Earthquake Research at Parkfield, California, 

for 1993 and Beyond- . 

Report of the NEPEC Working Group to Evaluate the 

Parkfield Earthquake Prediction Experiment 

National Earthquake Prediction Evaluation Council Working Group 

U.S. GEOLOGICAL SURVEY CIRCULAR 1116 
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Earthquake Research at 

Parkfield, California, 1993 and BeyondReport of the NEPEC Working Group to 

Evaluate the Parkfield Earthquake 

Prediction Experiment 

By National Earthquake Prediction Evaluation Council Working Group 

U.S. GEOLOGICAL SURVEY CIRCULAR 1116 

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

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

Free on application 

U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225 

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

not imply endorsement by the U.S. Government. 

Cover: A low-angle aerial view of Parkfield, California, looking NNE. In the foreground 

is the San Andreas fault (red line), the source of the Parkfield earthquakes. 
CONTENTS 

s~m~ 1 

Introduction 2 

Charge to the Working Group 3 

Assessment of the Prediction . 3 

What We Have Learned from the Experiment 4 

Lessons for the Scientific Community 4 

Improvements in Monitoring 4 

Scientific Results to Date 5 

Technology Transfer 6 

Lessons for the Response Community 6 

Parkfield Successes . . . . . . . 6 

Benefits from the Parkfield "A" -Level Alert, October 20-22, 1992 7 

Benefits Advancing Public Policy 8 

Problems Identified . . . . . . . . . 8 

Lack of Ongoing External Scientific and Emegency Response Review . 8 

Scientific Data Not Readily Accessible . 9 

Data Management . . . . . . . . . 9 

Overemphasis by the Public and Media of the Prediction Aspects of the Experiment 9 

How Should the Experiment Be Modified in the Future? . . . . . 9 

What are the Incremental Costs Associated with the Experiment? 9 

Should the Experiment be Moved? 10 

Long-Term Aspects of the Project . 10 

Commitment for Long-Term Management 10 

Acquisition of Land Rights at Parkfield 10 

Reasess Project Periodically to Modify, Upgrade and Acquire New Equipment 10 

Assessment of Costs and Productivity of the Experiment 11 

Recommendations for Future USGS Response Efforts 11 

Strengthen the USGS Response Role . . . . . . 11 

Review the Threshold Criteria Determining Alert Levels 12 

iii 
Fund Understanding of Societal Impacts of False Alarms . . . 

Improve Communication of the Hazard Potential to the Public 

Concluding Remarks 

References . . . . . 

FIGURES 

1. Modified Mercalli intensity pattern for the 1966 Parkfield earthquake. 

13 

13 

13 

13 

.......... 3 

2. Significant earthquakes have occurred on the Parkfield section of the San Andreas fault at fairly regular intervals- in 1857, 1881, 1901, 1992, 1934, and 1966. The next significant earthquake is anticipated to take place within the timeframe 1983 to 1993. . . . . . . . . . . . . . . . . . 3 

3. Conditional probability for the occurrence of major earthquakes along the San Andreas fault in the 

30-year interval from 1988 to 2018. 4 

4. Parkfield sign. 4 

5. De Bilt, the Netherlands, east-west (DBN-EW) and north-south (DBN-NS) component seismograms 

for the 1922, 1934, and 1966 Parkfield events. Amplitude and time scales are constant. . . . 5 

6. Surface creep of the San Andreas fault at Middle Mountain (in mm) and the rate of seismicity under Middle Mountain (cumulative number of events with M ~ 1.5, depths~ 6.5 km since 1980) are 

shown. The Coalinga M 6. 7 earthquake caused the fault to slip left lateral and stopped the seismicity for slightly over a year. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

7. This cross section along the San Andreas fault cmnpares the amount of slip in centimeters during the 

1966 mainshock (dark contours from Segall and Harris, 1986) to the seismic P wave velocities on the 

northeast side of the fault (shading and lighter contours in km/s from Michael and Eberhart-Phillips, 

1991). The mainshock hypocenter (large star) was under a body of low velocity material but most of 

the slip occurred to the southeast where higher velocity material exists. White areas are small background earthquakes that tend to occur in the lower velocity material and around the edges of the 

mainshock. . . . . . . 7 

8. Public Response Plans. 7 

9. Flow of information resulting from an A-level alert at Parkfield. Signals from USGS instruments at 

Parkfield {1) are telemetered to the USGS Western Region Headquarters in Menlo Park {2) for analysis by USGS scientists. The California Office of Emergency Services ( OES) in Sacramento {3) is notified by the USGS for all A-, B-, and C-level alerts. A-level alerts are immediately transmitted by 

OES to county governments (4) using established OES emergency communication channels. County 

officials also have established procedures to relay the warning to such groups as law enforcement 

agencies, fire departments, utility companies, and school districts. . . . . . . . . . . . . . 8 

iv 
10. The time of Parkfield status level, D through A, and their corresponding 72 hour probability of a M 6 

earthquake are shown for period from the start of the experiment in 1985 through 1993. Although 

this report described a single A-level alert in late 1992, the experiment went to its second A-level in 

November 1993. The 72 hour probability for each status level is discussed in Parkfield Earthquake 

Prediction Response Plan [1988]. . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

11. Scientists from the USGS and officials from the California OES met with reporters on the lawn of the 

Parkfield Cafe for regular briefings during the level-A alerts in October 1992. . . . . . . . 13 

v 
National Earthquake Prediction Evaluation Council Working Group 

Bradford H. Hager (Chair) 

C. Allin Cornell 

William M. Medigovich 

Kiyoo Mogi 

Robert M. Smith 

L. Thomas Tobin 

Joann Stock 

Ray Weldon 

vi 

Massachusetts Institute of Technology 

Stanford University 

Federal Emergency Management Agency 

Nihon University, Japan 

University of Utah 

California Seismic Safety Commission 

California Institute of Technology 

University of Oregon 
Earthquake Research at Parkfield, California, 1993 and 

Beyond - Report of the NEPEC Working Group to 

Evaluate the Parkfield Earthquake Prediction Experiment 

Summary 

During the past century, earthquakes of M ,....., 6 have 

occurred with remarkable regularity on the San Andreas 

fault at Parkfield, California. Events occurred in 1857, 1881, 

1901, 1922, 1934, and 1966. At least two of these events 

were preceded by large foreshocks and there is evidence for 

precursory creep of the shallow segment of the fault prior 

to the 1966 event. In 1984-1985, scientists developed and 

published a prediction, based on a model of "characteristic" 

earthquakes, that the next M ,....., 6 Parkfield event was 

expected in a time window centered on 1988, with 95% 

probability that the earthquake would occur by the end of 

1992 [Bakun and Lindh, 1985). 

Shortly after the publication of this prediction, with endorsement by the National Earthquake Prediction Evaluation Council (NEPEC), the United States Geological Survey 

(USGS) initiated the Parkfield Earthquake Prediction Experiment (the Experiment). With additional support from 

the state of California, the Experiment took on a public 

services aspect, as well as a geophysical aspect. 

By late summer, 1992, the predicted event had not yet 

occurred. NEPEC chartered a Working Group to evaluate 

the Parkfield Earthquake Prediction Experiment. This 

group was asked a series of questions which are summarized 

below, along with the responses of this Working Group. 

1) What is the current assessment of the prediction? 

a) Is it still considered likely that an M ,....., 6 earthquake 

will occur in the short term? 

