Seismic activity around shallow plate boundary near westernmost Nankai Trough revealed by ocean bottom seismometer observation

Abstract The Nankai Trough region has a history of devastating earthquakes. The Hyuga-nada region is situated in the westernmost Nankai Trough and has not experienced an earthquake exceeding magnitude 8. It is expected that there is a difference in a coupling between the subducting Philippine Sea Plate and the overriding plate. The region is known for its active region of low-frequency tremors and very low-frequency earthquakes. Several long-term ocean bottom seismometer (LTOBS) networks were deployed to monitor seismic activities in the region to reveal the characteristics of seismicity in the study region. Seafloor seismic observations were conducted on the seafloor in the Hyuga-nada region with periods of 2015–2016, 2017–2018, and 2022. Initially, the hypocenters of regular earthquakes were determined by using a location program that integrated absolute travel times and a 1-D velocity structure. Station corrections for travel times were applied to compensate for structural heterogeneity just beneath individual LTOBS. Subsequently, events were relocated using a double-difference technique to enhance the accuracy of the location. Focal mechanisms were estimated using the polarity data of the first arrivals. Comparison with a plate model indicated that these earthquakes occurred within the subducting Philippine Sea Plate. From the spatial–temporal distribution, hypocenters of earthquakes were concentrated in a small region with a size of a few km and occurred within a few days. We could clearly distinguish swarm activities from regular activities. During the observation period, there were two swarm activities in the study area. Fault plane solutions of regular events except swarms, are dominant with a normal fault type mechanism. On the other hand, swarms had a small number of events with a normal fault type. The swarm activities started slightly later than the very low frequency earthquake (VLFE) activities. The regions of activity for the swarms and the VLFE seemed to generally overlap in consideration of the low spatial resolution of the VLFE locations. Graphical Abstract