Ching-Yu Hu 1,2 , Masanao Shinohara 1 , Yusuke Yamashita 3 , Takashi Tonegawa 4 ,
Tomoaki Yamada 1,Takeshi Akuhara 1 , Kimihiro Mochizuki 1 


1 Earthquake Research Institute, The University of Tokyo, 2 Graduate School of Science, The University of
Tokyo, 
 3 Disaster Prevention Research Institute, Kyoto University, 4 Japan Agency for Marine-Earth　Science and Technology
Earth, Planets and Space, 76(1), 168. https://doi.org/10.1186/s40623-024-02122-4

　Seafloor seismic observations in the Hyuga-nada region were conducted during 2015–2016, 2017–2018, and 2022. Hypocenters of fast earthquakes were first determined using a 1-D velocity structure with station corrections applied for structural heterogeneity beneath each OBS station. Events were then relocated using the double-difference technique, and focal mechanisms were estimated from first-arrival polarity data.
　
　Relocation results indicates that 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 about 6 km by 12 km and occurred within a few days in December 2017 and January 2018, which could be classify as swarm activities from regular activities. Fault plane solutions of fast earthquakes 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.