Parkfield is still considered to be the most likely locality identified to "trap" a moderate sized earthquake. Empirically, no other location has demonstrated a sequence of 

earthquakes with as much regularity and as short an average 

recurrence time as the Parkfield sequence. The loading of 

the Parkfield segment of the San Andreas fault is unusually 

simple, with the creeping segment to the north leading to 

a continuous accumulation of strain on the locked segment 

near Parkfield. At this time, about as much strain has accumulated as was released in the previous event [Segall and 

Harris, 1986). 

Estimates of the probability of the Parkfield earthquake 

occurring in the near future have been generated by a number of scientists. These estimates are based on a number 

of assumptions about the statistical behavior of faults, but 

all update the probability estimate to include the informa1 

tion that, as of the time of the estimate, the event has not 

yet occurred. Estimates of the probability of occurrence are 

clustered around a value of approximately 10% per year. 

b) If the event does not occur by the end of 1992, what 

does that indicate about the original prediction? Was the 

basis for the prediction in error? 

The original prediction was based on a rather specific 

set of assumptions. These include that the loading rate is 

constant, that failure of the same patch of the fault occurs at 

or below a threshold stress level, and that the stress drop is 

identical for each event [Bakun and Lindh, 1985). It was the 

adoption of this specific model that allowed the 1934 event 

to be ignored when evaluating the expected time of the next 

"characteristic earthquake," leading to a small uncertainty 

in the expected time of the event. This model is now known 

to be too simple to apply to Parkfield. In retrospect, the 

original prediction should have included an assessment of 

the probability that the model assumptions were correct, 

in addition to considering the uncertainties related to data 

noise in fitting the assumed model. 

But failure of this specific prediction does not negate the 

consensus of the Working Group that Parkfield is still the 

most likely place in the United States to trap an earthquake 

and that there is a relatively high probability that this 

event will occur on a time scale of a decade or so. In 

addition, Parkfield is unique because the location of the 

likely nucleation point can be estimated. And there is a 

long baseline of measurements already established there. 

2) What have we learned during the experiment, 

both from the scientific and response community 

aspects? What have been the principal benefits 

that have come from both of these aspects of the 

experiment? 

While the major scientific impact of the experiment will 

not occur until the next Parkfield earthquake is trapped 

in the dense web of instrumentation operating there, there 

have already been important benefits reaped from the realtime exercise created by the response community interacting 

with the scientific comunity. Because of its low population 

density, Parkfield has been an ideal location to begin this 

process. The Experiment has been a public policy success, 

with positive implications for response and mobilization to 

possible future earthquake alerts elsewhere. Cooperation at 

Parkfield has produced the California Short- Term Earthquake Prediction Response Plan. The "A"-level alert in Oc-
tober, 1992, provided a realistic "fire drill" to test the implementation of this plan. The press has learned how to 

portray alert levels and associated probabilities. 

Several scientific results are also notable. For example, 

geodetic data have shown resolvable differences between the 

1966 and 1934 events; Parkfield events are similar, yet still 

show differences large enough to violate the specific assumptions of the model on which the original prediction was 

based. In addition, the fault zone and asperities have been 

imaged at unprecedented resolution, with the identification 

of a low-velocity zone, perhaps related to high fluid pressure. 

There has also been substantial technology transfer resulting from the Experiment, including increased expertise 

in siting of borehole instruments, experience with real-time 

seismic networks, and improved instrument design. These 

advances have made the installation of other instrumentation, such as that monitoring the Hayward fault, more cost 

effective. 

3) Where should the experiment go in the future? 

What modifications should be made to scientific 

monitoring? What research efforts should receive highest priority? Should there be any modification in the agreements that govern the interaction between the USGS and the State of California with regard to hazard warnings? 

The science of understanding the earthquake source is 

limited by the dearth of observations throughout the earthquake cycle. Parkfield is the most likely place yet identified 

to trap a moderate earthquake in a densely instrumented 

region and the best locale identified to answer a number 

of important scientific questions about the seismic source. 

Because the Parkfield segment of the San Andreas fault is 

loaded by the creeping section at one end, it provides a 

setting intermediate in scale between the simplicity of the 

laboratory and the complexity of most other faults. Substantial resources have been invested in setting up the Experiment and the marginal costs associated with continued 

operation of the experiment are minimal. Although the estimated annual probability of about 10% per year is the 

highest proposed for any specific location, and high enough 

to make the area scientifically and societally interesting, it 

is not high enough to ensure that an earthquake will occur 

on a time scale of a few years. Thus the USGS should view 

the Experiment as a long-term commitment. 

In this context, the Working Group recommends that 

USGS continue the Parkfield Experiment and assign it a 

high priority. This includes having a committed, longterm Project Scientist with sufficient resources available 

to deal with the scientific, response planning, and public 

relations priorities of the Experiment. These aspects of 

the Experiment are likely to be in a state of high activity 

simultaneously when alerts are called. 

A long-term plan is required for replacing failed or obsolete equipment, in particular, strain meters. This long-term 

plan should also include periodic peer review of the Experiment, with possible redirection and reordering of priorities. 

2 

In recognition of the long-term aspects of monitoring the 

preparation zone throughout the earthquake cycle, and the 

problems due to transients and costs associated with new 

installations, the USGS needs to ensure access to monitoring sites. The USGS should attempt to acquire control of 

land where instruments are located to avoid disruption when 

landowners change. 

There should also be a commitment to continue the public 

policy aspects of the Parkfield Experiment through the 

earthquake cycle. There is still much to be learned about 

public response to perceived false alarms, perception of risk 

assessment, and warning thresholds. 

Introduction 

Before the mid-1980's, the United States' earthquake prediction program was in a reconnaissance mode, with monitoring programs broadly distributed across regions of the 

nation that had high seismic activity. In 1984, scientists 

at the USGS began to plan spatially focused earthquake 

experiments, with a primary goal of evaluating possible precursory phenomena. The issue that had to be addressed at 

that time was where to use the limited resources available 

with the best chance for evaluating these phenomena. The 

immediate question that had to be answered was where to 

deploy the recently developed dual-frequency laser geodimeter. Sites under consideration included Parkfield and San 

Juan Bautista. 

In 1985, scientists at the USGS published a prediction 

that the next Parkfield earthquake was expected in a time 

window centered on 1988, with a 95% probability that the 

earthquake would occur by the 'end of 1992 [Bakun and 

Lindh, 1985]. This prediction was based on a specific model 

of the "characteristic" Parkfield earthquake, discussed in 

more detail below. NEPEC reviewed the prediction favorably and the USGS decided to locate their focused experiment in Parkfield. 

In April, 1985, the Director of the USGS sent a letter 

to the Governor of California informing him about the 

possibility of an earthquake at Parkfield. California and 

USGS each contributed $1M to begin instrumentation of the 

Parkfield area. Over 20 observational networks have been 

installed, including seismometers, creep meters, borehole 

strain meters, the two-color laser geodimeter, water wells, 

and magnetometers. Five of these networks are monitored 

in real time. 

There were two scientific goals for the Parkfield Earthquake Prediction Experiment: 1) To record the geophysical 

details before and after the expected earthquake; and 2) To 

issue a short-term prediction. In addition, with the involvement of the State of California, the Experiment took on 

an important public policy aspect, serving as a test bed for 

communication between earthquake scientists and public officials. The rural nature of the community made it an ideal 

location to carry out the Experiment. 

