[S-VC28] Active Volcanism

[SVC28-P02] Volcanic tremor source location at Tokachidake volcano from cross-correlation analysis of 2018-2020 seismic data

*Theodorus Permana¹, Hiroshi Aoyama¹ (1.Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University.)

Keywords:volcanic tremor, seismic data, cross-correlation, volcano monitoring, Tokachidake

Volcanic tremors are known to be related to the processes that involve the movement of magma and gas within a volcano. During the 1988-1989 eruptive activity at Tokachidake volcano, Hokkaido, Japan, volcanic tremors were observed accompanying explosive eruptions. Volcanic tremors were sometimes associated with other phenomena such as low-frequency earthquakes and tilt changes. Considering that past major eruptive activities have occurred with an interval of a few tens of years, and the risk the volcano imposed on the surrounding inhabited areas, monitoring of volcanic tremors is important among other phenomena that could provide us with insights into the ongoing volcanic activity at Tokachidake. However, volcanic tremors are challenging to locate due to their complex seismic waveforms showing unclear onset of seismic phases. We use cross-correlation analysis of seismic data that measures the travel time differences between seismic stations and avoid picking the arrival time of seismic phases. We compute stacked cross-correlation functions and find the most probable source location using a grid-search scheme. We analyzed tremors that occurred during 2018-2020, mainly those who are reported by the Japan Meteorological Agency (JMA). Specifically, we determine the source location of every 1-minute seismic data in WIN format used in Japanese monitoring system and focus on 1-minute data that contain volcanic tremors. We find that tremor source locations are located beneath the northwest flank of Tokachidake and the 62-II and Taisho craters at shallow depths of about 0.5 km from the ground surface. The distribution area of volcanic tremor sources is generally consistent with that of the hypocenter of volcanic earthquakes. We also show the possibility to detect more tremors at Tokachidake by measuring the spatial coherency of seismic signals. Our results show that the seismic correlation-based volcanic tremor location method is applicable at Tokachidake and could be used together with other monitoring techniques to gain insights into volcanic activity.