Hypocenter distribution of the main- and aftershocks of the 2005 Off Miyagi Prefecture earthquake located by ocean bottom seismographic data

Ryota Hino,Yoshiyuki Kaneda,Masanao Shinohara,Toshihiko Kanazawa,Masao Abe,Gou Fujie,Minoru Nishino,Kazuo Nakahigashi,Kouetsu Minato,Tomoaki Yamada,Eiichiro Araki,Kimihiro Mochizuki,Masayuki Tanaka,Shuichi Kodaira,Nariaki Okawara,Yojiro Yamamoto,Asako Kuwano,Gen Aoki

doi:10.1186/bf03352658

Abstract

Abstract The preliminary hypocenter distribution of the 2005 Off Miyagi Prefecture earthquake and its aftershocks is estimated using data from five ocean bottom and six onshore seismic stations located around the rupture area of the earthquake. The epicenter of the mainshock is relocated at 38.17°N, 142.18°E, and the focal depth is estimated to be 37.5 km. The aftershocks surrounding the mainshock hypocenter form several clusters that are concentrated along a distinct landward dipping plane corresponding to the plate boundary imaged by the previous seismic experiment. The strike and dip angles of the plane agree well with those of the focal mechanism solution of the mainshock. The size of the plane is about 20×25 km2 in the strike and dip directions, which is similar to that of the large coseismic slip area. The up-dip end of the planar distribution of the aftershocks corresponds to the bending point of the subducting oceanic plate, suggesting that the geometry of the plate boundary affects the spatial extent of the asperity of the 2005 earthquake

Highlights

The landward slope area of the Japan Trench subduction zone is characterized by active interplate seismicity, and large earthquakes with magnitudes of 7 and higher have occurred repeatedly. Yamanaka and Kikuchi (2004) have shown that these large interplate earthquakes are repeating ruptures of asperities - areas of large coseismic slip that are locked during interseismic periods along the surface of the subducting Pacific plate
No systematic differences are recognized between the relocated and the catalogue aftershock distributions, the epicenter of the mainshock is relocated at 38.17◦N, 142.18◦E to about 8 km west of the epicenter reported by JMA
The focal depth distributions according to the JMA catalogue and relocated by using the ocean bottom seismographs (OBSs) data are shown in Fig. 3(a) and (b), respectively

Summary

Introduction

The landward slope area of the Japan Trench subduction zone is characterized by active interplate seismicity, and large earthquakes with magnitudes of 7 and higher have occurred repeatedly. Yamanaka and Kikuchi (2004) have shown that these large interplate earthquakes are repeating ruptures of asperities - areas of large coseismic slip that are locked during interseismic periods along the surface of the subducting Pacific plate. The landward slope area of the Japan Trench subduction zone is characterized by active interplate seismicity, and large earthquakes with magnitudes of 7 and higher have occurred repeatedly. Yamanaka and Kikuchi (2004) have shown that these large interplate earthquakes are repeating ruptures of asperities - areas of large coseismic slip that are locked during interseismic periods along the surface of the subducting Pacific plate. In the middle part of the subduction zone, the M=7.5 class earthquakes off Miyagi Prefecture are known to occur with a recurrence interval of about 40 years. On August 16 2005, an earthquake of M=7.2 occurred off Miyagi Prefecture. The focal mechanism solution of the earthquake was of a thrust fault type (e.g. F-Net; NIED, 2005), indicating that this event was an interplate earthquake, but its seismic moment was 5.4×1019 Nm (e.g. F-Net; NIED, 2005), which was significantly smaller than that of the earthquake forecasted to occur. Okada et al (2005) proposed that the 2005 earthquake was a re-rupturing of one of the asperities of the preceding

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Conclusion