Coseismic Slip Distribution of the 1999 Chi-Chi, Taiwan, Earthquake Deduced from Inversion Analysis of GPS Data

by Shoichi Yoshioka

Abstract

The 1999 Chi-Chi earthquake (M_w = 7.6) was the largest thrust-type earthquake that occurred in Taiwan in the twentieth century. Following the event, large horizontal and vertical coseismic displacements were observed at GPS stations near the fault region. The observed maximum horizontal displacement, uplift, and subsidence were 9.1, 4.0, and 0.7 m, respectively. In order to estimate coseismic slip distribution, geodetic data inversion was carried out based on the GPS displacement data. Total numbers of horizontal and vertical data used here are both 118. Taking the shape of the surface rupture, aftershock distribution, and centroid moment tensor solutions into account, a gently curved fault plane with a fault size of = 70 km x 50 km, a dip angle of 31^o, and strike of N9^oE was constructed. The results show that the amount of average slip is 5.1 m, and the average slip direction is N55^oW on the fault plane. Seismic moment estimated from the geodetic data inversion is 5.4 x 10²⁰ N m. The amount of slip gradually increases toward the northwest on the fault plane, reaching 13.8 m. The spatial distribution of the coseismic slip is consistent with that estimated from inversion analysis of teleseismic and near-source strong-motion data, whereas the amount of slip of the former is larger than that of the latter. This may suggest that slow, large afterslips occurred on the northwestern part on the fault plane within one month of the event, and/or the fault dip adopted in this study was larger than the actual dip, indicating disagreement of coseismic fault plane with one of the nodal planes estimated from the CMT solutions and the aftershock distribution.

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