Abstract

The 2001 Hyogo-ken Hokubu earthquake (Mw 5.2) occurred in the northern part of Hyogo prefecture in southwestern Japan and some clusters of the following aftershock activity extended in a direction different from the mainshock strike direction. First, we investigate the rupture process of this event by inverting near-source strong-ground-motion waveforms. Though it is an M 5 class event, high-quality waveforms were recorded by the high-density, strong ground motion networks. To obtain accurate Green’s functions for the source inversion, we calibrate the crustal structure models by forward modeling of the observed waveforms of small events. The rupture process imaged from the inversion of the 0.4–2.0 Hz waveforms is relatively simple. A large slip area is located around the rupture nucleation point. Second, we examine the effect of the static stress changes caused by the mainshock and the large aftershocks on the following seismicity. We calculate the changes of Coulomb failure function (Δcff) using the heterogeneous slip distribution on the fault plane of the mainshock estimated from the waveform inversion and taking into account the effects of the following large aftershocks. In this way, we obtain a complex spatiotemporal distribution of Δcff in the focal area. Most of the aftershocks separated from the mainshock fault occurred in the positive Δcff region. This result suggests that these aftershocks were strongly affected by Δcff caused by the mainshock and the preceding large aftershocks.

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