INVERSION FOR THE RUPTURE DETAILS OF THE 1987 EAST-CHIBA EARTHQUAKE, JAPAN, USING A FAULT MODEL BASED ON THE DISTRIBUTION OF RELOCATED AFTERSHOCKS

The rupture of the 1987 east Chiba earthquake was investigated using an empirical Green's function method and a nonlinear inversion technique. In order to discuss what happened on the fault plane in more detail, we relocated the aftershocks as well as the main shock. Relocations were done using a joint hypocenter method in order to increase the accuracy of the relative locations. Based on this aftershock distribution, we propose a double-fault model. The main fault strikes N11-degrees-W with nearly vertical dip. A subsidiary fault is perpendicular to the main fault. The results of a source inversion using strong motion waveforms are as follows: (1) moment release was large both in the deep northern area of the main fault and on the subsidiary fault, where aftershock activity was weak; (2) rupture propagated on both main and subsidiary faults simultaneously, and rupture velocity decreased at the depth of around 35 km, near the upper boundary of the Philippine Sea plate; and (3) stress drop was high on the lower northern corner of the main fault where the rupture terminated and on the deep southern area of main fault where the rupture initiated. A negative correlation was found between moment release during the main shock and that during the aftershock sequence, but they did not compensate each other quantitatively. This suggests the existence of a heterogeneous strain distribution in the fault area either before the main shock or after its aftershock sequence.