Analysis of the Origin of Landslides 

in the New Madrid Seismic Zone

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-D
Cover. Part of a gray, shaded-relief, reduced-to-pole magnetic anomaly map. Map area 

includes parts of Missouri, Illinois, Indiana, Kentucky, Tennessee, and Arkansas. 

Illumination is from the west. Figure is from Geophysical setting of the Reelfoot rift and 

relations between rift structures and the New Madrid seismic zone, by Thomas G. 

Hildenbrand and John D. Hendricks (chapter E in this series).
Analysis of the Origin of Landslides in the 

New Madrid Seismic Zone

By Randall W. Jibson and David K. Keefer

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director

For Sale by U.S. Geological Survey, Map Distribution

Box 25286, MS 306, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Jibson, Randall W.

Analysis of the origin of landslides in the New Madrid seismic zone / by Randall W. Jibson and 

David K. Keefer.

p. cm.—(Investigations of the New Madrid seismic zone ; D)(U.S. Geological Survey 

professional paper; 1538)

Includes bibliographic references.

SupL of Docs, no.: 119.16 :1538D

1. Landslides—Missouri—New Madrid Region. I. Keefer, David K. II. Title. HI. Series. 

IV. Series: U.S. Geological Survey professional paper ; 1538. 

QE535.2.U6I59 1994 vol. D 

[QE 599.U5]

551.2'2'0 9778985—dc20 93-38068 

[551.3'07] CIP
CONTENTS

Abstract.......................................................................................................................... Dl

Introduction.................................................................................................................... 1

Geologic Setting............................................................................................................. 2

Seismic History.............................................................................................................. 3

Description of Landslides in the Area........................................................................... 3

Previous Work................................................................................................................ 4

Analysis of the Stewart and Campbell Landslides........................................................ 5

Geotechnical Investigation...................................................................................... 7

Static (Aseismic) Slope Stability Analysis............................................................. 8

Idealized Pre-Landslide Bluff Model............................................................... 8

Ground-Water Conditions................................................................................. 8

Method of Stability Analysis ........................................................................... 9

Results of Static Slope Stability Analysis........................................................ 10

Dynamic (Seismic) Slope Stability Analysis.......................................................... 11

Factor of Safety................................................................................................ 12

Thrust Angle..................................................................................................... 14

Earthquake Acceleration-Time History............................................................ 15

Calculation of the Newmark Landslide Displacement.................................... 17

Results of Dynamic Analysis of Stewart Landslide........................................ 18

Results of Dynamic Analysis of Campbell Landslide..................................... 18

Summary of Stability Analyses .............................................................................. 18

What if Seismic Conditions Are Unknown?................................................................. 18

Discussion...................................................................................................................... 20

Summary and Conclusions............................................................................................ 21

References Cited............................................................................................................ 22

	FIGURES

1. Map showing location of the study area........................................................................................................................... D2

2. Isoseismal map of the 1811-12 New Madrid earthquakes............................................................................................... 4

3. Idealized drawings of landslides along bluffs in study area............................................................................................. 5

4. Map showing major features of the Stewart landslide...................................................................................................... 6

5. Profile of Stewart landslide............................................................^ 6

6. Map of part of the Campbell earth-flow complex near the line of profile....................................................................... 7

7. Profile of Campbell landslide............................................................................................................................................ 7

8. Idealized model of bluff at Stewart site in drained conditions......................................................................................... 8

9. Idealized model of bluff at Campbell site in drained conditions ..................................................................................... 9

10. Ground-water conditions modeled in the slope stability analysis.................................................................................... 9

11. Factor of safety versus water-table level at the Stewart site............................................................................................. 10

12. Factor of safety versus water-table level at the Campbell site......................................................................................... 11

13. Critical slip surfaces in drained conditions at the Stewart site......................................................................................... 11

14. Critical slip surfaces in drained conditions at the Campbell site ..................................................................................... 12

15. Demonstration of the Newmark-analysis algorithm......................................................................................................... 12

III
IV CONTENTS

16. Idealized model of bluff at Stewart site in undrained conditions..................................................................................... D13

17. Idealized model of bluff at Campbell site in undrained conditions.................................................................................. 13

18. Critical slip surfaces in undrained conditions at the Stewart site..................................................................................... 14

19. Critical slip surfaces in undrained conditions at the Campbell site ................................................................................. 15

20. Newmark displacement as a function of Arias intensity for critical accelerations of 0.02-0.40 g ................................. 19

21. Critical acceleration contours from multivariate regression of Arias intensity and critical acceleration

	versus Newmark displacement.......................................................................................................................................... 19

	TABLES

1. Drained factors of safety from the Stewart and Campbell landslides................................................................................ D10

2. Undrained factors of safety from the Stewart and Campbell landslides............................................................................ 14

3. Strong-motion records used to model 1811-12 ground shaking........................................................................................ 16

4. Strong-motion records selected for analysis....................................................................................................................... 19

5. Most damaging earthquakes since 1811-12........................................................................................................................ 20
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE 

NEW MADRID SEISMIC ZONE

By Randall W. Jibson1 and David K. Keefer2

ABSTRACT

We develop a method for determining if a landslide or 

group of landslides of unknown origin was triggered by 

earthquake shaking. Important applications of this method 

are in seismic-hazard analysis and paleoseismology. If a 

group of landslides in a region can be shown to be statically stable but dynamically unstable, an earthquake origin 

can be inferred and the minimum shaking intensity 

required to have caused failure can be estimated. If such 

landslide features can be dated, a date for the triggering 

earthquake can be ascertained.

We analyze two landslides in the New Madrid seismic 

zone that represent the types of landslide features that 

previous research suggests may have been triggered by the 

1811-12 earthquakes. Slope stability models of aseismic 

conditions show that neither slide could have formed 

aseismically even in unrealistically high ground-water conditions. Dynamic stability analysis using Newmark's 

method shows that both slides would have experienced 

large inertial displacements during earthquake shaking similar to that which occurred in 1811-12; these displacements 

are large enough that catastrophic failure would result. 

Thus, these landslides probably formed during the 1811-12 

earthquakes.

Our analysis yields a general relationship between 

Newmark landslide displacement, earthquake shaking 

intensity, and the critical acceleration of a landslide. Using 

this relationship, we estimate the minimum shaking intensities required to trigger the types of landslides studied. 

Results indicate that an mb=5.8 or M=5.9 earthquake is the 

lower bound threshold at zero epicentral distance that could 

trigger catastrophic movement of typical block slides in the 

New Madrid seismic zone; for earth flows, 1115=5.4 or 

M=5.3 is the threshold earthquake. This approach can be

1 U.S. Geological Survey, Mail Stop 966, P.O. Box 25046, Denver 

Federal Center, Lakewood, CO 80225.

2 U.S. Geological Survey, Mail Stop 998,345 Middlefield Rd., Menlo 

Park, CA 94025.

applied in seismic-hazard analyses elsewhere to determine 

regional seismic slope stability.

INTRODUCTION

Among the most dramatic effects of the New Madrid 

earthquakes of 1811-12 were the numerous landslides 

along the bluffs bordering the Mississippi alluvial plain in 

western Tennessee and Kentucky. In his report of a field 

investigation of the New Madrid earthquakes conducted in 

1904, Fuller (1912, p. 59) stated:

Probably no feature of the earthquake is more striking than the landslides 

developed in certain of the steeper bluffs * * *. From the vicinity of 

Hickman in southwestern Kentucky at least to the mouth of the Obion 

River, about halfway across the state of Tennessee * * * the landslides are 

a striking feature. Skirting the edge of the bluffs, in the vicinity of Reelfoot Lake, a characteristic landslide topography is almost constantly in 

sight * * *.

Many landslide features are currently visible along 

the bluffs in the epicentral area of the 1811-12 earthquakes, but determining which, if any, of these features 

were triggered in 1811-12 is a complex and difficult problem. In our recent studies (Jibson and Keefer, 1988, 

1989), described in more detail later, we analyzed field 

and historical data and the regional distribution of landslides in the epicentral area. The data and conclusions 

from these studies, which were based on a consideration of 

the entire group of landslides at a regional scale, are consistent with many of these landslides having been triggered 

by the 1811-12 earthquakes. In the present paper, we use 

slope stability analysis to demonstrate that two representative individual landslides in the epicentral area could have 

resulted only from strong earthquake shaking such as that 

generated in 1811-12 and not from any other cause, such 

as increased ground-water levels. A detailed analysis of 

the stability of these individual landslides provides a 

firmer basis for our interpretation of the ground-failure 

effects of the 1811-12 earthquakes and will help us predict 

the conditions necessary to trigger widespread landsliding 

in future earthquakes in the New Madrid seismic zone.

Dl
D2 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

The successful application of the approach detailed in 

this paper also establishes a procedure for determining the 

probable cause of failure of landslides elsewhere. The ability to determine whether a landslide formed as a result of 

earthquake shaking or from other factors opens several 

opportunities for using landslide analysis to interpret the 

recent geologic record. For example, dating landslides that 

can be shown to have formed during earthquake shaking 

would be a valuable tool in paleoseismic studies, particularly in areas such as the Central United States, where surface exposures of seismogenic faults are rare or absent.

In this paper, we address a single fundamental question: Can we analyze a landslide of unknown origin and 

determine that (1) it could not have formed aseismically 

and (2) it must have formed as the result of earthquake 

shaking? To address this question, we review the geological and seismological context of the study, present the 

geotechnical data necessary for the analysis of the stability of the two representative landslides, and interpret the 

origin of these landslides in light of the results of the analyses. Further, we document and test a more general procedure for determining whether or not old landslides were 

triggered by earthquakes and propose a general quantitative relationship between earthquake shaking intensity and 

dynamic (seismic) slope stability.

GEOLOGIC SETTING

The area affected by major landsliding during the 

1811-12 earthquakes includes more than 300 km of bluffs 

forming the eastern edge of the Mississippi alluvial plain 

between Cairo, 111., and Memphis, Tenn. (fig. 1). The 

bluffs are along the eastern flank of the northern Mississippi Embayment, a broad, south-southwest-plunging syncline whose axis roughly coincides with the Mississippi 

River. Embayment deposits exposed in the bluffs thus generally dip to the west-northwest in most of the area. The 

region is seismically active, as evidenced by the 1811-12 

earthquake sequence and continuing seismicity.

