately 10 ka, as indicated by 

concordant 14C ages on charcoal from the basal part of 

the colluvial wedge (samples CAMS 838, CAMS 839, 

table 1; see fig. 2 for locations). The colluvium shown on 

the trench log is divided into two units, older and younger 

colluvium, on the basis of the occurrence of a buried soil 

horizon at the top of the older colluvium.

Four discrete displacements are recorded by the most 

recent colluvial deposits. Although earlier events are likely 

to have occurred, they are not preserved as discrete 

stratigraphic offsets in the sedimentary record. The first 

recorded episode of movement offset the uppermost layer 

of older colluvium (unit 4, fig. 2). Cumulative vertical 

separation on the two shears that offset the base of 

this layer is about 0.4 m, but total throw during the displacement event(s) may be obscured by later ground cracking and (or) subsequent soil-forming processes. This 

movement was followed by the deposition of unit 3. The 

shears that offset unit 4 extend into unit 3, indicating 

some secondary movement, but do not visibly offset the 

unit 3/4 contact, indicating that unit 3 postdates most of 

the observed displacement of unit 4. A 14C age of 

1,400±180 yr B.P. (sample CAMS 843, table 1) on soil 

from the upper part of unit 3 (fig. 2) indicates a latest date 

for the first recorded episode of movement. A tentative 

date for this event of about 2 ka was estimated by considering average sedimentation rates in the colluvial section 

(Nolan, 1992).

The second recorded episode of movement consisted of 

the displacement of unit 3, followed by the deposition of 

unit 2. A 14C age of 450±90 yr B.P. (sample CAMS 842, 

table 1) on charcoal from a backfilled older ground crack 

that penetrates unit 3 indicates a latest date for this event. 

The third and fourth episodes of movement are recorded 

in the offset of unit 2. The upper and lower contacts of 

unit 2 both show a cumulative offset of 0.6 m from corresponding sedimentary deposits on the footwall block (unit 

1). Because only 0.2 m of this offset is attributable to the 

1989 ground crack, this age implies an earlier event(s) 

constituting about 0.4 m of throw. This earlier event was 

probably the 1906 San Francisco earthquake, on the basis 

of indirect evidence (Nolan, 1992, p. 39). Cumulative
€148 LANDSLIDES

Table 1.—14C ages of samples from trenches 1 and 2

[Charcoal samples were pretreated by washing in O.IN HCI to remove carbonates, washing three to four times in 

O.IN NaOH until the solution remained translucent (to remove humic acid), and reacidifying in O.IN HCI. Soil 

fraction (density less than 2.0 mg/mL), primarily charcoal, was separated from bulk dried soil by using a solution of 

sodium polytungstate; the floated material was visually inspected for root hairs and washed once in O.IN HCI 

(samples were deemed too small for repeated washing). d13C values from Stuiver and Polach (1977). Ages calculated 

using Libby half-life of 5,568 yr, according to the conventions of Stuiver and Polach. ±, range of precision. Do., 

ditto]

Sample

C* A \/f C Q'iQ

CAMS 841

CAMS 842

CAMS 843 

CAMS 845

CAMS 1240

Material

— Do.—— — — —— — — — —

— Do. — — — — — — — — — -

—Do.-——————— ------

Low-density soil fraction —

Hn

—Do.——————————

d' 3 c

-25

-25

-25

-25

-25 

-25

-25

d 14c

-710.2

-712.2

-3.9

-53.9

-160

-220.5

±

8

s3.9

.7

10.2

18.2 

11.9

13.0

Age 

(yrB.P.)

9950

IVAAA/

<130

1400 

3170

9ftOO

±

£*T\J

110

180 

150

throw on the base of the younger colluvial section is 1.2 

m, indicating that the initial offset of unit 3 also consisted 

of 0.6 m of throw.

Evidence for repeated past displacements on the shear 

surface identified in the trench is unequivocal, although 

the precise timing of such displacements is only loosely 

constrained by analytic ages. The evidence indicates two 

recent movements (in 1989 and 1906), preceded by 

only two previous episodes of movement in approximately 

the past 2,000 years. The average recurrence interval 

for these events is therefore longer than the commonly 

cited recurrence intervals for major seismic events on the 

adjacent section of the San Andreas fault: 201 to 281 

years for 1906-type events and 88 to 100 years for Af=7 

earthquakes (Working Group on California Earthquake 

Probabilities, 1990). Each of the earlier recorded movements may consist of more than one individual movement; in fact, the 1989 and (inferred) 1906 events presently 

appear as a single stratigraphic offset. Also, events 

associated with small stratigraphic displacements may 

have occurred more frequently, but they left little record. 

A 14C age of 450±90 yr B.P. on a piece of charcoal lodged 

in a 1-cm-wide backfilled ground crack may indicate 

such an event. Each of the episodes of movement discussed above, however, is defined by a relatively large, 

discrete stratigraphic offset separated by long time periods, indicating a particular event or a series of closely 

timed events. These relations are incompatible with a history consisting only of more frequent, small displacement 

events.

We cannot be certain that all the displacements on the 

shear surface are associated with earthquakes. If the main 

shear surface in this trench is associated with a landslide, 

movement could be triggered by intense or prolonged rainfall. Thus, we cannot place upper limits on potential displacements. Nevertheless, information from the trench 

demonstrates that movements in the past few thousand 

years have been episodic and have involved limited, incremental displacements. The displacements observed in 

1989 and postulated for 1906 (0.2 and 0.4 m, respectively) provide a realistic range for coseismic offsets, because these events probably represent the upper limit of 

ground shaking intensity and duration in this locality.

TRENCH 2 

DESCRIPTION OF TRENCH SITE

Trench 2 was excavated across 1989 ground cracks that 

outlined the head of a preexisting landslide mass in the 

Burrell landslide, on the southwest flank of Summit Ridge 

(see pi. 5). The ground crack around this mass showed 7 

to 10 cm of horizontal extension and a few centimeters of 

vertical separation. The trench site is underlain by the 

mudstone unit of the Butano Sandstone (Clark and others, 

1989), a sequence of dark-gray, thin-bedded, nodular mudstone with interbedded arkosic sandstone. Bedding was 

not visible in the trench. The site is approximately 200 m 

southwest of the mapped trace of the San Andreas fault 

(see pi. 5).
10 

N.60°E. No vertical exaggera

Present-day and older 

crack fill

<130yrB.P.

1,400+180 yrB.P.

Pavement- remains of old 

alignment of Summit Road

EXPLANATION

-Contac

indistin

Contac

Artificial fill-- Silt and fine sand

Younger colluvium- Medium-grey-brown 

silt and very fine sand. Individual subunits 

are numbered.

Older colluvium- Light-grey-brown silt and 

very fine sand. Individual subunits are 

numbered.

Bedrock (Tertiary)- Sandstone and shale 

of the Vaqueros Sandstone

Fault; a

relative 

14C sa

10,000+110 yrBP.

Figure 2.—Log of trench 1, which was excavated across a ground crack along base of a 0.5-km-long, northwest-trending scarp at summit of the Santa Cruz Mo

dipping slip surface and a 4-m-thick colluvial wedge overlying downdropped block. Earliest Holocene 14C ages on soil from basal part of colluvium and offset strat

on this slip surface has been ongoing throughout the Holocene, including at least three discrete events in the past approximately 2,000 years. 
C150 LANDSLIDES

DESCRIPTION OF TRENCH EXPOSURE

The trench exposed a shear surface dipping 32° SW. 

and striking approximately N. 40° W. (fig. 3). Bedrock 

below the shear surface is dark-gray shale. Above the 

shear surface, bedrock consists of broken and disaggregated sandstone and shale that we interpreted as landslide 

debris (unit 8, fig. 3). Overlying this bedrock debris is a 

wedge of colluvial sedimentary deposits at the base of the 

landslide scarp (units 1-7, fig. 3). These colluvial sedimentary deposits consist of fine-grained sand and silt and 

varying amounts of pebble- to cobble-size, angular clasts 

of sandstone and siltstone.

SUMMARY OF MOVEMENT HISTORY

The colluvial wedge is offset by a steeply dipping slip 

surface that branches upward from the main slip surface 

(A, fig. 3). This slip surface is moderately well defined at 

the base of the colluvial section, where it offsets units 6 

and 7, but is visible only as a flexure in the coarser grained 

overlying units (fig. 3). The amount of flexure of unit 4, 

about 0.5 m, was estimated by comparing the flexured 

bed with the smoothly concave modern depositional surface; the estimated flexure is comparable to the amount of 

discrete offset on unit 7. Comparison of the geometry of 

unit 2 with the present surface profile shows about 0.2 m 

of flexure, about half as much as for unit 4, indicating 

that the total displacement of 0.5 m at the base of the 

section (units 6, 7) is due to at least two separate events. 

The upper surface of unit 1 shows no flexure and no 

ground cracks or other evidence for movement on this 

slip surface in 1989. These observations therefore suggest 

at least two pre-1989 movements of about 20 to 30 cm 

each after the initial landslide movement.

Displacement due to movement during the 1989 Loma 

Prieta earthquake occurred southwest of the colluvial 

wedge (slip surface B, fig. 3). Movement occurred on a 

fracture surface that shows a cumulative displacement (vertical separation) on a sandstone/shale contact of 24 cm. 

This displacement is larger than the 1989 vertical separation measured at this locality, indicating earlier 

movement(s), the timing of which is unconstrained.

A 14C age of 3,170±150 yr B.P. (sample CAMS 845, 

table 1) on soil from the basal part of unit 7 (fig. 3), 

approximately 6 to 10 cm above bedrock, dates the onset 

of colluvial deposition and thus approximates the date of 

the original landslide movement. This age is compatible 

with the state of preservation of the surficial landslide 

features.

A second soil sample from unit 3 (fig. 3) yielded a 14C 

age of 2,000+140 yr B.P. (sample CAMS 1240, table 1). 

The upper surface of unit 3 conforms to the geometry 

of overlying beds (unit 2) and therefore postdates the initial displacement of units 4 through 6, although it is 

unclear whether unit 3 was deposited entirely after the 

initial displacement of unit 4 or whether the lower part of 

unit 3 was already in place at the time of the initial movement. Nevertheless, this age suggests that the initial offset 

of units 4 through 6 occurred shortly before 2 ka. The 

second movement that offset unit 2 occurred after 2 ka. 

No information regarding the timing of earlier events on 

the fracture reactivated by the 1989 earthquake is available.

On the basis of these observations, we conclude that 

the Burrell landslide has been reactivated at least three 

times since its initial failure, including the movement in 

1989. This number of movements is a minimum because 

each episode of offset preserved in the stratigraphic record 

could have been produced by more than one movement 

on the shear surface and because some displacement events 

may have left no record in the part of the landslide examined in the trench. Cumulative throw on shear surfaces in 

the trench in the past 3,000 years (after the initial landslide formation) has been less than 1 m.

SUMMARY OF CASE HISTORIES

This section presents the results of a case-history study 

of coseismic ground cracking caused by the 1989 Loma 

Prieta earthquake. The purpose of this study was to evaluate whether the ground-cracking hazard posed to future 

development in the epicentral region can be mitigated by 

appropriate geologic investigation before site development. 

Specifically, we examined existing geologic-hazard investigations of damaged properties to answer two questions: 

(1) Can the locations of future coseismic ground cracks 

be reliably predicted; and (2) where potential ground-crack 

locations cannot be avoided, can probable displacements 

be constrained to provide mitigating design parameters?

We reviewed all geologic-hazard reports on the Summit Ridge area of the Santa Cruz Mountains filed with the 

County of Santa Cruz before mid-November 1990. These 

reports were prepared by private consultants for properties in steeply sloping terrain or in State-delineated hazard zones surrounding active or potentially active faults 

(Alquist-Priolo Special Studies Zones). Reports examined 

included those completed both before and after the earthquake. These reports were of varying quality, reflecting 

the varied backgrounds and experience of the geologists 

preparing them.

Of a total of 81 available reports, 23 included records 

of subsurface (trench) exposures and were thus selected 

for more detailed review. One additional geologic report, 

conducted for Loma Prieta Elementary School on the north 

side of Summit Road in Santa Clara County, was included 

in our review because its geologic setting is identical to 

that of adjacent parts of Santa Cruz County. The proper-
N.47°E.

6 MET

3,170t150yrB.P.

EXPLANATION

Colluvium- Grey-brown fine sand and silt, with 

varying amounts of pebble- to cobble-size, 

angular clasts of sandstone and shale. 

Individual subunits are numbered

Landslide debris derived from the Butano 

Sandstone- Sandstone (SS) and shale (SH), sheared 

and broken

Bedrock (Tertiary)- Shale 

of the Butano Sandstone

Contact, sharp

Contact, approximate

Contact, gradual

Landslide slip surface

located, dotted where 

\ direction of relative 

\

,-* • 14C sample location 

3,170t150yrB.P.

Figure 3.—Log of trench 2, which was excavated across a ground crack following crown of a preexisting landslide mass. Offset of layers within colluvial wedge a

cracks indicate that at least three reactivations of this landslide have taken place in the past approximately 2,000 years.
C152 LANDSLIDES

ties for which these reports were prepared are listed in 

table 2, and the locations of all 24 sites are shown on 

plate 5. All reports are on file with the County of Santa 

Cruz by assessor's parcel number (APN, table 2) and are 

available to the public.

Most of the reviewed reports contained either clear or 

probable evidence of ground cracking that predated the 

1989 Loma Prieta earthquake; of the 24 reports reviewed, 

14 contained definite evidence of previous ground cracking, 5 contained probable evidence for previous ground 

cracking, 3 were indeterminate, and 2 contained no evidence of previous ground cracking. Those sites considered to show definite evidence for previous ground 

cracking are those where relations shown in the trench 

logs require previous movements or where the author(s) 

specifically stated that previous movement was indicated 

by the exposure. The evidence for previous movement 

includes older ground cracks backfilled with surface sedimentary deposits, 1989 fissures that connected to shear 

surfaces with much larger stratigraphic separations at 

depth, and slip-parallel shear fabrics too well developed 

to have formed entirely in the recent slip event.

