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: We model the two most recent M w ~6 Parkfield, California, earthquakes, which occurred in 1966 and 2004, from a nonlinear global inversion of near-fault strong motion seismograms. Our rupture models are characterized by spatially variable slip amplitude and rake, rupture velocity, and risetime. The rupture models indicate that the two earthquakes generated slip in regions of the fault that are not identical, as earlier suggested. Given the sparse seismic data set available for the 1966 earthquake, we conduct a series of tests to verify our results: (1) we perform synthetic tests in order to study the resolution of the 1966 seismic data set; (2) we perform an inversion of the 2004 earthquake using a data set equivalent to the 1966 earthquake; and (3) we model the 1966 data set under the a priori assumption that it was similar to the 2004 earthquake. All of the tests, as well as independent observations, indicate that slip during the 1966 and 2004 Parkfield earthquakes occurred in different regions of the fault. This result implies that regions of a fault that are frictionally locked may remain locked even during a main shock (moderate-size earthquake). In this scenario, large earthquakes occur when all the locked regions of a fault are ``synchronized'' and ready to slip at the same time.
    Journal of Geophysical Research Atmospheres 01/2007; 112(B5). DOI:10.1029/2006JB004617 · 3.44 Impact Factor
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    ABSTRACT: Quantitative estimates of the crack density (ϵ), saturation rate (ξ) and porosity (ψ) parameters from seismic velocities (Vp, Vs) and Poisson’s ratio (σ) in the 2001 Bhuj earthquake area in western India indicate that the 2001 Bhuj earthquake hypocenter is associated with high-ϵ, high-ξ and high-ψ in the depth range of about 23–28 km, extending 15–30 km laterally. These anomalies may be due to a fluid-filled, fractured rock matrix, which might have contributed to trigger the 2001 Bhuj earthquake in the intraplate, stable continental region of the Indian Peninsula. This feature is similar to that of the 1995 Kobe earthquake [D. Zhao et al., Science 274 (1996) 1891–1894]. High-ψ areas are generally consistent with high-ϵ areas, but high-ξ areas have a wider distribution, indicating that micro-cracks exist in more localized areas within the crust, and that permeation of fluids in the hypocenter zone might have occurred extensively through the intergranular and fractured pores due to hidden intersecting fault geometry. Here we suggest the possibility that earthquake occurrence is closely related to in situ material heterogeneities, rather than stress conditions alone.
    Earth and Planetary Science Letters 07/2003; 212(3-4):393-405. DOI:10.1016/S0012-821X(03)00285-1 · 4.72 Impact Factor
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