Precise aftershock distribution and crustal structure in and around the northern focal area of the 2008 Iwate-Miyagi Nairiku Earthquake

Eiji Kurashimo,Naoko Kato,Susumu Abe,Takashi Iidaka,Shin Koshiya,Taku Kawanaka,Naoshi Hirata,Hiroshi Sato,Takaya Iwasaki

doi:10.5047/eps.2011.04.001

Abstract

The 2008 Iwate-Miyagi Nairiku Earthquake (Mj 7.2) occurred on June 14, 2008, at the eastern flank of the Ou backbone range, in the central part of northern Honshu, Japan. In the northern part of the focal area, seismic reflection/refraction experiments conducted in 2006 and 2007 revealed a series of west-dipping faults. To investigate the relation between these faults and aftershocks, we conducted a high-density seismic array observation across the northern focal area. The arrival times of earthquakes were used in a joint inversion for earthquake locations and velocity structure. The V p structure shows tilted blocks and the block boundaries coincide with the revealed west-dipping faults. These faults were interpreted to have formed during the Miocene extension. The aftershock distribution shows a concentration on a plane dipping westward approximately 40° beneath the eastern margin of the Ou backbone range. The zone of aftershock concentration can be correlated to the known range-bounding fault inferred from seismic profiling and does not coincide with the known active reverse fault (Dedana fault). These results indicate the significance of the potential source fault located along the boundary of the Ou backbone range.

Highlights

A large shallow earthquake occurred in the central part of northern Honshu (Iwate Prefecture), Japan, at 8:43 AM Japan standard time (JST, UT + 9 h) on June 14, 2008, with a Japan Meteorological Agency (JMA) magnitude (Mj) of 7.2 (Fig. 1)
We present a precise aftershock distribution and seismic velocity structure in and around the northern focal area of the 2008 Iwate-Miyagi Nairiku Earthquake derived from high-density aftershock observations and we discuss the relation between the source fault and the known geological structure
Similar correspondences have been found in other earthquake areas such as the 2003 northern Miyagi Earthquake (Okada et al, 2007) and the 2004 Mid-Niigata Prefecture Earthquake (Kato et al, 2010). These results suggest that the high-velocity region with low aftershock activity beneath the eastern margin of the Ou backbone range corresponds to the region of asperity

Summary

Introduction

A large shallow earthquake occurred in the central part of northern Honshu (Iwate Prefecture), Japan, at 8:43 AM Japan standard time (JST, UT + 9 h) on June 14, 2008, with a Japan Meteorological Agency (JMA) magnitude (Mj) of 7.2 (Fig. 1). The geological structure of the northern part of the focal area is adequately shown by seismic reflection profiles (Kato et al, 2006; Abe et al, 2008; Saito et al, 2008). We have conducted a highdensity, 40-km-long, seismic array observation across the northern focal area to investigate the aftershock distribution and crustal structure.

Results

Conclusion