The Lorna Prieta, California, Earthquake of 

October 17, 1989-Forecasts 

By RUTH A. HARRIS 

EARTHQUAKE OCCURRENCE 

WILLIAM H. BAKUN AND WILLIAM H. PRESCOTT, Coordinators 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1550-B 

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

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Thomas J. Casadevall, Acting Director 

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 

Manuscript approved for publication, February 9, 1998 

Text and illustrations edited by George A. Havach 

Library of Congress catalog-card No. 92-32287 

For sale by the 

U. S . Geological Survey 

Information Services 

Box 25286 

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Denver, CO 80225 
PREFACE 

There was a time when the weather belonged to the gods. Storms and 

drought were inflicted on man in punishment or for vengeance, man strove 

to avert them by sacrifice or prayer, and the priest was his intercessor. Now 

the weather belongs to nature, and the priestly robe has fallen on the Weather 

Bureau. Man's new agent, however, is not an intercessor; he does nothing to 

placate; he makes no attempt to control the source of nature; but inspired by 

science he foretells the coming changes so that his lay client may take warning and be prepared. The crops are harvested before the rain, the herds escape from the lowland before the flood, the ships reach harbor before the 

gale; and man chants a hymn of praise to science. 

There was a time when the earthquake was equally enveloped in 

mystery, and was forecast in the enigmatic phrases of the astrologer and 

oracle; and now that it too has passed from the shadow of the occult to the 

light of knowledge, the people of the civilized earth-the lay clients of the 

seismologist-would be glad to know whether the time has yet come for a 

scientific forecast of the impending tremor. The outlook for earthquake-forecasting is my theme to-day. 

-From G.K. Gilbert, "Earthquake Forecasts," Science, new series, v. 29, no. 734 (Jan. 22, 1909), p. 121 

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

EARTHQUAKE OCCURRENCE 

CONTENTS 

FORECASTS 

By Ruth A. Harris 

preface ........................................................................ 

Abstract ....................................................................... 

Introduction .................................................................. 

The forecasts---------------------------------------------------------------- 

Reid------------------------------------------------------------------ 

forecasts ......................................................... 

1906 slip models-geologic information----------------------- 

1906 slip models-geodetic information----------------------- 

San Andreas fault-slip-rate measurements---------------------- 

Was the 1989 Loma Prieta earthquake an anticipated event?--------- 

1 Definition of an earthquake advisory------------------------- 

2. The Lake Elsman advisory ..................................... 

3. Peck and Pearlman correspondences------------------------- 

Page 

I11 

B 1 

B 1 

B 5 

B 5 

B 6 

B6 

B7 

B7 

B7 

B7 

B8 

B 9 

B10 

B 10 

Bll 

B13 

B15 

B17 

B 17 

B18 

B 19 

B19 

B20 

B21 

B2 1 

B2 1 

B22 

B 22 

B23 

B25 

B25 

B26 

ABSTRACT 

The magnitude (M) 6.9 Loma Prieta earthquake struck 

the San Francisco Bay region of central California at 5:04 

p.m. P.d.t. on October 17, 1989, killing 62 people and generating billions of dollars in property damage. Scientists were 

not surprised by the occurrence of a destructive earthquake 

in this region and had, in fact, been attempting to forecast 

the location of the next large earthquake in the San Francisco Bay region for decades. This paper summarizes more 

than 20 scientifically based forecasts made before the 1989 

Loma Prieta earthquake for a large earthquake that might 

occur in the Loma Prieta area. The forecasts geographically 

closest to the actual earthquake primarily consisted of rightlateral strike-slip motion on the San Andreas fault northwest 

of San Juan Bautista. Several of the forecasts did encompass 

the magnitude of the actual earthquake, and at least one approximately encompassed the along-strike rupture length. The 

1989 Loma Prieta earthquake differed from most of the forecasted events in two ways: (1) it occurred with considerable 

dip-slip in addition to strike-slip motion, and (2) it was much 

deeper than expected. 

INTRODUCTION 

Was the 1989 Loma Prieta earthquake (fig. 1) a predicted 

event? Was it an expected or anticipated event? Was it formally forecasted, and, if so, how well, in what sense, and by 

whom? The answers to these question, as detailed below, 

show that this earthquake had been forecasted by many 

groups, in varying ways and with varying degrees of success 

and accuracy, during the eight decades that preceded it. Over 

this period, our understanding of earthquakes has evolved 

tremendously and, along with it, our ability to model and 

forecast some aspects of future earthquake activity. As our 

understanding of earthquakes has evolved, so have our ideas 

about how to define an earthquake forecast. A review of the 

published forecasts of the 1989 Loma Prieta earthquake 

makes this evolutionary path clear. Thus, this earthquake is 

an important milestone along this path whereby we can assess the progress, note the successes and failures, and speculate on the future of earthquake forecasting. 

An earthquake forecast is defined as a statement that an 

earthquake is expected to occur within a period of a few years 

to a few decades (Wallace and others, 1984). Scientifically 

based earthquake forecasting is not new to our lifetimes; instead, evidence points to earlier roots. In the late 19th century, the great earth scientist Grove Karl Gilbert (1 843-19 18) 

forecast where the next large earthquake would strike the 

Wasatch Front (Utah) and the east side of the Sierra Nevada 

(California) (Gilbert, 1884). In 1909, he delivered a lecture 

entitled "Earthquake Forecasts" as his presidential address 
B2 FORECASTS 

to the American Association of Geographers (Gilbert, 1909). 

His lecture, published in the journal Science, encompassed 

the specific details required for a successful forecast, the importance of local ground conditions (site effects) for assessing damage potential, and the insurance rates for an 

earthquake-prone region. Gilbert (1909, p. 133) declared that it 

was not difficult to predict the locations of future damaging earthquakes but that the timing of these events was still an enigma: 

In a word the determination of danger districts and danger 

spots belongs to the past, the present and the near future; the 

determination of times of danger belongs to the indefinite future. The one lies largely within the domain of accomplishment; 

the other still lingers in that of endeavor and hope.. . We may congratulate ourselves that it is not the place factor 

which lags behind, for knowledge of place has far more practical value than knowledge of time. In fact I see little practical 

value in any quality of time precision attainable along lines of 

achievement now seen to be open.. .Or suppose that prelude 

phenomena should be found to afford real warning; the forecaster on duty would still have to deal in probabilities, and when 

in doubt would often sound vain warnings, in the conscientious 

effort to escape the greater error of omission at the critical timePACIFIC 

OCEAN 

Figure 1 .-Active faults in California and the San Francisco Bay region. A, Active faults in California (from Jennings, 1992). Box outlines 

area of figure IB. North half of the San Andreas fault (heavy line; from Thatcher and others, 1997) broke in the great 1906 San Francisco 

earthquake from Cape Mendocino (M) to Juan Bautista (SJB) (Ellsworth and others, 1981). B, Active faults in the San Francisco Bay region 

and southeast (from Jennings, 1992). BM, Black Mountain; BV, Bear Valley; CF, Calaveras fault; GVF, Green Valley fault; HF, Hayward 
and again nervous strain would be wasted. And even if warning 

were definite, timely and infallible, so that peril of life could be 

altogether avoided, property peril would still remain unless construction had been earthquake-proof. If, on the other hand, the 

places of peril are definitely known, even though the dates are 

indefinite, wise construction will take all necessary precautions, 

and the earthquake-proof house not only will insure itself but 

will practically insure its inmates. 

During the 88 years since Gilbert's (1909) paper, numerous earthquake forecasts have been made for earthquakeprone regions around the world. These forecasts commonly 

are hotly debated. Earthquake prediction remains a topic of 

intense discussion (for example, Sykes, 1996; Geller, 1997; 

Geller and others, 1997; Scholz, 1997) as we enter the closing years of the 20th century. 

In the following sections, I review 18 studies published 

between 1910 and 1989 that variously offer or relate to scientific forecasts of the 1989 Loma Prieta earthquake. I attempt to reconstruct what some scientists were writing and 

saying before the earthquake, to review what data they relied 

upon and what assumptions they made, and to compare the 

various forecasts with what actually occurred during the earthfault; L, Livermore; LP, Loma Prieta; MR, Mussel Rock; P, Parkfield; PG, Pajaro Gap; RCF, Rodgers Creek fault; SAF, San 

Andreas fault; SAFC, creeping section of the San Andreas fault; SFB, San Francisco Bay; SGF, San Gregorio fault; SJ, San 

Jose; SJB, San Juan Bautista; W, Watsonville; WT, Wrights Tunnel. The Maacama fault is north of the RCF. 
FORECASTS 

Table 1 .-Summary of pre-1989 forecasts for large earthquakes in the Loma Prieta area 

[SAF, San Andreas fault; SJB, San Juan Bautista] 

Reference Location Magnitude Probability Time period Notes 

(M) (percent) 

Reid 

(1910) 

SAP 

Tocher 

(1959) 

Thenhaus and 

others (1 980) 

Ellsworth and 

others (1981) 

U.S. 

Geological 

Survey 

(1981) 

Moths and 

others (1 98 1) 

Lindh and 

others (1 982a, 

b) 

Lindh 

(1 983) 

Sykes and 

Nishenko 

(1 984) 

Scholz 

(1985) 

Keilis-Borok 

and others 

(1986b) 

Thatcher and 

Lisowski 

(1 987) 

Wesson and 

Nicholson 

(1 988) 

San Francisco Bay region, 

Monterey County and southern 

part of Santa Clara County 

San Francisco Bay area 

SAF 

SAP: Peninsula segment 

SAP: Entire 1906 rupture 

SAP: Pajaro Gap and NW 

(fig. 2) 

SAF: Pajaro Gap and 40 km 

NW. (40 km long) 

SAP: Peninsula segment 

SAP: San Juan Bautista segment 

(- 40-45 km long) 

(fig. 2) 

SAF: San Francisco to San Jose 

SAP: San Jose to SJB (75 km) 

(fig. 3) 

SAF: Black Mountain to SJB (75 

km) 

(fig. 3) 

7 x9 degree rectangle centered at 

37.5 deg. N., -1 19.5 deg. E. 

(fig. 4) 

SAP: SJB and NW. (90 km 

long) 

SAP: SJB and NW. (45 km 

long) 

SAP: SJB and NW. (30 km 

long) 

(fig. 6) 

SAP: Northwest of creeping 

section, near SJB, Watsonville, 

and Corralitos (NW. of SJB) 

(fig. 7) 

Important 

event 

Severe 

earthquake 

Large event 

6.4<Ms<1.0 

7.05aS7.6 

7.65a58.2 

Large event 

8.3 

6+ 

6+ 

7 

6.5 

--- 

Mw=7.0 

&=6.9 

27.5 

Annual 

--- 

0- 100 years 

Jan. 84-Dec. 

88 

Annual 

Annual 

Annual 

--- 

Annual 

--- 

--- 

982-2002 

982-2002 

983-2003 

983-2003 

--- 
THE FORECASTS 

Table 1 .-Summary of pre-1989 forecasts for large earthquakes in the Loma Prieta areaÃ‘Continue 

Reference Location Magnitude Probability Time period Notes 

(M) (percent) 

Keilis-Borok 

(in Updike, 

1989) 

Lindh 

(1988) 

Working 

Group on 

California 

Earthquake 

Probabilities 

(1988) 

James Davis 

(unpub. data, 

1988); 

California 

Office of 

Emergency 

Services 

3.8'-radius circle centered at lat 

37.5' N., long 1 19S0W. 

