An observation of rupture pulses of the 20 September 1999 Chi-Chi, Taiwan,earthquake from near-field seismograms

The ground-velocity recordings of the 20 September 1999, Chi-Chi, Taiwan ea rthquake recorded at stations near the ruptured fault trace show a simple, large-amplitude, and long-period pulse following the S wave, which is close ly associated with the surface faulting and the rupture process of thrust f aulting. The conspicuous pulse on the ground-velocity seismogram. following the S-wave arrival, called the S-1 phase, is interpreted as the superposit ion of the rupture pulses that nucleate at an asperity near and underneath the station and propagate up-dip and laterally along the fault plane toward the surface stations. The arrival times of the S-1 phase and the onsets of the permanent displacement at stations near and along the ruptured fault t race increase with hypocentral distance, suggesting that the rupture of the Chi-Chi earthquake might have initiated at the hypocenter of the mainshock and propagated both upward and laterally from south to north. On the basis of the travel-time differences between the S-1 phase and the direct S wave at the stations near and along the ruptured fault trace, the rupture veloc ities varied from 2.28 to 2.69 km/sec, with an average rupture velocity of about 2.49 km/sec. The rupture velocities decreased from south to north.