The "probable" category was applied to sites where 

information included in the trench logs or in the report 

text indicated previous ground-cracking episodes, although 

such information was not discussed by the author(s). 

The "indeterminate" category was applied to sites where 

information on the trench log was uninterpretable or 

judged to be incomplete. Of the two reports that contained no evidence for previous ground cracking, one 

omitted any trenches dug to intercept existing ground 

cracks or surface lineaments, and the other stated that no 

evidence of preexisting ground cracking existed where 

the cracks caused by the 1989 Loma Prieta earthquake 

crossed the trench, although the relations depicted in one 

trench indicated that preexisting cracks were present 

nearby.

The following sections review three case histories chosen to illustrate the type of subsurface relations observed 

in trenches and to describe the associations between subsurface expression and displacements during the 1989 

Loma Prieta earthquake. We note, however, that our conclusions are based on the entire data base, rather than on 

only the three case histories presented here. These case 

histories include one preearthquake investigation for Loma 

Prieta Elementary School and two postearthquake studies 

of residential properties. The figures accompanying this 

part of the paper are reproduced from the originals.

CASE HISTORY 1: PREEARTHQUAKE

INVESTIGATION FOR LOMA PRIETA

ELEMENTARY SCHOOL

Geologic investigations for Loma Prieta Elementary 

School were mandated by the Office of the California

State Architect, as directed by the Alquist-Priolo Act, to 

evaluate the potential ground-rupture hazard due to faulting. The school is located on the northeast side of Summit 

Road, approximately 4 km east of California Highway 17 

(case history 1, pi. 5). The investigation (Rogers E. Johnson 

and Associates, 1989) (loc. 24, table 2) consisted of more 

than 360 m of backhoe trenching.

Bedrock in the area is the Vaqueros Sandstone, a sequence of fine- to medium-grained sandstone with lesser 

interbedded shale and mudstone, of Oligocene and early 

Miocene age. The school site was extensively graded before construction, and so Quaternary deposits had been 

stripped from many of the areas trenched, leaving no datum with which to evaluate fault activity. Thus, several of 

the faults identified in the trenches could not be evaluated 

with regard to Holocene activity.

The locations of the school buildings and trenches, and 

of faults with demonstrable offset of Quaternary strata, 

are shown in figure 4. Pregrading aerial photographs of 

the site show a set of northeast-trending ridges and valleys approximately parallel to the nearby San Andreas 

fault. The identified faults coincide with topographic breaks 

that follow the margins or axes of valleys that existed 

before grading for the school.

Logs of trenches 3 and 4 are shown in figure 5. Trench 

3 shows a colluvium-fllled basin formed by downdropping 

and rotation of bedrock blocks along two well-defined 

slip surfaces. No slickensides were noted along these slip 

surfaces, although consistent normal separations on strata 

were observed. Both slip surfaces showed evidence for 

more than one episode of movement, as indicated by increasing stratigraphic separation on successively older layers. 14C ages on disseminated carbon from soil samples 

indicate that approximately 0.75 m of normal separation 

has occurred on the easternmost slip surface during the 

past 4.7 to 2.0 ka. This trench was discussed in more 

detail by Rogers E. Johnson and Associates (1989).

The log of trench 4 (fig. 5) depicts a type of subsurface 

expression of ground cracking common in the Summit 

Ridge area. The zone of offset is not marked by any clearly 

recognizable shear surface but, instead, by a near-vertical, 

indistinct contact between weathered bedrock and colluvium. The absence of a well-defined slip surface is due to 

extensional movement that produces open cracks without 

mechanical reduction of materials along the parting surface. In this trench, the contrast between weathered bedrock and colluvium is subtle, and the offset was not 

recognized during the initial trench logging. This exposure highlights the caution that must be exercised when 

examining subsurface exposures of extensional ground 

cracking.

Displacement of the ground surface during the 1989 

Loma Prieta earthquake occurred along the trend of the 

slip surface recognized in the middle of trench 3 (fig. 4). 

This deformation was not uniform along the projected 

surface fault trace but was greatest in the vicinity of class-
EVALUATION OF COSEISMIC GROUND CRACKING ACCOMPANYING THE EARTHQUAKE C153

Table 2.—Sites of properties where geologic-hazard reports were selected for detailed review

[All reports are public information on file with the County of Santa Cruz by assessor's parcel number (APN). Evidence for previous ground movement: D, definite; 

I, indeterminate; N, none; NGC, no ground cracks; P, probable. Do., ditto]

Locality (pl.6) APN Address Previous Movement Consultant

Postearthquake Reports

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

97-101-32

96-271-09

96-291-07

96-131-36

96-151-13

96-311-23

98-281-12

98-151-04

98-271-11

97-161-03

96-292-34

97-071-20

96-283-06

96-382-04

96-131-22

96-111-01

97-231-09

97-231-10

97-231-11

97-231-12

97-231-13

96-061-11

25295 Old San Jose Road

24620 Miller Road

23430 Sunset Drive

23080 Summit Road

23700 Morrell Cut Off

23484 Bel Aire Ct.

25200 Adams Road

15052 Stetson Road

25734 Adams Road

25507 Old San Jose Road

23048 Evergreen Lane

16000 Redwood Lodge Road

18338 Las Cumbres

22494 Citation Drive

23136 Summit Road

23201 Old Santa Cruz Highway

16010 Stetson Road

16020 & 16060 Stetson Road

16026 & 16044 Stetson Road

17774 Old Summit Road

N

D

I

P

D

I

P

I

D

NGC

D

P

D

D

D

D

D

D

D

D

Applied Soil Mechanics.

Upp Geotechnology.

J.W. Leonard, C.E.

Jo Crosby and Associates.

William Cotton and Associates.

Jo Crosby and Associates.

Michelucci and Associates.

Geoforensics.

Robertson Geotechnical.

Freeman-Kern Associates.

Upp Geotechnology.

Hydro-Geo Consultants.

William Cotton and Associates.

Geoforensics.

Foxx-Nielsen and Associates.

Pacific Geotechnical.

G.E. Weber and Associates.

Do.

Do.

Do.

Preearthquake Reports

21

22

23

24

96-281-13

96-121-21

95-021-21

23426 Sunset Road

Schultheis Road

Summit Road

Santa Clara 23845 Summit Road

P

P

D

D

K.W. Price.

G.E. Weber and Associates.

JCP Engineers.

Rogers E. Johnson and Associate

(1988).
EXPLANATION

TRENCH 4

Fault show

during tren

earthquak

Zone of su

the 1989 L

Trench loc

\

Zone of flexure, approx. 

6-in. of vertical separation

Maximum deflection in building

Axis of depressio

100 200 FEET

30 60 METERS

Figure 4.—Simplified site plan for Loma Prieta Elementary School, showing locations of paleoseismic trenches excavated before 1989 Loma Prieta earthquake

trenches, and ground displacement that occurred during the earthquake. Modified from Rogers E. Johnson and Associates (1989).
East

2180±70yrB.P.

4,680+110yrB.P.

EXPLANATION 

^^_^-- Contact, sharp 

_- — ---- Contact, approximate or indistinct 

,..,„.«""••'" Contact, gradual

/'' Fault, dashed where approximate, dotted where inferred; 

fy arrows indicate direction of relative movement

af Artificial fill

Qc Quaternary colluvium. Individual 

beds numbered

SS Sandstone of the Vaqueros 

Formation. Individual beds 

numbered

4,680+110yrB.P.

14C sample location and age

Southwest

8

Zone of very loose material, abundant 

burrows and deep roots

2500±110yrB.P.

0

1 —— 

0

10 ——— 1 ————— t —

3 

No vertical ex

Subvertical contact, oriented 

N.57°W. across trench

Figure 5.—Logs of trenches 3 (A) and 4 (B), showing evidence for active faulting found during preearthquake trenching investigation. Fault in center of trench 3 wa

earthquake, causing as much as 15 cm of down-to-the-northeast displacement under classroom unit 2. Modified from Rogers E. Johnson and Associates (1989). 
C156 LANDSLIDES

room building 2 and decreased to the northwest and southeast. The deformation occurred as flexure of the ground 

surface, northeast side down, probably as a result of normal 

slip at depth. Maximum vertical displacement was approximately 15 cm across a zone of flexure about 3 m wide. 

As a result of the geologic investigation, the consultant 

recommended that the school site be abandoned. This recommendation, however, was not followed by the school 

board, and so the school was in use, though not in session, when the earthquake struck. The administration building and classroom units 2 and 3 were moderately to 

severely damaged by ground cracking, but no structural 

collapse took place. The school was subsequently abandoned.

CASE HISTORY 2: POSTEARTHQUAKE 

INVESTIGATION FOR A FAMILY RESIDENCE

Weber and Associates (1990a) investigated the site of a 

single-family residence near the intersection of Summit 

Road and California Highway 17 (see pi. 5; loc. 20, table 

2). The house and property were severely damaged by 

strong seismic shaking and a broad, arcuate zone of ground 

cracking (fig. 6). The ground cracks had extensional displacements of 3 to 30 cm and southwest-side-down vertical separations of as much as 25 cm, reflecting movement 

of the earth materials toward the steep-walled drainage on 

the southwest.

Trenches 5B and 6 (fig. 7) were excavated across the 

ground cracks. In trench 5B, the ground cracks coincided 

with a large, colluvium-filled graben formed by normal 

displacement on bounding slip surfaces, indicating a history of displacement at this site. Previous episodes of 

movement were also indicated by trench 6, where 1989 

ground cracks were associated with larger displacements 

of bedrock stratigraphy and soil horizons, as well as older 

generations of infilled ground cracks (fig. 7).

In trench 5B, a 6-in.-wide ground crack at the surface 

could not be traced into the subsurface (fig. 7). The subsurface material here (unit 2) is a massive, very loose 

silty sand that evidently accommodated extension as distributed shear throughout, rather than as the single open 

crack seen at the surface. Similar behavior was observed 

in loose, granular sediment in other case-history localities. The disappearance of ground cracks, even very large 

ones, at shallow depth should not therefore be taken as 

conclusive evidence that no deeper seated failure has occurred. This point is particularly important at sites where 

the potential for catastrophic slope failure is being evaluated. In more cohesive materials, such as those in trench 

6, surficial cracks generally persist at depth (fig. 7).

Trench 5A is a northward continuation of trench 5B 

(figs. 6, 7). A second colluvium-filled basin was exposed 

in this trench. Although no ground cracks were observed 

at the surface in this locality (except for a few cracks due

to fill failure under the driveway), careful inspection of 

slip surfaces exposed in the trench revealed 3 to 6 mm of 

extension across shear surface a (fig. 7) that was not visible at the surface.

The processes responsible for forming and filling the 

colluvial basin in trench 5A began approximately 6 ka, as 

indicated by a 14C age on a bulk soil sample from the 

base of unit 2 (fig. 7). Like the Loma Prieta Elementary 

School site, this parcel was graded to form a level pad 

before development. Historical aerial photographs and old 

topographic maps, however, indicate a preexisting ridge 

through the central part of the site, corresponding to the 

bedrock high between the two grabens. The axes of the 

adjacent topographic troughs coincide with basins identified in the trenches.

Although one of the smaller ground cracks ruptured the 

foundation, the home on this site was destroyed primarily 

by seismic shaking. The ground-crack system trending 

through trenches 5B and 6 was considered particularly 

dangerous because the cracks dip toward a steep slope 

(50- to 80-percent gradient) and could therefore facilitate 

landsliding, with potentially catastrophic displacements.

CASE HISTORY 3: POSTEARTHQUAKE 

INVESTIGATION AT ROBINWOOD RIDGE

Robinwood Ridge is a long, narrow, northwest-trending ridge in the southeastern part of the study area (case 

history 3, pi. 5). Five contiguous parcels were investigated by G.E. Weber and Associates, Inc. (loc. 17, table 

2), covering a 0.5-km-long stretch of the ridgetop. The 

investigation included 12 trenches totaling about 150 m in 

length. The complete geologic maps and trench logs are 

too voluminous to reproduce here, but a small part of the 

mapped area (fig. 8) and two trench logs (fig. 9) are included for illustrative purposes. Observations in all the 

trenches are summarized below.

In general, 1989 ground cracks were associated with 

evidence of previous episodes of movement in the subsurface, commonly expressed as zones of fine, parallel, discontinuous fractures or shears that form a "shear fabric," 

with a grain parallel to the recent ground cracks. The 

shear fabric is commonly accompanied by older, infilled 

ground cracks that appear as thick, continuous veins or 

thin, discontinuous stringers of darker fill material. There 

may be little or no observable displacement of stratigraphic 

markers where the movement is primarily extensional, but 

large vertical separations occur elsewhere. The log of

Figure 6.—Area of case history 2 (see pi. 5 for location), showing pattern and dimensions of ground cracking that occurred during 1989 Loma 

Prieta earthquake. Trenches across ground-cracks showed evidence for 

many previous episodes of ground cracking in the same places. From 

G.E. Weber and Associates (1990a).
EVALUATION

OF COSEISMIC GROUND CRACKING ACCOMPANYING THE EARTHQUAKE C157

EXPLANATION 

"^ ' ">• Old landslide headscarp

Fill slope

Cut slope

Slope direction and grade in percent

Top of steep slope 

h=i2 Crack showing only horizontal movement, width in inches

h=i2 Crack showing horizontal and vertical separation, separations 

v=4 shown in inches

Landslide, of Quaternary age; arrow indicates direction of 

movement

N

50

Retaining wall 

>\ Trench location

15

100 FEET

1

30 METERS
C158 LANDSLIDES
I Trench 7

artificial 

fill

A1

/ 1989 ground crack

Robinwood Lane

<: //

1989 ground /——— older slip surface 

cracks *

Dirt Road

20

-f—

40 

-f12

60 FEET

18 METERS

No vertical exaggeration

EXPLANATION

Old landslide headscarp 

Fill slope 

Cut slope

'SO Slope direction and grade (in percent) 

h=i2 Crack showing only horizontal movement, width in inches

h=i2 Crack showing horizontal and vertical separation, separations 

v=4 shown in inches, barbs on down-dropped side

A

IT

A 1

Trench location

Line of Cross Section A-A'

Figure 8.—Area of case history 3 (see pi. 5 for location), at the crest of a narrow, steep-sided ridge, and cross section A-A'. Ground cracking occurred continuo

flank of ridge for a distance of more than 0.5 km. A total of 12 trenches across this series of cracks revealed evidence for multiple earlier movements on 1989 g

ground cracks that were not reactivated by 1989 Loma Prieta earthquake. From G.E. Weber and Associates (1990b).
C160 LANDSLIDES

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EVALUATION OF COSEISMIC GROUND CRACKING ACCOMPANYING THE EARTHQUAKE C161

trench 7 (fig. 9) shows features associated with both types 

of movement. At the northeast end of the trench (near the 

4-ft marker along the scale) is a zone characterized mainly 

by extensional deformation. The lower contact of unit 1A 

is not displaced across this zone but appears to be subsiding into a small graben created by extension. At the same 

time, several of the larger ground cracks mapped in the 

section curve abruptly into a ground-surface parallel orientation toward the bottom of the trench. These relations 

suggest formation of the graben by translation of a nearsurface layer moving approximately parallel to the ground 

surface. Infilled older fissures show that several episodes 

of movement have occurred.

