AN INVERSION METHOD TO DETERMINE STRESS DISTRIBUTION ON A FAULT PLANE USING GROUND MOTIONS

摘要

The rupture process of an earthquake has large effect on near-field ground motions. Because source-time functions are affected by the stress drop process, it is important to understand the dynamic rupture process of earthquake faults to estimate near-field ground motions. Kinematic inversion analyses have been carried out to obtain spatiotemporal slip distributions, from which the stress distributions on the fault plane have been inferred. In this study, an inversion method to infer stress drop process of an earthquake fault is developed. To express a spatiotemporal stress distribution on an earthquake fault during an earthquake, a fault is divided into subfaults and the shear stress on a subfault after the passage of the rupture front is assumed to change repeatedly to its residual stress. Stress changes of subfaults are inferred directly from the observed ground motions. Total rupture processes are composed of the stress change histories of the subfaults. To examine the performance of the method, a simulation test was conducted. A relatively simple model was used in the simulation and the computed synthetics were used as observed seismograms. The inversion was stabilized by adding smoothing constraints and ABIC was employed to objectively determine the optimal values of hyperparameters. The inferred time histories of stress changes on subfaults fit generally well with the assumed rupture process.