Recharge Zone of the Ogiri Geothermal Field, Analyzed the Microearthquake Data

Abstract

The Ogiri geothermal field is located on the western slope of Kirishima volcano, one of the largest Quaternary volcanoes in Japan. Based on the hypocenter distribution of microearthquakes, geothermal fluid seemed to recharge the reservoir from the east side of the Ogiri field. The image, however, was not clear due to insufficient seismic data in the east side region. Microearthquake swarms have been observed around the Kirishima volcano since August 2024 by the three seismic observation networks, whose data are available through internet. Few events occurred in the Ogiri field, but ray paths of P and S waves from the swarms covered the east side region, thus velocity model was revised. The new velocity model, which showed vertical and lateral changes in the region, resulted in lower uncertainty for locations of the microearthquakes. The relocated hypocenters as well as the new velocity model imply that geothermal fluid source charging the area lie to the east of the Ogiri field.