THE EARTHQUAKE PREDICTION EXPERIMENT AT PARKF!ELD, CALl FORN IA EvelynRoelofts John Langbein U.S. GeologicalSurvey, Vancouver,Washington U.S. GeologicalSurvey, Menlo Park, Cafifornia Abstract. Since 1985, a focused earthquake predic- with the existence of a "locked" patch on the fault tion experiment has been in progress along the San beneath Parkfield that has presently accumulated a slip Andreas fault near the town of Parkfield in central deficit equal to the slip in the 1966 earthquake. A California. Parkfield has experienced six moderate magnitude 4.7 earthquake in October 1992 brought the earthquakes since 1857 at average intervals of 22 Parkfield experiment to its highestlevel of alert, with a years, the most recent a magnitude 6 event in 1966. 72-hour public warning that there was a 37% chance of The probability of another moderate earthquake soon a magnitude 6 event. However, this warning proved to appears high, but studiesassigning it a 95% chance of be a false alarm. Most data collected at Parkfield occurring before 1993 now appear to have been over- indicate that strain is accumulating at a constant rate simplified.The identification of a Parkfield fault "seg- on this part of the San Andreas fault, but some inter- ment" was initially based on geometric features in the esting departures from this behavior have been re- surface trace of the San Andreas fault, but more recent corded. Here we outline the scientific arguments bear- microearthquakestudies have demonstratedthat those ing on when the next Parkfield earthquake is likely to features do not extend to seismogenicdepths. On the occur and summarize geophysical observations to other hand, geodetic measurements are consistent date. INTRODUCTION occur before 1993. The Parkfield Earthquake Predic- tion Experiment began in 1985, after Bakun and Earthquake prediction research seemed on the Lindh's conjecture was accepted by the National verge of a breakthrough in 1975, when Chinese seis- Earthquake Prediction Evaluation Council (NEPEC) mologistssuccessfully alerted Haicheng city of an im- and the California Earthquake Prediction Evaluation pending magnitude 7.3 earthquake [Deng et al., 1981]. Council (CEPEC) [Shearer, 1985a, b]. One million Public warnings were also achieved before the 1976 dollars of initial funding for instrumentation was pro- Songpan-Pingwu, China, earthquakes [Wallace and vided by the U.S. Geological Survey (USGS) and was Teng, 1980] and the 1978 Izu-Oshima, Japan, earth- matched by the state of California. The experimental quake (N. Nishide, oral communication, 1992). How- goals are to record geophysicalsignals accompanying ever, these early successeshave not been repeated. the earthquake process at Parkfield, to capture effects Many seismologistseven doubt the existence of mea- of strong motion in the near field, and, most ambi- surable phenomena preceding earthquakes that might tiously, to attempt to issue a warning up to 3 days justify short-termpredictions. Nonetheless, the search before the magnitude 6 event if seismicity and/or fault for earthquake precursorscontinues worldwide, much creep rate reach predefined threshold levels. of it in focused earthquake prediction study areas in Eight years into the experiment, 21 instrument net- Japan, Turkey, the former Soviet Union, and China. works designed to record preearthquake phenomena In the United States a focused earthquake prediction operate near Parkfield (Tables 1a and 1b) [Bakun, study is in progress at Parkfield, California. 1988b], five of which are monitored in real time. Ten The town of Parkfield, built practically on the San more networks wait to record strong motion, coseis- Andreas fault about halfway between San Francisco mic slip, and liquefaction (Table 1c) [Sherburne, 1988] and Los Angeles (Figure 1), experienced moderate when a moderate earthquake occurs. Investigators earthquakesin 1857, 1881, 1901, 1922, 1934, and, most from 13 institutions in addition to the USGS partici- recently, a magnitude 6 event in 1966. On the basis of pate. Including a seismic network that records and this sequenceof events, Bakun and Lindh [1985] esti- locates all events above magnitude 1.0, the instrumen- mated the recurrence interval for magnitude 6 earth- tation at Parkfield is unmatched in diversity and den- quakes near Parkfield as 22 -+ 3 years and conjectured sity by that of any other earthquake prediction study with 95% confidence that another such event would worldwide. This paper is not subjectto U.S. copyright. Reviewsof Geophysics, 32, 3 / August 1994 pages 315-336 Publishedin 1994 by the American GeophysicalUnion. Paper number 94RG01114 ß 315ß 316 ß Roeloftsand Langbein: EARTHQUAKEPREDICTION EXPERIMENT 32, 3 / REVIEWSOF GEOPHYSICS becausereports of earthquake precursorswill be more credible if they are corroborated by signals on other instruments and because long baselines of data may be ;• 5øBendin %,. necessary in order to determine whether small pre- 36ø0ß • FaultTrace earthquake signals are anomalous. A unique and important feature of the Parkfield experiment, which enabled it to receive start-up funds from California, is its prototypical plan for communi- cating earthquake hazard information to the California Office of Emergency Services (OES). Bakun et al. [1986, 1987] and Bakun [1988c] describe the plan in detail. "Alert/status" thresholds have been defined for six instrumentation networks (Table l a). The seismic and creep thresholds reflect phenomena preceding the 1966 Parkfield earthquake, but such data are not avail- able for the other instruments. Instead, their thresh- olds are set so that, statistically, signals unusual enough to meet them occur with specified frequency. When a signal exceeds a threshold (usually detected • .