Ground Motion and Source Kinematics Studies of the 21 September 1999 Taiwan Earthquake

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Ground Motion and Source Kinematics Studies of the 21 September 1999 Taiwan EarthquakePI: Douglas S. DregerProposal EAR-0105998The kinematic source process of the 1999 MW7.6 Taiwan (Chi-Chi) mainshock and large aftershocks, and the effects of three-dimensional velocity structure on the generation of strong ground motions are investigated. The Chi-Chi earthquake, which occurred in a heavily urbanized region, inflicted considerable damage and human suffering once again underscoring the need for better understanding of the processes that control the generation of strong ground motions. This earthquake is unique in terms of data quality and quantity. More than 400 three-component strong motion stations recorded the earthquake waves, and this high quality data set provides an opportunity to study the earthquake source and wave propagation at scales never before possible. Three fault models, each with a distinct geometry, have been proposed to describe the kinematics of thrust faulting in the Chi-Chi earthquake, and are being tested by fitting observed and synthetic seismic waveforms and ground deformation (GPS) data. These geometries are tested by performing kinematic inversions of the strong motion and GPS data sets to determine the optimal distribution of fault slip. Additionally, several large aftershocks exhibited either shallowly or steeply dipping reverse focal mechanisms. Of particular interest is whether any of the aftershocks actually slipped on the low angle planes indicating the presence of a controlling basal decollement. By investigating the source kinematics of the mainshock and the large aftershocks it is possible to map the slip distribution of the earthquake sequence thereby obtaining an image of the active fault surface. The active mountain building of the Taiwan arc-continent collision has resulted complex crustal structure, as evidenced in recent tomographic studies and the considerable topographic relief. Three component seismic waveforms, phase arrival times, and peak ground motion parameters for the mainshock and large aftershocks are being modeled to constrain the three-dimensional wave propagation, and effects of free-surface topography using an elastic, velocity-stress staggered-grid, finite difference algorithm. Finally, the sensitivity of simulated strong motion waveforms due to the coupled nature of extended finite-fault source representations and three-dimensional structure is being analyzed. The results from this study will form the framework for future waveform modeling studies to refine the 3D structure of Taiwan, and will also provide an essential examination of strong ground motion variability contributing to the US earthquake hazard mitigation effort.