In contrast to this zone of extensional deformation, a 

large, discrete, vertical displacement of soil horizon B 

(units IB, 1C) was also observed (at 8 ft along the scale). 

Unit IB is a well-developed argillic horizon that was partly 

stripped from the footwall after being offset; unit 1C is a 

relict of the partly stripped soil horizon B. No movement 

occurred on this slip surface during the 1989 Loma Prieta 

earthquake.

Some ground cracks crossed by the trenches were not 

observed in the subsurface, nor was any visible evidence 

of earlier episodes of ground cracking found at these localities. Such areas were associated only with smaller 

cracks showing less than 2.5 to 5 cm of total displacement in relatively loose, granular materials. In contrast, at 

some other localities where clear evidence for large, older 

ground cracks existed, no visible ground cracking occurred 

during the 1989 Loma Prieta earthquake.

In contrast to trench 7 and the other trenches included 

in the review, the crack observed in trench 8 (fig. 9) exhibits a large displacement at the surface (15 cm), yet any 

evidence for earlier movement would probably not have 

been observed, had trenching been conducted at this site 

before the earthquake. The slight shear fabric depicted in 

the trench log (fig. 9) is weakly expressed and could have 

formed in the most recent event. The geometry of the 

ground crack indicates that it results from nearly surface 

parallel slip (downhill translation) of the shallow weathered sandstone, rather than deep-seated offset. This type 

of failure appears to be associated with sites underlain by 

loose sand and was not observed at hard-bedrock sites. 

The adjacent residence was thrown off its foundation by 

shaking but was not damaged by ground cracking.

CASE HISTORIES: 

DISCUSSION AND CONCLUSIONS

The preceding case histories were selected to provide a 

brief overview of the types of subsurface expression associated with ground cracks caused by the 1989 Loma Prieta 

earthquake. On the basis of our review of these and other 

sites, we conclude as follows.

1. With few exceptions, ground cracks exhibiting large displacements resulting from the earthquake (more than 3-5 

cm of extension and (or) more than 1-3 cm of vertical 

displacement) showed clear evidence of earlier episodes of 

movement in the subsurface. Therefore, using appropriate 

investigative techniques, including geologic trenching of 

prospective sites, a site can be selected with alow potential 

for large-scale ground cracks. Ground cracks exhibiting 

smaller earthquake-related displacements were commonly, 

but not necessarily, associated with subsurface evidence of 

earlier episodes of movement. Therefore, the predictive 

reliability for small-scale cracks is lower.

2. Not all ground cracks with earlier displacements observed 

in the subsurface were reactivated by the earthquake. 

Therefore, the ground-cracking pattern resulting from the 

earthquake cannot, by itself, be used to evaluate the potential for future displacement.

3. Areas of repeated large-scale ground displacements, even 

those not reactivated during the earthquake, are commonly 

associated with visible topographic expressions. 

Precise predictions of displacement are probably impossible at most localities, except in the sense that areas 

not characterized by older ground cracks are unlikely to 

undergo displacements of more than 3 to 5 cm. Newmark 

(1965) described a method of quantifying seismically induced displacement of a soil mass, using the time-acceleration history of the seismic event; his method is now 

commonly used to estimate landslide displacements during earthquakes (Wilson and Keefer, 1983, 1985; Keefer, 

1991). According to Newmark, displacement is proportional to the intensity and duration of shaking above a 

certain minimum shaking threshold. Therefore, the displacements observed during the most intense expectable 

earthquakes are probably reasonable upper bounds for predicted displacements. Although data on displacements in 

identical ground-crack systems in 1989 and 1906 are 

sparse, they suggest that movement on ground cracks was 

greater in 1906 than in 1989 (for example, trench 1; 

Prentice and Schwartz, 1991). This information, which is 

consistent with the greater intensity and duration of the 

1906 earthquake, indicates that the 1906 displacements 

may be a reasonable upper limit for coseismic offset in 

the study area.

In some places, the magnitude of past displacements 

can be deduced from the geologic record exposed in the 

trench, such as in trenches 1 and 2. Where ground cracks 

connect to preexisting shear surfaces that dip downslope 

in steep terrain (for example, at Iocs. 2, 3, pi. 5), we 

assume that the shear surfaces can initiate landsliding with 

catastrophic displacements.

These conclusions indicate that a thorough subsurface 

investigation is prudent at any site located in an area with 

an elevated risk of coseismic ground cracking, such as on 

narrow-crested and (or) steep-sided ridges, or in areas of 

earlier landsliding. Such studies can identify building sites
C162 LANDSLIDES

free of past large-scale ground cracks and thus significantly lower the potential for future, large-scale displacement during earthquakes. Because smaller ground cracks 

are not so predictable, such investigations must be augmented by foundation design to accommodate small-scale 

displacements.

These evaluations must be integrated with a full geologic evaluation of the site because other site characteristics may complicate the analysis. A comparison between 

trenches 1 and 2 (figs. 2, 3) suggests that the potential for 

initiation of large-scale ground cracks at new sites is probably higher for reactivated landslides than for other types 

of ground-crack systems. Therefore, evaluation of a site 

on a preexisting landslide mass should be sensitive to the 

potential internal mechanics of the landslide mass.

GENERAL CONCLUSIONS

The results of both the trenching and case-history studies indicate that the ground cracks observed after the 1989 

Loma Prieta earthquake are recurrent phenomena which 

can be located according to recognizable surficial geologic features, including topographic lineaments, preexisting landslide masses, and preexisting shear surfaces. 

Trenches 1 and 2 both show evidence for two prehistoric 

episodes of movement during approximately the past 2,000 

years, with additional movement in 1989 and, probably, 

1906 in trench 1. The movement histories in these two 

trenches are compatible and may describe the same two 

prehistoric events. These data suggest major events, or 

groups of events, probably seismic, with a recurrence interval of about 1,000 years. We emphasize that these displacement histories record only "events," which are not 

necessarily linked only to seismic events or to every earthquake on the San Andreas fault.

The recurrence interval for episodes of movement indicated by the trenching data is significantly longer than the 

approximately 100- to 300-year recurrence intervals cited 

for various events on the adjacent segment of the San 

Andreas fault. The ground cracking may occur only with 

very specific seismic events, and so multisegment ruptures like the 1906 San Francisco earthquake or reverseoblique events like the 1989 Loma Prieta earthquake may 

be much rarer than the brief historical record would suggest.

The results of the our case-history review clearly indicate that appropriate geologic site investigations, in conjunction with site engineering, can significantly reduce 

the hazards posed to development by coseismic ground 

cracking. Where the ground cracking results from ridgetop 

spreading, potential ground-crack locations can be identified with confidence; where the ground cracking is due to 

landslide reactivation, predictive reliability may be less. 

Discrimination between these two types of ground cracking depends on correct geomorphologic evaluation of the 

site. This observation underscores the importance of a 

complete geologic characterization of a prospective building site, rather than relying simply on trenching.

The ground cracking recognized in the Santa Cruz 

Mountains may also be a significant geologic hazard in 

other areas. Not only are examples of coseismic ground 

cracking known from other earthquakes (Cotecchia, 1982; 

Philip and Meghraoui, 1983), but also abundant geomorphic evidence for ridgetop spreading has been observed in 

many seismically active regions throughout the world 

(Jahn, 1964; Beck, 1968; Radbruch-Hall and others, 1976; 

Mahr, 1977; Radbruch-Hall, 1978; Bovis, 1982). In the 

Summit Ridge area, preearthquake geologic mapping identified many large preexisting landslides on the flanks of 

the ridge (Cooper-Clark and Associates, 1975). These 

large, complex landslides, which commonly display relatively subtle morphology, were previously assumed to be 

ancient and inactive. Many of these preexisting landslides 

were reactivated during the 1989 Loma Prieta earthquake 

and may be viewed as active features that are evolving as 

a result of relatively regular, incremental displacements, 

coseismic or not. These observations highlight geologic 

hazards that may apply to any seismically active region of 

the world.

Local, regional, or State agencies must decide whether 

or not the hazard reduction afforded by geologic site investigations is sufficient to permit development. The results of this study, however, indicate that the cumulative 

risk to development posed by coseismic ground cracking 

can be significantly reduced by appropriate investigation 

and design.

REFERENCES CITED

Aydin, Atilla, Johnson, A.M., and Fleming, R.W., 1990, Field evidence 

for right-lateral/reverse tectonic surface rupture along the San Andreas/Sargent fault systems associated with the 1989 Loma Prieta 

earthquake [abs.]: Eos (American Geophysical Union Transactions), 

v. 71, no. 43, p. 1460.

Beck, A.C., 1968, Gravity faulting as a mechanism of topographic adjustment: New Zealand Journal of Geology and Geophysics, v. 11, 

no. l,p. 191-199.

Bovis, M.J., 1982, Uphill-facing (antislope) scarps in the Coast Mountains, Southwest British Columbia: Geological Society of America 

Bulletin, v. 93, no. 8, p. 804-812.

Clark, J.C., Brabb, E.E., and McLaughlin, R.J., 1989, Geologic map and 

structure sections of the Laurel 7 Il2r Quadrangle, Santa Clara and 

Santa Cruz counties, California: U.S. Geological Survey Open-File 

Map 89-679, 31 p., scale 1:24,000, 2 sheets.

Cooper-Clark and Associates, 1975, Preliminary map of landslide deposits in Santa Cruz County, California: Santa Cruz, Calif., Santa 

Cruz County Planning Department, scale 1:62,500.

Cotecchia, Vincenzo, 1982, Phenomena of ground instability produced 

by the earthquake of November 23, 1980 in Southern Italy: International Association of Engineering Geology International Congress, 

4th, New Dehli, 1982, Proceedings, v. 8, p. 151-164.
EVALUATION OF COSEISMIC GROUND CRACKING ACCOMPANYING THE EARTHQUAKE C163

Dietz, L.D., and Ellsworth, W.L., 1990, The October 17, 1989 Loma 

Prieta, California, earthquake and its aftershocks; geometry of the 

sequence from high-resolution locations: Geophysical Research Letters, v. 17, no. 9, p. 1417-1420.

Foxx, Nielsen, and Associates, 1988, Geologic fault investigation, lands 

of Donna Hines, Summit Road, Santa Cruz County, California: 

Santa Cruz, Calif. 32 p., 3 sheets.

G.E. Weber and Associates, 1990a, Geologic hazards evaluation for the 

Almaneih property, 17774 Old Summit Road, Los Gates, Santa 

Cruz County, California: Watsonville, Calif., 39 p., 3 sheets.

———— 1990b, Geologic hazards investigation, Gartner property, 16010 

Stetson Road, APN 097-231-09, Santa Cruz County, California: 

Watsonville, Calif., 35 p., 2 sheets.

John, Alfred, 1964, Slopes morphological features resulting from gravitation: Zeitschrift fur Geomorphologie, supp. v. 5, p. 59-72.

Keefer, D.K., ed., 1991, Geologic hazards in the Summit Ridge area of 

the Santa Cruz Mountains, Santa Cruz County, California, evaluated in response to the October 17, 1989, Loma Prieta earthquake; 

report of the Technical Advisory Group: U.S. Geological Survey 

Open-File Report 91-618, 427 p.

Lawson, A.C., chairman, 1908, The California earthquake of April 18, 

1906; report of the State Earthquake Investigation Commission: 

Carnegie Institution of Washington Publication 87, 2 v.

Mahr, Tibor, 1977, Deep-reaching gravitational deformations of high 

mountain slopes: International Association of Engineering Geology 

Bulletin, no. 16, p. 121-127.

McLaughlin, R.J., Clark, J.C., Brabb, E.E., and Helley, E.J., 1991, Geologic map and structure sections of the Los Gatos 7 l/2r Quadrangle, Santa Clara and Santa Cruz counties, California: U.S. Geological Survey Open-File Report 91-593, 48 p., scale 1:24,000, 3 

sheets.

Newmark, N.M., 1965, Effects of earthquakes on dams and embankments: Geotechnique, v. 15, no. 2, p. 139-160.

Nielsen, Foxx, and Associates, 1988, Geologic fault investigation, lands 

of Donna Hines, Summit Road, Santa Cruz County, California: 

Santa Cruz, Calif. 32 p., 3 sheets.

Nolan, J.M., 1992, An evaluation of coseismic ground-cracking accompanying the October 17, 1989 Loma Prieta, California earthquake; 

paleoseismic studies and case histories: Santa Cruz, University of 

California, M.S. thesis, 87 p., 8 sheets.

Phillip, Herve, and Meghraoui, Mustapha, 1983, Structural analysis and 

interpretation of the surface deformations of the El Asnam earthquake of October 10, 1980: Tectonics, v. 2, no. 1, p. 17-49.