2.5'-radius circle centered at lat 

38' N., long 122' W. 

2.5'-radius circle centered at lat 

36' N., long 120' W. 

(fig. 5) 

SAF: San Francisco Peninsula 

segment 

SAP: Loma Prieta segment 

(- 40-45 km long) 

SAF: Southern Santa Cruz 

Mountains segment (35 km long) 

SAP - SJB and NW. (90 km 

long) 

(fig. 8) 

SAF: Santa Cruz Mountains 

segment 

(fig. 9) 

until Dec. 91 -- 

Jul. 84-Jul. 89 -- 

Jan. 85-Jan. 3 

90 

5 days after -- each M5 event 

(1988; 1989) 

1. Business-insurance risk given as an average of one severe shock in 30 years. 

2. Business-insurance risk given as 30 years between groups of severe shocks, with the last 

"group" ending in 1906. 

3. Magnitude of forecast approximately correct. 

4. Rupture length of forecast approximately correct. 

5. Value in parentheses is the preferred value. 

6. Value in parentheses is based on repeat times derived from the historicallprehistoric record. 

7. Stated that the probability was the same as that of Lindh (1983) and Sykes and Nishenko 

(1984). 

8. Defined region where an MS5.7 earthquake might initiate (nucleate). 

quake. These forecasts vary widely, ranging from vague to 

specific, and many of them address only one or a few aspects 

of an anticipated event while saying nothing about other aspects. Because of these disparities, it is neither possible nor 

desirable to try to "rate" their comparative success on the 

basis of the occurrence of the 1989 Loma Prieta earthquake. 

Instead, I present these reviews to document the early evolution of earthquake forecasting in central California, to highlight what worked and what didn't in these early forecasting 

attempts, and to infer insights about possible future directions for predictive studies. 

and fault regions of future earthquakes. One study could even 

be considered an "anti-forecast," in that it stated where a large 

earthquake would be highly unlikely to occur during a specific time period. The studies reviewed in this section are 

listed in order of their publication date. My main purpose 

here is to present as objectively as possible what was thought 

and recorded before the 1989 Loma Prieta earthquake. Therefore, I include only documented information predating October 17, 1989. Relevant parameters of the various forecasts 

are summarized in table 1. 

REID 

THE FORECASTS 

Certain ideas that prevailed in the decades before the 

1989 Loma Prieta earthquake led some scientists to make 

detailed forecasts for earthquakes on specific faults, and others to speculate more generally about the possible time frames 

The great 1906 San Francisco earthquake ruptured more 

than 400 km of the San Andreas fault in central and northern 

California, from San Juan Bautista to Cape Mendocino (fig. 

1; Ellsworth and others, 1981; Thatcher and others, 1997). 

Earthquake forecasting in the San Francisco Bay region 
B6 FORECASTS 

came of scientific age shortly thereafter with Gilbert's (1909) 

paper and Harry Fielding Reid's (1859-1944) short section 

entitled "The Prediction of Earthquakes" in Lawson's (1908) 

comprehensive report on the 1906 earthquake (Reid, 19 10). 

Reid, a scientist at the Johns Hopkins University, was quite 

optimistic that future earthquakes could be predicted; he had 

just witnessed the clean signature of the 1906 earthquake on 

geodetic markers at the Earth's surface ,and used the data to 

develop his theory of elastic rebound. This pioneering theory, 

which assumes a simple mechanical model of accumulating 

elastic strain and sudden slip on faults, still forms a basis for 

our mechanical understanding of earthquakes today. Reid's 

observations led him to propose one of the first forecasts of 

future earthquakes on the San Andreas fault (Reid, 1910, p. 32): 

It seems probable that a very long period will elapse before 

another important earthquake occurs along that part of the San 

Andreas rift which broke in 1906; for we have seen that the strains 

causing the slip were probably accumulating for 100 years. There 

have been no serious earthquakes reported along this part of the 

rift, except at its southern extremity, since the country has been 

occupied by white men, altho strong earthquakes have occurred 

in neighboring regions. It seems probable that more consistent 

results might be obtained regarding the periodicity of earthquakes 

if only the earthquakes occurring at exactly the same place were 

considered in the series ... . It is quite possible, however, for strong earthquakes to occur 

on neighboring faults after short intervals. The ruptures of the 

Haywards [sic] fault in 1868 and of the San Andreas fault in 

1906 are a fair example, tho the interval is rather long .... 

In 1923 and 1924, Bailey Willis, president of the Seismological Society of America, wrote several papers outlining his thoughts on earthquake risk in California (for example, 

Willis, l923b, 1924). Among these papers, Willis (1924), calculated the likelihood of earthquakes in different parts of the 

State. He divided California into five earthquake provinces, 

then further subdivided three of these provinces, which he 

thought to be seismically active, into districts (see Wood, 

1916). The districts closest to Loma Prieta included the Bay 

district, which encompassed San Francisco Bay, and the 

Monterey-Santa Clara district, which encompassed parts of 

Monterey and southern Santa Clara Counties and included 

the towns of Gilroy, Hollister, San Juan Bautista, Salinas, 

Monterey, and Santa Cruz. 

Willis (1924) examined the historical record of earthquakes in these two districts from 1850 to 1906. He counted 

seven earthquakes exceeding intensity VIII for the Bay district (1858, 1865, 1868, 1889, 1892, 1898, 1906) and six 

earthquakes for the Monterey-Santa Clara district (1865, 

1868, 1890, 1897, 1903, 1906). He argued that liability (or 

damage) is confined to the area immediately surrounding a 

fault. Therefore, because the aforementioned earthquakes 

occurred on different faults-for example, in the Bay district, the 1858, 1865, and 1906 earthquakes were on the San 

Andreas fault or nearby minor faults, the 1868 and 1898 earthquakes were on the Hayward fault, and the 1889 and 1892 

earthquakes were on another fault-the calculated risk should 

not simply be seven shocks in 56 years. Instead, he proposed 

that the local risk should be one shock in 30 years, at least for 

business-insurance purposes. He added the suggestion that 

the 1906 earthquake had lengthened the interval of time to 

more than 30 years. 

Willis (1924) also calculated the insurance risk of an 

earthquake in the Monterey-Santa Clara district. For this 

district, he assigned a 30-year interval between (groups of) 

severe earthquakes. He obtained this number from two observations. First, there had been 18 years (1906-24) of relative 

quiescence since the 1906 earthquake. Second, excluding the 

1890 earthquake, which he stated was just a local event, widely 

felt earthquakes had occurred in 1865, 1868, and 1897. He 

used this collection of late-19th-century earthquakes to arrive at 29 years between 1868 and 1897. He then proposed 

that more damaging earthquakes would occur 10 years in the 

future (from the time of his writing in 1924). 

TOCHER 

Three decades after Willis' 1923 and 1924 papers, Don 

Tocher, then a seismologist at the University of California, Berkeley, studied the moderate (M=5.3) 1957 San 

Francisco earthquake, which damaged buildings in the San 

Francisco Bay region, and commented on its relation to 

the pattern of seismicity on regional faults (Tocher, 1959). 

The two earthquakes of greatest energy release since 1850 

(the beginning of the reliable historical record) were the 

1868 earthquake on the Hayward fault and the 1906 earthquake on the San Andreas fault (Tocher, 1959). He noted 

the following pattern: In the 18 years up to and including 

1868, 12 "strong" earthquakes had occurred, whereas in 

the 13 years after 1868 (1 869-8 I), no "strong" earthquakes 

had occurred. Then, in the 25 years up to and including 

1906, 26 "strong" earthquakes had occurred, whereas in 

the 47 years after 1906 (1907-53), only 10 "strong" earthquakes had occurred. Given this catalog of strong earthquakes in the San Francisco Bay region, Tocher suggested 

that it would be wise to prepare for an upcoming big event 

anytime the rate of "moderately strong" earthquakes approached one event per year. In the period 1954-57, the 

rate did approach one event per year, with one strong earthquake in 1954 (M=5.3), two in 1955 (M=5.5, 5.4), and 

one in 1957 (Mz5.3). After the series of events in those 4 

years, Tocher (1959, p. 48) wrote: "Whether or not these 

are the first of a series which will continue for a number 

of years and culminate in a major shock remains to be 

seen. The historic record is too short and too uncertain in 

its details for such a definitive statement; a major earthquake could hit San Francisco before these lines reach the 
THE FORECASTS \ B7 

printer, or there might not be such a shock in the next hundred years." 

THENHAUS AND OTHERS 

By the end of the 1970's, earthquake scientists and engineers were following Gilbert's (1909) advice and were 

collaborating to produce estimates of the expected damaging ground motion from future large earthquakes. Paul 

Thenhaus and his colleagues at the U.S. Geological Survey 

(Thenhaus and others, 1980) produced maps of maximum 

expected seismic horizontal ground motion for rock sites in 

coastal and offshore California. To calculate the ground motion, they needed, first, to estimate the potential sources of 

ground motion, that is, the distribution of moderate to large 

earthquakes on nearby active faults. They estimated annual 

occurrence rates of earthquakes for several seismogenic 

zones, including the San Andreas fault. Potential earthquake 

sources in the San Francisco Bay region were located in a 

zone encompassing much of the San Andreas fault, including segments in southern California (their zone 24) and another wider and more easterly zone (their zone 38) that 

encompassed many known active faults, including the Hayward fault. Thenhaus and others estimated annual earthquakeoccurrence rates for various magnitude intervals within each 

seismogenic zone. Thus, in zone 24, the San Andreas fault, 

an earthquake of 6A<M<7.0 was assigned an annual occurrence rate of 0.040 (event/year), an earthquake of 7.0<Ms<7.6 

was assigned an annual occurrence rate of 0.015, and an earthquake of 7.6SMs<8.2 was assigned an annual occurrence rate 

of 0.0057 (table 1). In comparison, in zone 38, an earthquake 

of 6.4<Ms<7.0 was assigned an annual occurrence rate of 

0.0057, an earthquake of 7.0<M<,7.6 was assigned an annual occurrence rate of 0.00165, and an earthquake of 

7.6<Ms<8.2 was assigned an annual occurrence rate of 

0.000475. 

ELLSWORTH AND OTHERS 

By the 1980's, the theory of plate tectonics had become 

widely accepted by the scientific community. In 198 1, William Ellsworth and his coworkers at the U.S. Geological Survey (Ellsworth and others, 1981) reevaluated the temporal 

pattern of earthquake activity surrounding the great 1906 San 

Francisco earthquake in light of the accepted scientific theory. 

In the half-century before the 1906 earthquake, many more 

strong earthquakes were felt and recorded than in the halfcentury after this earthquake, leading to an apparent quiescence after 1906. Then, starting in 1955, the rate of M>5 

earthquakes appeared once again to have increased (Tocher, 

1959). Ellsworth and others (1981) commented that this pattern was consistent with the idea of a seismic cycle (Fedotov, 

1965; Mogi, 198 I), consisting of a great earthquake, followed 

by a quiet period, followed by an increase in moderate to 

large events leading up to another great earthquake. 

Ellsworth and others (1981) observed that the rate of 

M>5 earthquakes was not yet comparable to the rate just before the 1906 earthquake and concluded that another great 

(M=8) earthquake was not imminent. They proposed that although the largest (great) event was unlikely to occur soon, 

slightly smaller, but still large (M=6-7) earthquakes similar 

to those that preceded the 1906 earthquake were likely to 

occur in the next 70 years, and they made an approximate 

forecast about the possible locations of these large events. 

