摘要：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.