Ponti, D.J., and Wells, R.E., 1991, Off-fault ground ruptures in the 

Santa Cruz Mountains, California; ridge-top spreading vs. tectonic 

extension during the 1989 Loma Prieta earthquake: Seismological 

Society of America Bulletin, v. 81, no. 5, p. 1480-1510.

Prentice, C.S., and Schwartz, D.P., 1991, Reevaluation of 1906 surface 

faulting, geomorphic expression, and seismic hazard along the San 

Andreas fault in the southern Santa Cruz Mountains; Seismological 

Society of America Bulletin, v. 81, no. 5, p. 1424-1479.

Radbruch-Hall, D.H., 1978, Gravitational creep of rock masses on slopes, 

chap. 17 o/Voight, Barry, ed., Rockslides and avalanches, 1. Natural phenomena (Developments in Geotechnical Engineering, v. 14A): 

Amsterdam, Elsevier, p. 607-657.

Radbruch-Hall, D.H., Varnes, D.J., and Savage, W.Z., 1976, Gravitational spreading of steep sided ridges ("Sackung") in western United 

States: International Association of Engineering Geology Bulletin, 

no. 14, p. 23-35.

Rogers E. Johnson and Associates, 1988, Fault investigation report, 

Loma Prieta Elementary School, Santa Clara County, California 

(phases 1 and 2): Santa Cruz, Calif. 28 p., 4 sheets

Sarna-Wojcicki, A.M., Pampeyan, E.G., and Hall, T.N., 1975, Map showing recently active breaks along the San Andreas fault between the 

central Santa Cruz Mountains and the northern Gabilan Range, California; U.S. Geological Survey Miscellaneous Field Studies Map 

MF-650, scale 1:24,000, 2 sheets.

Stuiver, Minze, and Polach, H.A., 1977, Discussion; reporting of 14C 

data: Radiocarbon, v. 19, no. 3, p. 355-363.

Wilson, R.C., and Keefer, D.K., 1983, Dynamic analysis of a slope 

failure from the 6 August 1979 Coyote Lake, California, earthquake: Seismological Society of America Bulletin, v. 73, no. 3, p. 

863-877.

———1985, Predicting areal limits of earthquake-induced landsliding, 

in Ziony, J.I., ed., Earthquake hazards in the Los Angeles region— 

an earth-science perspective: U.S. Geological Survey Professional 

Paper 1360, p. 317-494.

Working Group on California Earthquake Probabilities, 1990, Probabilities of large earthquakes in the San Francisco Bay region, California: U.S. Geological Survey Circular 1053, 51 p.

THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE

LANDSLIDES

ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN 

THE EPICENTRAL REGION, CENTRAL SANTA CRUZ MOUNTAINS,

CALIFORNIA

By William F. Cole, Dale R. Marcum, and Patrick O. Shires, 

Cotton, Shires and Associates, Inc.;

and

Bruce R. Clark, 

Leighton and Associates, Inc.

CONTENTS

	Page 

Abstract ----------------------------------------------------------------- C165

Introduction ——————————————————————————— 165

Topographic and geologic setting ——————————————— 166

Central Santa Cruz Mountains ——————————————— 166

Preexisting "ancient" landslides —————————————— 166

Landslide characterization ——————————————————— 169

Site selection ——————————————————————-— 169

Summary of subsurface conditions ———————————— 169

Regolith detachments———————————————— 169

Structural control on deeper failures ———————— 172

Key parameters in slope-stability analysis——————————— 172

Landslide geometry and ground-water conditions ———— 172

Earthquake ground motions ———————————————— 173

Unit weight and shear strength —————————————— 174

Methods of analysis —————————————————————— 175

Pseudostatic method ——————————————————— 175

Cumulative-displacement method ———————————— 175

Results —————————————————————— 176

Pseudostatic analysis ——————————————————— 176

Cumulative-displacement analysis ———————————— 177

Sensitivity analysis ————————————————————— 177

Conclusions ———————————————————————————— 181

Acknowledgments ———————————————————————— 183

References cited ———————————————————————— 183

Appendix: Description of landslide sites ——————————— 184

Lower Schultheis Road West landslide -————————- 184

Ditullio landslide ———————————————— 185

ABSTRACT

The reactivation of large landslides during the 1989 

Loma Prieta earthquake provided a unique opportunity to 

evaluate the seismic stability of preexisting landslides and 

to test the dynamic slope-stability methods currently used 

in geotechnical practice. For this study, we characterize 

two reactivated landslides, using the investigative techniques of geologic mapping, subsurface logging and sampling, and laboratory direct shear testing developed over 

decades of landslide investigation. We then compare actual landslide behavior during the earthquake with the results of several analytical methods, and test the sensitivity 

of key parameters in the calculations.

Results of both pseudostatic and cumulative-displacement analyses are less conservative than what actually 

occurred at the two landslide sites during the earthquake. 

We obtain a relatively high pseudostatic factor of safety 

of 1.1, using minimum shear strengths from direct shear 

tests and a seismic coefficient of 0.20. Cumulative-displacement calculations yield displacements within an order of magnitude of the 33 to 61 cm of displacement 

measured in the two landslides, generally lower than the 

actual field displacements.

Comparisons with actual measured landslide displacements show that the stability analyses are highly sensitive 

to the angle of internal friction, a parameter that is difficult to determine accurately in the laboratory. Backanalysis 

of the shear strength of the basal rupture surface for static 

stability can help define a realistic range of friction angles. 

The cumulative-displacement analyses are also sensitive 

to the acceleration-time history of ground motion at the 

two landslide sites. Models of earthquake-triggered landslide failure could be refined by using more rigorous physical models, but the resulting displacement calculations 

will not improve until earthquake ground motions at landslide sites can be better characterized.

INTRODUCTION

The combination of existing slope failures and a history of seismic ground shaking has led to the current practice of incorporating both static and seismic loading 

conditions into slope-stability studies in California. Although the analytical techniques and models used for staticC165
C166 LANDSLIDES

loading conditions have been standardized, uncertainty 

exists among practicing engineers and geologists regarding the accuracy of the seismic analytical methods. For 

typical residential- and commercial-development investigations, earthquake loading is simulated by applying a 

seismic coefficient to a static-limit-equilibrium calculation. The value of this coefficient is selected on the basis 

of experience and judgment; therefore, the values chosen 

to be appropriate for particular sites by various workers 

commonly differ. State-of-the-art slope-stability-analytical methods have been modified and used by scientists at 

the U.S. Geological Survey to predict hillslope behavior 

during earthquakes (Wieczorek and others, 1985; Wilson 

and Keefer, 1985); however, the ability of available analytical methods to predict the occurrence and amount of 

displacement of earthquake-triggered landslides is not well 

established. Very few case studies of actual earthquaketriggered landslides have been published as of this writing 

(1993) (for example, Clark and others, 1979; Wilson and 

Keefer, 1983; Keefer, 1991; Jibson and Keefer, 1993).

The 1989 M=7.0 Loma Prieta earthquake caused discrete ground cracking in the preexisting-landslide terrane 

of the central Santa Cruz Mountains. Much of this ground 

cracking occurred along preearthquake landslide boundaries, suggesting reactivation of old bedrock landslides in 

the region and providing an opportunity to compare the 

actual seismic stability of old landslides with the results 

of analyses for dynamic slope stability as currently conducted in geotechnical practice.

TOPOGRAPHIC AND GEOLOGIC 

SETTING

CENTRAL SANTA CRUZ MOUNTAINS

The Santa Cruz Mountains are part of the northern Coast 

Ranges of California. This rugged mountain range forms 

the spine of the San Francisco peninsula, extending approximately 135 km northwestward from the Pajaro River 

in the south to near the community of South San Francisco in the north (fig. 1). Elevations of ridgecrests range 

from about 450 to 1,150 m above mean sea level.

The study area lies between about 8 and 19 km south of 

the community of Los Gatos at elevations of 280 to 340 

m (fig. 1). The landslides chosen for study occurred in an 

area that underwent particularly intense ground cracking 

during the earthquake (fig. 2). The two sites selected for 

detailed study are situated 10 to 13 km northwest of the 

1989 main-shock epicenter.

The study area, which is from 1.5 to 3.0 km southwest 

of the mapped trace (that is, probable 1906 surface rupture) of the San Andreas fault zone, is underlain by tightly 

folded Eocene, Oligocene, and Miocene sedimentary rocks

(fig. 2). This Tertiary bedrock section consists of relatively soft, poorly indurated, marine clastic sedimentary 

rocks, about 7,600 m thick (Clark, 1981). In the study 

area, the rocks have been strongly folded and locally overturned into a series of northwest-southeast-trending anticlines and synclines. Specific Tertiary bedrock formations 

underlying the study areas are the Butano Sandstone, Vaqueros Sandstone, and San Lorenzo Formation.

PREEXISTING "ANCIENT" LANDSLIDES

The study area encompasses a widespread landslide terrane. In general, the geomorphology of this terrane is characterized by "stepped" topography, including very steep, 

curved scarps, flat-lying benches, and steep-walled, deeply 

incised creek canyons. Typically, several sequences of 

scarps with intervening flat-lying benches form the slope 

face between the ridgetops and creek canyons. Geomorphic features range in appearance from relatively fresh 

to subdued, strongly suggesting periodic movement of 

individual landslides within a large landslide complex. 

The large areal extent of these landslides and multiple 

scarps suggest large cumulative displacements of these 

features.

"Ancient landslides" are defined here to be landslides 

that originated thousands to tens of thousands of years 

ago, although the most recent movement may have been 

only years or decades ago. Indeed, geomorphic and subsurface geologic data suggest periodic and episodic reactivation of these landslides. Although the exact date of 

landslide initiation in the study area is unknown, other 

landslide complexes in California are associated with Pleistocene glacial and climatic changes during the past 100,000 

years (Stout, 1969).

The landslide masses are composed of weak earth materials derived from fractured, folded, and faulted Tertiary 

sedimentary rocks that have been subjected to periods of 

intense rainfall, ground-water fluctuations, and strong 

earthquake shaking. The sedimentary rocks consist primarily of fine-grained, thin-bedded, highly fractured shale, 

claystone, and siltstone that contain numerous shears, fractures, and other discontinuities. The resulting permeability of the near-surface materials enables them to collect, 

transmit, and store water from various sources (for example, rainfall, runoff, and lateral ground-water flow).

Figure 1.—San Francisco Bay region, Calif., showing location of study 

area (small shaded box), major faults (dashed where approximately 

located, dotted where concealed; half-arrows indicate direction of relative movement) and thrust faults (sawteeth on upper plate), and epicenter of 1989 Loma Prieta earthquake (star). Triangles, 

strong-motion-recording stations used in this study; LGPC, Los Gatos 

Presentation Center station.
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C167

EXPLANATION

Fault- Dashed where approximately located; 

dotted where concealed. Half arrows 

indicate direction of relative movement.

Strong motion recording stationLGPC, Los Gatos Presentation Center

Thrust fault- Sawteeth on upper plate 1989 Loma Prieta earthquake epicenter
C168 LANDSLIDES

.5 1.0 KILOMETER

EXPLANATION

TERTIARY BEDROCK UNITS

Tp Purisima Formation

Tl Lambert Shale

TV Vaqueros Sandstone

Ts San Lorenzo Formation

Tb Butano Sandstone

Preexisting landslide- Half arrows indicate 

direction of downslope movement.

* Reactivated-landslide study site

~CC 1989 coseismic ground fissures

— - - - - Contact- Dotted where concealed

>XV Strike and dip of bedding

—~ - • Fault- Approximately located; dotted where 

concealed.
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C169

LANDSLIDE CHARACTERIZATION 

SITE SELECTION

A general reconnaissance of more than 50 large earthquake triggered or reactivated landslides was performed 

after the 1989 Loma Prieta earthquake. Of the landslide 

sites, approximately 20 were selected for more rigorous 

inspection. Early inspections focused on identifying 

landslides that met the following criteria: (1) deep-seated 

failures that involved bedrock materials, (2) a well-defined pattern of ground cracks and surface displacements 

indicating probable reactivation of the basal rupture surface, (3) location within preexisting landslides or landslide complexes, and (4) terrain amenable to detailed 

surface and subsurface investigation and not so steep or 

heavily vegetated as to prevent drill-rig access. We also 

preferred sites where ground water was anticipated to be 

deeper than several meters, so that a geologist could safely 

enter and log large-diameter boreholes without the need 

for dewatering. Shallow ground water prevented full characterization of landslide parameters at 4 of the 20 selected 

sites.

Many of the earthquake-reactivated landslides appear 

to have been only partially mobilized, without shear failure along the entire basal rupture surface. The pattern of 

surface deformation at several sites, however, indicated 

that some landslides had been fully mobilized by the earthquake. Two of these landslides, here referred to as the 

Lower Schultheis Road West and Ditullio landslides, were 

characterized by a combination of field mapping and subsurface exploration. Large-diameter boreholes were excavated and logged by a geologist descending into the 

borehole to directly observe and sample landslide rupture-surface materials. Geologic and geotechnical conditions, deformational features, and the investigative 

procedures used at each site were described in detail by 

Cole and others (1991).

SUMMARY OF SUBSURFACE CONDITIONS

The preexisting-landslide masses are composed of three 

distinguishable rock and soil materials: (1) regolith, (2) 

sparsely to highly fractured bedrock units, and (3) clay 

shear and gouge material, including landslide rupture surfaces.

1. The regolith consists of oxidized and fractured rock and 

soil derived from the underlying bedrock that have been 

subjected to seismic shaking, cyclic changes in groundwater levels, and other weathering processes. Regolith at 

the landslide sites contains angular, weathered, gravel- to 

boulder-size sandstone and siltstone fragments in a matrix 

of clayey silt to silty sand. The upper approximately 4 m of 

regolith typically is a plastic silty clay.

2. Displaced sedimentary bedrock sequences of the San 

Lorenzo Formation are present at depth in the Lower 

Schultheis Road West and Ditullio landslides; the bedrock 

is highly fractured and oxidized above major shear surfaces at 7.3- and 18-m depth, respectively (figs. 3, 4). 

