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A High-Frequency Secondary Event During the 2004 Parkfield Earthquake

Bettina P. Allmann, & Peter M. Shearer

Published November 23, 2007, SCEC Contribution #1121

By using seismic records of the 2004 magnitude 6.0 Parkfield earthquake, we identified a burst of high-frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation. We imaged this event in three dimensions by using a waveform back-projection method, as well as by timing distinct arrivals visible on many of the seismograms. The high-frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions, indicating that slip may have grown abruptly at this point. The time history obtained from full-waveform back projection suggests a rupture velocity of 2.5 kilometers per second. Energy estimates for the subevent, together with long-period slip inversions, indicate a lower average stress drop for the northern part of the Parkfield earthquake compared with that for the region near its hypocenter, which is in agreement with stress-drop estimates obtained from small-magnitude aftershocks.

Key Words
United States, Parkfield earthquakes, three-dimensional models, magnitude, slip rates, inverse problem, elastic waves, waveforms, frequency, California, Parkfield earthquake 2004, San Andreas Fault, Monterey County California, Parkfield California, focus, seismic waves, arrival time, active faults, earthquakes, image analysis, faults

Allmann, B. P., & Shearer, P. M. (2007). A High-Frequency Secondary Event During the 2004 Parkfield Earthquake. Science, 318(5854), 1279-1283. doi: 10.1126/science.1146537.