The average height of the bluffs in this area is 35 m, 

though in some areas they are as high as 70 m. Bluff 

slope angles are from a few degrees to vertical; most are 

15°-25°. The bluffs trend north-northeast, approximately 

parallel with the Mississippi River. Locally, however, 

the bluff line is sinuous where the river has eroded arcuate meander scars.

The Eocene Jackson Formation (Conrad, 1856) forms 

the base of the bluffs throughout most of the area. Exposures are as thick as 45 m in the bluffs. The composition 

of the Jackson Formation is highly variable, but it generally 

consists of discontinuous layers of shallow-marine embayment deposits of clay and silt ranging from a few centimeters to several meters thick. In some areas, the Jackson 

Formation contains clean, uncemented sand layers as thick

92°

39°

90° 87°

38°

36C

35°-

34°

MISSOURI

CairS

ILLINOIS

v/ INDIANA

•- Evansville

KENTUCKY

Dyersburg

TENNESSEE

ARKANSAS

Memphis

MISSISSIPPI

100 KILOMETERS 

I

Figure 1. Study area (shaded) and estimated epicenters 

(diamonds), dates, and estimated body-wave magnitudes (1115) of 

the three largest earthquakes in the 1811-12 sequence (earthquake 

locations and magnitudes from Nuttli, 1983).

as several meters interbedded with soft clay layers. The 

Eocene beds generally dip a few degrees west-northwest 

(out of the bluff face in most of the area), but the amount 

and direction of dip vary locally; the amount of dip is generally less than 20°. The unconformity on top of the 

Eocene section is highly irregular, but, in areas where it 

has been mapped, it approximately parallels the present 

ground surface.

Lying unconformably on the Jackson Formation is as 

much as 20 m of Pliocene terrace gravel and sand of the 

Lafayette Gravel (McGee, 1891; Potter, 1955). The gravel 

and sand lenses commonly are uncemented but at some 

localities contain concretionary beds as thick as 2 m. The 

Lafayette Gravel is locally saturated where water tables 

are perched, and it probably is subject to large seasonal 

fluctuations in ground-water conditions. The unit pinches 

out in some areas.

The bluffs are capped by 5-50 m of Pleistocene loess 

lying unconformably on the Lafayette Gravel and Jackson 

Formation. The average thickness of the loess in the area is 

about 15m. Loess is glacially derived, eolian silt that commonly forms vertical faces owing to the presence of vertical fractures. Vertical slopes can be supported because the 

loess in the study area has cohesion imparted by a clay
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D3

binder, or calcareous cementation, or both (Krinitzsky and 

Turnbull, 1969).

SEISMIC HISTORY

The bluffs are in the epicentral region of the 1811-12 

New Madrid earthquake sequence, the most severe seismic 

event in historical times in the Central and Eastern United 

States. The three largest events, which occurred on 16 

December 1811, 23 January 1812, and 7 February 1812, 

had estimated magnitudes corresponding to 7.1-7.4 on the 

body-wave magnitude (mi,) scale (Nuttli, 1973) and 8.1-8.3 

on the moment magnitude (M) scale (Hamilton and 

Johnston, 1990). Thousands of aftershocks shook the area 

for many months, several of which were at least of moderate intensity (Nuttli, 1973).

The effects of the 1811-12 earthquake sequence 

have been described by Fuller (1912), Nuttli (1973), and 

Stearns and Wilson (1972); liquefaction effects were 

further documented by Saucier (1977) and Obermeier 

(1989). Figure 2 is a composite isoseismal map of the 

sequence, based on the Modified Mercalli intensity 

(MMI) scale, compiled by Stearns and Wilson (1972). 

Immediately apparent is the large area affected by the 

earthquakes. Nuttli (1973) estimated that 2,500,000 km2 

was affected at MMI>V, the threshold of structural damage, and that 600,000 km2 was affected at MMI>VII, the 

threshold of major damage. This compares to 150,000 km2 

affected at MMI>V and 30,000 km2 affected at MMI>VII 

for the 1906 San Francisco earthquake, which had about 

the same magnitude. The 1811-12 earthquakes were felt 

as far away as Detroit (900 km), Toronto (1,200 km), New 

York City (1,400 km), and Boston (1,700 km); in all they 

were felt over more than 5,000,000 km2 (Stearns and Wilson, 1972). In Washington, D.C. (1,100 km), people were 

awakened from sleep, dishes rattled, and suspended 

objects swung. Chimneys were damaged as far away as 

Richmond, Va. (1,100 km), Savannah, Ga. (1,000 km), 

and Cincinnati, Ohio (600 km); plaster cracked and fell in 

Columbia, S.C. (900 km); and many homes were badly 

damaged in St. Louis (250 km), all as a result of these 

earthquakes (Nuttli, 1973).

The epicentral area of greatest damage included 

approximately 130,000 km2 (Fuller, 1912). Within this 

region, uplift and subsidence on the order of a few 

meters occurred over hundreds of square kilometers, the 

most notable area of subsidence being Reelfoot Lake in 

northwestern Tennessee. Also, fissuring of the ground 

surface, caving of river banks, and extrusion of huge 

quantities of sand and water from the subsurface 

occurred. The last effect was the result of liquefaction of 

sand layers and is estimated to have inundated approximately 10,000 km2 with as much as 2 m of sand and

water. More than 650 km2 of timber was destroyed by 

flooding, violent ground shaking, or landsliding (Fuller, 

1912). Eyewitness accounts describe the entire land surface as being disrupted and, in many places, uninhabitable (Penick, 1981). Pertinent to this study are reports of 

landslides along the bluffs in western Tennessee and Kentucky from Hickman, Ky., at least to the Obion River, 

near Dyersburg, Tenn. (Fuller, 1912).

DESCRIPTION OF LANDSLIDES IN 

THE AREA

Along the bluffs in the study area, 221 large (greater 

than 50 m wide) landslides were identified on air photos, 

examined on the ground, classified morphologically (after 

Varnes, 1978), and plotted on an inventory map (Jibson 

and Keefer, 1988). Three classes of landslides were 

mapped: old coherent slides, earth flows, and young rotational slumps.

Old coherent slides (fig. 3) constitute 65 percent of the 

landslides in the study area. This group includes translational (block) slides and rotational slides (slumps), both of 

which remained fairly intact or coherent. These slides are 

termed "old" because they all are eroded and revegetated 

and show no sign of activity for at least the past several 

decades. They all appear to be of similar age based primarily on amount of scarp retreat, degree of erosion of ridges, 

and vegetation density and age on scarps and disrupted 

areas (Jibson and Keefer, 1988). Translational and rotational slides were grouped together because heavy tree 

cover and eroded features generally made it impossible to 

distinguish between them. These landslides are deep 

seated (typically deeper than 20 m) and have basal shear 

surfaces in the clayey Jackson Formation that form the 

base of the bluffs. The translational block slides (fig. 3A) 

are characterized by horst-and-graben topography, and toe 

areas commonly have compressional ridges in front of the 

landslide blocks that moved down and out (as far as 100 m) 

from the parent slope. Basal shear surfaces dip 4°-25°. 

The old rotational slumps (fig. 3#) are characterized by 

either single or multiple blocks that generally appear to 

have rotated a large amount relative to the younger slumps 

described subsequently.

Earth flows (fig. 3C) constitute 24 percent of the 

landslides. Characteristic features are gently hummocky 

topography and ridges of accumulated material in the toe 

area. A few deforested earth-flow complexes contain 

recently (0-5 yr) active flows. The large majority of the 

earth flows, however, including all those on forested 

slopes, are eroded and revegetated and have been inactive 

for at least the past several decades. As with the old 

coherent slides, the degree of erosion and revegetation 

suggests similar ages for the earth flows (Jibson and 

Keefer, 1988).
D4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Figure 2. Composite isoseismal map of the 1811-12 New Madrid earthquake sequence showing maximum Modified Mercalli intensity 

values. Adapted from Stearns and Wilson (1972).

The remaining 11 percent of the landslides are young 

rotational slumps. Young slumps are present only along 

bluffs where the Mississippi River has impinged since 

1820 (about 11 percent of bluff length). The young 

slumps are characterized by massive single slump blocks 

that form where the river has undercut the bluffs. They 

are differentiated from old slumps on the bases of less 

rotation, less scarp and head erosion, absence of multiple 

blocks, and, in some cases, absent or young vegetation 

(Jibson and Keefer, 1988).

PREVIOUS WORK

The two earliest field investigations of the landslides, 

conducted in 1891 (McGee, 1893) and 1904 (Fuller, 

1912), suggest that at least some of the old coherent slides 

in part of the area formed during the 1811-12 earthquakes. 

More recent studies (Jibson and Keefer, 1984, 1988; 

Jibson, 1985) used dendrochronology, geomorphology,

historical topographic maps, local historical accounts, and 

comparisons with landslides triggered by other earthquakes 

to show that the apparent ages and morphologies of most 

of the old coherent slides and earth flows are consistent 

with triggering during the 1811-12 earthquakes. The evidence further shows that the only ongoing, large, aseismic 

landslide activity in the area results from fluvial undercutting of near-river bluffs, which triggers slumps that are 

morphologically distinct from the old coherent slides on 

bluffs away from the river. Also, the landslides on bluffs 

away from the river all appear to be about the same age, 

which suggests a common triggering event; these landslides are unrelated to fluvial activity and have no active 

analogs in the area (Jibson and Keefer, 1988).