Below these shear surfaces, the bedrock is less fractured, 

denser, and relatively unoxidized. Interbedded sandstone 

and siltstone are present at the Lower Schultheis Road 

West landslide, and massive sandstone at the Ditullio 

landslide.

3. Thin seams of clayey gouge occur in the regolith and 

fractured bedrock at several landslide sites explored in the 

study area. The gouge is generally a wet silty clay with a 

soft to firm consistency and medium to high plasticity. The 

gouge seams are commonly from about 1 to 10 cm thick. 

Polished surfaces were observed in many of these seams. 

At the Lower Schultheis Road West landslide, several 

continuous and discontinuous gouge zones are present at 4- 

and 7-m depth. Thicker gouge zones are present within, 

and at the base of, the regolith. A relatively thin seam of 

gouge forms the contact between highly fractured rock and 

underlying unfractured rock. At the Ditullio landslide, a 2- 

to 5-cm-thick gouge seam separates oxidized, fractured 

rock from underlying unfractured rock at about 18-m 

depth; this gouge seam is associated with a zone of crushed 

siltstone and sandstone, as much as about 80 cm thick

(fig. 3).

The results of subsurface exploration indicate that the 

reactivated landslides occurred within preexisting landslide masses that are characterized by relatively coherent, 

rather than disrupted, landslide materials. Complex, 

shallow perched ground water and both shallow and 

deep preexisting rupture surfaces were penetrated. The 

shallow shear surfaces typically were observed at the base 

of the regolith at 5- to 15-m depth, and deeper shears 

were penetrated within the underlying fractured sedimentary bedrock. The primary geologic controls on these shear 

surfaces appear to be the regolith-bedrock contact and 

adversely oriented bedding planes and fractures in the 

bedrock.

Figure 2.—Geologic map of study area (see fig. 1 for location), showing 

locations of Lower Schultheis Road West (LSRW) and Ditullio landslides discussed in text. Geology from Clark and others (1989) and 

McLaughlin and others (1991); areas of preexisting landslides (shaded) 

from Cooper-Clark and Associates (1975) and this study; coseismic 

ground fissures from Spittler and Harp (1990).

REGOLITH DETACHMENT

On the basis of our downhole observations, the contact 

between overlying, oxidized regolith and underlying, 

unoxidized bedrock was commonly a well-developed shear
C170 LANDSLIDES

contact. The base of the overlying regolith is generally at 

5- to 15-m depth. The underlying bedrock, though part of 

a larger, ancient landslide mass, typically is unoxidized 

(or only locally oxidized) and closely fractured. The observed shear contact separating the two units is a saturated, clay-rich, soft, plastic gouge ranging from approximately 2 to 5 cm in thickness. At one of the landslides we 

explored but do not analyze here, a sheared water well

A

\

ANCIENT LANDSLIDE MASS

• 9A

\

REACTIVATED 

LANDSLIDE

\

\"

• 1

•3A

i=i 

T-5

EXPLANATION

1989 coseismic ground fissures- Dashed 

where approximately located. 

Hachures on downthrown side.

Strike and dip of bedding. 

Large-diameter borehole (this study)

Small-diameter borehole (Kingsley 

Associates, 1991)

Backhoe trench (Rogers Johnson and 

Associates, 1991)

T-4 10

_L

20 30 METERS

I

A Open ground fissure

340-

Topographic bulge 

(landslide toe)

ANCIENT LANDSLIDE MASS \

jj ^

SAN LORENZO FORMATION 

(interbedded sandstone and siltstone)

280

B N. 25° E.- 10 20 METERS

Figure 3.—Simplified geologic map (A) and cross section (B) of Lower Schultheis Road West landslide (see fig. 2 for location). Half-arrows 

indicate direction of downslope movement.
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C171

EXPLANATION

1989 coseismic ground fissuresHachures on downthrown side 

of vertical offset.

Large-diameter borehole (this study)

Cut slope

10 20 30 METERS

i ____ i

~ Sandstone, 

fractured, wet

330 - claV 9°u9e' 

plastic, moist

320 -

CO

£ 310

LU

~ 300

1

g; 290 _i

UJ

280 -

270 -

Clay gouge, gravelly, 

plastic, moist

Clayey siltstone, highly 

fractured, moist 

siltstone, crushed, 

highly sheared, moist

Sandstone, dense, 

unfractured, dry

Redwood Lodge Road

A'

Top of ridge 

Colluvium

Detailed log

LORENZO FORMATION

(interbedded sandstone and siltstone)

/

ANCIENT LANDSLIDE MASS

-\

B

N. 50° E.-

10 20 METERS

Figure 4.—Simplified geologic map (A) and cross section (B) of Ditullio landslide (see fig. 2 for location). Half-arrows indicate direction of 

downslope movement.
C172 LANDSLIDES

provided a piercing point on the reactivated rupture surface. A large-diameter borehole adjacent to the well revealed that this rupture surface coincided with the 

regolith-bedrock contact.

Although the geomorphology of the area is best explained by the presence of deep-seated landsliding, logging of large-diameter borings revealed that shallow 

landslide debris was much more disrupted than the deeper 

landslide material. The degree of development and fresh 

appearance of the shears separating the regolith from the 

underlying rock suggests that the Loma Prieta landsliding 

event mobilized these shears and that they had been mobilized frequently in the past as well.

also involved a much greater volume of the slope than the 

shallow ruptures observed at the regolith-bedrock contact. 

Such massive failures must have been generated by an 

event quite different in magnitude from the 1989 Loma 

Prieta earthquake. The largest measured displacements in 

this earthquake were less than 3 m (Keefer, 1991). Thus, 

it apparently reactivated only shallow segments of relatively small areal extent within deep-seated, large preexisting landslide masses.

KEY PARAMETERS IN SLOPESTABILITY ANALYSIS

STRUCTURAL CONTROL ON DEEPER FAILURES

Bedding-plane shears were observed at the Lower 

Schultheis Road West landslide and in trenches excavated 

in the headscarps of other landslides in the study area 

(Cotton and others, 1991; Keefer, 1991). An interpretation that old landslides failed repeatedly along a preexisting bedding-plane shear is supported by the difference 

in physical characteristics between the overlying landslide debris and underlying bedrock at the Lower 

Schultheis Road West landslide (fig. 4). Regional geologic data suggest that the upslope parts of several other 

landslides may be underlain by bedding planes dipping 

steeply downslope.

For the landslides to have fully failed in the past, the 

basal surfaces must eventually break out to the slope surface, whereas the bedding planes likely remain steeply 

dipping. Therefore, rupture surfaces that coincide with 

bedding planes in the upslope (headward) parts of a landslide mass dip less steeply than these bedding planes beneath much of the landslide mass. The presence of steeply 

dipping bedding planes could help to explain how many 

slope failures from the 1989 Loma Prieta earthquake were 

restricted to only the steeply dipping, upslope components 

of the landslides. Many of these landslides, however, do 

not appear to have been influenced by dip-slope conditions (D.K. Keefer, oral commun., 1992); thus, the limited movement of the landslides must be explained by 

other mechanisms.

Subsurface geologic evidence does not demonstrate conclusively whether the landslides were caused originally 

by seismic activity or by excessive water during wetter 

climatic conditions, or by some combination of these two 

mechanisms. Subsurface exposures, however, confirm the 

presence of multiple rupture surfaces of quite different 

geometry and stratigraphic position, arguing for the existence for multiple rupturing events since the landslides 

first formed. The deep-seated rupture surfaces that incorporated relatively fresh bedrock into the landslide mass

The key parameters in slope-stability analysis include 

landslide geometry, generally analyzed as a representative cross section; ground-water levels or pore-water pressures acting on the landslide, generally considered as a 

phreatic surface; the intensity of earthquake ground motion at the site, typically inputted in the form of a single 

seismic coefficient or an appropriate acceleration-time history; the unit weights of the landslide materials; and an 

average shear strength along the basal rupture surface.

LANDSLIDE GEOMETRY AND GROUND-WATER 

CONDITIONS

The geometry and ground-water parameters were determined directly from field measurements. Geologic cross 

sections were carefully constructed through the landslides 

on the basis of field measurements. Establishing that a 

landslide had actually moved downslope, in contrast to 

simply forming a scarp by settlement of loose graben debris, required careful examination but could be verified 

by the presence of sheared flanks or compressional-toe 

areas. Determining landslide depth is complicated by the 

fact that multiple shear surfaces were observed in boreholes penetrating the preexisting-landslide masses. In the 

absence of well-defined subsurface piercing points, the 

selection of which of the multiple shear surfaces was reactivated involves careful correlation between borehole 

logs and surface deformational features, as well as reasonable assumptions regarding landslide behavior. Only 

one well-developed shear surface was penetrated in the 

Ditullio landslide, thus making the determination of landslide depth straightforward. The selection of the activated 

Lower Schultheis Road West landslide rupture surface was 

based on the fresh appearance and continuity of the regolith-bedrock contact, correlation with the headscarp fissures and downslope zone of compression, and 

observations from other similar types of landslides in the 

study area.
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C173

The ground-water levels shown on the cross sections 

(figs. 3, 4) were those penetrated during subsurface exploration. Because the 1989 Loma Prieta earthquake occurred near the beginning of the 1989-90 rainy season 

during a period of prolonged drought, the conditions in 

boreholes are believed to represent ground-water conditions immediately before the earthquake. We realize that 

changes in water-well levels and surface streamflow after 

the earthquake suggest that trapped ground water may 

have been freed by fracturing during the earthquake and 

that water levels may have dropped significantly 

(Rojstaczer and Wolf, 1992). However, perched water in 

the analyzed landslides is unlikely to exit through new 

fractures extending into the landslide debris. Furthermore, 

we know of no reports or evidence of ground water flowing to the surface from within these small landslide masses 

after the earthquake. Nonetheless, we performed sensitivity analyses to test the effects of changing ground-water 

levels on the stability analyses (see subsection below entitled "Sensitivity Analysis").

EARTHQUAKE GROUND MOTIONS

Earthquake ground motions can be entered into the displacement calculations from published maps (K.L. Lee, 

unpub. data, 1977; Makdisi and Seed, 1977; Hynes-Griffin and Franklin, 1984; Lin and Whitman, 1986), as well 

as directly from acceleration-time histories recorded on 

strong-motion instruments located near the study area. We 

used data from the Corralitos station, part of the California Division of Mines and Geology's California Strong 

Motion Instrumentation Program (CSMIP), and the Los 

Gatos Presentation Center station (LGPC, fig. 1), operated by the Charles F. Richter Seismological Laboratory 

of the University of California, Santa Cruz. The Corralitos 

and LGPC stations are approximately 7 and 19 km, respectively, from the epicenter (fig. 1).

The intensity of ground motion at a particular site is 

influenced by several factors, including earthquake magnitude, epicentral distance, local geologic structure, source 

and rupture mechanism of the earthquake, wave-interference effects, geotechnical site conditions, and topographic 

amplification. The selection of appropriate accelerationtime histories for seismic stability is generally problematic because most landslide sites are at great distances 

from actual recording stations and the conditions at these 

sites generally differ from those at the recording stations. 

The large magnitude and complex rupture mechanism of 

the 1989 Loma Prieta earthquake, in combination with 

extremely irregular topographic conditions, indicate that 

ground motion in the study area may have varied considerably. Thus, selection of appropriate ground-motion parameters at the landslide sites involved an assessment of 

the site conditions at the two closest recording stations

(fig. 1). Several studies indicate that the earthquake had a 

strong azimuthal effect on ground motion (Beroza, 1991; 

Steidl and others, 1991; Wald and others, 1991). The earthquake ruptured bilaterally, and so the region northwest of 

the epicenter (including the LGPC station and both landslide sites) was influenced by northwest-propagating rupture, whereas the region southeast of the epicenter 

(including the Corralitos station) was more affected by 

southeast-propagating rupture (table 1). Furthermore, rupture was predominately reverse slip to the northwest of 

the hypocenter and predominately strike slip to the southeast. According to Wald and others, the bilateral rupture 

resulted in a larger overall stress drop and correspondingly higher ground motion to the northwest of the epicenter, and lower ground motion directly updip of the 

hypocenter near the Corralitos station. Campbell (1991) 

suggested that the northwest-trending geologic structure 

may explain the higher ground motion and lower attenuation rate northwest of the epicenter.

The Corralitos station (fig. 1) recorded a peak horizontal ground acceleration, &max, of 0.63 g (California Division of Mines and Geology, 1990), whereas the LGPC 

station appears to have recorded a /:max value of greater 

than 1.00 g (off scale). Because the recorder was not firmly 

anchored to the floor and an apparent baseline offset occurred during the strong ground shaking, seismologists 

have cautioned that the data from the LGPC station may 

not be a reliable indicator of the actual ground motion at 

this site (Karen McNally, oral commun., 1991). For the 

purposes of our analysis, however, the baseline correction 

for this offset (approx 0.03 g north-south), was considered insignificant relative to the /:max value. The amplitude of long-period waves on the corrected 

acceleration-time history for the LGPC station is similar 

to the modified record from the Corralitos station scaled 

to 1.00 g (fig. 5).

The site conditions at the LGPC station (fig. 1) are 

thought to more closely resemble those at the landslide 

sites than do those at the Corralitos station because (1) 

the LGPC station and the landslide site are at similar distances from the San Andreas fault and the Loma Prieta 

epicenter, (2) topographic conditions at the LGPC station 

and the landslide sites are similar in terms of slope 

position and elevation, and (3) the LGPC station is located along the same azimuth from the epicenter (320°) 

as the two landslide sites. In the absence of recorded data 

in the immediate vicinity of the landslide sites and because of very high ground motions in the surrounding 

region, the acceleration-time history of the LGPC station 

was selected initially to represent "bedrock" ground 

motions at the landslide sites during the earthquake. We 

also used the Corralitos station record, both unsealed 

(fcmax=0.63 g) and scaled (fcmax=1.00 g), to test the 

sensitivity of the calculations to ground motion (see subsection below entitled "Sensitivity Analysis").
C174 LANDSLIDES

Table I.—Distance of two strong-motion stations used in this study from earthquake and landslide 

features

[The Lower Schultheis Road West and Ditullio landslides are about 12 and 11 km, respectively, northwest (az 

320°) of the epicenter of the 1989 Loma Prieta earthquake. Both landslides are about 2 km southwest of the 

Loma Prieta rupture zone, as measured from the San Andreas fault zone]

Distance (km) to:

Strong-Motion 

station 

(fig- 1)

Corralitos

(elev., 320 m).