A formal series of alert levels has been set up, triggered 

by phenomena such as fault creep or earthquake activity in 
,. 0 

"..-...... 

No! Felt 

3 4" JI~JV Fe ll, no dam a g e 

0 

Scat ul a M1tes 

V Sma ll ob tec t s upset , liQuids disturbed 

Nt •Potl 

VJ Furn•lure mo ved . d ishes broken. books oH she lves nc~ 

VII Weak chimneys b r oken. damtge 10 w ea k masonry 

33" (N.B. Alono the l aul t tr ace near Parklield, there were 

conspicuous cra cks in !he ground and broken underground 

pipes tha i are usually assoclaled wit h Int ensity level IX ,) 

122° 121 " 120 ° 118 " 

Figure 1. Modified Mercalli intensity pattern for the 1966 

Parkfield earthquake. 

the "preparation zone." Alert levels range from D (triggered 

about 100 times since June, 1985) to A (triggered once, 

in October, 1992, by a M = 4.5 earthquake; this alert 

corresponds to an expected probability of 37% of the forecast 

event occurring within 72 hours). 

Charge to the Working Group 

In mid-1992, with the pending expiration of the prediction 

window, NEPEC chartered a working group to evaluate the 

original prediction. The charge to the working group was to 

evaluate the Parkfield Prediction Experiment in light of the 

following questions: 

1) What is the current assessment of the prediction? 

a) Is it still considered likely that an earthquake will occur 

in the short term? 

b) If the event does not occur by the end of 1992, what 

does that indicate about the original prediction? Was 

the basis for the prediction in error? 

2) What have we learned during the experiment, both from 

the scientific and response community aspects? What 

have been the principal benefits that have come from both 

of these aspects of the experiment? 

3) Where should the experiment go in the future ? What 

modifications should be made to scientific monitoring? 

What research efforts should receive highest priority? 

Should there be any modification in the agreements that 

govern the interaction between the USGS and the State 

of California with regard to hazard warnings ? 

3 

2000.---------------------------~ 

- - - 1993- - - - - - - - - - - - - - - -

---1983--- _)i!_n!!_a.!YJ@~~~- · 

1950-

1900-

/ 

/ 

, 

/ 

/ 

/• 

• / 

1850~--~--r-~---.--~------~~ 

Figure 2. Significant earthquakes have occurred on the 

Parkfield section of the San Andreas fault at fairly regular 

intervals-in 1857,1881,1901,1922,1934, and 1966. The 

next significant earthquake was anticipated to take place 

within the timeframe 1983 to 1993. 

Assessment of the Prediction 

The Working Group addressed two main questions in 

assessing the Parkfield Prediction. The more important 

one is whether Parkfield is still viewed as being the most 

likely place to trap a moderate earthquake. The secondary 

question is whether the specific prediction that expired at 

the end of 1992 was correct. 

Parkfield is thought to be more likely to experience a 

moderate earthquake than any other place in the United 

States. During the past century, earthquakes of M ~ 6 have 

occurred there with remarkable regularity. Events occurred 

in 1857, 1881, 1901, 1922, 1934, and 1966. The time between 

earthquakes ranged from 12 to 32 years, with an average 

of 22 years. The relatively short time between events may 

result from a near-uniform rate of loading of the Parkfield 

segment by slip on the creeping segment of the fault just to 

the northwest. 

With the demonstration that for the last two M = 6 Parkfield earthquakes the total (co- and post-seismic) surface displacements determined from geodesy are resolvably different 

[Segall and Du, 1993], it is now understood that the assumptions of the specific model on which the prediction was based 

do not hold. There is no consensus on the correct physical 

model to be used for evaluating the probability of the Parkfield event occurring within any given time window. The 

computation of statistical estimates of occurrence probabilities has become almost a cottage industry, with a variety 

of models produced using different assumptions about the 

nature of the appropriate statistics. Most recent statistical 

models update the estimated probability to include the information that the earthquake has not yet happened as of 

the time of the estimate. While the details vary, estimates 

of the probability of occurrence of the event cluster around 

a value of about 10% per year [e.g., Kagan and Jackson, 
1991; Savage, 1991]. Such a probability is high enough that 

continued monitoring of the Parkfield segment is important. 

In addition to Parkfield being the most likely place identified to trap a moderate earthquake, Parkfield has other features that make it a good place for a prediction experiment. 

First, a variety of events that may have been precursors occurred before previous M ~ 6 earthquakes there. At least in 

1934 and 1966, M > 5 foreshocks occurred. In 1966, cracks 

were observed in the ground and a pipe broke days before 

the event. The epicenters of the 1922, 1934, and 1966 events 

are all located close to each other, providing a clear target 

for siting instrumentation to trap the next event. 

From a social and economic perspective, Parkfield is an 

ideal location for a prediction experiment. The area is 

sparsely populated and the residents are well educated and 

relaxed about the occurrence of moderate earthquakes. 

The Experiment has made important contributions both 

to geophysical science and to public policy. But advances 

in our understanding suggest that the model on which the 

original prediction was based is too simplistic. The original 

model of constant loading rate, uniform moment release, 

and constant stress drop, controversial when it was first 

proposed, has now been shown not to apply to Parkfield. In 

retrospect, an estimate of the reliability of this hypothesis 

should have been included in the original calculation, and 

would have broadened the window of predicted recurrence. 

The relatively narrow window that was stated in the original 

prediction has led to expectations that the experiment would 

be over relatively quickly, leading to the misconception 

that the experiment has now somehow "failed" because the 

narrow time window has closed. 

LEVEL OF RELIABILITY 

(wlth Abeingmostreliable) 

• A 

• • • c 

D o 

0 ' 

CONDITIONAL PROBABILITY OF MAJOR 

EARTHQUAKES ALONG SEGMENTS OF THE 

SAN ANDREAS FAULT 

1988-2018 

Figure 3. Conditional probability for the occurrence of major 

earthquakes along the San Andreas fault in the 30-year 

interval from 1988 to 2018. 

4 

Figure 4. Parkfield sign. 

What We Have Learned from the 

Experiment 

Lessons for the Scientific Community 

Improvements in Monitoring 

The Parkfield project was the first concerted effort in the 

US to implement real-time monitoring of a variety of possible precursory signals and geophysical information in a single earthquake source region. This forced the development of 

a coherent plan for monitoring instrumentation: alignment 

arrays, a two-color laser geodimeter, creep meters, groundwater-level transducers, tilt meters, surface and borehole 

seismometers, volumetric strain meters, tensor strain meters, leveling lines, borehole temperature measurements, an 

earth resistivity monitoring network, Global Positioning System (GPS) geodetic measurements, magnetic field measurements, ULF electromagnetic measurements, ground water 

radon measurements, and soil hydrogen measurements. In 

addition, arrays to measure strong ground motion, liquefaction, coseismic slip, and pipeline response were installed. 

The result has been the most thoroughly instrumented earthquake source region in the world. The technology required 

to record and download these data (often in real time) has 

been a challenge to implement, and during the Parkfield experiment, the knowledge of how to run such a system has 

improved . 

