Intensive seismic activity around the Nankai trough revealed by DONET ocean-floor seismic observations

Masara Nakano,Shin’Ichiro Kamiya,Michihiro Ohori,Yoshiyuki Kaneda,Takeshi Nakamura

doi:10.5047/eps.2012.05.013

Abstract

Megathrust earthquakes anticipated in the Nankai trough will likely cause severe damage in central and western Japan. The Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET), a network of permanent ocean-bottom seismic stations for the early detection of earthquakes and tsunamis developed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), is in place above the expected source region of such earthquakes. Data from DONET sensors are transmitted in real time to our laboratory at JAMSTEC. Intensive ongoing seismic activity is being detected off the Kii Peninsula by DONET, mainly distributed in three clusters that overlap with the aftershock distribution of the 2004 off the Kii Peninsula earthquakes (MJMA = 7.1 and 7.4), and most of them are also aftershocks of the 2004 earthquakes. Some are linearly distributed on a different trend from the strike of the 2004 foreshock and mainshock fault planes. This result implies that the 2004 events triggered seismic activity on different faults on the subducting Philippine Sea plate. We also observed changes in seismic activity caused by the 2011 Tohoku-oki earthquake. These results could not have been obtained with on-land observations alone, which indicates the importance of DONET for monitoring seismic activity along the Nankai trough.

Highlights

Along the Nankai trough, southwest of Japan, where the Philippine Sea plate is subducting beneath the overriding Eurasian plate, megathrust earthquakes have occurred repeatedly at intervals of 100–150 years (e.g., Ando, 1975), causing severe damage in western and central Japan
DONET ocean-bottom seismic and water-pressure observation stations are connected with an optical fiber cable, and data from the sensors are transferred in real time to our laboratory at Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
The DONET ocean-bottom seismic observation network, deployed immediately above the source region of earthquakes along the Nankai trough, has much enhanced our ability to detect earthquakes off the Kii Peninsula compared with detection based on land observations alone

Summary

Introduction

Along the Nankai trough, southwest of Japan, where the Philippine Sea plate is subducting beneath the overriding Eurasian plate, megathrust earthquakes have occurred repeatedly at intervals of 100–150 years (e.g., Ando, 1975), causing severe damage in western and central Japan. The 1944 Tonankai (Mw = 8.1) and 1946 Nankai (Mw = 8.4) megathrust earthquakes ruptured the eastern and western segments of the trough, respectively. This repeated megathrust earthquake activity implies that another great earthquake may occur in the near future that would cause serious widespread damage in central to western Japan. DONET ocean-bottom seismic and water-pressure observation stations are connected with an optical fiber cable, and data from the sensors are transferred in real time to our laboratory at JAMSTEC.

Methods

Discussion

Conclusion