Loma

Prieta 

epicenter 

(azimuth)

7.0 (080°)

San 

Andreas 

fault

0.3

Loma

Prieta 

rupture 

zone

0.3

Lower Schultheis

Road West 

landslide 

(elev., 335 m)

16.4

Ditullio 

landslide 

(elev., 351 m)

15.9

Los Gatos 19.0 (320^ 

Presentation Center 

(elev., 366 m).

1.4 1.5 7.0

UNIT WEIGHT AND SHEAR STRENGTH

Z 0.5 

O

il o.o

aLU 

O

< -0.5

1.0

0.5

0.0

-0.5

34567 

TIME, IN SECONDS

B

01 23456789 10 

TIME, IN SECONDS

Figure 5.—Corrected north-south component of motion on strong-motion records of October 17, 1989, main shock from California Division 

of Mines and Geology station at Corralitos (A) and University of California, Santa Cruz, station at Los Gatos Presentation Center (B) (see fig. 

1 for locations). Record in figure 5A is scaled to 1.0 g.

Moisture content and density were determined in the 

laboratory on selected undisturbed samples of the landslide mass by measuring the weight and volume of material collected and sealed in brass tubes. For the Lower 

Schultheis Road West landslide, we identified two 

geotechnical units in the landslide mass. The soil (upper 

unit) has a moist unit weight of 2.06 g/cm3 and a saturated unit weight of 2.16 g/cm3 ; the regolith (lower unit) 

has a moist unit weight of 1.96 g/cm3 and a saturated unit 

weight of 2.08 g/cm3 . For the Ditullio landslide, only one 

geotechnical unit was identified, with a moist unit weight 

of 1.92 g/cm3 .

Shear strengths were determined initially by laboratory 

measurements on samples collected from subsurface rupture zones. Backcalculations were subsequently performed 

under static-loading conditions for comparison with the 

laboratory results. Collecting representative and oriented 

samples of landslide rupture surfaces is a difficult task. 

To obtain the best possible samples, we collected tube 

samples of the landslide rupture surfaces from largediameter boreholes. At each sample location, a geologist 

hand-excavated a horizontal shelf into the landslide material several centimeters above and parallel to the shear 

surface. Tube samples were pushed and driven through 

the landslide debris, through the shear surface, and into 

the underlying materials. The samples were sealed and 

transported to our laboratories for various direct shear 

tests (consolidated drained and consolidated undrained, 

peak, and residual) on "undisturbed" and remolded 

samples.

Because of the rapid failure of the landslides, we chose 

consolidated-undrained ("quick") direct shear tests, rather 

than triaxial strength testing, to represent the loading con-
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C175

ditions imposed by the earthquake. Although some drainage can occur during direct shear testing, the permeability 

of the gouge material from these landslides is very low, 

and the sample does not have sufficient time to drain 

during the relatively rapid test. Triaxial tests can measure 

pore pressures, but shear surfaces must be oriented at an 

angle to the sample axis to avoid stresses perpendicular to 

the shear surface. In addition, direct shear tests can be 

performed on relatively short samples, whereas triaxial 

samples must be about twice as long as the diameter. 

Thus, the triaxial procedure requires a longer sample, 

which involves more disturbance during sampling and is 

difficult where thin, soft gouge material lies between hard, 

brittle rock. We favor direct shear tests over triaxial tests 

for these reasons and because they offer the best chance 

for shearing the actual rupture surface in a similar mode 

of failure to that in the landslide.

The residual shear strengths measured in direct shear 

tests are approximately 70 to 75 percent of the peak values. This result approximates the reduction to 80 percent 

of peak shear strength estimated by Makdisi and Seed 

(1977) and recommended by Hynes-Griffin and Franklin 

(1984), owing to cyclic-loading effects. Therefore, we used 

the residual shear strengths to characterize the cyclic 

strength of the basal rupture surfaces. Two measures of 

residual shear strength, lower-bound and average or "best 

fit" curves, were drawn by linear regression through the 

data points (fig. 6). The shear-strength parameters, in terms 

of angle of internal friction (0) and cohesion (c), that 

describe the lower-bound and best-fit values are 0=25°, 

c=0 kg/cm2 and 0=24°, c=0.22 kg/cm2, respectively.

METHODS OF ANALYSIS

PSEUDOSTATIC METHOD

Pseudostatic analysis is a standard method of assessing 

seismic slope stability in the geotechnical industry. This 

method involves analyzing landslides by using a traditional method of limit-equilibrium-stability analysis, such 

as Spencer's or Bishop's method (Chowdhury, 1978; 

Huang, 1983). In addition to the static forces acting on 

the landslide body, a static force proportional to the weight 

of the landslide mass is then added as a permanent horizontal force to simulate the dynamic force of the earthquake. The magnitude of this force is the product of the 

weight of the landslide body and a seismic yield coefficient, k.

The k value significantly affects the results of the 

pseudostatic analysis, yet traditionally it has been arbitrarily chosen on the basis of experience and judgment. 

Lambe and Whitman (1969) stated that k values of 0.1 to 

0.2 are commonly assumed. K.L. Lee (unpub. data, 1977) 

indicated that the k value may fall in a somewhat broader 

range of 0.05-0.25. Chowdhury (1978) suggested that k 

values of 0.10 to 0.15 are typically used in the United 

States, whereas Huang (1983) suggested a k value of 0.27 

for design in western California. A k value of 0.15 is 

currently required by the California Division of Safety of 

Dams for analysis of earthfill dams. In California, k values of more than 0.20 are rarely used in practical design 

for residential and commercial sites. The diversity in chosen k values reflects the uncertainty that engineers have 

with regard to the pseudostatic approach and the selection 

of design-level earthquakes.

DC 2.0

LJJ

LJJ

S

il 1 '5

UJ D

fo

DC LLJ

W

1.0

Best fit 0 = 24° 

c = 0.22 kg/cm2

Lower bound

0 = 25°

c = 0 kg/cm2

0.5 1.0 1.5 2.0 2.5

NORMAL PRESSURE, 

IN KILOGRAMS PER SQUARE CENTIMETER

Figure 6.—Residual shear strength versus normal pressure in laboratory 

consolidated-undrained tests on landslide rupture surface material.

CUMULATIVE-DISPLACEMENT METHOD

As recognized by many previous workers, the 

pseudostatic method of analysis has some significant limitations. Newmark (1965) noted that the transitory vibrations of earthquake ground motions could cause the factor 

of safety (FS) of a slope to temporarily drop below 1 

several times during a seismic event. An accurate 

pseudostatic analysis of this situation would indicate failure. However, if the drop in the FS value is so transient 

that the induced deformations are small and the slope is 

essentially unchanged, then the term "failure" may not 

accurately describe the actual condition of the slope.

A method of measuring incremental displacements of 

the slope during an earthquake was developed by Newmark 

(1965) as an alternative to the pseudostatic approach for 

earth embankments. Commonly referred to as "Newmark 

analysis," "cumulative-displacement analysis," "permanent-deformation analysis," and "sliding-block analysis,"
C176 LANDSLIDES

this method sums the downslope movements of the landslide during each cycle of high acceleration to compute a 

cumulative displacement. Thus, this method evaluates performance rather than stability. The magnitude of the computed displacement can be used to assess the degree of 

damage. For example, if the displacements are several 

meters, deformation of the slope probably will be heavy; 

conversely, if the computed displacements are less than a 

few centimeters, deformation of the slope will be slight. 

This method requires a complete acceleration-time history 

as input, as well as a value for the yield acceleration (k ) 

of the landslide, defined as the ground acceleration required to bring the FS value to 1 in a pseudostatic analysis.

A basic assumption of the cumulative-displacement 

method is that the landslide mass behaves as a rigid body 

and can be modeled as a friction block on an inclined 

plane. Under static conditions, the block rests on the inclined plane without sliding. As the inclined plane is 

shaken, the block moves with the plane (without slipping) 

until the acceleration of the plane exceeds the k value of 

the friction surface, at which point the block begins to 

slide. When the acceleration of the plane decreases below 

the k value, the block continues to move because of momentum but decelerates because of friction until movement stops. The amount of displacement of the block is a 

function of the elapsed time while the k value was exceeded and the magnitude by which the k value was 

exceeded by the shaking. The displacement of the block 

during an acceleration pulse is then calculated by integrating the area under the velocity curve. Additional displacements caused by subsequent pulses of shaking above 

the k value are summed to give the cumulative displacement of the block.

For design problems, investigators wishing to estimate 

landslide displacements commonly use published curves 

or graphs of the integrated displacements determined for 

historical earthquakes of different magnitudes by previous workers, such as Makdisi and Seed (1977) or HynesGriffin and Franklin (1984). These curves were constructed 

by double-integrating acceleration-time histories as a function of the k value. Makdisi and Seed integrated the average acceleration-time histories calculated for 

embankments of varying heights that had been subjected 

to a range of earthquake-induced base accelerations. 

Hynes-Griffin and Franklin integrated the horizontal components of 348 actual and 6 synthetic earthquake acceleration-time histories. The investigator typically enters a 

curve with the k value of the landslide and the expected 

peak ground acceleration, and arrives at an estimate of the 

landslide displacement. Although these curves are relatively simple to use, they are by no means consistent with 

each other. A comparison of several widely used curves 

by Jibson (1993) demonstrated that the displacements estimated from different curves can vary by a factor of as 

much as 100.

Another method of using cumulative displacement during design to predict landslide movement is to select an 

actual earthquake acceleration-time history that could affect the site. The part of the acceleration record above the 

k value is then double-integrated to arrive at a displacement estimate. For this study, we chose to use the largestamplitude seismic record in the study area and a convenient 

computer program, DISPLMT (Houston and others, 1987), 

to carry out the computations. The effects of using various seismic records are discussed in the subsection below 

entitled "Sensitivity Analysis."

RESULTS

In our analysis of the Lower Schultheis Road West and 

Ditullio landslides, we approached the seismic-stability 

problem in two different ways. First, we calculated the 

pseudostatic stability and predicted displacements, using 

the parameters measured during our site investigation. For 

a typical geotechnical site investigation, either the best-fit 

or lowest shear strength determined from laboratory measurements would be used. This approach actually represented a hindcast, because accelerograph records from the 

1989 Loma Prieta earthquake (unavailable for a true prediction before the earthquake) were used rather than a 

record already on file before the earthquake. Second, we 

tested the sensitivity of the displacement results to reasonable variations in the shear-strength parameters. We 

analyzed the FS and k values by Spencer's method with 

the computer program PCSTABL developed at Purdue 

University (Carpenter, 1985).

PSEUDOSTATIC ANALYSIS

The pseudostatic FS and k values for the Lower 

Schultheis Road and Ditullio landslides, calculated from 

the laboratory-determined residual shear strengths, are 

listed in table 2.

FS values of about 1.1 are calculated for both sites if 

the lower-bound residual shear strengths and a relatively 

high k value (that is, 0.20 g) are used. In current 

geotechnical practice, an FS value of 1.1 is generally considered acceptable for the seismic stability of engineered 

slopes. Because the shear strengths used represent residual, 

rather than peak, values, some geotechnical practitioners 

would consider this analysis to be overly conservative. In 

practice, the best-fit, rather than the lower-bound, strengths 

might have been selected. These best-fit strengths would 

have resulted in computed FS values of 1.2 to 1.4, indicating considerable stability of both slopes under dynamic 

loading. Thus, our results demonstrate that, using the laboratory-determined residual shear strengths, the pseudostatic
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C177

Table 2.—Pseudostatic factors of safety (FS) and yield coefficients (k ) 

in the Lower Schultheis Road West and Ditullio landslides, calculated 

from laboratory-determined residual shear strengths

FS

for fc v=0.20 for FS=1.0

Lower Schultheis Road West landslide

Lower-bound shear strength..................... 1.1

Best-fit shear strength................................. 1.4

0.23

.35

Ditullio landslide

Lower-bound shear strength..................... 1.1

Best-fit shear strength ................................. 1.2

0.22 

.30

analysis would not have predicted the slope failures even 

if the lower-bound values were used. However, the FS 

value drops below 1.0 when a k value of about 0.25 g is 

used.

CUMULATIVE-DISPLACEMENT ANALYSIS

To estimate the displacements, we first used the curves 

of Makdisi and Seed (1977) and Hynes-Griffin and 

Franklin (1984), and then double-integrated a record from 

the 1989 Loma Prieta earthquake. The curves were entered with the k values calculated by the pseudostatic 

method, using kmax=l.O g. Makdisi and Seed presented 

upper- and lower-bound curves for M=6.5, 7.5, and 8.5 

earthquakes. Upper- and lower-bound curves were interpolated for a theoretical M=7.0 event. The k values listed 

in table 3 are derived from an interpreted geometric-mean 

curve drawn between the upper- and lower-bound M=7.0 

curves. Hynes-Griffin and Franklin presented mean, 

meanila, and upper-bound curves; the results listed in 

table 3 are from their mean curve.

Table 3 shows that the curves of Makdisi and Seed 

(1977) predict displacements in reasonable agreement with 

those measured after the earthquake, using the lower-bound 

residual shear strengths to characterize the landslide rupture surface. If the best-fit residual shear strengths are 

used, the predicted displacements are too low but are still 

within the same order of magnitude as those observed. 

The displacements calculated using the mean curve of 

Hynes-Griffin and Franklin (1984) underpredict the displacements by almost an order of magnitude, using the 

lower-bound residual shear strengths.

