Shallow tectonic tremor activities in Hyuga-nada, Nankai subduction zone, based on long-term broadband ocean bottom seismic observations

Abstract The study of slow earthquake activity, which occurs in the shallow and deep sides seismogenic zone, is crucial for understanding subduction zones, including variations frictional properties with depth interplate coupling. Observations at seafloor are necessary, particularly earthquakes occurring offshore areas; however, few observations such activity have been made. We conducted long-term seismic on Hyuga-nada region, located western end Nankai Trough, to characterize low-frequency tremor from 2014 2017. Although these lasted only a years, occurrence frequency tremors was lower than that major involving migration occurred once every two or more years. In contrast, minor duration days several times year. Major activities 2015 were accompanied by similar those 2013. 2013 characterized south north rate 30–60 km/day. However, west east migration, area extended further east. rates also much slower (several 20 km/day) These different likely reflect state coupling down-dip side area. Minor identified, triggered Nepal 2016 Kumamoto earthquakes. Activity mainly focal regions activities. Very-low-frequency (VLFEs) concurrently tremors, their epicenters coincided within margin error. VLFEs mostly peripheral concentration zones. This indicates heterogeneities present along plate boundary.