• Kilomete• by a computer program that can page a scientist), 35ø30' scientists monitoring all six of these networks are 1•ø• ' 120015' notified. There are five alert/status levels, with notifi- cation to OES at all but the two lowest levels. At the Figure 1. Parkfield experiment landmarks along the San highest level (level A) the notification to OES states Andreas fault in central California. In the inset California that there is a 37% chance that the magnitude 6 Park- map the solid rectangle is the location of map area; SF denotes San Francisco; LA denotes Los Angeles. Arrows field earthquake will happen within the next 72 hours. indicate the direction of motion of the Pacific plate relative to Before the Parkfield plan was fully exercised, response the North American plate. The San Andreas fault is dark- plans for volcanic unrest at Long Valley Caldera [Hill ened over the approximate extent of surface rupture associ- et al., 1991] and for earthquake hazard in southern ated with the 1966 Parkfield earthquake as mapped by Brown California [Jones et al., 1991] had already been mod- [1970]. eled after it. The NEPEC- and CEPEC-sanctioned prediction window for Parkfield officially closed on December 31, 1992, but the predicted magnitude 6 earthquake did not Since the initial two-million-dollar startup funding, occur. Focused study of Parkfield has generated con- virtually all work at Parkfield has been carried out troversy over the simple picture of cyclically recurring within the previously existing budget of the National seismicity that motivated the experiment. During the Earthquake Hazards Reduction Program (NEHRP). In interval of the experiment, California suffered two this sense, the Parkfield experiment constitutes a com- magnitude 7 earthquakes near major population cen- mitment to concentrate monitoring instruments in a ters that are less well instrumented than Parkfield. single area with a high probability of experiencing an Many have expressed concern that concentration of earthquake soon. Such a concentration is desirable earthquake prediction resources at Parkfield may be a TABLE la. PredictionMonitoring Networksat Parkfield With DefinedAlert/Status Levels as of November1992 Number Sensitivity/ Measurement Network of Sites Threshold Interval References Seismicity (USGS Calnet) 40 Magnitude 1.0 continuous Nishioka and Michael [1990] Continuous strain* 6 10-9 10 min Myren and Johnston [1989] Magnetic field 7 0.25 nT 10 min Mueller et al. [1994] Creep meters (Invar wire- 13 0.01 mm 10 min Schulz [ 1989] LVDT?) Groundwater level* 10 1 mm 15 min Roelofts et al. [1989] Two-color laser geodimeter 18 lines 1 mm 3 times/week Langbein et al. [1990] All of these networks are operatedby USGS, Menlo Park, California. Referencedescribes network and/or measurementtechnique. *The sensitivityin the table is appropriatefor signalswith durationsof 24 hours or less. Becauseof increasingEarth noise at longer periods, signalsmust be larger to be detected at longer periods (see, for example, Langbein et al. [1993]). ?LVDT, linear variable differential transformer. 32, 3 / REVIEWSOF GEOPHYSICS Roeloftsand Langbein:EARTHQUAKE PREDICTION EXPERIMENT ß 317 TABLE lb. Prediction-Oriented Networks at Parkfield Without Alert/Status Levels as of November 1992 Network and Number Sensitivity/ Measurement Operator of Sites Threshold Interval References Borehole tensor strain;*? 2 10-9 10 min Gladwin et al. [1987] University of Queensland, Australia Tilt;*? USGS 5 1 Ixrad 10 min Mortensen et al. [1978] Groundwaterradon; 2 USGS 2 1 pCi/L 10 min Noguchi and Wakita [1977] Soil hydrogen;2 USGS, Reston 7 10 ppm 10 min Sato et al. [1986] Borehole microtemperature; USGS 1 10-4 øC 12 min C. Williams (oral communication, 1992) Resistivity; UC, Riverside 6 dipoles 1% daily average Park [1991] 80-kHz magnetic field; University 1 0.5 •v 6s E. Wescott (oral of Alaska communication, 1992) 0.01- to 10-Hz magnetic field; 2 1 pT 30 min, average McGill et al. [1993], Stanford University Bernardi et al. [1991] Trilateration (4 to 33 km lines); 113 lines 3 mm + 0.2 mm/ yearly King et al. [1987] USGS km Small-aperture nets (3 to 4 km 4 3-4 mm yearly King et al. [1987] lines); USGS GPS; USGS, SIO 4 <1 cm north, east; 4 times per year Prescott et al. [1989] 1-2 cm vertical Rapid static GPS; JPL, USGS 107 2 mm yearly Hurst et al. [1992] Continuous GPS; SIO, JPL, USGS 1 1 cm continuous K. Hudnut (oral communication, 1992) Highway 46 GPS array; USGS, 16 1 cm 2 times per year K.