The results of the double-integrated acceleration-time 

history for the LGPC station, using the computer program 

DISPLMT of Houston and others (1987), are also listed

in table 3. The k value can be inputted as a constant, as a 

function of time, or as a function of displacement. Uphill 

movements of the landslide block are accounted for by 

assuming a relation between the upslope and downslope 

k values and the static FS value. Most accelerograms are 

asymmetric, and the program accounts for this feature by 

calculating displacements twice, assuming that each side 

acts downslope, and averaging the results. For simplicity, 

a constant k value was used.

The cumulative-displacement method predicts landslide 

displacements within an order of magnitude of the measured displacements, using the curves of Makdisi and Seed 

(1977). The curves of Hynes-Griffin and Franklin (1984) 

and the integrated record from the LGPC station (fig. 1) 

both predict displacements within the range of those measured, but only when the lower-bound residual shear 

strengths are used. When the best-fit residual shear 

strengths are used (corresponding to k =0.30-0.35 g), the 

predicted displacements are an order of magnitude less 

than those observed and an order of magnitude less than 

those calculated using fcy=0.22-0.23 g. This result indicates that the displacements calculated by integrating the 

record from the LGPC station are highly sensitive to the 

k value and thus highly sensitive to the shear strengths 

chosen for the landslide rupture surface. The results of 

cumulative-displacement calculations for the two reactivated landslides, using five different analytical methods, 

are listed in table 4. The method of K.L. Lee (unpub. 

data, 1977) produces the most conservative results. However, the median displacements from all methods range 

from 10 to 100 cm for fcmax=1.00 g, which is as close as 

we should expect from the level of uncertainty in the k 

value and shear-strength parameters, and the sensitivity of 

the predicted displacements to these measurements.

SENSITIVITY ANALYSIS

The sensitivity of the calculated displacements to variations in both shear strength and acceleration-time history 

is apparent from various analyses. The dependence on 

shear strength is of great concern because of the difficulty 

in obtaining well-constrained values. The shear strength 

of the rupture-surf ace materials can be calculated in two 

somewhat-independent ways: (1) by sampling and testing 

actual gouge materials in the laboratory and (2) by 

backanalyzing the landslide to determine the shear 

strengths required for failure.

We know that the present geometry of the landslide is 

stable under current ground-water conditions, which provides a minimum shear strength for the gouge. If we assume that the landslide would be unstable when ground 

water filled it to the surface, then a backanalysis of that 

saturated condition would provide a maximum shear 

strength for the gouge. An argument can be made that this
C178 LANDSLIDES

Table 3.—Seismic yield coefficients (ky) and displacements in the Lower Schultheis Road West and 

Ditullio landslides, calculated from laboratory-determined residual shear strengths

[All values assume a peak horizontal ground acceleration of 1.0 g. Shear-strengths: 0=25°, c=0 kg/cm2 for lowerbound values and 0=24°, c=0.22 kg/cm2 for best-fit values, where 0 is the angle of internal friction and c is the 

cohesion. Mean 1, geometric mean from Makdisi and Seed (1977); mean 2, mean from Hynes-Griffin and 

Franklin (1984), with displacements below 10 cm not plotted]

Displacement (cm)

for FS=1.0

Mean 1 Mean 2

Program 

DISPLMT Actual

Lower Schultheis Road West landslide

Lower-bound

Best-fit

.....0.23

35

36 10

17 <10

23

1

61

61

Ditullio landslide

Lower-bound 

shear strength..... ...... 0.22 38 10

Best-fit 

shear strength............. .30 24 <10

29 33-57 (avg 45)

3 33-57 (avg 45)

assumption is reasonable on the basis of the widespread 

presence of shallow slope failures within this terrain in 

the general vicinity during periods of very heavy rainfall. 

We have no evidence that the Lower Schultheis Road 

West or Ditullio landslide failed statically during recent 

years; however, we know that laboratory shear strengths 

generally are at the high end of the range. Thus, these two 

approaches can give us some confidence about the range 

of shear strengths that should be considered for these two 

reactivated landslides.

Representative samples of the material from rupture 

zones are generally available only if a geologist retrieves 

them by hand from within a large excavation, such as a 

trench or large-diameter borehole. Rupture zones are seldom captured intact in drill-core samples because they are 

softer and more clay rich than the surrounding rock. During coring, the gouge materials are commonly destroyed 

by small amounts of relative movement between the core 

segments above and below the shear surface, and the gouge 

can be washed away by circulating fluid.

When a sample is tested in the laboratory, precautions 

are needed to prevent disturbance of the gouge. Direct 

shear testing is generally favored because the thin rupture 

zone can be aligned with the shear surface to ensure that 

the gouge material itself is tested, rather than the harder 

rock above or below the rupture zone. As a result of the 

uncertainties associated with laboratory results, it is not 

uncommon during postfailure landslide studies to discover 

that laboratory-determined shear strengths overestimate the 

strengths computed for the landslide surface from 

backanalyses of landslide stability.

Backanalyses for stability also need to be evaluated 

carefully. A backanalysis based on a seismic failure is a 

circular argument, because we test our ability to predict 

seismic failure from independent measurements of key 

parameters, including shear strength and seismic ground 

motion. However, if we backanalyze a static (that is, 

ground-water driven) failure, we gain some insight into 

the range of possible shear strengths of the rupture-surface materials.

We backanalyzed the Lower Schultheis Road West and 

Ditullio landslides under static conditions to determine a 

reasonable range of shear strengths for examination in our 

sensitivity analysis. The backanalyses assume that the landslides failed under static conditions at some time in the 

past and that the rupture surfaces had reached their residual strengths (that is, c=0). The range of 0 values that 

would satisfy the backanalysis for possible ground-water 

levels is plotted in figure 7. This range was the basis for 

selecting angles of 17°-25° in our cumulative-displacement analyses.

Cumulative displacements as a function of the angle of 

internal friction, 0, and the yield acceleration, k , of the 

two reactivated landslides for a peak horizontal ground 

acceleration of 1.00 g are plotted in figure 8, which shows 

the results using the curves of K.L. Lee (unpub. data, 

1977), Makdisi and Seed (1977), Hynes-Griffin and 

Franklin (1984), and Lin and Whitman (1986), as well as 

from the program DISPLMT using the Corralitos and 

LGPC station (fig. 1) records scaled to &max=1.0 g.

Figure 8 demonstrates that the displacements calculated 

by all five methods are sensitive to the chosen 0 value.
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C179

Table 4.—Summary of cumulative-displacement results using five analytical methods for the Lower Schultheis Road West and Ditullio landslides

[k , seismic yield coefficient; fcmax, peak horizontal ground acceleration; M, earthquake magnitude; static FS, static factor of safety (3.6 for Lower Schultheis Road West 

landslide, 2.9 for Ditullio landslide); LSRW, Lower Schultheis Road West. Shear strengths for Lower Schultheis Road West landslide: 0=25°, c=0 kg/cm2 , £y=0.23 g for 

lower-bound values and 0=24°, c=0.22 kg/cm2 , k =0.35 g for best-fit values, where 0 is the angle of internal friction and c is the cohesion. Shear strengths for Ditullio 

landslide: 0=25°, c=0 kg/cm2, ^=0.23 g for lower-bound values and 0=24°, c=0.22 kg/cm2 , fcy=0.30 g for best-fit values]

Analytical method Input parameter Value Predicted displacements (cm)

Ditullio LSRW 

landslide landslide

ky for static FS Lower 

bound

Best 

fit

Lower 

bound

Corralitos station 

record

Program DISPLMT (Houston and

others, 1987): Double integration

of accelerations above ky (average (fcrnax=0-63 g)

of two runs with accelerations

Average of two runs

Best 

fit

Makdisi and Seed (1977), 

with Keefer and others (1991) 

for M= 7.0.

K.L. Lee (unpub. data, 1977) -----

Lin and Whitman (1986): Provides 

expected values, implying not 

especially conservative values.

Hynes-Griffin and Franklin (1984)

Field measurements -----------

V-UrrcUHU£> blcUlUIl ----- __________

record

Station LGPC record ----------------

(*max=1.00 g)

ky, fcmax-1-00 g. Upper bound ------ 

M=7.0 

Lower bound ------

Geometric mean - - - -

ky, fcmax =l-00&/ Upper bound ------ 

M=7.1 

Lower bound ------

Geometric mean - - - -

subsurface 

conditions Deep-cohesionlesssoil and stiff -soil 

sites.

ky, fcmax=l-00 gf Upper bound ------ 

confidence level 

Mean ?-----------

Mean ------------

29

82 

15 

37

179 

45

77

84 

67

42

73 

21 

11 <

3

50 

9 

22

97 

24 

42

42 

34

21

47 

12 

:10

33-57 

(avg 45)

23

76 

14

35

160 

40 

69

77 

62

38

68 

19 

10

61

1

40 

6 

17

68 

17 

29

29 

22

14

37 

<10

The curves of K.L. Lee (unpub. data, 1977) and Lin and 

Whitman (1986) are the most conservative, yielding predicted displacements near or slightly greater than the observed displacements when lower-bound laboratory shear strengths (c=Q, 0=25°) are used. The other methods predict displacements lower than the observed displacements
C180 LANDSLIDES

under the same conditions. However, these results also 

indicate that the laboratory shear strengths generally predict displacements lower than those actually measured for 

a seismic event as large as the 1989 Loma Prieta earthquake. The two program DISPLMT analyses for each landslide provide the clearest examples: The best comparison 

of predicted-to-observed displacements are for 0=18°-22° 

for the Lower Schultheis Road West landslide and 0=21°- 

24° for the Ditullio landslide. Use of the lower-bound 

laboratory shear strengths leads to predicted displacements 

that are about two-thirds less than those measured on the 

Lower Schultheis Road West landslide and about one-half 

less than those measured on the Ditullio landslide. The 

program DISPLMT results for the Corralitos and LGPC

6 Ground-water 

levels in meters 

below surface

Range of ground-water 

levels that may induce 

static failure

V19°,

5 10 15 20 25 30 

ANGLE OF INTERNAL FRICTION, IN DEGREES

35

•5 

ft

DC i

I

B

Ground-water levels in 

meters below surface

9112

Range of groundwater levels that 

may induce static 

failure

Y27°

10 15 20 25 30 

ANGLE OF INTERNAL FRICTION, IN DEGREES

35

Figure 7.—Factor of safety versus shear strength (expressed as angle of 

internal friction) at various ground-water levels in Lower Schultheis 

Road West (A) and Ditullio (B) landslides (see fig. 2 for locations).

station (fig. 1) records also differ markedly, demonstrating that displacements calculated by using integration 

methods depend heavily on the selected acceleration-time 

history.

Although we believe that ground motion at the landslide sites was much greater than that recorded at the 

Corralitos strong-motion station (fig. 1), we also calculated displacements by using the unsealed accelerationtime history at this station to determine the sensitivity of 

the results to this seismic record. Cumulative displacements calculated using five analytical methods for 

£max=0.63 g are plotted as a function of 0 value in figure 

9. A comparison of figures 8 and 9 demonstrates the importance of the acceleration-time history and peak ground 

acceleration on the calculations. Using the scaled Corralitos 

station record (£max=1.00 g), the measured displacements 

correspond relatively well to those calculated from the 

program DISPLMT; however, the unsealed Corralitos station record (&max=0-63 g) yields significantly smaller displacements for the same shear strengths.

The displacements that can be calculated for an M=7.0 

earthquake, using the three curves of Makdisi and Seed 

(1977) and Hynes-Griffin and Franklin (1984) for the 

Lower Schultheis Road West landslide, are plotted in figure 10. Because of the bilateral rupture, the 1989 Loma 

Prieta earthquake was of considerably shorter duration than 

most other earthquakes of similar magnitude. Therefore, 

we might expect the measured displacements to fall in the 

lower part of the range of calculated displacements. Figure 10 shows, however, that they do so only for the smallest 0 values. If we assume that the appropriate 0 value 

should be about 21°, then the measured displacements fall 

in the middle of the range of displacements in the data 

sets of both Makdisi and Seed (1977) and Hynes-Griffin 

and Franklin (1984).

In summary, the mean to upper-bound curves for the 

four chart methods, as well as the DISPLMT integration 

method, appear to predict displacements that are within 

an order of magnitude of the displacements measured on 

the two reactivated landslides when a £max value of 1.00 g 

is used in conjunction with low shear strengths. It is remarkable, however, how broad the range of calculated 

displacements can be for an earthquake of a specific size. 

Adaptation of specific strong-motion records to the actual 

field conditions at a particular site, to account for the 

effects of attenuation, distance, topography, or directivity, 

is subject to uncertainties that can change the predicted 

displacements by a factor of at least 3. For example, the 

displacements calculated using the unsealed Corralitos 

and LGPC station records scaled to the same peak acceleration (£max=1.00 g) vary by a factor of as much as 

2 (fig. 8). At this time, it is unclear how the specific 

record should be chosen or scaled (if at all) for local 

conditions.
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C181

CONCLUSIONS

1. Preexisting landslides commonly were only partially 

reactivated. The pattern of ground cracking within preexisting ancient landslides indicates that only part of many 

preexisting landslides were mobilized by the ground motion from the 1989 Loma Prieta earthquake. In many 

places, ground cracks coincided with preexisting-landslide boundaries, even where these boundaries were relatively subdued. The absence of ground cracks displaying 

shear movement on the flanks of many of these landslides 

was interpreted to reflect mobilization of only the upslope 

segments of the basal rupture surfaces of the landslides 

during earthquake shaking.