In a statistical analysis of the regional distribution of 

the three types of landslides along the bluffs, we (Jibson 

and Keefer, 1989) used discriminant analysis and multivariate linear regression to detect correlation between landslide distribution and slope height and steepness, strati-
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D5

Disturbed material 

Loess

Lafayette

Gravel 

Jackson

Formation

Figure 3. Idealized drawings of types of landslides along the 

bluffs. These landslides all have eroded, revegetated features, and 

no active analogs are present in the area. A, Old coherent translational block slide. B, Old coherent rotational slump. C, Earth 

flow.

graphic variation, slope aspect, and proximity to the hypocenters of the 1811-12 New Madrid earthquakes. The 

discriminant analysis shows no correlation between young 

rotational slumps and the earthquakes. Bluffs having old 

coherent slides and earth flows, however, are significantly 

closer to the estimated hypocenters of the 1811-12 earthquakes than bluffs without these slides (Jibson and Keefer, 

1989). Multiple regression analysis, which simultaneously 

combined all the factors cited above, likewise indicates no 

correlation between the distribution of young rotational 

slumps and the earthquake locations. The distribution of 

old coherent slides and earth flows, however, strongly

correlates with the proximity to the hypocenters of the 

1811-12 earthquakes, as well as with slope height and 

aspect (Jibson and Keefer, 1989). The results of these statistical analyses thus show that the old coherent slides and 

earth flows in the area are spatially related to the 1811-12 

earthquake hypocenters.

The methods and conclusions from these previous 

studies do not address the questions of whether any specific 

landslide in the area was actually triggered in 1811-12 or 

what conditions led to the failure of a given landslide. The 

following sections, based on more recent studies (Jibson 

and Keefer, 1992, 1993), describe an approach to address 

these questions.

ANALYSIS OF THE STEWART AND 

CAMPBELL LANDSLIDES

We chose two landslides for detailed analysis: the 

Stewart landslide, a translational block slide about 11 km 

north of Dyersburg, Tenn., and the Campbell landslide, an 

earth flow about 10 km west of Dyersburg (fig. 1). These 

slides are representative of the two major types of old landslides in the area that previous research indicates may have 

been triggered by the 1811-12 earthquakes.

The Stewart landslide averages 800 m wide by 400 m 

long and covers approximately 0.3 km2. Large parts of the 

slide are heavily forested, which precludes detailed topographic mapping. Figure 4 is a sketch map of the landslide 

compiled using 1:24,000-scale topographic map contours, 

profiles measured in 2-m increments, 1:20,000-scale air 

photos, and field observations. Figure 5 shows a profile of 

the Stewart slide; subsurface data are from drilling along 

the line of profile. The broad, bowl-shaped scarp forms a 

notable reentrant in the local bluff line, and below the scarp 

several large horst-and-graben blocks form prominent but 

discontinuous ridges and troughs that interfinger with one 

another and create a complex topography. The grabens 

commonly have sloping bottoms that allow drainage; however, one ephemeral sag pond is in a closed depression on a 

graben block. Some of the smaller horst blocks show evidence of headword rotation of as much as 10°, but the 

larger blocks, as well as most of the smaller ones, did not 

rotate and apparently translated with little internal deformation on a gently sloping basal shear surface. The horst 

block shown on the profile (fig. 5), for example, is displaced downward only 3 m but moved horizontally about 

50 m. Thus, the basal shear surface dips less than 4°. 

Below the main landslide blocks, a toe area of subdued, 

hummocky topography formed from subsidiary slumping 

from the displaced bluff face (fig. 5) and compression of 

the material at the base of the slope when the landslide 

blocks moved down and out from the parent slope.

The Campbell landslide is a large complex of coalescing earth flows that extends almost 4 km along the bluffs.
D6 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

EXPLANATION

"'" Ridge crest line 

Rotary-drill hole 

— — Profile 

'f** Sag pond

Figure 4. Map showing major features of the Stewart landslide. 

Contours in feet above mean sea level (1 ft = 0.305 m).

Heavy tree cover makes detailed mapping infeasible; a 

sketch map of the part of the complex near the line of profile is shown in figure 6, and a profile is shown in figure 7.

Individual earth flows in the complex average about 400 m 

long, and the complex covers about 1.5 km2.

The Campbell slide has subtle features: gently hummocky slopes, between 5° and 10°, lie downslope from a 

20° scarp near the top of the bluff. Discreet lobes and an 

irregular scarp visible on air photos indicate that landsliding initiated at several localities and coalesced to form a 

continuous complex. All of these lobes appear to be the 

same age; no evidence exists of episodic movement on the 

slide except in deforested areas where smaller active earth 

flows are present (Jibson and Keefer, 1988). Along the 

base of the bluff, a gentle compressional ridge formed 

where material moved downslope to an area flat enough to 

inhibit movement and cause accumulation (fig. 7). Two 

lobes of material are visible in the profile; the break in 

slope between the toe and the head scarp is probably a secondary scarp. Some of the larger swales on the complex 

have been dammed for stock ponds, but no original sag 

ponds are present. The location of the disturbed zone in 

the drill holes (fig. 7) and the surface morphology of the 

slide indicate that sliding probably occurred along a shear 

surface that dips subparallel with the ground surface at 

about 5° from horizontal.

The intent of the analyses of these representative landslides is to determine the probable conditions leading to 

failure. Thus, we analyze both slides in static (aseismic) 

conditions to determine the likelihood of failure caused by 

changes in ground-water conditions in the absence of earthquake shaking. We then analyze both slides in dynamic 

(seismic) conditions to determine if failure would have 

occurred in earthquake shaking similar to that produced in 

1811-12. If the analyses show that (1) the bluffs are stable 

in aseismic conditions even in the worst conceivable 

ground-water conditions and (2) the bluffs would fail catastrophically in severe earthquake shaking, then we can conclude that these landslides did indeed form as a result of 

the 1811-12 earthquakes.

EXPLANATION

Loess

Lafayette Gravel 

Jackson Formation

R-3

R-4

-20

-40

-60

250 200 150 100 50

METERS

Figure 5. Profile of Stewart landslide showing subsurface stratigraphy (from drill holes shown on fig. 4) and diagrammatic representation 

of failure surfaces (dashed lines). Intact stratigraphy is shown at R-l.
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D7

———— Profile 

TI i rj Scarp 

»R-7 Rotary-drillhole

EXPLANATION

— — — Boundary of earth-flow lobe

—*• Predominant direction of 

movement of lobe

Figure 6. Map of the part of the Campbell earth-flow complex 

near the line of profile. Contours in feet above mean sea level (1 ft 

= 0.305 m).

GEOTECHNICAL INVESTIGATION

The purpose of the geotechnical investigation was to 

procure soil samples and conduct laboratory and in situ 

engineering tests to determine the soil properties that 

affect slope stability. Fundamental properties required for 

stability analyses are unit weight and shear strength of the 

soil. Index properties such as grain-size distribution, plasticity, water content, and color are determined to classify 

and correlate soil layers between sample sites.

We drilled nine hollow-stem rotary boreholes on the 

two landslides from which we procured two types of samples: split-spoon samples from standard-penetration testing (SPT) and 13-cm-diameter undisturbed piston cores.

Split-spoon samples are obtained by hammering a steel 

sampler into the bottom of the borehole; such samples typically are heavily disturbed by the sampling process and 

are used primarily for determining index properties. Piston 

cores are procured by carefully pushing a thin-walled steel 

tube into the bottom of the hole; piston cores provide 

nearly undisturbed soil samples that are used to measure 

soil unit weight and shear strength. Additional samples 

were carved from surface outcrops and obtained by hand 

augering; these samples were used primarily to determine 

index properties, but some block samples were of sufficient quality to allow shear-strength testing. A detailed 

description of the sampling techniques used is given by 

Jibson (1985).

Soil shear strength commonly is characterized in one 

of two ways, depending on the conditions being considered 

(Newmark, 1965; Lambe and Whitman, 1969). In static 

(aseismic) conditions, drained or effective shear strength is 

characterized using two components: the angle of internal 

friction (<J>') and the cohesion (cf). The pore-water pressure 

is assumed to be measurable or estimable and is accounted 

for explicitly in the stability analysis, and no pore-water 

pressures in excess of hydrostatic are present. In dynamic 

(seismic) conditions, undrained or total shear-strength 

parameters are used. During earthquakes, slope materials 

behave in a so-called undrained manner because excess 

pore-water pressures induced by dynamic deformation of 

the soil column cannot dissipate during the relatively brief 

duration of the shaking. Undrained strength also is called 

total strength because the respective contributions of friction, cohesion, and pore pressure are not differentiated and 

the total strength is expressed as a single quantity.

Drained shear strengths of undisturbed soil samples 

were measured by direct shear and consolidatedundrained (CUTX) triaxial shear (Jibson, 1985). Direct 

shear tests were run slowly enough to allow full drainage 

(no excess pore pressure), and the frictional and cohesive 

components of the shear strength were measured directly 

under different normal loads. In triaxial tests, no drainage

EXPLANATION

Loess

Lafayette Gravel 

Jackson Formation

R-5

350 300 200 

METERS

Figure 7. Profile of Campbell landslide showing subsurface stratigraphy (from drill holes shown on fig. 6) and diagrammatic representation of failure surfaces (dashed lines). Intact stratigraphy is shown at R-8.
D8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

is allowed, but pore pressures are measured throughout the 

test and can be mathematically removed from the results to 

estimate the drained conditions.

Undrained shear strengths were measured primarily 

by CUTX tests. CUTX results were supplemented by a 

variety of other methods for measuring undrained strength 

where undisturbed samples were unavailable; these other 

methods included vane shear, penetrometer, and correlation 

with SPTblow counts (Jibson, 1985).

STATIC (ASEISMIC) SLOPE STABILITY 

ANALYSIS

IDEALIZED PRE-LANDSLIDE BLUFF MODEL

To analyze the long-term stability of the bluffs at the 

Stewart and Campbell sites, we constructed idealized models of the pre-landslide bluffs for stability analysis. Undisturbed bluffs adjacent to the slides were examined to 

estimate the pre-landslide slope morphology, and stratigraphic data from the drilling were used to construct 

detailed profile models of the bluffs for two-dimensional 

analysis. Geotechnical properties of the stratigraphic layers in the idealized model were assigned using the results 

of the shear-strength tests; layers where no shear-strength 

tests were performed were assigned strengths based on 

stratigraphic and index-property correlation with layers 

where shear strengths were measured directly (Jibson, 

1985).

