Article

The Parkfield, California earthquake experiment: An update in 2000

01/2000; 79.

ABSTRACT The US Geological Survey, in cooperation with other institutions, continues to monitor the San Andreas Fault (SAF) near Parkfield, California, hoping to capture high resolution records of continuous de-formation before, during and after a magnitude 6 earthquake, as well as the details of its rupture initia-tion and strong ground motion. Despite the failure of the prediction that the next M 6 Parkfield earthquake would occur before 1993, Parkfield still has a higher known probability (1 to 10% per year) than anywhere else in the US of a M 6 or greater earthquake. Park-field instrumentation is still largely in place, although there have been losses due to attrition as well as improvements made possible by new technology. Most Parkfield data sets are now available via the Internet, and all others may be obtained upon request from individual investigators. Detailed seismic monitoring has shown that events with identical seismograms, recurring in exactly the same locations, account for a high proportion of the background seismicity at Park-field. Geophysical studies have revealed that fault zone seismic and electrical properties are consistent with high fluid content. The rate of interseismic slip on the SAF changed significantly in late 1992 or early 1993, during a period of relatively high seismic acti-vity. The strain-rate change, measured by borehole tensor strainmeters and the two-colour electronic dis-tance-measuring network, was also manifested as shortened recurrence intervals of repeating micro-earthquakes. Whether or not the accelerated defor-mation turns out to be an intermediate-term precursor to the next M 6 Parkfield earthquake, docu-menting the variation of interseismic strain rates with time has important implications for fault dynamics and seismic hazard estimation. Two possible instances of pre-earthquake signals have been recorded at Parkfield: water-level and strain changes over a period of three days prior to the nearby 1985 M w 6.1 Kettleman Hills, California, earthquake and anoma-lous electromagnetic signals prior to the M 5 earth-quake near Parkfield on 20 December 1994. Future work planned at Parkfield includes a National Science Foundation proposal to construct an SAF Observa-tory at Depth (SAFOD), as part of the Earthscope initiative. The Observatory will consist of a 4-km-deep borehole to penetrate the SAF and a shallow micro-earthquake cluster on Middle Mountain, directly above the hypocenter of the 1966 Parkfield earthquake.

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    ABSTRACT: In this paper (paper 1), we quantitatively show that the heterogeneous pattern of b values (of the Gutenberg-Richter relation) in the Parkfield segment of the San Andreas fault is to a high degree stationary for the past 35 years. This prepares the grounds for paper 2, where we test the hypothesis that our model of spatially varying b values forecasts future seismicity more accurately than the approach in which one assumes a constant b value equal to the average regional value. The method we develop to measure stationarity in the presence of spatial heterogeneity consists of the following steps: (1) Determine the optimal dimensions of the sampling volume by mapping b values with a wide range of radii and selecting the largest radius that gives the most detailed resolution of the b value heterogeneity. Along the selected fault segment, the high data density permits the definition of the dominant dimensions of the seismotectonic fabric, which is about 8-10 km. (2) Map the difference in b value between two periods, selecting numerous possible catalog divisions. (3) Identify significant changes of b values by the Utsu test (Utsu, 1992). Along the studied fault segment of 110 km length, only one patch of radius 5 km showed a significant increase in b, from below average to above, as a function of time. This change in b initiates around 1993 and thus correlates in space and time with a well-documented episode of creep at depth. Using the derived spatial variable b value distributions, we find that the highest probability for earthquakes with magnitude M >= 6 is in the Middle Mountain asperity, where the 1966 Parkfield earthquake nucleated and where all M >= 4.5 events in the data set occurred. In contrast, if only the regional average b value of 0.92 is used to predict future seismicity, the creeping segment north of Parkfield should produce major earthquakes most frequently, a conclusion that contradicts the observations.
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