Temporal-Spatial Rupture Process of the 1999 Chi-Chi Earthquake from IRIS and GEOSCOPE Long-Period Waveform Data Using Aftershocks as Empirical Green's Functions

Abstract

A large earthquake ( M w 7.6) occurred near Chi-Chi, Taiwan, on 20 September 1999 (UTC) and was followed by many moderate-size aftershocks in the following days. The two largest aftershocks with magnitudes M w 6.1 and 6.2, respectively, were used as empirical Green's functions (EGFs) to retrieve the source time functions (STFs) and further image the temporal-spatial rupture process of the mainshock. For each station, two types of STFs were retrieved, one from P phases and another from S phases. A total of 178 STF individuals were retrieved for source-process analysis of the event. From the STFs retrieved, firstly, similarities appeared on the STFs from most stations except that several STFs in special azimuths looked different or odd because of the focal mechanism difference between the mainshock and the EGF aftershocks; and secondly, systematic shape-variation with azimuth appeared. The analysis of the STFs indicated that this event consisted of two subevents; on the average, the second event was about 7 sec later than the first one, and the first event was about 15% larger than the second one in the moment-release rate. The total duration time of earthquake rupture process was about 26 sec. From the image of the static slip distribution, there were two slip-concentrated areas on the fault plane, and their centers were about 45 km away from each other. The maximum slip of about 6.5 m appeared on the northern one. The rupture area, with the slip greater than 0.5 m, was about 80 km long and 60 km wide. The maximum stress drop was about 25 MPa (250 bar), and the average stress drop on the entire fault was 9.2 MPa (92 bar). From the snapshots of the temporal-spatial variation of slip and slip rate, the rupture initiated at the southern end and stopped at the northern end of the fault, which suggested an overall unilateral rupture at a rupture velocity of about 2.5 km/sec. The depth of the initiation locus was about 20 km. The rupture duration times on the subfaults were estimated. A good correlation was noticed between the rupture duration time and slip amplitude of the subfaults: the larger the slip amplitude, the longer the rupture duration time of the subfault. The maximum duration time for the subfaults was 16 sec, which was about two-thirds the total duration time of the earthquake rupture process. In the beginning and near the end of the source process, the rupture propagated like a self-healing pulse of slip. Manuscript received 21 May 2001.