Dynamic Triggering of Microearthquakes by P- and S-waves from the 1999 Chi-Chi, Taiwan Earthquake

High-frequency band-pass filtering of acceleration records from the 1999 Chi-Chi, Taiwan earthquake (Mw = 7.5) resolves the continuous signal into a series of relatively short duration discrete energy bursts. We hypothesize that these bursts originate at small shallow events near the individual stations that have been dynamically triggered by the P- and S-waves emanating from the Chi-Chi mainshock. Bursts are observed only during the seismic signal and not in the noise before or after. We show that the bursts are not due to band- pass filtering of instrumentally generated step functions. Our local event hypothesis is supported by the observations of bursts up to 170km from the epicenter. If the bursts originated on the Chi-Chi fault plane, as originally hypothesized by Chen et al. [2006], then they should not be observable at this distance for any reasonable value of crustal attenuation. Moment magnititude calculations for three of the largest amplitude bursts without any distance correction gives Mw = 1.5 1.9, providing a reasonable estimate of event sizes. By observing which of the 200 stations on Taiwan recorded bursts and which did not, we determined that the stresses threshold for triggering is in the range of 10-20 gals for P-wave and 40-100 gals for S-waves. Comparison of these thresholds with those determined for surface waves [Hill et al. (1993), Brodsky et al. (2000), Miyazawa & Mori (2006)] and earth tides [Wilcock (2001), Cochran et al. (2004)] finds that the threshold stress scales as the inverse square root of the wavelength of the dynamic trigger. This scaling is reminiscent of the scaling of the strength of a laboratory sample with the square root of its size, and suggests a similar relation between the wavelength of the dynamic stress and the size of the local events that can be triggered.