Figure 8 shows the idealized model of the prelandslide bluff at the Stewart site in drained conditions; 

also shown are the unit weight, drained cohesion intercept, 

and drained friction angle for each layer defined. The bluff 

is 45 m high as measured from the profile (fig. 5). Undisturbed bluffs adjacent to the Stewart slide slope about 20° 

and have simple, uniformly sloping faces. The profile can 

be approximated by two horizontal lines representing the

base and top of the bluff connected by a straight linesegment dipping 20° that represents the bluff face. The 

stratigraphy of the pre-landslide bluff is best illustrated in 

hole R-l (fig. 5) because this hole was placed above the 

scarp of the Stewart slide in intact material. In the plane of 

the model, the beds are almost horizontal because the bluff 

faces north, whereas the dip of the Jackson Formation is to 

the west and is no more than a few degrees. The bluffs in 

the area have a thin veneer of colluvial loess, but this 

would have little effect on slope stability, and so the model 

shows each unit exposed in the bluff face.

Figure 9 shows the model of the pre-landslide bluff in 

drained conditions for the Campbell site. The intact bluff 

adjacent to the Campbell slide has an average slope angle 

of about 15°, and the profile (fig. 7) indicates a bluff height 

of 55 m. The structure of the bluff at the Campbell site is 

somewhat different than that at the Stewart site because the 

strata dip out of the bluff face at about 5°. Fewer layers 

are defined for the Campbell slide because drilling results 

indicate that formations at this site are more uniform.

GROUND-WATER CONDITIONS

Lack of published data or observations makes modeling ground-water conditions along the bluffs difficult. 

Several potential ground-water conditions were analyzed 

(fig. 10): (1) a water table at the top of the bluff, with 

seepage along the entire bluff face (the most critical situation), (2) a water table at the top of the Lafayette Gravel, 

with seepage on the bluff face only below this level, (3) a 

water table at the top of the Jackson Formation, (4) a 

water table at the base of the bluffs (the least critical situation), and (5) a water table sloping upward from the base 

of the bluffs to the top of the Jackson and a second water 

table perched on the relatively impermeable Jackson Formation that saturates the Lafayette Gravel. The first condition is a more critical situation than can realistically exist

Layer

1 

2 

3

4 

5 

6 

7 

8 

9 

10

Alluvial silt

| 1 __

Unit 

weight 

(kN/m3 )

19.3 

19.6 

19.8 

22.0 

18.1 

18.1 

17.3 

18.1 

18.1 

18.1

— — —— ——

Drained 

friction 

angle

34° 

35° 

30° 

32° 

15° 

20° 

12° 

20° 

15° 

12°

Jackson

__ ——— —— •

Drained 

cohesion 

(kPa)

0.25 

0.50 

0.43 

0.32

Z.UO ^^^

1.81 ^^^^ 2 

0 71 Loess^^^^^

2.?7 ... . ^~ 3

2.06 Lafayette GraveJ^-^"'^ 4

2.52 ^ ^ 5 ————————————————————————— 7_b —————————————————————

^^^ 9

10

300 METERS

Figure 8. Idealized model of pre-landslide bluff at Stewart site in drained conditions.
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D9

Layer

Unit 

weight 

(kN/m3 )

Drained 

friction 

angle

Drained

cohesion

(kPa)

300 METERS

Figure 9. Idealized model of pre-landslide bluff at Campbell site in drained conditions.

in the bluffs and thus provides a worst-case bounding condition. Local hydrologists and geologists who have studied ground-water conditions in the area and who have 

unpublished water-level data from wells along the bluffs 

(Robert Davis, Army Corps of Engineers, Memphis District, oral commun., 1983; William Parks, U.S. Geological 

Survey, Memphis, oral commun., 1983) indicate that condition five is the most likely.

METHOD OF STABILITY ANALYSIS

We used the computer program STABL (Siegel, 

1978) to determine the stability of the modeled bluffs in

aseismic conditions. STABL randomly generates any 

number of circular, wedge-shaped, or irregular slip surfaces of any given geometry and calculates the factor of 

safety1 for each randomly generated surface. The program 

plots the 10 most critical surfaces of each given type along 

with their factors of safety. STABL also calculates safety 

factors for slip surfaces having predetermined geometries.

1 The factor of safety (FS) is the ratio of the sum of the resisting forces 

that act to inhibit slope movement to the sum of the driving forces that tend 

to cause movement. Slopes having factors of safety greater than 1.0 are thus 

stable; those having factors of safety less than 1.0 should move.

Most likely ground-water condition

Alluvial silt

Loess

Perchedx

•^^%^^^'0^^%0;^^';;0^^^0^ 'Lafayette Gravel

5 Jackson Formation rr-^rrrv^v^'^^'^N-'^^^^x^^

^\N\^X^^^N\X^^^NXNNX^^^NN\^^^N\X;^^^NX\X^^ JackSOn Formation

Alternate ground-water conditions

Figure 10. Ground-water conditions modeled in the slope stability analysis. Upper drawing shows most likely ground-water conditions; pattern indicates saturated strata. Lower drawing shows four other possible ground-water conditions. Inverted triangles indicate 

piezometric surfaces for various ground-water conditions (1-5) described in text.
D10 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

The locations of the actual failure surfaces were estimated from drilling data and from the surface geometry of 

the landslides; safety factors were calculated for these surfaces in each ground-water scenario. STABL generated a 

wide variety of circular and irregular surfaces, and wedgeshaped surfaces were generated in each of the layers in the 

Jackson Formation and Lafayette Gravel because the landslide geometries and the drilling data indicate that the basal 

shear surfaces are beneath the loess.

RESULTS OF STATIC SLOPE STABILITY ANALYSIS

Determining the stability of the bluff from the factor 

of safety (FS) requires some judgment. Gedney and 

Weber (1978) recommend that engineered slopes have 

safety factors between 1.25 and 1.50. We use this range 

as the criterion to evaluate slope stability: between FS 

1.00 and 1.25, slopes are considered marginally stable; 

between FS 1.25 and 1.50, slopes are considered stable; 

and above FS 1.50, slopes are considered very stable.

Results of the drained stability analyses of the Stewart 

and Campbell landslides are summarized in table 1. For 

the Stewart site, the lowest factor of safety in the most critical ground-water situation is 1.32, which indicates that the 

bluff there is stable in aseismic conditions even in the most 

critical ground-water condition. In the most likely groundwater condition, the minimum factor of safety at the Stewart site is 1.82: the bluff is very stable. The factor of safety 

of the observed failure surface in the most likely groundwater condition is 1.88. For the Campbell site, the minimum factor of safety in the most critical ground-water situation is 1.30, also in the stable range. In the most likely 

ground-water situation, the minimum factor of safety is 

2.05, in the very stable range.

Figures 11 and 12 show the effects of ground-water 

level on factor of safety for differently shaped failure surfaces for the Stewart and Campbell slides, respectively. 

Visual extrapolation of the lines shows that an artesian piezometric surface tens of meters above ground level at the 

top of the bluff would be needed to reduce the factor of 

safety to 1.0 at both sites. Such an artesian condition is 

implausible because (1) the regional geology and topography preclude such a condition and (2) a piezometric 

surface far above the bluff top that dips steeply to the bluff 

base is physically unrealistic. Thus, the Stewart and 

Campbell landslides could not have formed aseismically in 

any conceivable ground-water condition.

Figures 13 and 14 show, for the Stewart and Campbell sites, respectively, the locations of the most critical 

slip surfaces of various shapes and of the observed slip 

surfaces. For the Stewart site, all the surfaces have 

grossly similar shapes, but the randomly generated

Table 1. Static factors of safety from stability analyses of the 

Stewart and Campbell landslides in drained conditions.

[Most critical surface for each ground-water condition shown in bold type]

Type 

of

failure 

surface

Location of piezometric surface

Base

of

bluff

Top of Top of

Jackson Lafayette

Fm. Gravel

Top

of

bluff

Sloped

and 

perched

Stewart landslide

Circular

Irregular

Wedge, layer 5

Wedge, layer 6

Wedge, layer 7

Wedge, layer 8

Wedge, layer 9

Wedge, layer 10

Observed surface

1.90

1.95

4.06

4.24

2.46

3.81

2.83

2.40

1.96

1.66

1.69

3.98

4.03

2.28

3.39

2.48

2.10

1.73

1.61

1.64

3.76

3.80

2.14

3.23

2.38

2.03

1.67

1.35

1.32

2.83

2.79

1.47

2.51

1.88

1.68

1.40

1.82

1.87

4.03

4.23

2.45

3.72

2.71

2.25

1.88

CampbeU landslide

Circular

Irregular

Wedge, layer 5

Wedge, layer 6

Wedge, layer 7

Observed surface

2.33

2.40

2.44

3.32

2.41

2.88

1.77

1.82

2.40

2.99

1.87

2.86

1.66

1.71

2.17

2.78

1.76

2.60

1.35

1.35

1.30

1.96

1.32

1.53

2.05

2.41

3.33

2.09

2.75

Top of Top of

Lafayette Jackson

i I ,1

10 20 30 

WATER -TABLE LEVEL, IN METERS 

BELOW TOP OF BLUFF

40

Figure 11. Static, drained factor of safety versus water-table level 

for several failure geometries at the Stewart site.
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE Dll

Top of Top of

LafayetteJackson

1,1

10 20 30 40 

WATER-TABLE LEVEL, IN METERS 

BELOW TOP OF BLUFF

50

Figure 12. Static, drained factor of safety versus water-table 

level for several failure geometries at the Campbell site.

surfaces all lie well above the observed failure surface. 

For the Campbell site, the two most critical slip surfaces 

lie well below the observed surface and have factors of 

safety much lower than the observed surface and its adjacent randomly generated surface. This disparity between 

the most critical randomly generated slip surfaces and the 

observed surfaces further confirms that the existing landslides at the Stewart and Campbell sites did not form in 

aseismic, drained conditions.

DYNAMIC (SEISMIC) SLOPE STABILITY 

ANALYSIS

We use the dynamic displacement analysis developed 

by Newmark (1965), now used widely in engineering practice (Seed, 1979), to evaluate the seismic stability of the 

bluffs. Newmark's method models a landslide as a rigid 

friction block of known critical acceleration—the acceleration required to overcome frictional resistance and initiate 

sliding—on an inclined plane. The analysis calculates the 

cumulative displacement of the block as it is subjected to 

the effects of an earthquake acceleration-time history, and 

the user judges the significance of the displacement.