The monitoring done in Parkfield since the start of the 

earthquake experiment has yielded a very detailed baseline 

of behavior of all of these phenomena, which will be invaluable for comparison with post-earthquake observations. The 

baseline observations have permitted a good understanding 

of the transient results of rainfall events, slow strain events, 

and tides. In addition, they have provided observations 
regarding the response of the Parkfield region to smaller 

earthquakes nearby, and to larger earthquakes at some distance from the region (e.g., Kettleman Hills; Lorna Prieta; 

Coalinga; and Landers). These observations are extremely 

relevant to the design of monitoring installations and establishment of alert levels in other regions; they will also 

contribute to the revision of alert levels for Parkfield. 

Scientific Results to Date 

The scientific experiments at Parkfield fall into three classes: those designed to monitor possible earthquake precursors, those designed to monitor the behavior of the region 

around the anticipated rupture nucleation point, and those 

designed to study the effects of earthquake-induced ground 

shaking on both natural and manmade structures. Many 

of the experiments will not yield their full scientific value 

until the earthquake has occurred, when the difference in 

baseline before and after the earthquake can be determined, 

and the effect of the earthquake on manmade structures can 

be analyzed. However, some of the instruments installed 

at Parkfield have shown temporal variations (particularly in 

levels of seismicity) that have led to important scientific advances even during the current pre-earthquake monitoring 

period. 

DBN-EW 

Figure 5. De Bilt, the Netherlands, east-west (DBN-EW) 

and north-south (DBN-NS) component seismograms for the 

1922, 1934, and 1966 Parkfield events. Amplitude and time 

scales are constant. 

One such example is the borehole seismic array that has 

been installed at Parkfield. Because of the extremely low 

noise present on the downhole seismometers, a complete seismic catalog down to magnitude near zero has been recorded 

for the region. These seismic observations have permitted 

the recognition of slow, microseismic slip events [Malin and 

5 

Alvarez, 1992) and the identification of periods of higher 

seismicity that may be related to the locations of future 

slip events [Roeloffs and Langbein, 1994). Such events would 

have been impossible to identify with conventional surface 

seismic arrays, and have illustrated the importance of including downhole installations in other prediction arrays. 

Because of the focus on Parkfield as a center of earthquake 

prediction efforts, the seismic history of the region has been 

scrutinized during the years since 1985. This has led to 

major advances in our understanding of the details of the 

previous Parkfield earthquakes. For example, differences 

in the extent of the rupture plane between the 1934 event 

and the 1966 event are now recognized [Segall and Du, 

1993). The pre-1930 historical events in the region have 

been more closely studied, resulting in the recognition of 10 

previously unrecognized events with magnitudes above 5.5 

within 100 km of Parkfield [e.g., Toppozada et al., 1990). 

This new information has led to debate about just how 

similar the 1934 event and the 1966 event really are, and 

whether all of the earlier events were on the same fault patch, 

casting doubt on the validity of applying the "characteristic 

earthquake" hypothesis, in its simplest form, at Parkfield. 

The focus on Parkfield has also resulted in reexamination 

of the predicted earthquake recurrence interval there based 

on the simplistic idea of uniform loading rate. For example, 

earthquakes on nearby faults, such as the 1983 Coalinga 

event, may have an effect [e.g., Simpson et al., 1988; Tullis 

et al., 1990). In addition, the results of viscoelastic relaxation following the great 1857 earthquake may lead to a 

decrease in recurrence time during the longer-term San Andreas seismic cycle [Ben-Zion et al., 1993). The possibilities 

that the segment boundaries at Parkfield are not geometrically well defined [Nishioka and Michael, 1990) nor visible as 

steps in the seismically defined fault zone [Eberhart-Phillips 

and Michael, 1992; Michael and Eberhart-Phillips, 1991) and 

that successive ruptures may overlap in spatial extent has 

been recognized, along with the tendency for earthquakes 

to cluster [Kagan and Jackson, 1991). The rate of strain 

accumulation since the 1966 event has also been examined, 

and used to constrain the moment deficit since 1966, which 

can be used for estimates of recurrence time [see summary 

by Roeloffs and Langbein, 1994). 

Partially due to the focus on Parkfield, understanding of 

the statistics of "earthquake prediction" (in terms of estimation of recurrence times and their uncertainties) has advanced considerably. If we were to calculate the mean recurrence time and probability of the Parkfield earthquake as of 

today, we would do it by a more sophisticated technique, and 

using different uncertainties, than used in 1984. We would 

also recognize that various models of earthquake recurrence 

are possible. The question of whether to include the 1934 

event in the probability calculation, and how to estimate 

the variability in the recurrence time ("shape factor") has 

led to further evaluations of the details of probabilistic calculations [Nishenko and Ruland, 1987; Savage, 1991; Roeloffs 

and Langbein, 1994). These advances in understanding of 

methods of estimation of earthquake probabilities have been 
applied to other regions along the San Andreas fault system 

[e.g., Working Group on California Earthquake Probabilities, 1988, 1990; NEPEC/CEPEC/SCEC Working Group, 

1992; Jones et al., 1991] and elsewhere, including volcanic 

unrest at Long Valley Caldera [Hill et al., 1991]. 

Regional studies of crustal structure around Parkfield have 

been carried out as part of the Experiment. These studies 

are aimed at ( 1) characterizing the three-dimensional velocity structure for improved hypocentral determinations and 

(2) identification of temporal and/or spatial variations in 

seismic velocity that may be related to fault zone material 

properties or fluid pressure buildup in the fault zone. The 

three-dimensional velocity structure obtained by EberhartPhillips and Michael [1992] and Michael and Eberhart-Phillips [1991] suggests the presence of a body with low compressional wave velocity and low resistivity, near the northeast side of the fault, beneath Middle Mountain. They infer 

that this material may contain high fluid pressure. Low 

shear wave velocity is also inferred at depths of 5-9 km near 

the 1966 hypocenter, possibly indicating high pore pressure 

[Michelini and McEvilly, 1991]. In addition, the quarterly 

vibroseis (polarized shear wave) investigations have shown 

temporal variations in seismic velocity that may be correlated to resistivity changes and/or slow creep events [Karageorgi et al., 1992]. 

Left Lateral I Slip 

Coalinga_____. 

M6.7 

················"'········ 

crer ... ;:--.; ............. . 

!····· .............. : ..... :·"seismicity 

•··· 81 82 86 87 88 

YEARS 

Figure 6. Surface creep of the San Andreas fault at Middle 

Mountain (in mm) and the rate of seismicity under Middle 

Mountain (cumulative number of events with M ~ 1.5, 

depths ~ 6.5 km since 1980) are shown. The Coalinga 

M 6. 7 earthquake caused the fault to slip left lateral and 

stopped the seismicity for slightly over a year. 

Possible precursory phenomena have been scrutinized. The 

Parkfield project required a careful evaluation of various 

possible precursors, in order to decide what to measure and 

record at the start of the experiment. During the course of 

the experiment, it has become clear that some possible precursory phenomena were not being studied (e.g., the ULF 

electromagnetic signals) and that some other measurements 

related to precursory phenomena may be relatively problematic (e.g., rainfall-induced creep events). Theoretical modeling of expected strain accumulation prior to rupture [e.g., 

6 

Tullis and Stuart, 1992] has forced a careful look at the time 

scales and spatial scales over which precursory signals might 

be visible. This has shown that, for certain scenarios, we 

would expect to capture precursory signals on the currently 

designed and located instruments, but for other scenarios we 

would not be able to resolve these precursory signals. Thus, 

we have improved our knowledge of optimal experiment design for this location, and can now apply these techniques 

to future experiment design in other regions. 

