More than six 6 and greater aftershocks of the 1999, Chi-Chi, Taiwan earthquake (=7.6) were well-recorded. In this study we invert for the finite source rupture process for one of the largest aftershocks (23.86N, 121.01E) that occurred on 1999/9/25 at 23:52:49.5 UTC, 5 days after the mainshock. Through inversions we hope to discriminate the causative fault plane from the auxiliary plane. There are 3 scenarios for the ruptured plane: down-dip extension of the mainshock on the detachment, backthrust above the detachment, and a basement-involved fault below the detachment (Figure 28.1). Although the hypocenter is located in the vicinity of the proposed major shallow east-dipping fault, previous data could not exclude the possibility of a high-angle, west-dipping conjugate fault (backthrust). If true, a backthrust scenario will give us an important constraint on the deep crustal geometry under Taiwan. In addition, recent seismicity studies (e.g. Carena et. al., EOS 82(47), p1176, 2001) show a steep, west-dipping fault below the detachment and the aftershock might have occurred on this fault, if the aftershock focal depth is actually deeper than reported. This scenario implies that large seismic strains can be stored in the footwall of the detachment and future geodynamic studies might need to consider a deformable footwall block that can generate 6 earthquakes. If the rupture was on the proposed detachment, we can delineate its attitude and slip distribution, which can be compared to the mainshock.
We used strong motion data from the Central Weather Bureau of Taiwan (Lee at al., 2001) to invert the representation theorem for finite source parameters by using the method of Hartzell and Heaton (1983). We used a linear least-squares inversion of observed velocity seismograms to compute the spatio-temporal slip distribution. To improve inversion stability, we have applied the following additional constraints: slip-positivity, Laplacian smoothing, and moment minimization. We have performed a grid-search over a range of focal parameters to find the optimal orientation using the variance reduction measure. In total, 1036 inversions were performed and we chose 5°/30°/100° (variance reduction 73 percent) as our preferred model.
One surprising outcome from this study is that the strike of our preferred focal mechanism (5°) is different from that of teleseismic results (28°). An initial teleseismic moment tensor inversion for the Chi-Chi mainshock also gave a strike of 26°, compared with the strike of 5° derived from the mainshock surface rupture, thus this discrepancy could be systematic and relate to complex crustal velocity structures underneath Taiwan. The dip is 30° to the east.
The relatively large moment release from this aftershock indicates that its effects should be incorporated into the ongoing aftershock/afterslip studies. Our results might help recalibrate the coseismic/postseismic GPS data. Here we forward modeled the GPS displacements using our slip model in an elastic half space (c.f. Okada, 1992). For stations near the aftershock epicenter, most of the horizontal surface displacements are about 1/500 of the observed GPS data from the mainshock. The small displacements are due to the greater depth of this aftershock. However, the displacements can still be up to 3.3 cm at some GPS stations, and thus need to be taken into account in afterslip studies.
We thank Roland Burgmann and David Schmidt for their constructive discussions. This research is partially funded by NSF Grant EAR-0105998.
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