Laboratory model tests (Goodman and Seed, 1966) 

and analysis of earthquake-induced landslides (Wilson and 

Keefer, 1983) confirm that Newmark's method fairly accurately predicts slope displacements if slope geometry and 

soil properties are known and earthquake ground accelerations can be estimated. Newmark's method is a significant 

improvement over traditional pseudostatic stability analysis, which defines any exceedence of the critical acceleration, no matter how brief, as failure.

Newmark (1965) showed that the critical acceleration 

is a function of the static factor of safety and the landslide 

geometry:

ac=(FS-l)g sin a (1)

where

ac is the critical acceleration in terms of g, the acceleration due to Earth's gravity; 

FS is the static factor of safety; and 

a is the angle (herein called the thrust angle) from the 

horizontal that the center of mass of the potential 

landslide block first moves.

We used an algorithm to apply Newmark's method 

that is similar to that described by Wilson and Keefer 

(1983). Figure 15A shows a strong-motion record having 

a hypothetical ac of 0.2 g superimposed. To the left of

-215 METERS

Figure 13. Critical slip surfaces and their factors of safety for static, drained conditions at the Stewart site for the most likely groundwater condition. Solid line is actual failure surface.
D12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

-275 METERS

Figure 14. Critical slip surfaces and their factors of safety for static, drained conditions at the Campbell site for the most likely groundwater condition. Solid line is actual failure surface.

point X (fig. 15), the accelerations are less than ac, and no 

landslide displacement occurs. To the right of point X, 

those parts of the strong-motion record lying above ac are 

integrated over time to derive a velocity profile of the 

block. Integration begins at point X (figs. 15A, B), and the 

velocity increases to point Y, the peak velocity for this 

pulse. Past point Y, the ground acceleration drops below 

ac, but the block continues moving because of its inertia. 

Friction and ground motion in the opposite direction 

cause the block to decelerate until it stops at point Z. All 

pulses of ground motion exceeding ac are similarly integrated to yield a velocity profile (fig. 15B), which, in turn, 

is integrated to yield the cumulative displacement of the 

block (fig. 15Q.

For simplicity, we modified the algorithm of Wilson 

and Keefer (1983) to prohibit upslope displacement. This 

prohibition was justified by Newmark (1965), as well as 

other investigators (Franklin and Chang, 1977; Chang and 

others, 1984; Lin and Whitman, 1986), because the resistance of a slope to sliding is highly asymmetrical: ac in 

the upslope direction is generally so much greater than ac 

in the downslope direction that it can be assumed to be 

infinitely large. In most cases, the upslope ac is greater 

than the peak earthquake acceleration, and no error is 

introduced by prohibiting upslope displacement.

Conducting a Newmark analysis requires three pieces 

of information: the static factor of safety and the thrust 

angle of the potential landslide, both needed to calculate 

the critical acceleration, and an earthquake accelerationtime history.

FACTOR OF SAFETY

During earthquakes, slope materials behave in a socalled undrained manner; thus, layered models of the 

bluffs in undrained conditions were constructed. Because 

undrained shear strength depends in large part on consolidation stress, layers of roughly similar thickness were 

constructed that reflect the increase in shear strength with 

depth even for relatively homogeneous materials.

Different shear-strength parameters were used for various layers depending on the ground-water conditions. For

the Lafayette Gravel and the Pleistocene loess, drained 

strengths were used above the water table, undrained 

strengths below the water table. The Jackson Formation 

was considered saturated in all conditions because of its 

high clay content and consequent low permeability, and 

undrained shear strengths were used in all cases. The high

Q 50

81

50

B

10

oc 

zP o

C/3

5 10, 2 3 

TIME, IN SECONDS

Figure 15. Demonstration of the Newmark-analysis algorithm 

(adapted from Wilson and Keefer, 1983). Points X, Y, and Z are 

discussed in text. A, Strong-motion record with critical acceleration (dotted line) superimposed. B, Velocity of landslide block 

versus time. C, Displacement of block versus time.
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D13

Layer

1

2

3

4

5

6

7

9

10

11

12

13

| 1 __ ____

Unit

weight

(kN/m3)

19.3

19.6

19.6

19.6

22.0

22.0

18.1

1 Q 1

17.3

lo. I

18.1

1Q 1

18.1

—— ——— ——

Undrained

shear

strength

(kPa)

8.41

4.63

16.01

4.63

3.93

4.46

2.54

3.GU ^^* 

2 42 ^^

c Q4 Loess^ —————————————————— 3 —————————— O.U* ^^^ A

3.17 ________ -^- ———————————————————— ^ ———————————————

3.78 Lafayette Gravel^ ——————————————— - —————— g ———————————————

438 ^ 7

____________ ^ ———————————————————————————————————————— Q _CJ ———————————————————————————————

^-^ 12

——— 13

•300 METERS

Figure 16. Idealized model of pre-landslide bluff at Stewart site in undrained conditions.

undrained shear strength of the cemented loess layer makes 

the situation where the water table is at the top of the bluffs 

less critical than other situations.

STABL was used to generate potential failure surfaces 

and to determine the most critical failure surfaces in the 

same manner as described above for the drained stability 

analysis. Figures 16 and 17 show the undrained bluff models for the Stewart and Campbell sites, respectively; unit 

weights and undrained shear strengths for each layer are 

shown. Table 2 summarizes the results of the undrained 

stability analyses. The lowest factors of safety for the 

Stewart and Campbell sites are 1.53 and 1.32, respectively; 

both bluffs were statically stable in undrained conditions.

Figures 18 and 19 show the locations of the critical 

undrained failure surfaces for the Stewart and Campbell 

sites, respectively, in the most likely ground-water

condition. For the Campbell slide, the most critical surface, 

a wedge failure in layer five, coincides almost exactly with 

the observed failure surface. For the Stewart slide, all the 

slip surfaces, including the observed failure surface, plot 

very close to one another and have similar factors of safety. 

Both circular surfaces have large radii and approximate 

planar basal shear surfaces. The landslide geometry suggests (fig. 5) that translatory movement likely occurred 

along a weak layer within the Jackson Formation; thus, a 

wedge failure in one of the deeper layers might have been 

expected to produce the lowest factor of safety. A thin, 

weaker layer that was not detected in the drill holes may 

exist in the Jackson Formation along which sliding 

occurred, but the close proximity of the observed failure 

surface to the most critical surfaces generated by STABL 

indicates that the model generally agrees with observations.

Layer

Unit 

weight 

(kN/m3)

Undrained

shear

strength

(kPa)

300 METERS

Figure 17. Idealized model of pre-landslide bluff at Campbell site in undrained conditions.
D14 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Table 2. Static factors of safety from stability analyses of the 

Stewart and Campbell landslides in undrained conditions.

[Most critical surface for each ground-water condition shown in bold type]

Type

of

failure

surface

Location of piezometric surface

Base

of

bluff

Top of

Jackson

Fm.

Top of

Lafayette

Gravel

Top

of

bluff

Sloped

and

perched

Stewart landslide

Circular

Irregular

Wedge, layer 5

Wedge, layer 6

Wedge, layer 7

Wedge, layer 8

Wedge, layer 9

Wedge, layer 10

Observed surface

1.72

1.64

2.81

3.23

2.19

3.18

2.00

1.99

1.74

1.72

1.64

2.81

3.23

2.19

3.18

2.00

1.99

1.74

1.64

1.55

2.50

2.96

1.99

3.05

1.89

1.88

1.66

1.99

2.16

3.59

3.84

2.81

3.57

2.41

2.25

2.12

1.62

1.53

2.49

2.93

1.97

3.02

1.87

1.87

1.65

Campbell landslide

Circular

Irregular

Wedge, layer 5

Wedge, layer 6

Wedge, layer 7

Wedge, layer 8

Wedge, layer 9

Observed surface

1.90

1.79

1.44

1.85

1.84

1.76

1.67

2.63

1.90

1.79

1.44

1.85

1.84

1.76

1.67

2.63

1.79

1.74

1.32

1.76

1.77

1.71

1.64

1.57

2.55

3.12

3.68

3.19

2.97

2.60

2.39

5.64

1.79

1.74

1.32

1.76

1.77

1.71

1.64

1.57

The fact that the most critical computer-generated surfaces closely parallel the observed failure surfaces indicates 

that the bluff models are realistic and that the bluffs at the 

Stewart and Campbell sites probably failed in undrained 

conditions.

THRUST ANGLE

The thrust angle is the direction the center of gravity 

of the slide mass moves when displacement first occurs. 

For a planar slip surface parallel with the slope face (an 

infinite slope), this angle is the slope angle. For rotational 

movement, Newmark (1965) showed that the thrust angle 

is the angle between the vertical and a line segment connecting the center of gravity of the slide mass and the center of the slip circle.

Figure 18 shows geometric constructions of the 

thrust angles of the most critical surfaces for the Stewart 

slide. Thrust angles for these surfaces all are 15°-16°. 

The thrust angle of the observed surface is difficult to estimate because of the irregular shape of the surface and the 

consequent complex movement. We calculated an average 

inclination for the observed failure surface by weighting 

the inclinations of the line segments forming the observed 

surface by their relative lengths. This yields an average 

inclination of 16°, consistent with that of the other generated surfaces.

Centers of slip circles

215 METERS

Figure 18. Critical slip surfaces and their factors of safety in static, undrained conditions at the Stewart site for the most likely groundwater condition. Geometric construction to determine the thrust angle is shown. Solid line is actual failure surface.
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D15

275 METERS

Figure 19. Critical slip surfaces and their factors of safety in static, undrained conditions at the Campbell site for the most likely 

ground-water condition. Solid line is actual failure surface.

For the Campbell slide, the thrust angle is the inclination of the basal shear surface along which sliding 

occurred in layer five. The simple morphology of the slide 

and the coincidence of the most critical slip surface with 

the observed one justify this approach. Layer five dips 5°; 

this is used as the thrust angle.

