2025 Volume 78 Pages 1-13
Seismic observation was conducted on Hachijojima Island, the second largest volcanic island among the Izu Islands, Japan. This temporary observation took place twice, with each session occurring within a 7-months period in 2019 and 2021. We have densely installed 46 temporary stations across the island, in addition to 9 permanent stations, to collect seismic data. In this paper, our primary focus is to describe the seismic observation and establish an earthquake catalog, with initial evaluation of seismicity in relation to the magmatic system and tectonic setting. The arrival times of P- and S- wave were manually picked using WIN System, a processing system that handles multi-channel seismic waveform data. Subsequently, the hypocenters of the local earthquakes were located directly during manual pick using the ‘hypomh’ program within WIN System. A total of 179 local earthquakes were localized where 119 took place in 2019 and the remaining 60 occurred in 2021. It was observed that most earthquake events' hypocenters were situated primarily in the northwestern region of the island, located at the northern edge of the rift margin. These earthquake events appear to be associated with the long-distance lateral magma transport in the middle to lower crust at depths of 10-20 km, which is driven by the regional tectonic conditions within the deeper crust. The catalogue obtained from this study can be utilized for investigating the subsurface structure around Hachijojima Island.
