Lawrence M. Baker

Rupture Propagation of the 2004 Parkfield, California, Earthquake from Observations at the upsar

Using a short-baseline seismic array (U.S. Geological Survey Parkfield Dense Seismograph Array [upsar]) about 12 km west of the rupture initiation of the 28 September 2004 M 6.0 Parkfield, California, earthquake, we have observed the movement of the rupture front of this earthquake on the San Andreas fault. The sources of high-frequency arrivals at upsar, which we use to identify the rupture front, are mapped onto the San Andreas fault using their apparent velocity and back azimuth. Measurements of apparent velocity and back azimuth are calibrated using aftershocks, which have a compact source and known location. Aftershock back azimuths show considerable lateral refraction, consistent with a high-velocity ridge on the southwest side of the fault. We infer that the initial mainshock rupture velocity was approximately the Rayleigh speed (with respect to slower side of the fault), and the rupture then slowed to about 0.66 β near the town of Parkfield after 2 sec. The last well-correlated pulse, 4 sec after S, is the largest at upsar, and its source is near the region of large accelerations recorded by strong-motion accelerographs and close to northern extent of continuous surface fractures on the southwest fracture zone. Coincidence of sources with preshock and aftershock distributions suggests fault material properties control rupture behavior. High-frequency sources approximately correlate with the edges of asperities identified as regions of high slip derived from inversion of strong-motion waveforms.

Stability of coda Q in the region of Parkfield, California: View from the U.S. Geological Survey Parkfield Dense Seismograph Array

Many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate-to-large earthquakes. Parkfield, California, on the San Andreas Fault, is a promising location for studying premonitory changes in coda Q, Qc, because a large earthquake is likely to occur there. We have investigated Qc using recordings from the U.S. Geological Survey Parkfield Dense Seismograph Array, which is a digital array with 14 triaxial sensors and an aperture of about 1 km. For each earthquake we can measure Qc from up to 42 recordings. Their average is more stable than the measurement from a single station. Using clustered seismicity, we have developed criteria for selecting events and reducing scatter in the measurement. The Qc value determined from a seismogram depends on the position and length of the analysis window. Thus Qc should always be measured from the same length window starting at the same lapse time regardless of the source location. In addition, the band-limited signal-to-noise ratio at the end of the analysis window is important. Qc determined in two frequency bands, 4–8 Hz and 8–16 Hz, from a tight cluster of 26 events which occurred between December 1989 and January 1994 has not changed, despite M 4.7 and M 4.6 events in October 1992 and November 1993. Qc measured from local events (Δ < 60 km) in three frequency bands shows larger scatter but has also not changed during this period. For monitoring Qc, observations should include array averaged measurements from a single lapse time. Because Qc measurements made using an analysis window that starts at a constant multiple of the S wave lapse time depend on epicentral distance, a procedure combining the evaluation of the time and distance dependences of Qc also gives stable observations.