They proposed as potential sites for these earthquakes the 

most rapidly slipping faults in the region, including the San 

Francisco Peninsula segment of the San Andreas fault, and the 

San Gregorio, Hayward, Calaveras, Rodgers Creek, Maacama, 

and Green Valley faults (fig. 1 B). 

U.S. GEOLOGICAL SURVEY 

After the spectacular and devastating 1980 eruption of 

Mount St. Helens in Washington State, President Carter called 

for an immediate assessment of the possible consequences 

of and preparedness for a major California earthquake. As a 

result of this directive, earthquake scientists at the U.S. Geological Survey (198 1) prepared a report on the potential losses 

from some scenario earthquakes in populated regions of California. These scenario earthquakes were used to estimate the 

probable intensities and strong ground motions that could 

occur. Their report also presented probability estimates for 

the scenario earthquakes. The postulated events involving the 

San Francisco Bay region included an M=8.3 event on the 

northern section of the San Andreas fault (a repeat of the 

great 1906 San Francisco earthquake) and an M=7.4 earthquake on the Hayward fault. These two scenario earthquakes 

were each assigned an annual probability of occurrence of 

0.0 1, with a "moderate" probability in the next 20 years (from 

1981), according to their report. In comparison, an M=8+ 

event on the southern section of the San Andreas fault was 

assigned an annual probability of occurrence of 0.05, with a 

"high" probability for the next 20 years, and an M=7.0 event 

on the Rose Canyon fault near San Diego, Calif., was assigned an annual probability of occurrence of 0.0001, with a 

"very low" probability in the next 20 years. 

MOTHS AND OTHERS 

In a talk presented at the annual meeting of the American Geophysical Union in San Francisco, Barbara Moths and 

her colleagues at the U.S. Geological Survey (Moths and others, 1981) estimated the seismogenic potential of the San 

Andreas fault just north of San Juan Bautista near the southernmost reach of the 1906 San Francisco earthquake (fig. 2). 

Inferred long-term fault-loading rates were straining the shal- 
B8 FORECASTS 

low part of the San Andreas fault that was believed to have 

been locked (was not slipping) since 1906. Recurrence calculations based on slip during the 1906 earthquake appeared to 

allow for an M=6-6.5 earthquake on the San Andreas fault 

north of San Juan Bautista. Moths and others compared the 

seismicity pattern near San Juan Bautista with that of another 

seismically section of the San Andreas fault near Parkfield, 

Calif., some 150 km to the south. Parkfield had been the site 

of five M=5.5-6.0 earthquakes since 1881, most recently in 

1966, whereas San Juan Bautista experienced three or four 

M=6-7 earthquakes during the 19th century. Between these 

two sections, the San Andreas fault slips freely, producing 

earthquakes no larger than M=5. Moths and others noted that 

both "locked" sections were next to the creeping section of 

the San Andreas fault and that shallow diffuse microseismicity appeared in the adjacent creeping section, whereas deeper 

earthquake clusters occurred in the locked sections. On the 

basis of slip calculations and similarities in the two seismicity patterns, Moths and others made a long-term forecast of 

an M=6+ event north of San Juan Bautista. They suggested 

that this event would initiate near Pajaro Gap and rupture into 

the locked section of the San Andreas fault (fig. 2). 

LINDH AND OTHERS 

Allan Lindh of the U.S. Geological Survey composed a 

series of publications on earthquake forecasts for the San Francisco Bay region, starting with simple back-of-the-envelope 

calculations relating 1906 slip to the cumulative strain on the 

shallow part of the San Andreas fault since 1906 (Lindh and 

others, 1982a, b), and combining this information with seismicity pattern recognition. The conclusions were then expanded into probability estimates for some of California's 

active faults (Lindh, 1983, 1988). 

Lindh and others (1982a, b) discussed the seismic potential of the southernmost section of the San Andreas fault that 

had last ruptured in the great 1906 San Francisco earthquake. 

This section, which extends just northwestward of San Juan 

Bautista, was believed to be the site of three or four A426 

earthquakes in the 110 years before 1906, but it had been 

quiet since 1906. They proposed that the 19th-century earthquakes could have been accounted for by repeated slip on the 

50-km-long section of the fault just northwest of San Juan 

Bautista. Geodetic triangulation data provided evidence that 

the strain was accumulating across that zone at a rate equivaFigure 2.-San Francisco Bay (SFB) region, showing extent of best forecast (heavy double line) by Lindh 

(1983). Moths and others (1981) also predicted an earthquake starting at Pajaro Gap (PG) and extending 

northwestward. Northwest limit of rupture was not specified but would have been similar to Lindh's, given 

Moths and others' forecast that earthquake would be of M=6+. Dotted rectangle shows actual extent of Loma 

Prieta rupture (horizontal and projected vertical), as inferred by Steidl and Archuleta (1996) and Wald and 

others (1996). BM, Black Mountain; LP, Loma Prieta; PG, Pajaro Gap; SAF, San Andreas fault; SJ, San Jose; 

SJB, San Juan Bautista; W, Watsonville; WT, Wrights Tunnel. Star, epicenter of 1989 Lorna Prieta earthquake. 
THE FORECASTS B9 

lent to 2 cmlyr, given a fault that was also locked interseismically down to 10-km depth (Prescott, 1980). They reasoned that sufficient strain was accumulating to produce an 

M=6.5 earthquake approximately every 30 years. This 

interevent timing approximately agreed with what appeared 

to have occurred during the 19th century. 

Lindh and others (1982a, b) estimated that (by 1981) the 

section of the San Andreas fault northwest of San Juan Bautista 

had accumulated sufficient strain to reproduce the 1.5 m of 

slip released there in the great 1906 earthquake. Given this 

model, they proposed that a large (E6) earthquake could 

occur in this region at any time. 

In addition to these slip-based calculations, Lindh and 

others (1982a, b) also commented on the seismicity pattern 

for smaller earthquakes. A sequence of M=4 earthquakes had 

been occurring since 1979 on the San Andreas fault northwest of San Juan Bautista, after a 40-year-long drought of 

such events. Microearthquakes associated with these M=4 

events highlighted two sections of the San Andreas fault where 

they postulated future large earthquakes might occur. Like 

Moths and others' (1981) study, Lindh and others (1982a, b) 

noted a similarity between the section of the fault northwest 

of San Juan Bautista and the active, moderate-earthquakeproducing section 150 km to the southeast, near Parkfield. 

Lindh and others (1982a, b) identified two San Andreas fault 

zones with high seismogenic potential: a 10- to 15-km-long 

zone between San Juan Bautista and Pajaro Gap, and a 40- 

km-long zone northwest of Pajaro Gap that they suggested 

(fig. 2) appeared most likely to produce a large earthquake. 

They presented a fault cross section with the proposed rupture area outlined (Lindh and others, 1982a, b). 

Lindh (1983) estimated the probability of large earthquakes along the San Andreas fault system between the Imperial Valley and Cape Mendocino. His evaluation differed 

from most other studies of the time in that it attempted to 

identify where (temporally) a fault segment was in its recurrence cycle and used that information to modify the earthquake probabilities. According to his calculations, recent 

historical occurrences of large earthquakes could actually 

decrease the probability estimate. 

For his estimates, Lindh (1983) used geologically or 

geodetically derived estimates of the repeat times of large 

earthquakes along specific segments of active faults and combined this information with the historical record of when the 

latest large earthquake occurred on those fault segments. The 

fault segmentation was assigned on the basis of several criteria, generally the locations of historical earthquakes. All of 

the results assumed that each fault segment would be most 

likely to fail in a "characteristic" earthquake. Lindh (1983) 

assumed that the San Juan Bautista and San Francisco Peninsula segments of the San Andreas fault failed in M=6-7 events, 

similar to those that occurred during the 19th century. 

The statistical model used by Lindh (1983) assumed that 

earthquake occurrence along a given fault segment could be 

characterized by a Gaussian distribution with a mean recurrence time and standard deviation, the latter of which he took 

to be 30 percent of the mean recurrence time. He assigned a 

probability for future time periods, starting with the date of 

the latest event. For fault segments where the geologic and 

geodetic estimates gave much different recurrence intervals, 

the more conservative value was used, although both values 

were presented. 

Lindh's (1983) results for some San Francisco Bay region faults are as follows. The San Juan Bautista segment of 

the San Andreas fault was assigned an annual probability (in 

1982) of 1.2 to 3.7 percent (1.2 percent was the more conservative value), with a cumulative 10-year probability of 14 to 35 

percent (14 percent was the more conservative value) and a 

cumulative 20-year probability of 30 to 64 percent (30 percent was the more conservative value). The San Francisco 

Peninsula segment of the San Andreas fault was assigned annual, cumulative 10-year, and cumulative 20-year probabilities of 0.16 to 0.5 percent, 2 to 6 percent, and 5 to 14 percent, 

respectively (where the more conservative, preferred value 

was the first one in the range). In summary, Lindh's (1983) 

evaluation showed the San Juan Bautista segment (fig. 2) of 

the San Andreas fault with the highest likelihood of an upcoming large earthquake among San Francisco Bay region 

fault segments. 

Although Lindh's (1988) was mostly an update of Lindh's 

(1983) report, it did assign Lindh's (1983) San Juan Bautista 

segment a new name, calling it the Loma Prieta segment, after Willis' (1923a) California fault study. Lindh (1988) also 

stated that probabilities as high as 1.6 percent in 1 year were 

now applicable to an MS6.5 event, an increase over the 1.2 

percent quoted in his earlier work, simply because 5.5 years 

had elapsed (and was therefore added to the interevent period) since the previous calculation. 

SYKES AND NISHENKO 

Lynn Sykes and Stuart Nishenko of the Lamont-Doherty 

Geological Observatory (Sykes and Nishenko, 1984) estimated the probabilities of large earthquakes on the San 

Andreas, San Jacinto, and Imperial faults between 1983 and 

2003. They used as inputs to their calculations the time since 

the latest large earthquake (where known), model estimates 

for the average recurrence interval, and the standard deviation of the time between earthquakes. They calculated conditional probabilities by using two methods, one based on the 

estimated repeat times and the other based on the displacement during the latest large earthquake divided by the rate of 

fault motion. The faults were segmented mostly on the basis 

of the locations of historical earthquake ruptures but also on 

the basis of changes in fault strike, changes in the amounts of 

slip in the previous earthquake (where known), and tectonic 

complexity. 

Looking at the San Andreas fault in the San Francisco 

Bay region, Sykes and Nishenko (1984) defined a San Fran- 
B 10 FORECASTS 

cisco Peninsula segment of the San Andreas fault (their zone 

3) that extended from San Francisco to San Jose. Estimated 

repeat times derived from historical or prehistoric earthquakes 

gave a probability of 2 percent for the period 1983-2003. 

When they divided the 1906 earthquake slip in this region, 

2.5 to 4 m (depending on location along strike of the fault), 

by the fault-slip rates obtained from geologic and geodetic 

measurements, they obtained a conditional probability of 0.6 

to 8.0 percent for a large earthquake on this segment in the 

period 1983-2003. 

Sykes and Nishenko (1984) defined another segment (fig. 

