[1] We inverted the high-resolution spatiotemporal slip distribution of the 21 September 1999 Chi-Chi, Taiwan, earthquake utilizing data from densely distributed island-wide strong motion stations for a three-dimensional (3-D) fault geometry, and 3-D Green's functions calculations based upon parallel nonnegative least squares inversion. The 3-D fault geometry, consistent with high-resolution reflection profile, is determined from GPS inversion and aftershocks distribution. This 3-D fault model presents the dip angle gradually becoming shallower from south to north along the fault and near flat at the deeper portion of the fault. The 3-D Green's functions are calculated through numerical wavefield simulation from three-dimensional heterogeneous velocity structure derived from tomography studies. The Green's functions show significant azimuthal variations and suggest the necessity of lateral heterogeneity in velocity structure. Considering complex fault geometry and Green's functions in full 3-D scale, we invert the spatial/temporal slip distribution of the 1999 Chi-Chi earthquake using the best available and most densely populated strong motion waveform data. We perform the inversion under a parallel environment utilizing multiple-time window to manage the large data volume and source parameters. Results indicate that most slip occurred at the shallower portion of the fault above the decollement. Two major asperities are found, one in the middle of the fault and another one at the northern portion of the fault near the bend in the fault trace. The slip in the southern portion of the fault shows a relatively low slip rate with longer time duration, while the slip in the northern portion of the fault shows a large slip rate with shorter time duration. The synthetics explain the observations well for the island-wide distributed strong motion stations. This comprehensive study emphasizes the importance of realistic fault geometry, 3-D Green's functions, and parallel inversion technique in correctly accounting for both the detailed source rupture process and its relationship with the strong ground motion of this intense earthquake.

[1] We applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the Philippine Sea plate. Our results indicate that the crustal stress along the Ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction, including the Tokara channel. An extensional stress regime is observed along the entire Ryukyu back arc, implying that back-arc rifting may have extended northward to Kyushu. A triangular area near the southernmost terminus of the Ryukyu arc is characterized by a unique stress signature. The eastern boundary of this Ryukyu-Taiwan Stress Transition coincides with the 123°E meridian where the Gagua ridge intercepts the Ryukyu trench; whereas its western boundary agrees remarkably well with the border between the postcollision and waning-collision domains in northern Taiwan. The Taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the Lukang Magnetization High (LMH), suggesting that the LMH may be critical in controlling the local stress distribution. The stress signature of the Luzon arc-Taiwan collision reaches as far south as 19.5°N. The tectonic stress along the Manila trench-Luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in-slab membrane stress. Since this extensional regime is observed only south of ∼22°N, it probably marks the northern limit of the contemporary boundary between the subduction along the Manila trench and the collision in Taiwan.

[1] We simulate the strong ground motion of 1999 Chi-Chi, Taiwan earthquake (M w = 7.6) by considering a three-dimensional source rupture model in a full waveform three-dimensional wave propagation study. The strong ground motion records during the 1999 Chi-Chi earthquake show various characteristics at different sites in Taiwan. We adopt a three-dimensional source model derived from an inversion study with identical path effects as considered in this three-dimensional forward study. Comparisons between the simulation results and observed waveforms from dense island-wide strong motion stations demonstrate that the fault geometry, lateral velocity variation, and complex source rupture process greatly influence the distribution of strong ground shaking. The simulation has reproduced the heavy damage area that is mainly concentrated in the hanging wall, especially close to the surface break of the Chelungpu fault. The source directivity effect is also reproduced and shows serious shaking along the northward rupture direction. Low-velocity material in the shallow part of the Western Plain is found to generate significantly amplified ground motions. In the Central Range, the shaking is relatively weak owing to the energy radiation characteristics of a low-angle thrust of the Chelungpu faulting system. The wavefield is then amplified by a high-velocity gradient under the Coastal Range. Our simulation results in the frequency range of 0.01-0.5 Hz give good agreement with the extensive strong motion observations of the Chi-Chi earthquake. We find that adequate source representation, good three-dimensional crustal velocity structures, and careful numerical work are necessary to make the ground motion prediction feasible.

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