Deep Fracture Structure of the Otake Geothermal Field, Japan, Inferred by Rupture Behaviors of the Largest Event at the Field

Abstract

There are geothermal systems in the steep slopes around the Shishimuta subsiding zone, which was originated of a caldera volcano. The Otake geothermal field, Japan, is located at the southern margin of the Shishimuta subsiding zone. The geological structure in the Otake geothermal field is characterized by the presence of NW-SE trending fault systems. In August 2025 an earthquake with a magnitude of 4.9 (USGS scale) occurred at the Otake geothermal field. It is the largest event at the field. We made waveform cross correlation and high precision relative relocation of seismicity triggered by the largest event using publicly available seismic data. In Japan, we can effectively use the nation-wide seismic network, seismic station of which are basically deployed in the 20-km mesh. Generally, it is too sparse to analyze fracture structure in geothermal fields. A local volcanic network, however, is working near the Otake field. Therefore, we can make use of the digital waveform data recorded at the seismic station which situated in a few kilometers from the Otake field and study the fracture structure of the field. An empirical Greens function analysis was applied to the largest event of the seismicity, and revealed particular characteristics of the fracture structure in the deeper part of the geothermal reservoir. We developed a kinematic source model for the largest event, The source model as well as accurate hypocenter relocations was effective to image the deep fracture structure of the geothermal reservoir.