• journal_title：Bulletin of the Seismological Society of America
• Contributor：Kou-Cheng Chen ; Bor-Shouh Huang ; Jeen-Hwa Wang ; Win-Gee Huang ; Tao-Ming Chang ; Ruey-Der Hwang ; Hung-Chie Chiu ; Chu-Chuan Peter Tsai
• Publisher：Seismological Society of America
• Date：2001-
• Format：text/html
• Language：en
• Identifier：10.1785/0120000716
• journal_abbrev：Bulletin of the Seismological Society of America
• issn：0037-1106
• volume：91
• issue：5
• firstpage：1247
• section：Articles

The ground-velocity recordings of the 20 September 1999, Chi-Chi, Taiwan earthquake recorded at stations near the ruptured fault trace show a simple, large-amplitude, and long-period pulse following the S wave, which is closely associated with the surface faulting and the rupture process of thrust faulting. The conspicuous pulse on the ground-velocity seismogram following the S-wave arrival, called the S₁ phase, is interpreted as the superposition 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 S₁ phase and the onsets of the permanent displacement at stations near and along the ruptured fault trace 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 S₁ phase and the direct S wave at the stations near and along the ruptured fault trace, the rupture velocities 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.