Technology Transfer 

The Parkfield experiment required a working collection of 

monitoring instrumentation designed to observe long-term 

and short-term changes in the fault near the inferred point of 

rupture initiation. As the experiment progressed, equipment 

failure and changes required modification of both the hardware (e.g., the cable connections on some downhole equipment) and the science plan (e.g., the alert levels triggered 

by creep events during heavy rainfall). Many of the bugs 

related to real-time operation of this monitoring system, 

and related to real-time operation of this monitoring system, and to design and installation improvements, are now 

worked out. Similar monitoring systems are now being installed along the Hayward fault system, near San Francisco, 

and along parts of the southern California fault systems. 

There has been a great savings in cost for these installations because of the experience gained at Parkfield. The 

real-time aspect of such data collection has been greatly advanced by the expertise gained in the Parkfield experiment, 

so that Parkfield serves as a starting model for the design of 

similar, but younger and more sophisticated, systems. 

Lessons for the Response Community 

Parkfield Successes 

The Parkfield Earthquake Prediction Experiment has been 

a success from the response community perspective as demonstrated by the response of emergency management services, in conjunction with the scientific and media communities, to the October, 1992, "A"-level alert. During the Experiment, the emergency response community has been able 

to prepare and exercise its response plan for the "A"- and 

"B" -level alerts, including the notification of local governments. Cooperation at Parkfield between the USGS and the 

California Office of Emergency Services (OES) has resulted 

in the State of California Governor's Office of Emergency 

Services Parkfield Earthquake Prediction Response Plan 

[OES, 1988], which was the basis for the response actions 

during the "A" -level alert. Based on the Parkfield Earthquake Prediction Response Plan, California OES developed 

and published the California Short- Term Earthquake Prediction Response Plan [OES,1990]. This second document 

describes state agency and county government actions to 

be taken in response to any scientifically-driven earthquake 

alert or prediction at any other location in the state. 
Middle Mtn Gold Hill 

DISTANCE (KM) SE 

I 

f---

(1._ 

~1 

Figure 7. This cross section along the San Andreas fault compares the amount of slip in centimeters during the 1966 

mainshock (dark contours from Segall and Harris, 1986) to the seismic P wave velocities on the northeast side of the fault 

(shading and lighter contours in km/s from Michael and Eberhart-Phillips, 1991 ). The mainshock hypocenter (large star) 

was under a body of low velocity material but most of the slip occurred to the southeast where higher velocity material 

exists. White areas are small background earthquakes that tend to occur in the lower velocity material and around the 

edges of the mainshock. 

The Experiment has brought scientists together with state 

and local officials, emergency managers, and the media, in 

a productive, mutually beneficial relationship. The state established the first scientifically-based state emergency management protocol for a specific predicted earthquake. This 

interaction has permitted emergency managers to understand the earthquake hazard as well as to understand 

""' ""'" ~ ~ _., __ .. __ _ ,._..., ___ ....,. _ ... _ _._,._. ·-............ .,., ... , ..... 

""'1 .... -$~~<>< 

Figure 8. Public Response 

5o<OO<)'

w.,.,.ws,_,oJSJ..,d.p,," 

I«~yoodfoi>loc:PoN:y 

Plans. ~ 

7 

the perspective of the scientific community regarding earthquake prediction. USGS scientists now understand the importance of providing timely interpretation of earthquake 

data to the state and local emergency managers and the 

public. 

Benefits From the Parkfield "A"-Level Alert, October 2Q-22, 1992 

The "A"-level alert was mostly a positive experience. 

California OES received an initial alert from the USGS 

regarding a "B"-level alert 18 minutes after the initial 

earthquake. Six minutes later, the USGS notified OES of 

the "A"-level alert, triggered by the M = 4.7 earthquake 

at Middle Mountain. Eight minutes later, OES broadcast 

the alert to state agencies and local governments over the 

California Warning System and the California Mutual Aid 

System. Kern County was the first county to activate 

its Emergency Operations Center, 47 minutes after the 

OES alert. OES completed its alert of local government 

and response officials in less than one hour following the 

earthquake. OES staff went to the USGS offices at Menlo 

Park, and to the town of Cholame, near Parkfield. 

Planning and exercising of the Parkfield Earthquake Prediction Response Plan allowed for this timely activation and 

mobilization of local government and OES. OES' overall assessment of the notification is that they "could not have done 

it much quicker." The operational response plan worked well 

because it prescribes simple and straightforward actions. As 

a result of evaluation of the state and local response to the 

"A"-level alert, the process of formal notification and validation did not need to be changed. However, key OES staff 

now carry the USGS alert pager, which will notify OES staff 

simultaneously, along with the USGS personnel, of Parkfield 

earthquake activity. 
Interaction with the press was one of most valuable benefits of the "A"-level alert. OES was able to explain the purpose of the alert and associated response to the media. The 

seven counties' response was overwhelmingly positive. Local 

officials and politicians appreciated receiving the warning 

San 

MENLO 

Figure 9. Flow of information resulting from an A-level alert 

at Parkfield. Signals from USGS instruments at Parkfield (1) 

are telemetered to the USGS Western Region Headquarters 

in Menlo Park (2) for analysis by USGS scientists. The California Office of Emergency Services ( OES) in Sacramento 

{3) is notified by the USGS for all A-, B-, and C-level alerts. 

A-level alerts are immediately transmitted by OES to county 

governments (4) using established OES emergency communication channels. County officials also have established 

procedures to relay the warning to such groups as law enforcement agencies, fire departments, utility companies, and 

school districts. 

and the continuous flow of information. The county afteraction reports were very positive - nothing of substance was 

reported as negative. 

The widespread news coverage of the Experiment and 

the October "A"-level alert has sensitized the public in 

California to the possibility of future earthquake alerts. 

For example, news media gave the alert front-page and 

lead-story status in the San Francisco Bay Area. Just as 

significant, the media also announced the end of the alert 

window three days later. To prompt media closure of the 

72-hour alert window, the USGS, OES, and the California 

Division of Mines and Geology (CDMG) conducted a news 

conference. Fortuitously, the next week's "B"-level alert, 

October 26, 1992, following the "A"-level alert, emphasized 

to the public that different levels of alert exist with different 

8 

associated earthquake probability percentages. Again, the 

media did a good job in portraying these alert levels with 

their associated probabilities. The alerts emphasized the 

prediction issue and hazard reduction issues, both of which 

will be raised in other contexts during future events. 

The "A"-level alert underscored the key steps that local, 

state, and federal government officials must take when a 

prediction or alert is issued to the public. As a result, 

some local governments are now in the process of developing 

formal short-term earthquake prediction response plans. 

Benefits Advancing Public Policy 

The Experiment resulted in the development of the following beneficial response-related actions integrating scientific 

information with public policy: 

1) It offered the first opportunity for the California Earthquake Prediction Evaluation Council (CEPEC) to validate an earthquake prediction; 

2) It prompted the California OES to write and issue a 

public information brochure to the people of the area; 

3) It prompted the National Science Foundation to fund a 

study to gauge the effectiveness of this public information brochure; 

4) It prompted the California OES to write and issue 

written press statements on the alert prior to the "A"-

level alert in October, 1992; 

5) It contributed to the development of the City of Los 

Angeles' Earthquake Prediction Response Plan; 

6) It led to the passage of California Assembly Bill 938 in 

1985 (California Public Resources Code 2800 et seq.), 

which authorized California to contribute $1 million in 

state funds to the Experiment; 

7) It attracted international public and private scientific 

participation in the Experiment through the CDMGsponsored Turkey Flat Strong Motion Experiment; 

and 

8) It offered the first opportunity to engage the liability 

immunity provisions of California's law for state and 

local agencies and officials involved in response activities. 

