Complex source processes and the interaction of moderate earthquakes during the earthquake swarm in the Hida-Mountains, Japan, 1998

Abstract

This paper presents a set of detailed source models of 18 moderate (M4-5) earthquakes that were part of the swarm activity beneath the Hida-Mountains in central Japan in 1998. To reveal the detailed rupture process, this study uses a waveform inversion procedure with the empirical Green function (EGF). Most of our models show a complicated rupture process that includes several separate rupture areas and that involves ruptures that, in some events, seem to repeat at almost the same location. In our models, we measured as characteristic values the maximums of slip, slip-rate, stress drop, and average rupture propagation velocity. In comparison with the models for larger events determined by similar methods, the maximum slip scales with the cubic squares of the seismic moment for large stress drop events only. The slip-rate has an upper limit of about 1–2 m/s, and the stress drop is distributed widely from 1 to 50 MPa. The average stress drops, estimated using the corner frequency, are almost uniform at about 1 MPa. The first event in each swarm cluster tends to have a large maximum stress drop, which reflects the change in the stress field during swarm activity. The average rupture velocity, determined by the least squares method, is relatively slow and shows a negative relationship with the maximum stress drop. This finding cannot be explained by simple crack models and instead suggests a very complicated mechanism like the interactive fault system. The interaction between the slip areas of neighbor events resulted in a non-overlapping slip distribution. This finding indicates that previous rupture areas restrict the size and location of later events even in such complex swarm activities.