Ryo Kurihara, Kazushige Obara, Akiko Takeo, Yusaku Tanaka

Deep low-frequency (DLF) earthquakes occur beneath the Kirishima volcanoes in southwest Japan at depths of 10–30 km. In this study, we aim to reveal the relationship between DLF earthquakes and volcanic activity including eruptions by relocating the hypocenters of the earthquakes using the network correlation coefficient method and detecting the earthquakes comprehensively using the matched filter technique. Hypocenters of DLF earthquakes are found to be concentrated in some separated small clusters within depths of 10–15 and 20–27 km. Activation of deeper DLF earthquakes had been observed for approximately two years from December 2009, during which various styles of eruptions occurred. Such a two-year increase in DLF seismicity was well correlated with crustal deformation because of the volume change of a magma reservoir. The waveforms and hypocenters of DLF earthquakes during the activation period were different from those during other time periods. The activated DLF earthquakes mostly had low dominant frequencies and were located in four deeper clusters. The activation of each cluster was switched three times at the transition of the eruption styles. These results suggest that DLF earthquakes might occur near magma sills and could be triggered by fluid flow in the changing paths by complex eruption processes. In addition, the waveforms and hypocenters of DLF earthquakes associated with the 2018 eruptions are different from those associated with the 2011 eruptions. The fluid paths of the 2018 eruptions might be different from those of the 2011 eruptions.