(California Government Code Section 955.1, et seq.) 

We believe that the California OES and the USGS plan 

to continue supporting the Experiment. The seven counties 

also are willing to continue participation in the prediction 

aspects. 

Problems Identified 

Although the Parkfield Earthquake Prediction Experiment has been very successful, there have been a few problems that the NEPEC Working Group has identified. These 

should be addressed as the Experiment evolves. 

Lack of Ongoing External Scientific and Emergency 

Response Review 

Perhaps because of initial haste prompted by anxiety that 

the anticipated earthquake might occur before the Experi-
ment was fully operational, there has been little provision 

for ongoing review. The annual expenditure and the importance of this project to the prediction community requires 

periodic external review (suggested annually) to assure its 

viability and credibility. An assessment of the progress and 

quality of the project by a panel of experts would ensure that 

appropriate scientific and response objectives are kept at the 

forefront of the project and that the Experiment changes in 

response to new developments, as well as doing much to 

dispel criticism of the experiment. The review might be 

done by a subset of the NEHRP Review Panel, which could 

oversee the general worth of the project in the context of 

the NEHRP program and give advice on the direction and 

disposition of financial support. 

Scientific Data Not Readily Accessible 

There is a perception that some investigators funded 

under the Parkfield project have been too busy, inadequately 

supported, or perhaps reluctant to release their data in a 

timely manner. These delays have reduced the usefulness 

of the data to the Parkfield team. An implicit philosophy 

of the Experiment is that all investigators must work in a 

cooperative effort in order to provide the data in a timely 

fashion for time-dependent decisions regarding designations 

of alerts and to assure timely review of data. If data 

gathered within the Parkfield experiment are important 

enough to be funded, they are important enough to be made 

available on a short enough time frame to contribute to the 

decision making process. 

Data Management 

The question of data accessibility is also related to the 

philosophy of data management. It is problematic that 

there is not now a modern data base management scheme 

implemented for the Parkfield data. Individuals who wish 

to examine the data must be familiar with the location(s) 

of particular data files in USGS computers (which require 

secure passwords) in order to retrieve the data. The user 

then must plot the data with his or her own software and 

has little information on attributes or an understanding of 

how the data may relate to the earthquake process. This is 

a serious limitation and does not allow ready access to the 

user. 

While we realize that data acquired in sophisticated experiments must be carefully processed and scrutinized by 

individual investigators, we suggest that because of the specific NEHRP financial commitment and national stature of 

the Parkfield Experiment that a modern Data Base Management System be implemented for archiving and managing 

the Parkfield data. This requires that all on-line recorded 

data be available immediately after processing and that offline and low-rate data, requiring editing and processing, 

should be archived within a sufficiently short time to be useful to the decision makers. This aspect is also important for 

providing on-line access to designated users. A user-friendly 

Geographical Information System available with sub-licenses 

9 

to the participating Principal Investigators would also be 

useful to manage, correlate, and display the archived data. 

We also note that some additional data, now collected 

at Parkfield, should be considered for transmission and 

recording at Menlo Park. These include the electrical 

resistivity array and the ULF EM measurements. 

A possible restriction in the Parkfield data distribution 

scheme is that the downhole and acoustic data, which 

are recorded on tape at Parkfield, are then sent to Duke 

University and UC Berkeley for analysis. Because of these 

time-consuming steps they are not available to the USGS 

team in a timely or efficient manner. A plan should be made 

to make these data more readily available to the USGS in 

Menlo Park quickly enough to be useful in a short-term alert 

and prediction mode. 

Overemphasis by the Public and Media on the Prediction Aspects of the Experiment 

The original objectives as stated by Bakun and Lindh 

[1985) were that the Parkfield experiment was to monitor 

the details of the final stages of the earthquake preparation 

process. The instrumental aspect of the Parkfield project 

was designed primarily as a surveillance project. However, 

with the involvement of California funding, the Experiment 

took on a short-term earthquake prediction objective that 

is perceived by the scientific and emergency response community as the "national" prediction experiment. 

The general public now perceives the Experiment primarily as a short-term earthquake forecasting project with an 

inherent expectation to accurately predict an earthquake, 

while the scientific community views it not only as a shortterm prediction experiment, but also as an effort to trap 

a moderate earthquake within a densely instrumented network. It is important to educate the public that there is 

great value to this monitoring effort even if the prediction 

effort is unsuccessful. 

How Should the Experiment be Modified in 

the Future? 

The Working Group recommends that the Experiment 

continue as a specific coordinated scientific effort in monitoring through the earthquake cycle, as well as in earthquake 

prediction. Although the annual probability of the expected 

characteristic event occurring, about 10% per year, is sufficiently high that the Experiment should continue, it is not 

so high that we can expect the Experiment to be over on 

a time scale of a few years. Thus, it is prudent to take 

a long-term perspective in contemplating the future of the 

Experiment. Issues to be addressed include costs, relocating 

the Experiment, mechanisms for ongoing evaluation of the 

Experiment, and future USGS response efforts. 

What are the Incremental Costs Associated 

with the Experiment? 

In addressing possible modifications of the Experiment, 
it is important to place the budget in context. The internal 

budget for the Parkfield Experiment is $1.4 million/year, 

but this amount is much more than the true incremental 

cost of the Experiment. The great majority of these expenditures are for salaries of scientists (prorated based on 

estimates of the fraction of time spent on the Experiment). 

Since these salaries would continue to be paid if the Experiment were ended, termination of the Experiment would not 

allow salary costs to be cut, although the effort could be 

deployed elsewhere. Real-time monitoring is highly automated, making use of computer systems that already exist. 

In the judgment of the Working Group, turning off the Experiment completely would save less than $200,000 per year 

in the internal program. The external program has a budget 

of rv$400,000 per year. Thus the total incremental cost of 

the Experiment is approximately $600,000 per year -just 

over 1% of the USGS NEHRP budget. 

Should the Experiment be Moved? 

USGS is under tremendous pressure to "do something" 

in more heavily populated areas, such as near the Hayward 

fault. The argument is made "How can we justify spending 

money trying to predict an earthquake in an area with as few 

taxpayers as Parkfield, when so many more people would be 

affected by an earthquake on the Hayward fault?" In answering this question it is important to remember that we 

do not at this time know which, if any, of the instruments 

monitoring pre-earthquake activity will measure premonitory signals. Thus,· we do not know the requirements for 

instrumentation to install to be useful for earthquake prediction in heavily populated areas. It is extremely important 

to answer this question as quickly as possible. Impatient as 

we may be with the lack of the expected event, Parkfield 

remains the most likely place identified to trap a moderate 

earthquake. It is probable that precursors would only be 

detected by instruments located quite close to the earthquake preparation area. Parkfield represents one of only 

a few places where this preparation area has been specifically identified. In addition, the long baseline of measurements already made at Parkfield represents an investment 

that should be used, not walked away from. 

