Source Processes of 15 September 1998 M 5.0 Sendai, Northeastern Japan, Earthquake and Its M 3.8 Foreshock by Waveform Inversionby Tomomi Okada, Norihito Umino, Yoshihiro Ito, Toru Matsuzawa, Akira Hasegawa, and Makoto KamiyamaAbstractA 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 Greenʼs-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. |