Source processes of 15 September 1998 M 5.0 Sendai, Northeastern Japan, earthquake and its M 3.8 foreshock by waveform inversion

A moderate earthquake (M 5.0) occurred on 15 September 1998 in Sendai City, northeastern Japan, causing slight damage in a portion of the city. It was located at about 12-km depth on the Nagamachi-Rifu fault, an active reverse fault passing through the center of the city. We investigate rupture processes of the earthquake and its largest foreshock (M 3.8) by an empirical Green's-function method. We use waveform data recorded by nearby dense strong-motion arrays and by short-period and broadband stations. Waveform inversion results for the mainshock show that the spatial extent of its rupture area is about 3 × 3 km², which is roughly consistent with that of the aftershock area. Most of the moment release was contained in two asperities. Aftershocks precisely relocated by a homogeneous-station method show that their activity is high in areas with a relatively small amount of moment release of the mainshock. Rupture of the largest foreshock (M 3.8) that occurred 6 min before the mainshock started approximately at the mainshock hypocenter. Its rupture area, with a radius of about 300 m, does not strongly overlap with the asperities of the mainshock.