3) of the San Andreas fault that extended from San Jose to 

San Juan Bautista (their zone 4) and was differentiated from 

their zone 3 on the basis of a change in the amount of slip 

during the 1906 earthquake. They used a 1906 coseismic displacement of 0.6 to 1.4 m and the same fault-slip rates as for 

zone 3 to obtain a recurrence interval of 50 to 115 years. The 

wide range in this recurrence interval also led to a wide range 

in conditional probability of 19 to 95 percent for an M=7.0 

earthquake on this segment in the period 1983-2003. Sykes 

and Nishenko (1984) determined this conditional probability 

on the basis of the historical earthquake sequence by noting 

that this segment may have broken before 1906, in an h427 

earthquake in 1838 and in two M=6.0 earthquakes in 1865 

and 1890. They suggested that, owing to their size, the 1865 

and 1890 earthquakes probably ruptured only part of zone 4. 

By assuming that the 1838 earthquake ruptured zone 4, they 

calculated a 68-year repeat time (from 1838 to 1906), which 

gave a 7 1-percent probability of an M=7.0 earthquake. They 

mentioned that part of zone 4 could rupture in a moderate 

earthquake like that in 1865. 

Farther south, the segment of the San Andreas fault from 

San Juan Bautista to Bear Valley (zone 5 of Sykes and Nishenko 

(1984)) also bears mentioning. Sykes and Nishenko postulated 

that the aseismic slip in zone 5 is similar to that at Parkfield, 

which breaks regularly in M=5.5-6 events, and that zone 5 might 

rupture in conjunction with zone 4 mentioned above, as may 

have occurred in 1838 (Sykes and Nishenko, 1984). 

SCHOLZ 

Chris Scholz of the Lamont Doherty Geological Observatory (Scholz, 1985) suggested that changes in fault geometry can control earthquake rupture propagation and discussed 

how slip during the great 1906 San Francisco earthquake may 

have been affected in this manner. He observed that the 1906 

coseismic slip was greater to the northwest than to the southeast of a 9' bend in the San Andreas fault (on the San Francisco peninsula) near Black Mountain. The 75-km-long 

section of the fault between Black Mountain and San Juan 

Bautista (fig. 3) slipped only 1 to 1.4 m, whereas farther northwest on the San Francisco Peninsula the slip was 2.5 to 4 m. 

By assuming a long-term slip rate of 15k2 mrnlyr, Scholz 

calculated that a rupture of this 75-km-long slip-deficient 

segment could occur in 60 to 110 years. Using one of his 

scaling laws (Scholz, 1982), which related slip to rupture 

length, Scholz (1985) estimated a slip of 0.9 m, which would 

result in an Mn=6.9 earthquake. The rupture length of Scholz's 

(1985) forecasted earthquake was 30 km longer than that of 

Lindh's (1983) and the same as that of Sykes and Nishenko's 

(1984) forecasted events. Scholz (1985) stated that the 20- 

year probability estimates were the same as the former studies' because only the rupture length was changed from one 

of the previous calculations. 

KEILIS-BOROK AND OTHERS 

Of all of the proposed scientific prediction methods used 

for forecasting earthquakes in California, the Russian algorithm M8 has perhaps engendered the most protracted dialog. Algorithm M8 (Keilis-Borok and others, 1986b; 

Keilis-Borok and Kossobokov, 1988) received much attention after its authors, scientists at the Academy of Sciences 

of the USSR's Institute of Physics of the Earth in Moscow, 

used it to make forecasts for large to great earthquakes in 

California during the 1980's. At the Geneva Summit Meeting in 1985, Chairman Gorbachev passed information to 

President Reagan about forecasts for M28.0 and M27.5 earthquakes in California (Vladimir Kossobokov, written 

commun., August 1997) within the next 3 years (see app. 3). 

Keilis-Borok and others' (1986b) algorithms were designed to diagnose a "time of increased probability" (TIP), 

using a set of traits described as (1) level of seismic activity, 

(2) temporal variation of clustering, (3) space-time clustering, (4) concentration in space, and (5) long-range interaction. The two algorithms for the diagnosis of TIP'S were an 

older algorithm called CN, which was designed to predict, 

and was based on, earthquakes of M26.4 in California and 

Nevada, and a newer algorithm called M8, which was designed to predict worldwide M28 earthquakes. Keilis-Borok 

and others (1986a) applied algorithm M8 (and CN) to earthquake catalogs for several regions and claimed success in 

some of these retrospective tests. Using algorithm M8, KeilisBorok and others (1986a, table 14) also made a forward-looking earthquake forecast for a part of California and Nevada 

for an m7.5 earthquake in a 7O-latitude by 9'-longitude rectangle centered at lat 37.5' N., long 1 19S0 W. (fig. 4). The 

time interval for the TIP was January 1, 1984, to December 

31, 1988. (Keilis-Borok and others, 1986a, lists January 1, 

1988, as the end of the TIP, but numerous published correspondences soon after the manuscript's release corrected this 

date to December 1988, in accordance with their 5-year TIP'S 

for other regions.) 

On December 16, 1986, Leon Knopoff of the University of California, Los Angeles, having recently returned 

from a meeting in Moscow, sent a letter to Lynn Sykes, 

the chairman of the National Earthquake Prediction Evaluation Council (NEPEC). In his letter (Updike, 1989), 
THE FORECASTS Dl 1 

Knopoff noted that Keilis-Borok wanted his forecast entered into the lists of earthquake predictions considered 

by NEPEC. In June 1988, Keilis-Borok had an opportunity to present his research to NEPEC (Updike, 1989, app. 

A). Keilis-Borok's handouts showed the M27.5 TIP for 

California extended to July 1, 1992. The geographic area 

of the TIP was also changed from that shown by KeilisBorok and others (1986a): Instead of the previous rectangle, it was now a circle, still centered at lat 37.5' N., 

long 119.5' W., and encompassing much of California and 

western Nevada. 

Keilis-Borok's NEPEC presentation included a figure describing M27.0 forecasts for the Western United 

States (Keilis-Borok and Kossobokov, 1988; Updike, 1989, 

app. A) in which two plotted TIP'S encompassed the Loma 

Prieta area. The more northern TIP was a circular region 

of radius 2.5' centered at lat 38' N., long 122' W. The 

TIP started in July 1984 and extended through July 1989, 

and so it did not include the time of the 1989 Loma Prieta 

earthquake. The more southern TIP was a circle of the 

same radius centered at lat 36O N., long 120' W. This documented (Updike, 1989, app. A) TIP started in 1985 and 

ended in 1990, encompassing both the location and date 

of the 1989 Loma Prieta earthquake (fig. 5). 

THATCHER AND LISOWSKI 

Wayne Thatcher and Michael Lisowski of the U.S. Geological Survey (Thatcher and Lisowski, 1987) investigated 

the long-term (210 yr) earthquake potential of the San Francisco Peninsula segment of the San Andreas fault (which they 

defined as the 125-km-long segment southeast of San Francisco, from Mussel Rock to San Juan Bautista; see fig. 15). 

They addressed the seismic potential of parts of this segment, 

including the 90-km-long section of the San Andreas fault 

northwest of San Juan Bautista, in addition to smaller subsections of 30 and 45 km (fig. 6). Much of the 90-km-long 

fault reach is correlated with the segment discussed by Sykes 

and Nishenko (1984). 

Using geodetic observations made before and after 1906, 

Thatcher and Lisowski (1987) evaluated the slip that occurred 

during the great 1906 San Francisco earthquake and compared it with the slip that had not occurred since 1906. 

Figure 3.-San Francisco Bay (SFB) region, showing extent of closest forecasts (heavy double line) by 

Sykes and Nishenko (1984) and Scholz (1985). Dotted rectangle shows actual extent of Lorna Prieta rupture 

(horizontal and projected vertical), as inferred by Steidl and Archuleta (1996) and Wald and others (1996). 

BM, Black Mountain; LP, Lorna Prieta; PG, Pajaro Gap; SAF, San Andreas fault; SJ, San Jose; SJB, San 

Juan Bautista; W, Watsonville; WT, Wrights Tunnel. Star, epicenter of 1989 Lorna Prieta earthquake. 
B12 FORECASTS 

Thatcher and Lisowski's modeled 1906 slip (2.6k0.2 m) differed by a factor of 2 from the 1906 slip (~1.5 m) observed 

at the Earth's surface. They argued that this discrepancy was 

due to distributed surface slip, which was not easily observed, 

and that their geodetically determined value was correct. 

Assuming 2.6 m for the 1906 coseismic slip and taking the 

long-term fault-slip rate for this segment of the San Andreas 

fault to be 15 mmlyr, Thatcher and Lisowski used a timepredictable model of earthquake recurrence (Shimazaki and 

Nakata, 1980). They concluded that the probability of an M=7 

earthquake on the southern 90 km of the San Francisco Peninsula segment of the San Andreas fault (the 90-km-long 

section of the San Andreas fault northwest of San Juan 

Bautista, fig. 6) during the upcoming several decades was 

quite low. Using the probability methods of Sykes and 

Nishenko (1984), they estimated a 20-year conditional probability of 6 percent for the period 1986-2006. 

Thatcher and Lisowski (1987) also considered two other 

scenarios: an M=6.5 earthquake on the southernmost 45 km 

of the 1906 rupture and an event of unspecified magnitude 

on the southernmost 30 km of the 1906 rupture (fig. 6). They 

concluded that the geodetic data were insufficient to resolve 

Figure 4.-California, showing 7'-by-9' rectangular area centered at lat 37.5' N., long 119.5' W., in which Keilis-Borok and others (1986a) 

used algorithm M8 to predict an m7.5 earthquake. Star, epicenter of 1989 Loma Prieta earthquake; heavy line, part of the San Andreas fault 

that ruptured in great 1906 San Francisco earthquake. 
THE FORECASTS B13 

the deep 1906 slip for the northern part of the 45-km-long 

rupture. They also examined the possibility of a 30-km-long 

rupture just northwest of San Juan Bautista. Their calculations, which assumed 1.2 m of slip in 1906, in combination 

with a 15-mrnlyr long-term fault-slip rate, did permit an upcoming Mz6.5 event, but they declared that Lindh's (1983) 

20-year conditional probability of 30 percent was unmerited. 

Their conclusion was based on 0.5 m of fault offset inferred 

from an 1890 coseismic measurement of the Pajaro River 

bridge, 11 km northwest of San Juan Bautista. They believed 

that the 1890 earthquake already slipped that segment of the 

San Andreas fault and therefore an imminent event was unlikely. 

WESSON AND NICHOLSON 

Robert Wesson and Craig Nicholson of the U.S. Geological Survey presented their work at an international conference on intermediate-term earthquake prediction held in 

California in November 1986. In summarizing their conference presentation, Wesson and Nicholson (1988) combined 

Figure 5.-California, showing 2.5' circular area centered at lat 36' N., long 120' W., in which V.I. Keilis-Borok (in Updike, 1989, app. A) 

used algorithm M8 to predict an m7.0 earthquake. Star, epicenter of 1989 Loma Prieta earthquake; heavy line, part of the San Andreas fault 

that ruptured in great 1906 San Francisco earthquake. 
B 14 FORECASTS 

observations from a decade of California earthquakes with 

physical arguments about fault behavior and postulated a set 

of forecast rules that they used to highlight 16 sites in California which they thought had a likelihood of nucleating an 

iE5.7 earthquake during the decade 1987-96: 

A sudden increase in seismic activity in a previously 

quiet area must be taken seriously as an intermediate-term 

precursor and should be reflected as a probability gain in 

assessing the intermediate-term seismic potential of the surrounding area. 

Larger earthquakes in California along faults exhibiting fault creep tend to occur at the ends of creeping sections, 

or where the creep rate is markedly reduced. 

