Microearthquake activity has been reported to be one of the potential indicators of fractured structure in many geothermal fields. Typically, microearthquake activity is observed by the local seismic network which was deployed at the geothermal field. In Japan, we can effectively use the nation-wide seismic network, seismic station of which are basically deployed in the 20-km mesh, for such analysis. Hypocenters of earthquakes with magnitude greater than 0.5 are routinely determined to the third decimal place in degree both in latitude and in longitude. The resolution of the hypocentral location is too low to delineate fracture structure. We, however, can relocate microearthquake hypocenters accurately enough to delineate fracture structure. All the digital data from the nation-wide networks have been available for all researchers since June 2002. Generally, microearthquake activity in geothermal fields are characterized by many low magnitude events. Most microearthquakes are analyzed to be point sources. Only a few larger events have the detectable source faults. The nation-wide seismic network provides us the opportunity for re-analyzing the long-term accumulated data. In September 2022 a microearthquake with a magnitude of 3.5 occurred at the Kakkonda geothermal field, in Japan. It is the largest event at the field. We made waveform cross correlation and high precision relative relocation of both background seismicity and seismicity triggered by the largest event. We also developed a kinematic source model for the largest event, The source model as well as accurate hypocenter relocations was effective to image the fracture structure of the geothermal reservoir.

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