2. Seismic-displacement calculations are highly sensitive 

to selected yield coefficients and shear strengths. The

results of the cumulative-displacement analyses indicate 

that calculated displacements are highly sensitive to the 

seismic yield coefficient, which is a function of the shear 

strength of the basal rupture surface. The determination of 

a representative strength across the basal rupture surface, 

therefore, is crucial to the success of the analysis. Sampling 

and testing of thin shear surfaces separating hard-rock 

strata is difficult, and standard "small diameter" exploration methods have a very low probability of allowing 

identification, adequate sampling, and preservation of thin 

rupture surfaces. Direct observation and careful hand extraction of oriented samples from large excavations

300

250

200

100

50

SEISMIC YIELD COEFFICIENT 

0.10 0.15 0.20 0.25

\

\ \

15 17 19 21 23 25 

ANGLE OF INTERNAL FRICTION, IN DEGREES

27

300

250

200

150

100

50

0.05

SEISMIC YIELD COEFFICIENT 

0.10 0.15 0.20 0.25

B \ \ \ \\

\\ Measured range of 

displacements (33 - 

57 cm, avg. 45 cm)

15 -\7 19 21 23 25 

ANGLE OF INTERNAL FRICTION, IN DEGREES

27

EXPLANATION

• • • • Program DISPLMT (Houston and others, 1987), scaled 

Corralitos Station record

Program DISPLMT (Houston and others, 1987), unsealed Los 

Gatos Presentation Center Station record,

Mean ± 10{Hynes-Grirrin and Franklin, 1984)

———— Mean for theoretical M =7.0 earthquake (Makdisi and Seed, 

1977)

— * * * Geometric mean (K.L. Lee, unpublished data, 1977)

— — Stiff soils or deep cohesionless soils (Lin and Whitman, 1986)

Figure 8.—Predicted displacement versus shear strength (expressed as angle of internal friction) calculated by five cumulative-displacement methods, 

using records from stations at Corralitos and Los Gatos Presentation Center (see fig. 1 for locations), assuming a peak horizontal ground acceleration of 

1.0 g. A, Lower Schultheis Road West landslide. B, Ditullio landslide.
C182 LANDSLIDES

(trenches or large-diameter boreholes) are recommended 

to observe the landslide geometry and collect the appropriate materials for testing.

3. Laboratory strengths may be too high to accurately 

predict stability and displacements. The pseudostatic 

method of analysis incorrectly predicted the stability of the 

Lower Schultheis Road West and Ditullio landslides, using 

lower-bound laboratory shear strengths and a seismic yield 

coefficient of 0.20, but would have predicted the failures if 

a higher seismic yield coefficient had been used. The 

magnitudes of the deformations on the Lower Schultheis 

Road West and Ditullio landslides generally were underestimated, using various cumulative-displacement methods 

and laboratory shear strengths.

4. Backanalyzed shear strengths can be used to improve 

the accuracy of calculated cumulative displacements. 

Backanalyses give a range of reasonable shear strengths 

for landslide rupture materials in the form of a range of 

angles of internal friction of 17°-25°. Backanalyzed shear 

strengths can be used with laboratory results to better 

estimate the average shear strength of the basal rupture 

surface. In this study, use of an angle of internal friction of 

about 21° produces displacements that generally are consistent with measured displacements, and appears to correctly predict the onset of instability when using the 

pseudostatic analysis.

5. Ground-water levels significantly influence slope stability. Despite a prolonged, 4-year-long drought, shallow 

ground water was present in many of the landslides investigated. Ground-water zones appear to be perched on 

impermeable rupture surfaces. Laboratory results indicate 

high angles of internal friction and relatively low cohesion. 

The presence of higher ground-water levels in the landslides before the earthquake would have increased the 

landslide displacements. Measurements of ground-water 

levels are important to make in the field and to include in 

calculations.

6. Displacement calculations are also sensitive to the selected acceleration-time history. The range of displacements calculated from published compilations of earthquake records is clear testimony to the degree of sensitivity 

to this parameter. Because the effect of the long-period part 

of the seismic record is significant, the peak ground acceleration, though important, may not be as critical as other 

factors, such as duration of shaking, attenuation with 

distance from the source, and seismic focusing effects 

(including topographic amplification and directivity). Seismic-stability analyses should be based on more than one 

seismic record whenever practical. 

This study emphasizes that available methods of cumulative-displacement analysis are useful, but investigators 

should select variables with care so as not to underestiSEISMIC YIELD COEFFICIENT

150

,100

Q. UJ w o 50

O "Z.

0.10 0.15 0.20 0.25 -

15 17 19 21 23 25 

ANGLE OF INTERNAL FRICTION, IN DEGREES

27

SEISMIC YIELD COEFFICIENT

150

- 0.05

B

2 fc UJ 5

o £

w O en 5 2 50

0.10 0.15 0.20 0.25

Measured range of 

displacements (33 - 57cm, 

avg 45 cm)

15 17 19 21 23 25 

ANGLE OF INTERNAL FRICTION, IN DEGREES

27

EXPLANATION

Program DISPLMT (Houston and others, 1987), unsealed 

Corralitos Station record Geometric mean (K.L. Lee, unpublished data, 1977)

Mean + lcr(Hynes-Griffin and Franklin, 1984) — — Stiff soils or deep cohesionless soils (Lin and Whitman, 1986)

———— Mean for theoretical M =7.0 earthquake (Makdisi and Seed, 

1977)

Figure 9.—Predicted displacement versus shear strength (expressed as angle of internal friction) calculated by five cumulative-displacement methods, 

using peak horizontal ground acceleration of 0.63 g from station at Corralitos (see fig. 1 for location). A, Lower Schultheis Road West landslide. B, 

Ditullio landslide.
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C183

mate the stability of preexisting landslides. With a thorough characterization of landslide geometry, local geologic conditions, and engineering properties, and a 

selection of conservative strength parameters for the basal 

rupture surfaces, existing analytical techniques are sufficient for order-of-magnitude predictions when the lowest 

laboratory shear strengths are used.

ACKNOWLEDGMENTS

This research was partly supported by U.S. Geological 

Survey grant 14-08-0001-G1860, Cotton, Shires and Associates, Inc., and Leighton and Associates, Inc. We thank 

Randy Jibson, Dave Keefer, and Ray Wilson for their 

thorough and provocative reviews. We also thank Phil 

Buchiarelli, Chester Burrous, and Rick Lozinsky for assisting with this study.

SEISMIC YIELD COEFFICIENT 

0.10 0.15 0.20 0.25

C/D

£ 150

fc

H

LLJ 

O

p. 100

LLJ 

HI

O_

OT 5 5n bu

n

i i ii

;v •

n--.

^ W Y i ——————————— i-

^ . V Measured 

^ T v displacement 

W T y (61 cm)

\. v;"^v :

^~-^_ ^ \f

! ^S& :

15 17 19 21 23 25 27 

ANGLE OF INTERNAL FRICTION, IN DEGREES

EXPLANATION

Upper bound (Hynes-Griffin and Franklin, 1984)

Mean ± lcr(Hynes-Griffin and Franklin, 1984) 

Deviation

Mean (Hynes-Griffin and Franklin, 1984)

•* •* •* Upper bound for theoretical M =7.0 earthquake (Makdisi and 

Seed, 1977)

———— Mean for theoretical M =7.0 earthquake (Makdisi and 

Seed, 1977)

* * * * Lower bound for theoretical M =7.0 earthquake (Makdisi and 

Seed, 1977)

Figure 10.—Predicted displacement versus shear strength (expressed as 

angle of internal friction) calculated by two cumulative-displacement 

methods for Lower Schultheis Road West landslide (see fig. 2 for location).

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C184 LANDSLIDES

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cross sections, borehole logs, and laboratory procedures, 

were presented by Cole and others (1991).

LOWER SCHULTHEIS ROAD WEST LANDSLIDE

LOCATION AND SCOPE OF INVESTIGATION

The site of the Lower Schultheis Road West landslide, 

referred to as "Upper Laurel" by Cole and others (1991), 

is near Laurel, Calif., along the north-facing flank of an 

east-west-trending spur ridge (fig. 2) that is underlain at 

depth by siltstone and sandstone of the San Lorenzo Formation. A surveyed topographic map of the entire 7.7-ha 

parcel was provided by the property owner. Surface mapping and profiling of the landslide was performed at a 

scale of 1:240, using a semi-total-station (STS) survey 

instrument for horizontal and vertical control. Two largediameter boreholes, LD-1 and LD-2 (fig. 3), were excavated and logged to 11.3- and 8.5-m depth, respectively.

SURFACE CONDITIONS

The geomorphology of the ridge and adjacent valley 

indicate that a large, ancient landslide underlies most of 

the north-facing hillside. A system of arcuate ground fissures near the ridge crest indicates that the earthquake 

reactivated a part of this preexisting landslide. The system of cracks defines a graben that follows the ridge crest 

for approximately 60 m before curving downslope to form 

a broad arc (fig. 4). The head of the reactivated landslide 

mass was downthrown about 25 cm and offset laterally 

about 61 cm. No ground deformation was observed along 

the probable lateral margins. The probable toe of the landslide is indicated by a zone of compression coinciding 

with a slight topographic bulge. The reactivated landslide 

appears to be about 61 m wide by 76 m long. The total 

area of the reactivated landslide is approximately 0.4 ha.

SUBSURFACE CONDITIONS

APPENDIX: 

DESCRIPTION OF LANDSLIDE SITES

The two reactivated landslides discussed in this paper 

were chosen from an initial group of more than 50 relatively large landslides in the study area. Detailed surface 

and subsurface data were collected from five of these landslides. This appendix provides descriptions of the two landslides discussed above. More complete descriptions of 

these sites, as well as two additional sites, including maps,

The two large-diameter boreholes, LD-1 and LD-2 (fig. 

3), were located 4.6 and 12.2 m, respectively, downslope 

from the headscarp crack. Both boreholes penetrated a 

near-horizontal, well-developed shear surface at 4.9-m 

depth; this shear surface separates overlying, oxidized regolith from underlying, unoxidized fractured rock. A 

deeper, steeply dipping shear surface was penetrated in 

borehole LD-1 at 7.3-m depth; this deeper shear surface 

is a sheared siltstone interbed within massive sandstone 

that separates the overlying fractured rock from dense, 

relatively intact rock. The shallower shear surface appeared
ANALYSIS OF EARTHQUAKE-REACTIVATED LANDSLIDES IN THE EPICENTRAL REGION C185

to be thicker, more moist, and generally better developed 

than the deeper shear surface within more indurated rock.

Subsurface data indicate that the earthquake did not 

reactivate the uppermost part of the preexisting landslide 

mass. Trenches excavated by Rogers E. Johnson and Associates (1991) demonstrate that the deeper (7.3 m deep) 

rupture surface exposed in borehole LD-1 (fig. 3) extends 

along a 30° plane to the ridge crest and passes undisturbed beneath the 1989 graben that forms the upslope 

boundary of the 1989 reactivated landslide (fig. 4). The 

reactivated headscarp (1989 graben) is 5.9 m downslope 

from the intersection of the buried slide surface with the 

present ground surface. The slide surface at this point is 

3.2 m deep. Thus, the earthquake apparently failed to reactivate an upper wedge of preexisting landslide material. 

(The deeper shear surface is a bedding-plane fault that 

probably formed during folding of an underlying bedrock 

anticline; the shallower shear surface exposed in boreholes LD-1 and LD-2 was not exposed in the trenches.) 

Rogers E. Johnson and Associates found a similar relation 

between a buried rupture surface and an arcuate, 1989 

earthquake-triggered landslide headscarp in the same vicinity. The preexisting rupture surface forming the base 

of the dormant wedge of preexisting-landslide material 

may have acquired more cohesion over time, owing to 

root reinforcement and clay development during pedogenesis. The associated increase in shear strength may have 

helped to maintain stability in the shallow (upper 3 m) 

part of the landslide. The maximum thickness of the stable 

wedge coincides with the approximate base of significant 

roots, suggesting that root reinforcement could have increased resistance to sliding in this wedge. Terwilliger 

and Waldron (1991) discussed the role of root reinforcement on the stability of shallow landslides.

Ground water was penetrated in the two large-diameter 

boreholes, as well as in boreholes drilled by Kingsley 

Associates (1991). Isolated zones of ground water were 

perched on impermeable shear surfaces. Significant 

amounts of ground water also were penetrated below the 

deepest shear surface (8-10 m below the ground surface).

DITULLIO LANDSLIDE

LOCATION AND SCOPE OF INVESTIGATION

The site of the Ditullio landslide is on Redwood Lodge 

Road, about 1.2 km southeast of the Lower Schultheis

Road West landslide. The Ditullio landslide is near the 

top of a southwest-facing slope within and near the southern margin of a previously mapped, very large landslide 

complex (fig. 2). The Ditullio landslide is 92 to 122 m 

long by 76 m wide, covering an area of about 0.6 to 0.8 

ha. Surface mapping and profiling of the landslide was 

performed at a scale of 1:240, using an STS survey instrument for horizontal and vertical control.

SURFACE CONDITIONS

The headscarp is characterized by echelon and parallel 

fissures that form a 76-m-long, arcuate zone of ground 

cracks. Individual cracks within this zone indicate 0.3 to 

0.6 m of extension and 0.3 m of vertical downdropping. 

The zone coincides with the apparent headscarp of an 

ancient landslide. The locations of the toes of both the 

ancient and 1989 landslides are unknown; however, this 

ancient landslide probably is part of a landslide complex 

that extends about 350 m downslope to the creek.

Several linear, echelon cracks that extend 52 m 

downslope from the scarp along the probable north boundary of the landslide may represent shearing along its right 

flank; however, these cracks could also be due to settlement of a septic leachfield. No evidence of offset or deformation was observed along the left flank. The 

earthquake-triggered landslide does not appear to cross 

Redwood Lodge Road, located 61 to 92 m downslope 

from the headscarp (fig. 3).

SUBSURFACE CONDITIONS

A large-diameter borehole, DT-1 (fig. 4), was located 

about 40 m downslope from the headscarp cracks. Borehole DT-1, which was drilled and downhole logged to 

22.5-m depth, exposed a well-developed, slickensided 

shear surface at about 18-m depth. The gradational oxidized-unoxidized contact is at 20.7-m depth; however, 

oxidized sandstone bedrock immediately below the landslide plane is very dense. Figure 4, which includes a detailed log of the landslide rupture surface, illustrates the 

relation between the 1989 reactivation, the underlying ancient landslide material, and deeper bedrock structure.

No ground water was penetrated in borehole DT-1, even 

after remaining open for 12 days.

U.S. GOVERNMENT PRINTING OFFICE: 1998 - 673-048 / 20098 REGION NO. 8


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