While there are benefits to having the monitoring effort 

visible to the public, evaluating the possibility of precursors 

should have highest priority. The best place to find out 

if they exist is at Parkfield, where the target area is well 

defined. It is important that adequate resources be made 

available to keep the Experiment alive and evolving, not 

placed in mothballs. 

Long-Term Aspects of the Project 

Commitment for Long-Term Management 

There has been an apparent lack of commitment by the 

USGS for long-term management and identification of importance for the Parkfield project. The project has included 

10 

several Project Chiefs whose terms have been less than that 

of their corresponding USGS Branch Chiefs, with some in 

that position for as little as one year. The Experiment is 

sufficiently important that it deserves a commitment by the 

USGS for a long-term management team with the requirement that those individuals become totally familiar with the 

objectives and results, and have a good working relationship 

with participants. A Parkfield Project Chief should remain 

in that position for sufficient time to be able to make critical 

decisions based on a broad experience of observations and 

predictive models. Further she/he should have the status of 

major contributor of the NEHRP program and be involved 

in its planning and implementation. 

Acquisition of Land Rights at Parkfield 

There is apparently a problem with long-term rights of 

access to instrumentation sites at Parkfield. We believe 

that the importance of this experiment merits a cooperative commitment from federal, state, and county officials to 

assure land accessibility and that an effort should be made 

with private land owners to gain long-term commitments 

and accessibility. This aspect of the project would be enhanced by a local coordinating committee with members 

from the Parkfield community. This may require consideration of the instrumentation sites as designated land within a 

state or federal easement program, as a national earthquake 

study area, or other designated scientific establishment similar to the Stanford Linear Accelerator, the Superconducting 

Super-Collider site, etc. 

Reassess Project Periodically to Modify, 

Upgrade and Acquire New Equipment 

The scientific objectives of the Parkfield experiment are 

now reasonably well supported with modern instrumentation. It is imperative that these instruments, particularly 

the strain meters, be maintained, upgraded, and replaced 

when they fail. In addition, there are some methodologies 

that deserve additional consideration at Parkfield: 

Broad Band Seismological Studies - An important issue 

is that there is only one broad-band, continuously recording 

system at Parkfield, installed in late 1991; it is not apparent 

to the Working Group how data from this instrument are 

used by those involved in the Parkfield Experiment. In 

the past few years, broad-band seismic data, 50 Hz to 

0.03 Hz, recorded on wide-dynamic range digital recorders 

(either on site or via digital transmission) have been shown 

to be very useful for retrieving source and transmission 

properties of moderate to large earthquakes. In the recent 

earthquake for which the "A"-level alert was issued, broadband techniques could have been used to rapidly obtain 

accurate source properies (e.g., rupture geometry, time and 

space properties, relationships to geometrical features of the 

proposed nucleation site at Middle Mountain, etc.). The 

Committee was unaware of to what extent, and how rapidly, 

broad-band data had been used in evaluating this event. 
With the availability of new wide dynamic range broad 

band seismometers (such as Guralps, Streckheisens or equivalent) and 24-bit recording for example with ReiTeks, there 

is a wealth of new information that can be gained by large 

dynamic range-wide band technology. For example, attenuation (weak vs. strong ground motion) over broad ranges 

of magnitudes and distances, source properties of small to 

moderate events, and accurate foreshock and aftershock assessment may yield important information on earthquake 

precursors. We suggest that a component of wide-dynamic 

range broad band coverage of the Parkfield area be implemented. 

While real-time recording is perceived as costly to some, 

modern seismological instrumentation has been designed for 

on-line recording and is standard throughout the world's 

seismological community. These data would cost the same, 

if not more, if they were recorded on site or at Menlo Park 

in a time-delay mode. Moreover, if they were recorded onsite and transmitted to Menlo Park, then the timely nature 

of the data would be negated for warnings and alert. We 

suggest that real-time recording of seismological and related 

high sample-rate data be continued. 

Parkfield Network Calibration- A general problem common to most short period seismic networks, including CALNET and hence Parkfield, is the lack of a systematic absolute calibration of the complete seismometer-recording systems for true ground motion. The USGS indicates that calibration pulses are recorded daily on the short period seismometers, but in reality little is done with the calibration 

data. The short period seismometers are calibrated only 

when they are brought to Menlo Park for bench tests where 

they can be absolutely calibrated (presumably very infrequently, i.e., every few years). If there is a large earthquake 

at Parkfield and the short period instruments have not been 

calibrated accurately, a major source of information, namely 

accurate ground motions (for the on-scale events for even 

small to moderate events), will be lost. This information 

is especially useful for assessing large ground motions and 

attenuation. 

Network and other instrument calibrations should become 

part of the Parkfield archive accessible to all users. Additional information in a data base should include a history of 

each station, model numbers of components, maps of station 

locations, instrument modifications, calibration constants, 

etc. The public must be assured that the data acquired 

from this and any similar project are validated. 

Paleoseismicity- The basic premise of the Parkfield experiment is the recurrence of "characteristic" M rv 6 events. 

However, because there is little trenching information in the 

Parkfield area (in part due to geography and other logistical 

problems) the long-term Holocene record has not been evaluated. Thus, we do not know if the typical event size and 

recurrence interval assumed from the six historic events also 

dominates the longer-term seismic patterns. For example, 

if paleoseismological studies revealed the presence of larger 

events, this would change our view of the typical Parkfield 

event. This information is crucial for making statistical as11 

sessments of earthquake probability and for evaluating the 

characteristic earthquake model. Some studies have been 

carried out addressing the feasibility of trenching near Parkfield. These studies need to be documented and critically 

assessed. 

Assessments of Costs and Productivity of 

the Experiment 

The Experiment should continue to evolve, and external 

review should be part of this process. We suggest that the 

project be reviewed periodically by a panel of independent 

scientists. An annual report, discussing productivity and 

cost, should be prepared and submitted to this panel. The 

report should contain a summary of the scientific and emergency response aspects of the project, as well as a listing of 

products (reports, catalogs, data archives, papers, presentations, etc.) supported in part or totally by the Parkfield 

experiment. This part of the report should be presented at 

a national meeting such as the AGU or SSA, with consideration of publication in a journal such as Eos, to inform the 

community of the status of the project. 

An accurate assessment of the cost of the Parkfield experiments should be included in the report to the panel, assessing individual project costs and related salaries. A review of 

funds for USGS employees and contract employees should be 

scrutinized and the USGS management should ensure that 

funds for the Parkfield project are expended according to 

their intended use. Equipment acquisitions should be listed 

and a comparison of operating costs ofthe network and other 

systems should be made and reported periodically to the 

review panel. Further, the report should provide a list of 

instruments or projects that were considered each year for 

implementation, continuation, or deletion. 

Recommendations for Future USGS Response Efforts 

Strengthen the USGS Response Role 

The USGS should recognize and provide support for the 

Experiment as a scientific experiment in the broader integrated context of an actual public policy activity. In general, the USGS must recognize the importance of developing 

written plans to provide the scientific and public information support that is needed during all stages of public alerts. 

The USGS should also consider institutional recognition for 

scientists who are committed to devoting portions of their 

careers to public policy and education without jeopardizing 

potential career advancement. The USGS should consider 

formal and institutional recognition of the important public 

policy role that scientists can play for all phases of natural 

hazards prediction and response. 

