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Dynamic driving of Small Shallow Events During Strong Motion

Adam D. Fischer, & Charles G. Sammis

Published 2009, SCEC Contribution #1224

High-pass filtering (>20Hz) of acceleration records from the 1999 Chi-Chi Taiwan and 2004 Parkfield, California earthquakes reveal a series of bursts that occur only during strong shaking. Initially interpreted as originating from asperity failure on the Chelungpu fault, bursts observed during the Chi-Chi earthquake were subsequently determined to be a local effect within about 1 km of the seismic stations. Similar bursts were observed at the UPSAR array during the Parkfield earthquake and were constrained to originate less than 20m from the instruments. Such small shallow events can not result from the triggered release of stored elastic energy because rate-and-state friction rules out stick-slip instability on such small, shallow patches. Our hypothesis is that the bursts are not triggered, but are driven by simultaneous shear and tensile stresses near the surface during the strong motion. At 2 Hz, SV to P wave mode conversion at the free surface produces tensile stresses to depths of 70m. Where standard triggering releases stored elastic energy and adds to the incident wavefield, this new driving mechanism takes energy out of the 2Hz strong motion and reradiates it at high frequencies. It is thus an attenuation mechanism which we estimate can contribute 3% to the net attenuation in the very shallow crust.

Fischer, A. D., & Sammis, C. G. (2009). Dynamic driving of Small Shallow Events During Strong Motion. Bulletin of the Seismological Society of America,. doi: 10.1785/0120080293.