EARTHQUAKE ACCELERATION-TIME HISTORY

Choosing a strong-motion record to represent the 

ground motions from the 1811-12 earthquakes is difficult 

because most available records are for California earthquakes, which probably differ in many respects from large 

earthquakes in the Central United States (e.g., Nuttli, 

1983). Differences in the propagation of strong ground 

motion, however, may not be as great as previously 

believed and appear to be significant only at great epicentral distances (>150 km) for very large earthquakes (Hanks 

and Johnston, 1992). Estimating ground-motion characteristics of the 1811-12 earthquakes at the Stewart and 

Campbell sites and comparing these estimates with existing earthquake records provides a basis for choosing an 

input ground motion. Peak ground acceleration (PGA), 

duration, and shaking intensity are used for this comparison, as described below.

Nuttli and Herrmann (1984) used instrumental data 

from several Central U.S. earthquakes to develop the following equation to relate mean horizontal PGA for soil 

sites in the Central United States to earthquake magnitude 

and source distance:

log a=0.57+0.50 rrib-0.83 

where

1/2-0.00069 R (2)

& is the PGA in centimeters per second squared, 

rrib is the body-wave magnitude, 

R is the epicentral distance in kilometers, and 

h is the focal depth in kilometers. 

For the 23 January and 7 February 1812 earthquakes, Nuttli's (1973) estimated epicenters (see fig. 1)

and magnitudes (rrib 7.1 and 7.4, respectively) are used to 

calculate epicentral distance. For the 16 December 1811 

earthquake, Obermeier's (1989) analysis of the distribution of earthquake-triggered liquefaction effects indicates 

an epicenter about 15 km southwest of Nuttli's estimated 

location; we use this location and Nuttli's (1973) estimated 

magnitude (mi, 7.2). We use 20 km, the approximate maximum depth of instrumentally recorded earthquakes in the 

seismic zone, for the focal depth. These input parameters 

yield PGA values at the Stewart and Campbell sites 

between about 0.4 g and 0.7 g (table 3).

PGA measures only a single point (the maximum 

acceleration, of whatever frequency) in an accelerationtime history and is thus a rather crude single measure of 

earthquake shaking intensity. A more comprehensive and 

quantitative measure of total shaking intensity developed 

by Arias (1970) is useful in seismic-hazard analysis and 

correlates well with the distribution of earthquake-induced 

landslides (Harp and Wilson, 1989). Arias intensity is the 

integral over time of the square of the acceleration, 

expressed as

la=n/2g (3)

where

Ia is the Arias intensity, expressed in units of velocity, and

a(f) is the ground acceleration as a function of time. 

Wilson and Keefer (1985) developed a simple relationship between Arias intensity, earthquake magnitude, 

and source distance:

(4)

where

Ia is in meters per second,

M is moment magnitude, and

R is earthquake source distance in kilometers. 

Equation 4 was developed from California earthquakes and may underestimate the shaking intensity in the 

Central United States. Table 3 shows Arias intensities
D16 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Table 3. Strong-motion records used to model ground shaking from the 1811-12 earthquakes at 

the Stewart and Campbell landslides.

[Characteristics of the 1811-12 earthquakes estimated as described in text All strong-motion records are from U.S. Geological 

Survey recording stations except for the Tabas, Iran, record (Hadley and others, 1983). M is moment magnitude (estimates for 

the 1811-12 earthquakes from Hamilton and Johnston, 1990); & is peak ground acceleration; 7* is duration of strong shaking as 

defined by Dobry and others (1978); Ia is Arias (1970) intensity; Off is Newmark (1965) displacement (range shown covers range 

of critical accelerations discussed in text)]

Earthquake

Recording site, component

M R & 

(km) fe)

T 

_(§)

'a

(m/s)

DN 

(cm)

Stewart landslide

16 Dec 1811, New Madrid, Mo.,

Stewart landslide site (estimated) 8.2 68

15 Oct 1979, Imperial Valley, Calif.,

El Centre differential array, 360° 6.5 7

24 Nov 1987, Superstition Hills, Calif., 

Superstition Mountain site 8,135° 6.5 6

23 Jan 1812, New Madrid, Mo.,

Stewart landslide site (estimated) 8.1 24

9 Feb 1971, San Fernando, Calif.,

Pacoima Dam, 164° 6.6 3

16 Sept 1978, Tabas, Iran,

74° 7.4 3

7 Feb 1812, New Madrid, Mo.,

Stewart landslide site (estimated) 8.3 44

24 Nov 1987, Superstition Hills, Calif., 

Superstition Mountain site 8,135° 6.5 6

16 Sept 1978 Tabas, Iran,

74° 7.4 3

2.7* 

0.39 20-40 2.7-5.5f

0.49 6.6

0.90 12.2

2.1 6-8

6.8 23-25

17.4* 

0.74 18^10 8.9-19.7t

1.22 6.7

0.71 16.1

9.1 50-55

10.0 39-44

8.2* 

0.71 25^10 ll.3-18.lt

0.90 12.2

0.71 16.1

6.8 23-25 

10.0 39-44

Campbell landslide

16 Dec 1811, New Madrid, Mo.,

Campbell landslide site (estimated) 8.2 59

24 Nov 1987, Superstition Hills, Calif., 

Parachute test site, 315° 6.5 1

24 Nov 1987, Superstition Hills, Calif., 

Superstition Mountain site 8,135° 6.5 6

23 Jan 1812, New Madrid, Mo.,

Campbell landslide site (estimated) 8.1 27

24 Nov 1987, Superstition Hills, Calif., 

Superstition Mountain site 8,135° 6.5 6

16 Sept 1978, Tabas, Iran,

74° 7.4 3

7 Feb 1812, New Madrid, Mo.,

Campbell landslide site (estimated) 8.3 49

24 Nov 1987, Superstition Hills, Calif., 

Superstition Mountain site 8,135° 6.5 6

16 Sept 1978, Tabas, Iran,

74° 7.4 3

3.6* 

0.44 20-40 3.5-7.0t

0.52 10.9

0.90 12.2

3.0 124-206

6.8 137-215

13.7* 

0.70 18-40 7.9-17.6t

0.90 12.2

0.71 16.1

6.8 137-215

10.0 278-486

6.6* 

0.65 25^10 9.5-15.2t

0.90 12.2

0.71 16.1

6.8 137-215 

10.0 278-486

* Estimated using equation 4. 

t Estimated using equation 5.

estimated from equation 4 using moment-magnitude esti- Arias intensity also correlates closely with the combimates from Hamilton and Johnston (1990) and source dis- nation of PGA and duration. R.C. Wilson (U.S. Geological

tances based on earthquake locations as described above. Survey, written commun., 1988) developed an empirical
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D17

equation using 43 strong-motion records to predict Arias 

intensity from PGA and a specific measure of duration:

7a=0.9 Td2 (5)

where

Ia is in meters per second, 

a is the peak ground acceleration in g's, and 

T is the duration (hereafter called Dobry duration) in 

seconds, defined by Dobry and others (1978) as 

the time required to build up the central 90 percent of the Arias intensity.

Estimating Arias intensities from the 1811-12 earthquakes 

using this method requires an estimate of the duration of 

strong shaking.

Because duration is a difficult parameter to estimate, 

we use a variety of methods and compare the results to 

estimate a probable range of durations of strong shaking. 

Dobry and others (1978) proposed an empirical relationship between duration and magnitude:

log7-=0.432M-1.83 (6)

where

Tis Dobry duration in seconds, and

M is unspecified earthquake magnitude (probably

local magnitude, ML).

In the magnitude range of interest, ML values are about 0.1 

magnitude points greater than 1% values. Thus, equation 6 

yields Dobry durations of 20, 18, and 25 s, respectively, for 

the three 1811-12 earthquakes (table 3).

Krinitzsky and Marcuson (1983) plotted bracketed 

durations (time between first and last acceleration pulses 

above 0.05 g) against MMI for different site conditions. 

Although few data plot in the MMI X-XI range of interest 

here, their curves suggest a bracketed duration between 40 

and 60 s. Bracketed durations at the 0.05-g level are about 

50 percent longer than Dobry durations in the magnitude 

range of interest (Dobry and others, 1978); thus, the lower 

value of 40 s is a reasonable estimate of the Dobry duration 

for all three 1811-12 earthquakes using this method.

We also examined estimated seismic source durations 

to verify that the ranges estimated above are realistic. 

Somerville and others (1987) developed theoretical and 

empirical relationships between earthquake moment and 

source duration for earthquakes in eastern North America. 

Using moment estimates for the 1811-12 events of 1028-2- 

1028-5 dyne-cm (A.C. Johnston, Memphis State University, 

written commun., 1991) yields source durations of 26-33 s, 

within the ranges estimated above.

We applied equation 5 to the PGA values and the 

upper and lower bound durations estimated above to estimate Arias intensities at the Stewart and Campbell sites for

the three 1811-12 earthquakes. Resulting Arias intensity 

estimates are shown in table 3. Estimates for the 16 

December 1811 and 23 January 1812 earthquakes using 

equation 4 are within the range of intensities estimated 

using equation 5, and intensity estimates for the 7 February 

1812 earthquake are somewhat lower. Both methods thus 

provide compatible intensity estimates that can be used to 

characterize shaking conditions at the two sites.

Although strong motion has not been recorded for 

earthquakes in the magnitude range of the 1811-12 events, 

several existing strong-motion records have shaking characteristics similar enough to estimated shaking characteristics of the 1811-12 events to be useful. We examined an 

extensive catalog of digitized strong-motion records, primarily from California earthquakes, and selected two 

records for each site for each of the three 1811-12 earthquakes (a total of six records for each site). Records were 

selected to match, as closely as possible, the estimated 

range of Arias intensities and PGA's from the 1811-12 

events so as to bracket the likely range of shaking conditions that actually occurred. None of the available strongmotion records have Arias intensities greater than 10 m/s; 

therefore, where estimated Arias intensities exceed this 

level, the available record having the greatest Arias intensity is used. Table 3 shows the records selected and compares some of their characteristics with those estimated 

for the Stewart and Campbell sites from the 1811-12 

earthquakes.

