High-pass filtering (>20 Hz) of acceleration records from the 1999 Chi-Chi, Taiwan, and 2004 Parkfield, California, earthquakes reveals 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 U.S. Geological Survey Parkfield seismic array during the Parkfield earthquake and were constrained to originate less than 20 m from the instruments. Such small shallow events cannot 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 70 m. Where standard triggering releases stored elastic energy and adds to the incident wave field, this new driving mechanism takes energy out of the 2 Hz strong motion and reradiates it at high frequencies. It is thus an attenuation mechanism that we estimate can contribute 3% to the net attenuation in the very shallow crust.

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