The USGS should establish a formal protocol describing 

agency functions and personnel functions required to adequately support the media, the state and local governments, 

and the emergency management community, after issuing 

earthquake predictions. This protocol should be written in 
.5 - A -

.1 - !-- 8 -

- 1- c 

.01 -

-

~ D 

-

'--- I I I .001 

1986 1987 1988 1989 1990 1991 1992 1993 

Figure 1 0. The time of Parkfield status level, D through A, and their corresponding 72 hour probability of a M 6 earthquake 

are s~own for period from th~ start of the experiment in 1985 through 1993. Although this report described a single A-level 

alert 1n late 1992, the expenment went to its second A-level in November 1993. The 72 hour probability for each status 

level is discussed in Parkfield Earthquake Prediction Response Plan (1988]. 

the form of a Standard Operating Procedure which describes 

the tasks to be performed by dedicated USGS personnel 

during earthquake alerts. The USGS should develop these 

Standard Operating Procedures by querying representatives 

of the media, state and local governments, and emergency 

management who depend on earth science information during alert periods. In the case of Parkfield, the USGS should 

be committed, as an institution, to preparing for an "A"-

level alert. To date, the USGS has relied on individual scientists to provide a presence and continuity for interaction 

with state and local government officials. Even though individual Parkfield scientists have had a continuous commitment to providing earth science information to the media 

the USGS was unable to provide adequate representation t~ address media questions regarding the prediction during the 

"A" -level alert. 

The USGS has strengthened its response role since the 

October, 1992 "A"-level alert by recognizing that the Parkfield Chief Scientist, Project Chief, and the Public Information Officer roles must be adequately covered by three 

scientists rather than one scientist, as has been the case for 

most of the duration of the Experiment. The Parkfield scientists at the USGS, Menlo Park, have already implemented 

this additional coverage of duties at least until the closure 

of the fiscal year, September 30, 1993. For additional "A"-

level alerts, one scientist will run the field laser experiments, 

one scientist will coordinate the Menlo Park operations, and 

a three-scientist team will mobilize to Parkfield to provide 

earth science information to the media. 

Review the Threshold Criteria Determining Alert 

Levels 

The USGS should review the probability percentages, 

which act as threshold criteria for the Experiment alert 

levels, with respect to the following three concerns: 

1) the level of accuracy and the statistical uncertainty 

associated with these probability percentages; 

12 

2) the effect of "false alarms" on the credibility of the 

earthquake prediction process; and 

3) the appropriate actions for the public and response 

community as reflected by these probability percentages 

and alert levels. 

An "A"-level alert for the Experiment has been assigned 

a 37% probability for the occurrence of an earthquake of 

M rv 6 within 72 hours. The public and the media infer 

that the accuracy of this percentage is very high, reasoning 

that it has not been stated as, e.g., 36% or 38%. Since 

the uncertainty in this estimate is much larger than the 

reliability associated with a probability expressed to two 

significant digits, the scientists are implying to the public 

unrealistic accuracy. This leads to the misperception that 

scientists believe their probabilities to be more accurate than 

they are. 

There is also a problem in the public perception of the Experiment created by the highest level alert - the "A" -level 

alert. The alert scale is saturated at a low probability level. 

It is easy (albeit incorrect) to associate the highest available 

alert level with a high absolute level of probability and the 

need for media attention. When the expected event does 

not follow the posting of the highest available alert level (as 

should usually be the case for the alert levels as defined), 

the alert is easy to perceive as a false alarm. Will the frequency of perceived false alarms associated with future alerts 

diminish the intended response and preparedness actions of 

the public, as well as begin to discredit the scientific credibility of the earthquake prediction process? The false alarm 

rate needs to be more fully understood by local governments 

and the public. Perhaps the most straightforward remedy 

would be to define a new alert scale, e.g., I-V, with the current "A" -level assigned level II. (Level I might be triggered, 

for example, by the "early warning" system under study.) 

Although this is more of a public relations issue than a science issue, public relations are an important aspect of the 

Experiment. 
An "A"-level alert results in notification of the state and 

local governments and the public, and triggers response 

agency mobilization. Are the public and response actions 

appropriate? Should future notification and mobilization 

be triggered at higher probability percentages to minimize 

false alarms and justify mobilization? For example, should 

an "A"-level alert be the basis for canceling employees' 

leave, or lead to the evacuation of hazardous buildings for 

one week? Are the social, economic, and political impacts 

appropriate? The USGS must work closely with state 

and local governments and private industry to review and 

possibly devise alert levels which are sensitive to the actions 

that the alerts inspire. 

Fund Understanding of Societal Impacts of False 

Alarms 

Repeated future alerts at Parkfield, which result in perceived false alarms, could have a negative "cry wolf" effect 

on the public. Since the Experiment will continue to teach 

us about the impact of perceived false alarms on the public, 

funding should be provided by some agency to determine the 

societal impact of false alarms. The societal impact of earthquake prediction and associated public alerts also includes 

unknown political and economic consequences. California 

OES believes that emergency managers still have a great 

deal to learn about the implications of "A"-level alerts in 

terms of what occurs as the public and emergency management goes through these cycles. 

Improve Communication of the Hazard Potential to 

the Public 

The USGS should support studying how to most effectively communicate the predicted earthquake hazard to the 

potentially affected population, as well as to populations 

located beyond the area of concern. During the "A"-level 

alert, some people who were unfamiliar with the Experiment 

and located long distances from Parkfield became unduly 

concerned for their safety. The alert stirred some to call for 

a shut down of Diablo Canyon nuclear power plant, located 

50 miles southwest of Parkfield. Residents were concerned 

as far away as San Francisco. The OES Earthquake Safety 

Information Center Hotline, located in Pasadena, received 

numerous calls from residents in southern California counties. During alerts, the message to the public might also include appropriate warning and preparedness information to 

those located beyond the areas likely to be affected. Since 

there is virtually no part of California that is immune to 

earthquake damage, the public needs to receive the message 

that they can reduce potential damage from future earthquakes by mitigating risk and that they cannot "go to some 

place where they will be safe from the shaking." 

The USGS might convey information using a map format 

to define areas of expected strong ground shaking and 

provide damage estimates out to the probable limits of 

felt ground shaking. Map displays that show where the 

earthquake will not be felt should be offered to the publ;c 

13 

Figure 11. Scientists from the USGS and officials from 

the California OES met with reporters on the lawn of the 

Parkfield Cafe for regular briefings during the level-A alerts 

in October 1992. 

as well. The USGS should investigate how to better communicate risk information to the public, the media and local 

government officials, not just earth science specialists. 

Concluding Remarks 

Parkfield remains the best identified locale to trap an 

earthquake. The consensus is that the annual probability 

for the expected "characteristic" event is about 10% per 

year. At this level, the Working Group concludes that the 

Experiment should be continued, both for its geophysical 

and its public response benefits. 

Although this probability is relatively high, it is not so 

high that we can have confidence that the event will occur 

on a time-scale of a year a two. Thus the Experiment should 

be viewed with a long-term perspective. The Experiment 

should not stagnate: rather it should continue to evolve. 

This will require adequate resources. In view of the longterm commitment required, consideration should be given 

to a separate NEHRP funding status. Further, the project 

should be periodically reviewed for its merits and progress. 

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14 

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fault at Parkfield, California, as revealed by inversion of 

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1990. 
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