CALCULATION OF THE NEWMARK LANDSLIDE 

DISPLACEMENT

The static factor of safety from the undrained slope 

stability analysis and the thrust angle from the landslide 

geometry are combined in equation 1 to calculate the critical acceleration. The critical acceleration is then specified 

in the computer program that double integrates the strongmotion record to calculate the Newmark displacement.

The significance of the Newmark displacements 

must be judged in terms of the probable effect on the 

potential landslide mass. For example, Wieczorek and 

others (1985) used 5 cm as the critical displacement leading to catastrophic failure of landslides in San Mateo 

County, Calif.; Keefer and Wilson (1989) used 10 cm as 

the critical displacement for coherent landslides in southern California. When displacements in this range occur, 

most soils lose a significant amount of their shear strength 

and are in a residual-strength condition. Laboratory shearstrength tests on samples from the Stewart and Campbell 

sites indicate that residual strength is reached after a total 

shear displacement of about 6 cm (Jibson, 1985); therefore, the 5-10 cm range is reasonable for these landslides. If this amount of displacement is exceeded, static 

factors of safety using residual shear strengths can be
D18 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

calculated to determine the stability of the landslide mass 

after the earthquake shaking (and consequent inertial landslide displacement) ceases.

RESULTS OF DYNAMIC ANALYSIS OF 

STEWART LANDSLIDE

For the Stewart landslide, we calculated critical accelerations based on a thrust angle of 16° and on the factors of 

safety of the two circular slip surfaces in the perched and 

sloped ground-water conditions (FS=1.62, 1.64) because 

they most closely coincide with the observed surface and 

have the lowest factors of safety (fig. 18). Equation 1 

yields critical accelerations of 0.17-0.18 g for these input 

parameters. Newmark displacements were calculated for 

these two critical accelerations using the six strong-motion 

records listed in table 3.

Results of the Newmark analyses are shown in table 3. 

Displacements are between 6 and 55 cm and thus overlap 

the critical 5-10 cm range. Displacements generated by 

the model earthquakes for the 16 December 1811 event are 

between 6 and 25 cm; in this range, the likelihood of catastrophic failure is uncertain, but enough displacement probably occurred during this earthquake to at least partly 

reduce soil shear strength and actually reduce the critical 

acceleration of the slide mass in future earthquakes. The 

model earthquakes for the 23 January and 7 February 1812 

events generated Newmark displacements be tween 23 and 

55 cm, amounts that would almost undoubtedly reduce soil 

shear strengths to residual levels.

Static factors of safety for the Stewart slide were calculated using residual strengths in both drained and undrained conditions (Jibson, 1985), and, in all ground-water 

conditions, factors of safety were substantially less than 

1.0. Therefore, if the soils in the bluff reach residual 

strength, catastrophic failure will occur even in the absence 

of earthquake shaking. Because Newmark displacements in 

successive earthquakes are cumulative, the entire 1811-12 

sequence would have generated a minimum of about 1 m 

of displacement, which certainly would have reduced the 

strength of the bluff materials to residual levels and caused 

catastrophic failure.

RESULTS OF DYNAMIC ANALYSIS OF 

CAMPBELL LANDSLIDE

We computed the Newmark displacement of the 

Campbell slide using the following input parameters: (1) 

the static factors of safety from the undrained analysis in 

the perched and sloped ground-water condition for the 

observed failure surface (FS=1.57) and the most critical 

generated surface (FS=1.32) (fig. 19), (2) a thrust angle 

of 5°, and (3) earthquake acceleration-time histories from

table 3. The critical accelerations computed (equation 1) 

for the two failure surfaces are 0.03 and 0.05 g.

Results of the Newmark analysis are shown in table 3. 

Displacements are between 124 and 486 cm. Such large 

displacements undoubtedly would lead to reduction of soil 

shear strength to residual levels. The effect of the much 

lower critical acceleration of the Campbell slide relative to 

the Stewart slide is evident in the substantially larger Newmark displacements; any of the earthquakes modeled 

would generate large inertial displacements that would 

reduce soil strength to residual levels. Static slope stability 

analyses using residual strengths in both drained and 

undrained conditions all yield factors of safety well below 

1.0; therefore, catastrophic failure would occur after large 

earthquake-induced displacements in the slope.

SUMMARY OF STABILITY ANALYSES

The static stability analyses of the Stewart and Campbell landslides show that failure could not have occurred in 

any conceivable ground-water condition in the absence of 

earthquake shaking. The dynamic analyses show that 

earthquake shaking similar to that in 1811-12 would have 

induced large displacements almost certainly leading to 

catastrophic failure.

WHAT IF SEISMIC CONDITIONS ARE 

UNKNOWN?

If we knew nothing about the ground shaking in 

1811-12, or if we did not even know if earthquakes had 

ever occurred in that region, could analysis of these landslides tell us anything of their possible origin? The static 

stability analyses clearly show that failure in aseismic 

conditions is highly unlikely, and an earthquake origin 

could be hypothesized on that basis alone. The dynamic 

analysis could then be used to estimate the minimum shaking intensities necessary to have caused failure.

Such an approach requires a general relationship 

between critical acceleration, Arias intensity, and Newmark displacement. To estimate Newmark displacement 

associated with a given Arias intensity and critical acceleration, we selected 11 strong-motion records having Arias 

intensities between 0.2 and 10.0 m/s (table 4), which span 

the range between the smallest shaking intensities that 

might cause landslide movement and the largest shaking 

intensities ever recorded. For each strong-motion record, 

we calculated the Newmark displacement for several critical accelerations between 0.02 and 0.40 g, the range of 

practical interest for earthquake-induced landslides. The 

resulting data are plotted in figure 20. Data points for 

each critical acceleration plot fairly linearly in the log-log 

space of Arias intensity versus Newmark displacement. 

Best-fit lines from regression models for each value of
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D19

Table 4. Strong-motion records selected for analysis.

[M is moment magnitude, & is peak ground acceleration, T is duration as defined by 

Dobry and others (1978), and /„ is Arias intensity. All strong-motion records are from 

USGS recording stations except for the Tabas, Iran, record (Hadley and others, 1983)]

1000

Earthquake

Recording site, component

M & T Ia 

(g) (s) (m/s)

15 Oct 1979, Imperial Valley, Calif,

6.5

5.8

7.5

Coachella Canal, station 4, 135°

6 Aug 1979, Coyote Lake, Calif., 

Coyote Creek, San Martin, 250°

21 Jul 1952, Kern County, Calif, 

Taft School, 111°

6 Aug 1979, Coyote Lake, Calif,

Gilroy array, San Ysidro School, 270° 5.8

15 Oct 1979, Imperial Valley, Calif, 

Calexico Fire Station, 225° 6.5

1 Oct 1987, Whittier Narrows, Calif, 

Bulk Mail Center, 280° 6.0

15 Oct 1979, Imperial Valley, Calif,

El Centre differential array, 360° 6.5

24 Nov 1987, Superstition Hills, Calif, 

Parachute test site, 225° 6.5

15 Oct 1979, Imperial Valley, Calif, 

Bonds Corner, 230° 6.5

9 Feb 1971, San Fernando, Calif, 

PacoimaDam, 164° 6.6

16 Sep 1978, Tabas, Iran,

74° 7.4

0.13 10.4 0.20

0.21 3.8 0.25

0.14 17.7 0.46

0.23 8.5 0.60

0.28 11.1 0.86

0.45 5.5 1.23

0.49 6.6 2.12

0.46 10.1 4.15

0.79 9.8 6.00

1.22 6.7 9.08

0.71 16.1 9.96

1000

100 -

g < ai-

^ LU

<2 I

31°£ o

ARIAS INTENSITY, IN METERS PER SECOND

Figure 20. Newmark displacement as a function of Arias 

intensity for critical accelerations of 0.02-0.40 g. Best-fit 

regression lines for each critical acceleration shown.

critical acceleration have excellent fits (/?2 values between 

0.81 and 0.95), and the lines are roughly parallel and 

proportionately spaced, which suggests that a multivariate

°- 5 

w 1

ARIAS INTENSITY, IN METERS PER SECOND

Figure 21. Critical acceleration contours derived from multivariate regression of Arias intensity and critical acceleration 

versus Newmark displacement (equation 8).

model would fit the data well. Therefore, we constructed 

a multivariate regression model having the following form:

(7)

where

Off is Newmark displacement in centimeters,

Ia is Arias Intensity in meters per second,

ac is critical acceleration in g's,

A, B, and C are the regression coefficients, and

o is the estimated standard deviation of the regression

model. 

The resulting model has an R2 of 0.87:

,4601og/fl-6.642ac+l .546±0.409 (8)

Figure 21 shows critical acceleration lines derived 

from equation 8. This model yields the mean value of 

Newmark displacement when o is ignored; the variation 

about this mean, represented by o, results from the stochastic nature of earthquake ground shaking. Thus, even 

two strong-motion records having identical Arias intensities can produce significantly different Newmark displacements for slopes having the same critical acceleration.

The threshold earthquake shaking intensity necessary 

to trigger the Stewart and Campbell landslides can be estimated by judging the amount of Newmark displacement 

that would reduce shear strength on the failure surface to 

residual levels and cause catastrophic failure (the critical 

displacement). As discussed previously, critical displacements of about 10 cm are probably realistic for these types 

of slides, based on previous studies (Wieczorek and others, 

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

laboratory shear-strength testing of soil samples from our 

sites (Jibson, 1985), and field studies of landslides in the
D20 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

region (Jibson and Keefer, 1988). Inserting a displacement 

value of 10 cm and the critical accelerations of the Stewart 

(0.17-0.18 g) and Campbell (0.03-0.05 g) landslides into 

equation 8 yields a lower bound Arias intensity of about 

2.6 m/s to trigger the Stewart slide and about 0.6 m/s to 

trigger the Campbell slide.