Large earthquakes tend to occur adjacent to previous large earthquakes, representing either an extension of 

the rupture, a migration, or perhaps a response (commonly 

delayed by creep) to static-stress change. Therefore, the 

occurrence of any significant earthquake increases the intermediate-term probability of future earthquakes on adjacent fault segments and in the surrounding area. 

There is marginal evidence for periods of high regional 

deformation. Therefore, any evidence for a period of similar 

high regional deformation (a regional strain event or the 

occurrence of an earthquake of M25.7) increases the intermediate-term probability of future earthquakes throughout the region. 

Using these forecast rules, Wesson and Nicholson (1988) 

identified 16 sites in California for their earthquake potential. 

On the basis of their previous experience with the 1975-86 

earthquakes, they concluded that about a fourth of the "independent" (excluding aftershocks, and counting only one earthquake in an earthquake sequence) earthquakes in the decade 

1987-96 would occur without recognizable intermediate-term 

precursory phenomena, and that about a third would occur at 

the ends of creeping fault segments and about a third of their 

identified sites would be the locations of significant earthquakes in the decade 1987-96. They thus estimated that the 

probability of occurrence for any one of their forecast sites 

was about 113. Of their 16 sites, 4 were located in the San 

Francisco Bay region: at the north and south ends of the Hayward fault, on the central Calaveras fault and east of the 

Calaveras fault south of Livermore Valley, and at the north 

end of the creeping section of the San Andreas fault near San 

Juan Bautista (fig. 7). 

Figure 6.-San Francisco Bay (SFB) region, showing locations of closest forecasts (heavy double line) by 

Thatcher and Lisowski (1987) for ruptures starting at San Juan Bautista (SJB) and extending 30,45, and 90 

krn northwestward. Dotted rectangle shows actual extent of Lorna Prieta rupture (horizontal and projected 

vertical), as inferred by Steidl and Archuleta (1996) and Wald and others (1996). BM, Black Mountain; LP, 

Lorna Prieta; PG, Pajaro Gap; SAF, San Andreas fault; SJ, San Jose; SJB, San Juan Bautista; W, Watsonville; 

WT, Wrights Tunnel. Star, epicenter of 1989 Lorna Prieta earthquake. 
THE FORECASTS 

WORKING GROUP ON 

CALIFORNIA EARTHQUAKE PROBABILITIES 

In early 1988, the Working Group on California Earthquake Probabilities, consisting of Duncan Agnew (University of California, San Diego); Clarence Allen and Kerry Sieh 

(California Institute of Technology); Lloyd Cluff (Pacific Gas 

& Electric Co.); James Dieterich, William Ellsworth, Allan 

Lindh, David Schwartz, Wayne Thatcher, and Robert Wesson 

(U.S. Geological Survey); Stuart Nishenko (now U.S. Geological Survey); and R. Keeney (University of Southern California) published a consensus report for NEPEC on the 

probabilities of large earthquakes on the San Andreas fault 

system (Working Group on California Earthquake Probabilities, 1988). Their probability estimates were based on a timedependent model in which the probability increases as a 

function of time after the latest large earthquake. They estimated the 30-year probability for one or more large earthquakes on any of the fault segments that they evaluated in 

the San Francisco Bay region at 0.5, the 20-year (1988-2008) 

probability at 0.3, and the 10-year (1988-98) probability at 

0.2. These probability estimates were for an M=7 earthquake, 

and the faults included in their study were the entire Hayward fault (including a northern and southern segment), the 

San Francisco Peninsula segment of the San Andreas fault 

that extends 90 km from Men10 Park to San Juan Bautista 

(fig. 8), a 30- to 35-km-long section of the San Francisco 

Peninsula segment called the Southern Santa Cruz Mountains segment just northwest of San Juan Bautista (fig. 8), 

and the 360-krn-long North Coast segment of the San Andreas 

fault that extends northward from the San Francisco Peninsula segment. The Hayward fault segments were the presumed 

sites of large earthquakes in 1836 and 1868; the San Andreas 

fault segments were defined on the basis of the slip believed 

to have occurred in 1906. 

The North Coast segment of the San Andreas fault was 

assigned the lowest probability of all the San Francisco Bay 

region fault segments studied. The Working Group on California Earthquake Probabilities (1 988) assumed 4.5i-1.0 m 

of slip on this segment in 1906 and estimated its long-term 

slip rate at 16k2.5 mmlyr. They calculated a probability of 

less than 0.1 for a large earthquake on this segment in the 

30-year period 1988-20 18. 

The San Francisco Peninsula segment of the San Andreas 

fault produced much debate among the scientists in the Working Group on California Earthquake Probabilities (1 988), cenFigure 7.-San Francisco Bay (SFB) region, showing locations of closest forecast (circle) by Wesson and 

Nicholson (1988) for the initiation point of an m5.7 earthquake between 1987 and 1996. Dotted rectangle 

shows actual extent of Lorna Prieta rupture (horizontal and projected vertical), as inferred by Steidl and 

Archuleta (1996) and Wald and others (1996). BM, Black Mountain; LP, Lorna Prieta; PG, Pajaro Gap; SAF, 

San Andreas fault; SAFC, creeping section of the San Andreas fault; SJ, San Jose; SJB, San Juan Bautista; 

W, Watsonville; WT, Wrights Tunnel. Star, epicenter of 1989 Lorna Prieta earthquake. 
B16 FORECASTS 

tering on the amount of slip during the 1906 earthquake. The 

amount of slip in the southernmost part of the 1906 rupture 

was discussed by Lindh (1983), Sykes and Nishenko (1984), 

Scholz (1985), and Thatcher and Lisowski (1987). The geodetic (Thatcher and Lisowski, 1987) and geologic (Lindh, 

1983; Sykes and Nishenko, 1984; Scholz, 1985) 1906 slip 

estimates for the southernmost part of the San Francisco Peninsula segment differed by a factor of 2. Because the probability estimates depended heavily on this value, the Working 

Group on California Earthquake Probabilities discussed the 

ramifications and ultimately assumed a value of 2.6k0.6 m 

for the 1906 slip, arguing that this value was within two standard deviations of the maximum fault offset (geologic) measurements of less than l .5 m. A long-term fault-slip rate of 

16k2.5 mdyr was also selected. Geologic and geodetic data 

gave rates of 8 to 15 mmlyr, but they chose a slightly higher 

rate to allow for the possibility that the long-term fault-slip 

rate of 33 mdyr recorded across the San Andreas fault system 100 krn farther south was distributed in the north only 

along the Hayward, Calaveras, and San Andreas faults. This 

distribution would have determined a long-term fault-slip rate 

of 20 mm/yr for the San Andreas fault north of San Juan 

Bautista. 

Assigning a 16k2.5 mdyr long-term fault-slip rate and 

2.6k0.6 m of 1906 slip produced a 30-year probability of 0.2 

for the San Francisco Peninsula segment of the San Andreas 

fault. This was the probability finally adopted by the Working Group on California Earthquake Probabilities (1988). They 

also noted that reducing the 1906 slip estimate to 1.5k0.5 m 

raised this probability to 0.4; however, this calculation was 

not formally adopted for the official publication. 

The Working Group on California Earthquake Probabilities (1988) discussed the possibility of a somewhat smaller 

(M=6.5-7) earthquake rupturing the Southern Santa Cruz 

Mountains segment of the San Andreas fault (fig. 8). Although 

they did not want to project high probabilities for this segment on the basis of what they considered to be incomplete 

information, they did suggest that 1906 slip must decrease 

southward along this segment, with a value of at least 1.2 m 5 

km northwest of San Juan Bautista. They used an average 

Figure 8.-San Francisco Bay (SFB) region, showing locations of closest forecasts (heavy double line) by 

Working Group on California Earthquake Probabilities (1988). One forecast was for a 30- to 35-km-long 

rupture, starting at San Juan Bautista (SJB); arrow at 35-km mark shows northwest end of the Southern 

Santa Cruz Mountains segment of the San Andreas fault. Another forecast was for a 90-km-long rupture, 

also starting at SJB, of the entire San Francisco Peninsula segment of the San Andreas fault. Dotted rectangle 

shows actual extent of Loma Prieta rupture (horizontal and projected vertical), as inferred by Steidl and 

Archuleta (1996) and Wald and others (1996). BM, Black Mountain; LP, Loma Prieta; MP, Men10 Park, PG, 

Pajaro Gap; SAF, San Andreas fault; SJ, San Jose; SJB, San Juan Bautista; W, Watsonville; WT, Wrights 

Tunnel. Star, epicenter of 1989 Loma Prieta earthquake. 
THE FORECASTS B17 

value of 2.Ok0.5 m for 1906 slip on the Southern Santa Cruz 

Mountains segment and a long-term fault-slip rate of 16i-2.5 

mmlyr. They calculated a 30-year probability (1988-2018) 

of 0.3 for an earthquake on this segment, but they assigned it 

the lowest level of reliability. 

DAVIS (LAKE ELSMAN EARTHQUAKES) 

In 1988 and 1989, two M=5 earthquakes occurred in a 

region proposed by several forecasters to have the potential 

of producing a large earthquake on the San Andreas fault (fig. 

9). These earthquakes were named the "Lake Elsman earthquakes" by the U.S. Geological Survey, and the "Lake Elsman 

and Lexington Reservoir earthquakes" by the State of California. Given the recent forecast by the Working Group on 

California Earthquake Probabilities (1988), the State of California decided to take seriously the potential implications of 

the Lake Elsman earthquakes. For the first time in San Francisco Bay region history, the California Office of Emergency 

Services issued an official short-term advisory regarding a 

potential upcoming large earthquake. This advisory assigned 

a slightly increased likelihood of an M=6.5 event on the Santa 

Cruz Mountains segment of the San Andreas fault in the next 

5 days. Appendix 2 describes the incidents that followed the 

first M=5 earthquake, as transcribed in a draft report by the 

California State Geologist James Davis (unpub. data, 1988). 

On August 8, 1989, another M=5 Lake Elsman earthquake occurred (fig. 9). The Office of Emergency Services 

issued another 5-day (short term) advisory similar to that issued in June 1988. Five days later, this advisory also expired. 

KERR 

In August 1989, Richard Ken", a writer for Science magazine, reported on the relation of the two Lake Elsman earthquakes to the forecasts made by Lindh and others (1982a, b) 

and Lindh (1983, 1988) for the 45-km-long segment of the 

San Andreas fault south of Lake Elsman (Kerr, 1989a). He 

Figure 9.-San Francisco Bay (SFB) region, showing locations of epicenters of 1988 and 1989 Lake Elsman 

earthquakes (small stars) north of the Southern Santa Cruz Mountains segment of the San Andreas fault 

(heavy double line; Working Group on California Earthquake Probabilities (1988).). 1988 Lake Elsman 

earthquake was located at lat 37.13' N., long 121.90' W., at 13.4-km depth; 1989 Lake Elsman earthquake 

was located at lat 37.14' N., long 121.93OW., at 13.9-km depth (data from Northern California Earthquake 

Data Center, March 1997). Dotted rectangle shows actual extent of Loma Prieta rupture (horizontal and 

projected vertical), as inferred by Steidl and Archuleta (1996) and Wald and others (1996). BM, Black Mountain; 

LP, Loma Prieta; PG, Pajaro Gap; SAP, San Andreas fault; SJ, San Jose; SJB, San Juan Bautista; W, Watsonville; 

WT, Wrights Tunnel. Large star, epicenter of 1989 Lorna Prieta earthquake. 
B18 FORECASTS 

remarked that the Lake Elsman earthquakes were two of the 

three largest earthquakes since 1914 on the more than 400 

km of the San Andreas fault that last broke in 1906,. Each 

Lake Elsman earthquake ruptured an estimated 1-km patch 

of fault at 14-km depth. The location was right at the end of 

Lindh's forecasted rupture segment, and he was quoted as 

saying: "Now we've had two of these magnitude 5's where 

we didn't have anything. I thought it was a dangerous segment before anything happened; I can only be reinforced in 

that feeling now" (A.G. Lindh, in Kerr, 1989a). Kerr (1989a) 

also noted that the 30-percent probability assigned (pre-Lake 

Elsman) to the Santa Cruz Mountains segment of the San 

Andreas fault by the Working Group on California Earthquake 

Probabilities (1988) was, after the Lake Elsman earthquakes, 

looking a bit low. 

