Urgent aftershock observation of the 2004 off the Kii Peninsula earthquake using ocean bottom seismometers

Shin’Ichi Sakai,Koichiro Obana,Yoshiyuki Kaneda,Masanao Shinohara,Shuichi Kodaira,Hiroko Hagiwara,Toshihiko Kanazawa,Tomoaki Yamada

doi:10.1186/bf03352577

Abstract

Abstract The 2004 off the Kii Peninsula earthquake occurred on September 5, 2004. Knowing the precise aftershock distribution is important for understanding the mechanism of this earthquake. However, the hypocenter of the main shock was located more than 100 km offshore from the nearest station of the land observation network. In the three days after the main shock, we started ocean bottom seismometer (OBS) observation in order to determine the precise distribution of the aftershocks. We assumed a seismic velocity structure for the hypocenter calculation, based on the results of previous seismic refraction study. The station corrections were incorporated to locate the hypocenter precisely. The hypocenters located within an area covered by five OBSs show relatively small errors. It is found that the OBS-located hypocenters are located about 5.5 km east-southeast from those by JMA and the depth range of the aftershocks is about 5–25 km just beneath the Nankai trough axis. The aftershock hypocenters can be grouped into two clusters at different depths of about 10 km and about 20 km. It is inferred that the main shock also has a depth of 5–25 km. Since this extent of the main shock was larger than one of the oceanic crust of the Philippine Sea plate, the fault plane of the main shock extended at the upper most mantle of the Philippine Sea plate. Although we cannot assign the actual fault plane of the main shock form our observation results, it is clarified that intra-plate earthquakes occurred near the trench region. Our OBS result supports that the main shock was the earthquake not at the plate boundary but within the bending Philippine Sea plate near the trough axis.

Highlights

An earthquake with a local body wave magnitude (MJMA)of 7.4 occurred near the Nankai trough axis, approximately 100–150 km offshore from the Kii peninsula, Japan, at 14:57, September 5, 2004 (UT)
We describe our ocean bottom seismometer (OBS) observation and discuss the tectonic implications of the aftershock distributions located with the OBSs
Seismic Observation In order to retrieve the precise distribution of the aftershock activity of the 2004 off the Kii Peninsula earthquake as quickly as possible, we started to deploy the OBSs on the source region three days after the main shock using a helicopter chartered by Earthquake Research Institute (Fig. 2)

Summary

Introduction

Of 7.4 occurred near the Nankai trough axis, approximately 100–150 km offshore from the Kii peninsula, Japan, at 14:57, September 5, 2004 (UT). Information of the focal process of the 2004 off the Kii Peninsula earthquake is necessary to study its influence on the next Tonankai Earthquake with M8 It is, difficult to locate precisely the aftershocks using data from the land seismic network alone, because the epicenters are located 100 km or more away from the observation station. 2. Seismic Observation In order to retrieve the precise distribution of the aftershock activity of the 2004 off the Kii Peninsula earthquake as quickly as possible, we started to deploy the OBSs on the source region three days after the main shock using a helicopter chartered by Earthquake Research Institute (Fig. 2). During this analysis, the RMS of the O-C times for each hypocenter determination improved from 0.075 s to 0.030 s for P-wave arrival and from 0.272 s to 0.116 s for S-wave arrival. The hypocenters determined by the OBSs can be classified into two different depth groups: the shallower group of the hypocenters is concentrated mainly at 5–15 km depth in the approximate NW-SE direction, nearly perpendicular to the trough axis;

Discussion

Conclusion