The threshold earthquake magnitude (M) needed to 

trigger these landslides can be estimated using equation 

4. If we assume a minimum earthquake source distance 

of 5 km (the focal depth at the epicenter), the Arias intensities estimated above yield lower bound threshold 

moment magnitudes (M) of 5.9 for the Stewart landslide 

and 5.3 for the Campbell landslide. Lower bound threshold body-wave magnitudes (mb) can be estimated using 

equations 2, 5, and 6 in combination, which yield lower 

bound nib values of 5.8 for the Stewart slide and 5.4 for 

the Campbell slide. Although these magnitudes are much 

smaller than those we believe would have been recorded 

in 1811-12, they nonetheless provide a lower bound 

threshold in the absence of any additional information on 

the seismic shaking.

DISCUSSION

Several damaging earthquakes have occurred in the 

New Madrid seismic zone since the 1811-12 sequence. 

Nuttli (1982) described the 20 most damaging events since 

1812 and estimated their epicentral locations and bodywave magnitudes. Could any of these more recent earthquakes have triggered the landslides we studied? We 

answer this question by using equation 8 to estimate the 

effects from these earthquakes at the Stewart and Campbell 

landslide sites.

We selected the earthquakes from Nuttli (1982) that 

would have produced the greatest shaking intensities at 

the Stewart and Campbell sites. We selected the largest 

earthquake (mb=6.2) and then eliminated all earthquakes 

of lesser magnitudes that were farther from our sites. For 

earthquakes closer to our sites than the 1115=6.2 event, we

selected the next largest magnitude and eliminated all 

earthquakes of lesser magnitude that were farther from 

our sites. Using this iterative approach, we selected 3 of 

the 20 earthquakes that would have produced the greatest 

shaking intensities at the Stewart and Campbell sites 

(table 5).

We used equations 4 and 5 to estimate the range of 

possible Arias intensities that would have been produced 

from each of these earthquakes at each of our sites. Equation 4 requires the magnitude and distance, which are 

taken from Nuttli's (1982) estimates. Equation 5 requires 

knowing the PGA and the duration of strong shaking 

(defined as previously by Dobry and others, 1978); we 

estimated the maximum PGA using equation 2 and the 

duration using equation 6. Table 5 shows characteristics 

of the three selected earthquakes and the estimated range 

of PGA's and Arias intensities at the Stewart and Campbell sites. All of the earthquakes would have produced 

PGA's at the Stewart site much less than the critical acceleration necessary to initiate landslide movement (0.17- 

0.18 g); therefore, none of these earthquakes could have 

triggered any landslide movement there. At the Campbell 

site, the PGA's are all at or slightly above the critical 

acceleration (0.03-0.05 g), and thus we need to determine 

if the estimated Arias intensities would have been sufficient to generate enough displacement to cause catastrophic failure.

Combining the greatest estimated Arias intensity for 

the Campbell site from table 4 (0.077 m/s) with the 

range of critical accelerations for the Campbell slide 

(0.03-0.05 g) in equation 8 yields Newmark displacements of 0.4-0.5 cm, an order of magnitude smaller than 

the critical displacement levels of 5-10 cm required to 

cause catastrophic failure. Thus, we conclude that no 

earthquakes since 1812 could have triggered catastrophic 

movement of the Campbell slide.

Equation 8 and figure 21 can be applied to estimate 

the dynamic performance of any slope of known critical 

acceleration because they are derived from generic 

values of critical acceleration that are not site specific.

Table 5. Most damaging earthquakes since 1811-12.

[mb is body-wave magnitude from Nuttli (1982); ML is local magnitude and M is moment magnitude, both converted from mb 

as suggested by Heaton and others (1986); 7* is duration estimated from equation 6; R is epicentral distance to each site using 

locations from Nuttli (1982); & is peak horizontal ground acceleration estimated from Nuttli and Hemnann (1984); Ia is Arias 

intensity]

Stewart landslide

Date

31 Oct 1895 

4Novl903 

21 Aug 1905

"h,

6.2 

5.3 

5.0

ML

6.6 

5.6 

5.4

M

6.8 

5.5 

5.4

T 

(s)

10.5 

3.9 

3.2

R 

(km)

% 

86 

75

a 

(g)

0.09 

0.04 

0.03

jl j2 

a a 

(m/s) (m/s)

0.077 0.054 

0.006 0.003 

0.003 0.004

Campbell landslide

R 

(km)

103 

94 

82

a 

(g)

0.09 

0.03 

0.03

V 

(m/s)

0.077 

0.003 

0.003

1? 

(m/s)

0.047 

0.003 

0.003

1 Estimated using equation 4.

2 Estimated using equation 5.
ANALYSIS OF THE ORIGIN OF LANDSLIDES IN THE NEW MADRID SEISMIC ZONE D21

Thus, several types of regional hazard analyses for earthquake-triggered landslides can be developed using equation 8. A common type of regional earthquake-hazard 

analysis involves estimating the effects from a proposed 

model earthquake of given magnitude and location. In 

such a scenario-based hazard analysis, the seismic stability of slopes of known critical acceleration can be estimated by using (1) equation 4 to estimate the Arias 

intensity from the postulated earthquake magnitude and 

location and (2) equation 8 to calculate the Newmark displacement. An alternative approach is to use (1) equation 2 (or a similar attenuation equation appropriate for 

the region of interest) to estimate the PGA, (2) equation 

6 to estimate the duration, (3) equation 5 to calculate the 

Arias intensity from the PGA and duration, and (4) equation 8 to calculate the Newmark displacement.

In previous applications of the Newmark method to 

regional seismic-hazard analysis (Wieczorek and others, 

1985; Wilson and Keefer, 1985; Keefer and Wilson, 

1989), estimates of the amount of Newmark displacement that would result in damaging or catastrophic 

failure—critical displacements—have been made for different slope materials. As stated previously, critical 

displacements of 5-10 cm were assigned for materials in 

San Mateo County, Calif., and other similar materials in 

southern California. Thus, if critical displacements for 

slope materials in an area can be estimated using laboratory testing or field observations, then equation 8 can be 

used to estimate (1) the threshold Arias intensity required 

to initiate failure of slopes of given critical acceleration 

and (2) the critical acceleration below which slopes will 

fail for a given shaking intensity. These approaches can 

be applied in several ways to estimate regional seismic 

slope stability. This represents an incremental improvement over the previous methods, which assumed a uniform level of ground shaking throughout a region 

(Wieczorek and others, 1985).

If the Stewart and Campbell landslides are typical of 

block slides and earth flows in the New Madrid region, 

then the shaking-intensity and magnitude thresholds discussed previously could be considered lower bound conditions for triggering large landslides of these types in future 

earthquakes. Thus, an Arias intensity of about 2.6 m/s, 

corresponding to a minimum M=5.9 or mb=5.8 earthquake, 

would be necessary to trigger block slides similar to the 

Stewart slide. For earth flows similar to the Campbell 

slide, an Arias intensity of 0.6 m/s, corresponding to a minimum M=5.3 or mb=5.4 earthquake, would be required. 

Obviously, larger earthquakes are required to trigger landslides at greater epicentral distances and across a large 

area.

In this paper, we have developed and successfully 

applied a method for determining if a landslide or group of 

landslides of unknown origin was seismically triggered. 

An important potential application of this method is in

paleoseismology. If a group of landslides in a region can 

be shown to be statically stable but dynamically unstable, 

an earthquake origin can be inferred. If such landslide features can be dated, a date for the triggering earthquake can 

be determined. Dynamic analysis of such landslides can 

yield minimum shaking intensities that would have been 

required to trigger failure. In areas such as the Central and 

Eastern United States, where exposures of recent faults are 

absent or yield equivocal interpretations, groups of old 

landslides that can be shown to be triggered by earthquake 

shaking might be used for paleoseismic dating. If more 

than one generation of such features exists, earthquake 

return periods could be established. The possibility of such 

an application exists in the New Madrid seismic zone, 

where we (Jibson and Keefer, 1988) have mapped many 

features of questionable origin, some of which may be an 

older, more eroded set of landslides.

SUMMARY AND CONCLUSIONS

Static and dynamic analyses of two large landslides 

that are representative of coherent block slides and earth 

flows in the New Madrid seismic zone show that neither 

could have moved in the absence of earthquake shaking, 

even in unrealistically high ground-water conditions. 

During earthquake shaking similar to that generated in 

1811-12, both would have experienced large displacements leading to catastrophic failure. Analysis of shaking intensities generated by the largest earthquakes since 

the 1811-12 sequence indicates that no earthquakes since 

then could have triggered the observed slide movement. 

Therefore, we conclude that the Stewart and Campbell 

landslides formed during the 1811-12 earthquakes. These 

analyses confirm the results of our previous studies 

(Jibson and Keefer, 1988, 1989) that indicated spatial 

dependence of the landslides on the locations of the 

1811-12 earthquakes.

Back calculations of threshold shaking intensities 

required to cause catastrophic failure indicate that earthquakes of mb=5.4 or M=5.3 are required to trigger earth 

flows similar to the Campbell slide and that earthquakes of 

nib=5.8 or M=5.9 are required to trigger coherent slides 

similar to the Stewart slide.

The method for determining the likely conditions— 

seismic or aseismic—that triggered failure of these landslides can be applied to landslides in other areas. In some 

cases, it may impossible to determine an earthquake origin 

for a landslide or group of landslides if failure in aseismic 

conditions cannot be ruled out. In cases where an earthquake origin can be demonstrated for a landslide, establishing the age of the sliding will indicate the timing of the 

triggering earthquake. Back analysis of minimum shaking 

intensities will establish a lower bound for the triggering
D22 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

earthquake. Hence, the methods described in this paper 

can find useful application in paleoseismic studies.

The empirical relationship developed between Newmark displacement, Arias intensity, and critical acceleration also can applied generally to a wide variety of 

seismic-hazard problems related to slope instability. 

Specifically, if a model earthquake of designated location 

and magnitude is postulated, then the Newmark displacement of slopes of known critical acceleration can be estimated. If the critical displacement is known, then the 

shaking intensity required to cause failure of a slope of 

known critical acceleration can be estimated, or, for a 

given shaking intensity, the critical acceleration below 

which slopes will fail can be determined.

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication August 11,1993 

Edited by Richard W. Scott, Jr. 

Graphics prepared by Wayne Hawkins 

Photocomposition by Marie Melone

U.S. GOVERNMENT PRINTING OFFICE: 1994-573-038/86018