THE 1989 LOMA PRIETA EARTHQUAKE 

The Loma Prieta earthquake occurred at 5:04 P.d.t. on 

October 17, 1989, killing 62 people and generating billions 

of dollars in property damage (Plafker and Galloway, 1989). 

Although no scientific agreement exists on the exact form of 

some details of the main shock, many of its features have 

been generally agreed upon by those who have studied the 

earthquake. Spudich (1996) summarized the main shock as 

an Ms=7. 1 (M=6.9) earthquake that occurred on a fault dipping steeply northwest which was approximately 35 km long 

(fig. 10). The hypocenter was located at lat 37.036ON., long 

121.883' W., at 19-km depth. The slip was mostly confined 

to depths between 7 and 20 km and was oblique right lateral, 

with a considerable vertical component. Large amounts of 

slip concentrated in two regions, located northwest and southeast of the hypocenter. This distribution of slip caused seismic energy to radiate both southeastward and northwestward, 

thereby lessening the ground-shaking intensity in each direction from that of a unilateral rupture. The northwestern slip 

region had a considerable reverse component, whereas the 

southeastern slip region was predominantly strike slip. 

Dietz and Ellsworth (1997) analyzed both pre- and postLoma Prieta seismicity for the 20 years before and a few years 

after the earthquake. They found that the aftershocks and the 

main-shock rupture plane were not highlighted by the preceding 20 years of seismicity. Using aftershock patterns, they 

Figure 10.-San Francisco Bay (SFB) region, showing location of epicenter of 1989 Loma Prieta M=6.9 

(M=7.1) earthquake (star). Rupture was approximately 35 km long on a steeply dipping fault and generated 

both strike-slip and dip-slip motions. Shaded rectangle shows extent of rupture (horizontal and projected 

vertical), as inferred by Steidl and Archuleta (1996) and Wald and others (1996). BM, Black Mountain; LP, 

Lorna Prieta; PG, Pajaro Gap; SAP, San Andreas fault; SJ, San Jose; SJB, San Juan Bautista; W, Watsonville; 

WT, Wrights Tunnel. 
COMPARISON OF THE FORECASTS WITH THE 1989 LOMA PRIETA EARTHQUAKE B 19 

also showed that the Lorna Prieta main-shock rupture plane 

was distinct from a San Andreas fault which extended vertically from the Earth's surface, at least in the northwestern 

part of the rupture zone. Southeast of the hypocenter, Dietz 

and Ellsworth's aftershock locations show the main-shock 

fault plane merging with the San Andreas fault near Pajaro 

Gap. These results led them to propose that the Loma Prieta 

main shock occurred on an ancestral, formerly strike slip section of the San Andreas fault that is now acting as a rightlateral reverse fault. 

COMPARISON OF THE FORECASTS 

WITH THE 1989 LOMA PRIETA 

EARTHQUAKE 

Most pre- 1989 forecasts for large earthquakes in the Loma 

Prieta area and for the entire San Francisco Bay region were 

for events on the major strike-slip faults. It was assumed (for 

example, Ellsworth and others, 1981) that these large earthquakes would occur as events which were (apparently) similar to most of the large earthquakes that had already occurred 

in the late 19th and 20th centuries. Discussions surrounding 

large earthquakes that might rupture the San Andreas fault 

had primarily focused on the horizontal strain budget (for 

example, Reid, 19 10; Moths and others, 198 1 ; Lindh and others, 1982a, b; Lindh, 1983,1988; Sykes and Nishenko, 1984; 

Scholz, 1985; Thatcher and Lisowski, 1987; Working Group 

on California Earthquake Probabilities, 1988) and its relation 

to the slip that occurred during the 1906 earthquake. Many of 

the forecasts for "large" earthquakes in the San Francisco Bay 

region also assumed that such an earthquake would rupture 

the entire plate boundary (Sykes and Nishenko, 1984). Thus, 

in these two respects, the 1989 Loma Prieta earthquake differed from the forecasts: It was an oblique-slip event on a 

dipping fault, with both strike-slip and reverse-slip components; and it occurred much deeper than any of the forecasted 

events, with little, if any, slip on shallow parts of the fault. 

The rupture length and magnitude of the 1989 Loma 

Prieta earthquake differed from those in many of the forecasts. Most of the forecasted events closest geographically to 

the earthquake involved rupture scenarios for a nearby segment of the San Andreas fault (Reid, 1910; Thenhaus and 

others, 1980; Ellsworth and others, 198 1 ; U.S. Geological 

Survey, 198 1; Moths and others, 1981; Lindh and others, 

1982a, b; Lindh, 1983, 1988; Sykes and Nishenko, 1984; 

Scholz, 1985; Thatcher and Lisowski, 1987; California Office of Emergency Services, 1988,1989; James Davis, unpub. 

data, 1988; Wesson and Nicholson, 1988; Working Group on 

California Earthquake Probabilities, 1988). The rupture 

lengths ranged from hundreds of kilometers (U.S. Geological Survey, 1981) to 30-35 km (Working Group on California Earthquake Probabilities, 1988). Of all the forecasted 

rupture lengths, Lindh's (1983) most closely matched that of 

the actual earthquake (Sykes, 1996), with the forecasted earthquake extending approximately 40 to 45 km, starting near 

Pajaro Gap. The magnitude of the earthquake was, however, 

greatly underestimated by Lindh (1983) and more accurately 

captured by workers who overestimated the rupture length 

(Sykes and Nishenko; 1984; Scholz, 1985; Working Group 

on California Earthquake Probabilities; 1988) and thereby 

forecasted an M=7 earthquake (Sykes, 1996). 

Time-dependent probabilities for the forecasted rupture 

scenarios were assigned by Thenhaus and others (1980), U.S. 

Geological Survey (198 I), Lindh (1983, updated in 1988), 

Sykes and Nishenko (1984), Thatcher and Lisowski (1987), 

California Office of Emergency Services (1988,1989), James 

Davis (unpub. data, 1988), Wesson and Nicholson (1988), and 

Working Group on California Earthquake Probabilities (1988); 

these probabilities were for times ranging from 5 days and 

upward (table 1). A few of the forecasts that did encompass 

the correct geographic region did not include the date of the 

earthquake; these forecasts included the two 5-day advisories 

by the State of California (California Office of Emergency 

Services, 1988,1989; James Davis, unpub. data, 1988) and at 

least one of the forecasts using algorithm M8 (V.I. KeilisBorok, in Updike, 1989). 

Some of the forecasts that did encompass the earthquake's 

date and magnitude were for very large geographic regions. 

Thenhaus and others (1980) calculated a probability estimate 

for a range of earthquake magnitudes, including that of the 

1989 earthquake, but the source nucleation region was highly 

uncertain, encompassing hundreds of kilometers of the San 

Andreas fault. V.I. Keilis-Borok's (in Updike, 1989) M>7 

forecast, which did not present any probability estimates, was 

for an even-larger area, encompassing more than 200,000 

km2 (fig. 5). 

POST-LOMA PRIETA INFORMATION 

THAT MIGHT HAVE LED 

TO DIFFERENT FORECASTS 

Some of the debates surrounding the characteristics of 

the 1989 Loma Prieta earthquake regard its relation to the 

1906 earthquake rupture and the San Andreas fault. Several 

workers have worried that accepting the 1989 earthquake as a 

forecasted event could lead to an incorrect downweighting of 

the likelihood of future events on the San Andreas fault (for 

example, Segall and Lisowski, 1990; Beroza, 1991 ; Thatcher 

and others, 1997). Their arguments have primarily been based 

on the depth of the Loma Prieta slip and on its rake. Other 

workers have proposed that the 1906 surface slip measurements used to make the forecasts were just plain incorrect 

and that the adoption of "correct" values would have led to 

much lower probability assignments for the Loma Prieta area 

(Thatcher and others, 1997). In an attempt to resolve these 

controversies, geologists and geophysicists have studied the 

1989 Loma Prieta earthquake intensively. Some have compared their results with the data that are still available from 
B20 FORECASTS 

the 1906 rupture. The information about 1906 slip in the Loma 

Prieta area is summarized in table 2. 

1906 SLIP MODELSGEOLOGIC INFORMATION 

After the 1989 Lorna Prieta earthquake, geologists investigated sites where the effects of both the 1906 and 1989 

earthquakes might be accessible (Prentice and Schwartz, 

1991; Prentice and Ponti, 1997). One study site was Wrights 

Tunnel (figs. 1, 2), an abandoned railroad tunnel approximately 50 km northwest of San Juan Bautista that crosses 

the San Andreas fault in the southern Santa Cruz Mountains. 

Deformation of Wrights Tunnel during the 1906 earthquake 

was recorded soon after the great earthquake, and some of 

this information was used as a basis for constructing several 

of the forecasts for future events on the San Andreas fault. In 

preparing their forecasts, Lindh and others (1982a, b), Lindh 

(1983,1988), Sykes and Nishenko (1984), and Scholz (1985) 

all adopted values for the 1906 slip in Wrights Tunnel from 

the report by Lawson (1908). 

After the 1989 Lorna Prieta earthquake, Prentice and 

Schwartz (1991) and Prentice and Ponti (1997) searched the 

literature for information about Wrights Tunnel and also performed field investigations in the tunnel. In contrast to some 

previous assumptions about distributed slip across a broad 

shear zone during the 1906 earthquake (for example, SarnaWojcicki and others, 1975; Thatcher and Lisowski, 1987), 

Prentice and Ponti's study of Wrights Tunnel concluded that 

1906 faulting was confined to a zone less than 400 m wide 

and that 60 to 85 percent of the coseismic slip occurred across 

a single line. They inferred that at least 1.7 to 1.8 m of 1906 

coseismic slip took place across the San Andreas fault in 

Wrights Tunnel, a value that differs from the 1.4 m assumed 

by Sykes and Nishenko (1984) and Scholz (1985) and from 

the 1.5 m assumed by Lindh and others (1982a, b). (Lindh, 

1988, assumed 1.5k0.5 m.) Higher 1906 slip values might, 

conceivably, have decreased the probabilities assigned by 

those workers. 

In another study, Schwartz and others (in press) investigated a site in the Santa Cruz Mountains for evidence of large 

earthquakes on the San Andreas fault. They found indications of the 1906 surface rupture, as well as a previous surface-rupturing earthquake that they estimated occurred in 

the mid-1600's. The date (range) on this penultimate earthquake approximately matched the timing of surface-rupturing events inferred at several sites farther north along the 

Table 2.-Published estimates of 1906 coseismic slip on the San Andreas fault in the Loma 

Prieta area 

Reference Description Horizontal slip 

Lawson (1 908) .----------------------- Displacement in Wright's Tunnel, as 

described by G.A. Waring. 

