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

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 (M JMA = 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.

Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

“…Before the events, earthquakes occurred in several clusters located southwest of the activity obtained in this study (Obana et al 2004. Although the overall distribution is similar to that obtained in our previous study (Nakano et al 2013), the new dataset improved our capability to detect earthquakes, especially south of the observation network, and we have recognized several new features described below.…”

“…2), more than five times the number determined in our previous study (Nakano et al 2013). The seismic activity occurred mainly below the Nankai trough and Kumano forearc basin and extended over distances of about 100 km in both the N-S and E-W directions.…”

“…3b, d). This cluster lies outside the 2004 aftershock region; Nakano et al (2013) attributed this cluster to activity along pre-existing weak structures in the incoming PHS plate. We note that there are some ambiguities in the source depths of earthquakes in clusters B and C, which may reflect the assumed velocity structure.…”

“…They considered that the ridge affects mechanical coupling at the plate boundary. In a previous study (Nakano et al 2013), based on data from January to August 2011, we showed that DONET ocean-floor observation data provide a better detection capability and higher precision of earthquake source locations below the Nankai trough than analyses based only on onshore observation data. However, DONET was still under construction during most of the analysis period; construction of the final planned station was finished in the end of June 2011.…”

Anomalously large seismic amplifications in the seafloor area off the Kii peninsula

“…Since most of shallow events below DONET are of a magnitude less than 4 (Nakano et al 2013) and low signal to noise ratio is expected for low-frequency components, we have not included near-field small events in the present study. Previous studies by Hayashimoto and Hoshiba (2012, unpublished results) and Hayashimoto and Hoshiba (2013) investigated the difference in magnitudes for events including distant ones with an epicentral distance of more than 200 km between those estimated from displacement data at seafloor stations and the JMA catalogue magnitudes.…”