Coseismic slip distribution of the 2005 off Miyagi earthquake (M7.2) estimated by inversion of teleseismic and regional seismograms

Tadashi Yaginuma,Akira Hasegawa,Norihito Umino,Yuji Yagi,Tomomi Okada,Toru Matsuzawa

doi:10.1186/bf03352659

Tadashi Yaginuma, Akira Hasegawa + Show 4 more

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https://doi.org/10.1186/bf03352659

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Abstract

Abstract A large earthquake (M7.2) occurred along the plate boundary off Miyagi Prefecture (Miyagi-Oki), northeastern Japan, on August 16, 2005. In this area, large earthquakes (~M7.5) have occurred repeatedly at intervals of about 37 years, and more than 27 years have passed since the last event occurred. To estimate the relationship between this earthquake and the previous events, we determined coseismic slip distribution by this 2005 Miyagi-Oki earthquake by adopting the seismic waveform inversion method of Yagi et al. (2004) and compared it with that of the previous 1978 Miyagi-Oki earthquake. We performed two cases of the inversions; inversion using only far-field seismograms and that using far-field seismograms and local seismograms simultaneously. Both results show that a large slip occurred near the hypocenter and rupture extended to the westward deeper portion. Considering that the rupture area of the 2005 event partly overlapped with the southeastern part of that of the 1978 event, suggests this result the possibility that plural asperities exist which cause the sequence of Miyagi-Oki earthquakes and that the 2005 event ruptured one of such asperities, although the previous 1978 event ruptured all the asperities at one time.

Highlights

A magnitude 7.2 interplate earthquake occurred at 11:46 on August 16, 2005 at a depth of approximately 40 km in the Miyagi-Oki region, northeastern Japan
Recent investigations on interplate earthquakes suggest the existence of asperities along the plate boundary east off northeastern Japan (Nagai et al, 2001; Okada et al, 2003; Yamanaka and Kikuchi, 2004; Igarashi et al, 2003; Matsuzawa et al, 2004; Uchida et al, 2005; Hasegawa et al, 2005)
The results indicate that the rupture process consisted of a main rupture in the vicinity of the hypocenter, followed by propagation of the rupture toward the west. This is compatible with the directivity seen in the teleseismic waveforms and the spread of the aftershock distribution

Summary

Introduction

A magnitude 7.2 interplate earthquake occurred at 11:46 on August 16, 2005 at a depth of approximately 40 km in the Miyagi-Oki region (offshore of Miyagi Prefecture), northeastern Japan. The last Miyagi-Oki earthquake (M7.4) occurred on June 12, 1978 and caused major loss of life (28 deaths) and 1325 people injured. A magnitude 7.1 intraslab earthquake occurred nearby on May 26, 2003 (Okada and Hasegawa, 2003). Recent investigations on interplate earthquakes suggest the existence of asperities along the plate boundary east off northeastern Japan (Nagai et al, 2001; Okada et al, 2003; Yamanaka and Kikuchi, 2004; Igarashi et al, 2003; Matsuzawa et al, 2004; Uchida et al, 2005; Hasegawa et al, 2005).

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