Lawson (1908) ........................ Displacement in Wright's Tunnel 

measured by E.P. Carey. 

~indh and others (1982a, b)---------- Assumed from Lawson (1908) 

Lindh (1983, 1988) ------------------- Assumed from Lawson (1908) and 

Thatcher and Lisowski (1987). 

Sykes and Nishenko (1984)--------- Assumed from Lawson (1908). 

Scholz ( 1985) ---------- - --------- ------ Assumed from Lawson (1908). 

Thatcher and Lisowski (1987)------- Inferred from geodetic data 

Working Group on California None: assumed value is intermediate 

Earthquake Probabilities (1988) between those of Lawson (1908) and 

Thatcher and Lisowski (1987). 

Segall and Lisowski (1990)---------- Inferred from geodetic data 

Prentice and Ponti (1997)------------ Modeling and reevaluation of Wright's 

Tunnel-data provided by historical 

documents. 

Thatcher and others (1997)----------- Inferred from geodetic data 

5 feet 

4.5 feet 

1.5 m 

1.5k0.5 m 

0.6- 1.4 m 

1.4 m 

2.6 m 

2.0k0.5 m 

2.5k0.4 m 

21.7-1.8 m 

2.73k0.35 m 
WAS THE 1989 LOMA PRIETA EARTHQUAKE AN ANTICIPATED EVENT? B21 

San Andreas fault. Schwartz and others did not find evidence 

for large surface-rupturing earthquakes between the mid1600's and 1906. 

Sykes and Nishenko (1984) used an earthquake that occurred in 1838 as a "prototype" for their Loma Prieta forecast, and Lindh and others (1982a, b) used an earthquake that 

occurred in 1865 as a "prototype" for their Loma Prieta forecast. Schwartz and others (in press) did not see evidence in 

their geologic trenches for either 1838 or 1865 surface rupture, but it is unclear how and whether this new information 

might have changed either Lindh and others' or Sykes and 

Nishenko's forecasts, especially that of Lindh and others 

(1982a, b), who depicted the 1865 earthquake as an event 

that did not rupture to the Earth's surface. 

1906 SLIP MODELSGEODETIC INFORMATION 

After the 1989 Loma Prieta earthquake, Segall and 

Lisowski (1990) and Thatcher and others (1997) analyzed 

geodetic data comparing the 1989 and 1906 earthquakes and 

examined the relation between these two earthquakes. 

Thatcher and Lisowski (1987) had originally proposed, on 

the basis of detailed modeling of triangulation data, that the 

1906 earthquake in the vicinity of Loma Prieta peak (figs. 1, 

2) slipped approximately 2.6 m down to a depth of approximately 10 km. After the 1989 Loma Prieta earthquake, Segall 

and Lisowski (1990) analyzed the triangulation data available for the 1906 coseismic period and determined that the 

geodetic station at Loma Prieta peak was displaced slightly 

more than 1 m parallel to the trace of the San Andreas fault, 

indicating more than 2 m of strike-slip motion on that part of 

the fault in 1906. Assuming that fault slip extended from the 

Earth's surface down to 10-km depth, Segall and Lisowski 

(1990) calculated a 1906 fault slip of 2.5k0.4 m of strikeslip motion. They observed that in 1989 the Loma Prieta station moved 0.15 m parallel to the San Andreas fault and 0.1 1 

m perpendicular to the fault, markedly different from the 1 

m of fault-parallel motion observed for the 1906 earthquake. 

On the basis of their analysis of the 1989 data, Segall and 

Lisowski concluded that the 1989 earthquake slipped much 

differently from the 1906 earthquake, in that it produced both 

dip-slip and strike-slip motion and occurred on a different 

fault plane. 

Thatcher and others (1997) reevaluated the triangulation data set used to calculate the deep-fault-slip values for 

the 1906 earthquake; they especially scrutinized resolution 

problems that might affect their slip results. Most of their 

analyses resulted in calculations producing a minimum value 

of slip. For 1906 slip in the vicinity of Loma Prieta, their 

triangulation data inversion resulted in a slip of 2.73k0.35 m 

(again assuming that the fault slipped down to 10-km depth), 

close to the value previously obtained (Thatcher and 

Lisowski, 1987) and still higher than all values of 1906 surface slip reported for the Loma Prieta area. 

Thatcher and others (1997) also examined several variations from a vertical San Andreas fault rupturing in 1906. 

Included among the test cases were a uniformly dipping fault 

near Loma Prieta and a fault that dips at depth but becomes 

vertical near the Earth's surface. These test cases produced 

1906 strike-slip motion on the San Andreas fault ranging from 

2.3 to 3.1 m in the vicinity of Loma Prieta. In all of the cases, 

there still appeared to be substantial differences between the 

1906 geodetically modeled slip and the modeled slip that 

occurred during the 1989 earthquake. In light of this modeling effort, and because the geodetically derived 1906 slip 

was still, after careful consideration, much greater than the 

surface slip used in the forecasts (Lindh, 1983, 1988; Sykes 

and Nishenko, 1984; Scholz, 1985; Working Group on California Earthquake Probabilities, 1988), Thatcher and others 

(1997) concluded that the 1989 earthquake was not correctly 

forecasted by those who used the smaller slip values. 

SAN ANDREAS FAULT-SLIP-RATE 

MEASUREMENTS 

Unlike the 1906 slip measurements, which have been 

refined in the wake of the 1989 Loma Prieta earthquake, estimates of the slip rate on the San Andreas fault have not changed 

significantly. Williams (1995) inverted geodetic data and, assuming a 12-km locking depth, calculated a slip rate of 15.6k 1.2 

rnmlyr for segments of the San Andreas fault near Loma Prieta. 

His value is similar to the values of 15k2 to 16k2.8 mmlyr 

used by the forecasters and most likely would not have resulted in any significant changes to the forecast probabilities. 

WAS THE 

1989 LOMA PRIETA EARTHQUAKE 

AN ANTICIPATED EVENT? 

Debate continues about whether or not the 1989 Loma 

Prieta earthquake was actually predicted (for example, Ken, 

1989b; Segall and Lisowski, 1990; U.S. Geological Survey, 

1990; Beroza, 199 1; Prentice and Schwartz, 199 1 ; Kanamori 

and Satake, 1996; Sykes, 1996; Thatcher and others, 1997; 

Geller, 1997; Schwartz and others, in press). Both protagonists and antagonists have presented data and concepts that 

were developed before the earthquake, data and concepts that 

were developed after the earthquake, and various ideas about 

the position and timing of large earthquakes in central California tectonics. Some geologists and geophysicists who studied the big-picture history and mechanics of the San 

Andreas fault system have presented testimony that the 

1989 Loma Prieta earthquake was predicted. It was an 

event that occurred on the San Andreas fault zone (Wallace, 
B22 FORECASTS 

1990) in the Santa Cruz mountains, and it released much of 

the horizontal strain accumulated since the 1906 earthquake 

(Ellsworth, 1990; Working Group on California Earthquake 

Probabilities, 1990; Hanks and Krawinkler, 199 1 ; Sykes, 1996; 

R.E. Wallace, written commun., 1997). For this conception, 

several of the forecasts were successful. 

Alternatively, other geologists and geophysicists have 

looked at a different definition of forecasts for large earthquakes in the San Francisco Bay region. They have depicted 

the San Andreas fault as a single vertical planar feature that 

broke with fairly shallow right-lateral slip in 1906, and have 

required the forecasts to be events that would produce similar 

action at the same depth (Segall and Lisowski, 1990; Beroza, 

1991 ; Prentice and Schwartz, 199 1; Thatcher and others, 1997; 

Schwartz and others, in press). According to this perspective, 

none of the forecasts were successful, with the authors not 

delivering judgment on two of the forecasts, M8 (Keilis-Borok 

and Kossobokov, 1988; Updike, 1989) and Wesson and 

Nicholson (1988). From this discussion, it can be seen how 

different views of earthquake faulting and fault-zone mechanics heavily influenced previous assessments of the Loma Prieta 

forecasts. At the same time, this debate would not have occurred if none of the forecastshad encompassed the 1989 

Loma Prieta earthquake's time, rupture extent, and magnitude. 

SUMMARY 

Before the 1989 Loma Prieta earthquake, more than a 

century of earthquake history and geomorphic evidence 

from long-term geologic fault slip in the San Francisco 

Bay region had already provided evidence that earthquakes 

do and will occur on regional faults. In the decades before 

the 1989 Loma Prieta earthquake, scientists were therefore well aware of the potential for large, destructive earthquakes. Some attempted to take the next step, to forecast 

where the next large earthquake might occur. 

Reid (1 9 10) presented the first known scientifically 

based forecast for a large San Francisco Bay region earthquake. His forecast was followed in the next 79 years by 

at least 20 more forecasts. Although Reid's forecast was 

not particularly specific as far as a time was concerned, 

some subsequent forecasters came closer to the mark, including Moths and others (198 I), Lindh (1983), Sykes and 

Nishenko (1984), and Scholz (1985), all of whom used 

estimates of the slip during the 1906 earthquake to calculate when the next large earthquake might occur on the 

San Andreas fault northwest of San Juan Bautista. Using 

a slightly different tactic, Wesson and Nicholson (1988) 

presented a forecast that geographically and temporally 

encompassed the 1989 Loma Prieta earthquake, looking 

at patterns of previous seismicity and fault activity to determine where future earthquakes might occur. On a much 

bigger scale and over a much larger forecast area, KeilisBorok and Kossobokov (1988) used seismicity patterns to 

forecast M>7 and m7.5 earthquakes in the Loma Prieta area. 

Although none of the forecasts of the 1989 Loma 

Prieta earthquake can be rigorously tested, because most 

of them are incomplete, the forecasts can be qualitatively 

judged on those aspects that came closest to the actual 

event, including the rupture length, which was most closely 

estimated by Lindh (1983), and the magnitude, which was 

accounted for by many of the forecasters, including 

Thenhaus and others (1980), Sykes and Nishenko (1984), 

Scholz (1985), Keilis-Borok and Kossobokov (1988), and 

Wesson and Nicholson (1988). In contrast to those who 

presented scenarios of probable large earthquakes, 

Thatcher and Lisowski (1987) showed that an earthquake 

on the San Andreas fault near Loma Prieta was unlikely. 

Thus, the 1989 Loma Prieta earthquake did not follow all of the specifications set out for it by the forecasters. Instead, it occurred much deeper than expected, and 

with a significant reverse-slip component. There is still discussion about whether or not the 1989 earthquake occurred 

on the same fault that ruptured in the great 1906 earthquake. 

Segall and Lisowski (1990), Dietz and Ellsworth (1997), 

Thatcher and others (1997), and Schwartz and others (in press) 

present evidence that the 1989 and 1906 earthquakes could 

not have occurred on the same fault plane. This evidence has 

not, however, dissuaded other workers (for example, Hanks 

and Krawinkler, 1991; Sykes, 1996) who have viewed the 

forecasts more favorably, noting that the 1989 earthquake released the horizontal strain accumulated since 1906 and thus 

that it was an expected (and forecasted) event. 

ACKNOWLEDGMENTS 

Thanks to Art McGarr, Walter Mooney, and Tom 

Holzer for the assignment to write this review. I learned 

much about the elusive task of forecasting earthquakes and 

about how good foresight is much harder to attain than 

good hindsight. Thanks to numerous people for their guidance on this project. A1 Lindh, Bill Ellsworth, and Bob 

Wallace provided me with many excellent references, both 

published and unpublished. John Filson found a reference 

to the M8 transmittal between Gorbachev and Reagan. 

