A High-Frequency View of the 1999 Chi-Chi, Taiwan, Source Rupture and Fault Mechanics

Abstract

High-frequency bandpass filtering of broadband strong-motion seismograms recorded immediately adjacent to the fault plane of the 1999 Chi-Chi, Taiwan, earthquake reveals a sequence of distinct bursts, many of which contain quasi-periodic subbursts with repeat times on the order of a few tenths of a second. The subevents that produce these bursts do not appear in conventional slip-maps, presumably because of the low-pass filtering used in the waveform inversions. The origin times, locations, and magnitudes of of about 500 of these subevents were determined. Those closest to the hypocenter appear to have been triggered by the P wave, while the earliest subevents at greater distances are consistent with a rupture front propagating at about 2.0 km/sec. Later subevents at a given distance follow Omori9s law and may be interpreted as aftershocks that begin before the Chi-Chi rupture has terminated. The frequency-magnitude distribution of these subevents has a b -value equal to 1.1. The subevents are clustered in space, with most clusters located at shallow depth along the Chelungpu surface rupture. The larger subevents tend to occur at greater depth, while the small subevents are only located at shallower depths. Cluster locations generally coincide with the large-amplitude slip-patches found by source inversions at lower frequencies. Relocation of the deeper subevents suggests a nonplaner rupture surface where dip increases with depth at the southern end.