HR: 1340h
AN: S43A-1053    [Abstracts]
TI: A Closer Look at High Frequency Bursts Observed During the 1999 Chi-Chi, Taiwan Earthquake
AU: * Fischer, A D
EM: adfische@usc.edu
AF: University Of Southern California, 3651 University Ave, Lost Angeles, CA 90089
AU: Sammis, C
EM: sammis@usc.edu
AF: University Of Southern California, 3651 University Ave, Lost Angeles, CA 90089
AB: High frequency, band-pass filtered waveforms from the 1999 Mw = 7.6 Chi-Chi, Taiwan earthquake show a multitude of distinct, short duration energy bursts. Chen et al. (B.S.S.A., 2005, accepted for publication) assumed the sources of these bursts were slip patches on or near the Chelungpu fault plane and devised a location algorithm that resolved about 500 unique events. Based on these locations and the relative sizes of the bursts they reached the following conclusions: 1) The earliest bursts occur directly up-dip from the hypocenter and appear to have been triggered by the P wave. 2) The first bursts at greater distances from the epicenter appear to be triggered by the propagating rupture front. 3) Later events at all distances follow Omori's law if time is measured from the arrival of the rupture front at each distance. 4) The size distribution of the bursts is described by the Gutenberg-Richter distribution over a range of two magnitude units. 5) Most shallow events are small. All deep events are large. In this study, we test the hypothesis that the high frequency bursts originate on the fault plane, and are not path or site effects. For several events, the vector displacements were measured at a number of 3-component stations and compared with the predicted radiation pattern from double-couple point source at the observed hypocenter in a homogeneous half-space. We also show that small events occurring at depth would be resolved by the array, and hence conclusion (5) above is not an artifact of attenuation. We present further evidence that the events are not site effects by showing they are not correlated with the amplitude of direct P and S. Because the time and magnitudes of the bursts appear to obey both Gutenberg-Richter and Omori laws, we further investigate their relationship with the normal aftershock sequence following the mainshock.
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: Fall Meeting 2005