Volodya Kossobokov cited a reference for the M27.0 preLoma Prieta M8 forecast, and Bernard Minster and Nadya 

Williams helped me understand the details. Jack Healy discussed the complexities of M8 forecasts in general and 

located a copy of the Trieste publication. Bob Simpson 

generously plotted the California fault maps, and Craig 

Nicholson expeditiously sent a figure from 1986 for me 

to evaluate. Bill Bakun, Tom Hanks, Tom Holzer, Volodya 

Kossobokov, A1 Lindh, Art McGarr, Will Prescott, Paul 

Spudich, Lynn Sykes, Wayne Thatcher, and Bob Wallace 

all provided helpful detailed comments on a draft version 

of the manuscript. Paul Reasenberg and Craig Nicholson 
REFERENCES CITED B23 

thoroughly and thoughtfully reviewed the manuscript and 

suggested numerous helpful improvements. 

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APPENDIX 2. THE LAKE ELSMAN ADVISORY B25 

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APPENDIX 1. 

DEFINITION OF 

AN EARTHQUAKE ADVISORY 

In his July 12,1988, draft after the first M=5 Lake Elsman 

earthquake, California State Geologist James Davis (unpub. 

data, 1988) defined the terms earthquake "advisory" and earthquake "warning or alert." He stated that an advisory is a commentary on earthquake potential which has less urgency in 

terms of public safety than an alert or warning. The advisory 

provides information about a scientific conclusion that has 

not developed consensus on the likelihood that the damaging 

earthquake will occur, so that a quantifiable probability cannot be agreed upon, although the time frame of concern may 

be short, as in a few days. An advisory can also be for a longer 

time period (for example, now to decades in the future), or 

for an event that may not cause extensive enough damage to 

merit full-scale response. 

In the case of the Lake Elsman advisories (starting one, 

June 27; ending one, July 2), there was not a short-term prediction, just the potential for a moderate event in a relatively 

sparsely populated region. Similar advisories were issued in 

June 1985 for San Diego and in July 1986 for Chalfant Valley. 

APPENDIX 2. 

THE LAKE ELSMAN ADVISORY 

The first M=5 Lake Elsman earthquake occurred just before noon on June 27, 1988 (fig. 9). Preliminary locations 

requested by California State Geologist James Davis (unpub. 

data, 1988) soon after the earthquake placed the event at 35- 

km depth at lat 37'7.7' N., long 121'54.2' W., about 35 krn 

northwest of San Juan Bautista. (Later estimates placed the 

event much shallower, at 14.4-km depth.) That afternoon, A1 

Lindh of the U.S. Geological Survey informed Davis that the 

earthquake was on the San Andreas fault at the northern margin of the Working Group on California Earthquake Probabilities' (1988) Southern Santa Cruz Mountain segment. The 

(first) Lake Elsman earthquake also appeared to have been 

the largest earthquake on this part of the San Andreas fault 

since 1906. Lindh told Davis that this Lake Elsman earthquake could be a foreshock preceding an M=6-6.5 earthquake 

on thesanta Cmz Mountains segment of the San Andreas 

fault. 

Brian Tucker of the California Division of Mines and 

Geology and Jim Davis decided to call the California Office 

of Emergency Services to discuss whether or not the release 

of an earthquake advisory (which is a commentary on earthquake potential (see app. 1; Wallace and others, 1984; James 

Davis, unpub. data, 1988) to local governments was the appropriate next step. Meanwhile, several conference calls were 

coordinated to discuss whether or not the M=5 earthquake 

might set off any of the forecasted earthquake scenarios. In 

the scientific discussions, it was decided that it would be difficult to assign a meaningful probability gain that everyone 

could agree on and that they were not dealing with earthquake 

prediction as such. The scientists did agree, however, that the 

likelihood of a larger subsequent earthquake was probably 

higher than usual, owing to the location of the earthquake on 

a segment of the San Andreas fault that had built up considerable strain since 1906. The final consensus by the scientists 

was that the Office of Emergency Services should decide what 

type of message would be used best by local agencies and 

that this message should be low key, without a news release. 

As a result, an advisory was edited by the State and the Office 

of Emergency Services and sent by the Office of Emergency 

Services over the telephone to the local governments of Santa 

Clara, Santa Cmz, San Benito, and Monterey Counties. This 

was the first time in San Francisco Bay region history that the 

State Office of Emergency Services had ever sent an official 

short-term advisory regarding a potential upcoming large 

earthquake. 

The advisory stated that there was a slightly increased 

likelihood of an M=6.5 event on the Santa Cruz Mountains 

segment of the San Andreas fault in the next 5 days. The 5- 

day time period was based on a southern California-based 

study by Jones (1985) stating that there exists a 6-percent 

chance of a larger earthquake within 5 days after an h423 earthquake. The advisory mentioned that agencies in the specified 

areas should maintain preparedness and awareness measures. 

On June 28, after the advisory had been received by local governments, several State officials decided that a news 

release would be desirable, and so one was drafted and sent 

out. The public was now also aware of the advisory. 

On June 30, State officials started preparing a notice for 

July 2 stating that the 5-day advisory had expired. Meanwhile, 

extremely low aftershock activity occurred near the Lake 

Elsman main shock. On July 2, the State issued the official 

expiration advisory. 

On August 8, 1989, another M=5 earthquake occurred 

(fig. 9). The Office of Emergency Services issued another 5- 

day (short term) advisory similar to that issued in June 1988. 

Five days later, this advisory also expired. 
FORECASTS 

APPENDIX 3. 

PECK AND PEARLMAN CORRESPONDENCES 

Memorandum 

United States Department of the Interior 

OFFICE OFTHE SECRETARY 

WASHINGTON, D.C. 20240 DEC 3 1985 

To : Executive Secretary~National Security Council 

Fromi Executive- Assistant to the Secretary 

Subject; Recent .reference to California earthquakes by a Soviet official 

At a recent meeting a Soviet official indicated that their studies showed 

that during the next 3 year6 there is a 60 percent probability of a 

magnitude 8-8.5 earthquake end a 70 - 75 percent chance of a magnitude 

7-7.5 earthquake in southern California. 

Under the aegia of the National Earthquake Hazards Reduction Program, 

the Geological Survey carries out research and other activities on earthquake 

hazards assessment and earthquake prediction. Within this program and 

through an agreement on "Cooperation in Environmental Protection" between 

the United States and thesoviet Union signed in 1972, the Geological Survey 

has cooperated in earthquake prediction studies with the Academy of 

Sciences of the Soviet Union. For the past several years this cooperation 

has been in the exchange of scientists to carry out joint studies and to 

discuaa recent results. During these exchanges, Soviet scientists have 

worked with colleagues at California universities on earthquake prediction 

problems. 

Based on detailed geological study of active faults and seismicity patterns 

in California, the Geological Survey has made earthquake hazards aSSeaSment8 

of various regions. The salient features of these aaseesments are that 

there is about A 50 percent chance of a major (magnitude 8+) on the 

southern San Andrea8 fault northeast of LO6 Angelef within the next 30 

years and a 90 percent chance of a moderate earthquake (magnitude 6) 

along the same fault east of Pam Roblee within the next 7 years. This 

latter area is the site of an intensive earthquake prediction experiment 

in an attempt to give warning within a few days or hours of the anticipated 

event. A succeaaful outcome of this experiment would have wider application. 

There are many other active faults in California capable of producing 

damaging earthquakes in the near future, but the two regions mentioned 

above are the most well understood. 

It is believed that the recent reference to earthquakes in California 

has ceaulted from a Soviet approach to earthquake prediction based on 

statistical analyses to the historical record of earthquakes in a given 

region. This technique haa had some success in retrospectively identifying 

'times of increased probability," periods of a few years, when earthquakes 

of magnitude 6 or greater have occurred in California. United States 

scientists have not acclaimed this work because of its lack of geologic2il 

basis and geographical specificity. The recent reference appears to be 

cast with much greater precision than previous results. 

It is recommended that follow-up contact be made with Soviet scientists 

through the existing agreement to determine and evaluate the basis for 

the recent statement. 
APPENDIX 3. PECK AND PEARLMAN CORRESPONDENCES B 27 

United States Department of the Interior 

GEOLOGICAL SURVEY 

RESTON. VA. 22092 

In Reply Refer TO: 

'Flail Stop 905 

Memorandum 

To : Executive Assistant to the Secretary 

From: Director, Geological Survey 

Subject: Summary of a meeting at the White House, December 5, 1985, on 

Soviet prediction of southern California earthquake 

Attendees: Billy Martin, NSC 

Rod McDanlel, NSC 

Ty Cobb, NSC 

Max Robinson, State/Soviet Desk 

Jack Blanchard, State/OES 

Gary Waxnoneky, EPA 

Dallas Peck, USGS 

John Filson, USGS 

Background: Chairman Gorbachev passed to President Reagan at the Geneva 

Summit Meeting a prediction by Soviet scientists that there was a 60 

percent probability of a magnitude 8-8.5 earthquake and a 70 - 75 percent 

probability of a magnitude 7-7.5 earthquake occurring in southern California 

in the next 3 years. Admiral Poindexter requested that there be some 

f 01 low-up. 

The subject meeting focused on our reaction to the Soviet prediction, 

the state of us/Swiet cooperation in earthquake prediction research, 

and possible ways to pursue the matter. 

Waxmonsky, Executive Secretary of the US/Soviet cooperative in Environmental 

Protection summarized the history of that program since it was established 

in 1972. At the time of the recent Geneva Summit, Lee Thomas of EPA was 

heading a delegation to Moscow to meet with the Soviets to review that 

agreement and discuss future activities. This agreement contains a 

section on earthquake studies that includes a project on earthquake 

predict ion. 

Filson, who ie a oochairman of the earthquake project, then summarized 

some of the joint work under the project, which has included exchange 

visits of a soviet mathematician named Keilis-Borok. Keilis-Borok recently 

worked with Profensor Leon Knopoff of UCLA to predict retrospectively 

times of heightened seismic activity in California through statistical 

analyses of paat earthquake occurrence. Review of his approach by leading 

American scientists has yielded mixed but mostly mildly favorable reviews. 

Keilis-Borok may be the originator of the prediction that was passed to 

President Reagan. 
B28 FORECASTS 

Executive Assistant to the Secretary 2 

Filson summarized the current USGS outlook on southern California earthquakes. 

Baaed on the recurrence intervals of past earthquakes in southern California 

and detailed geologic studies of the faults there, we have estimated the 

probabilities of future earthquakes along segments of the San Andrea6 

and other faults. The probability of a magnitude 8 earthquake near 

Lo6 Anqeles is estimated to be 1-3 percent a year or about 50 percent in 

the next 30 years~an estimated probability far smaller than that advanced 

by the Soviets. 

After discussing what-the next steps should be, MeDaniel and Martin 

requested that Peck and Filson draft a memo (attached) to Martin for 

your signature summarizing the above material and proposing scientistto-scientist discussions on the subject under the US/Soviet agreement. 

After receipt of a copy of this memo, Robinson and Blanchard are to 

prepare a memo suggesting steps to take to carry the matter forward. We 

were requested to keep Judge Clark informed on the matter. After receipt 

Of the memos, the NSC will advise us how to proceed. 

 all as L. Peck 

xk U.S. GOVERNMENT PRINTING OFFICE: 1998-673-048 / 20113 Region No. 8