A continuous water injection test was conducted to halt the reduction of steam production in the Okuaizu Geothermal Field, Japan. One of the possible effects associated with water flows is creation of tensile seismic events. Those events are likely triggered by inflow into or outflow from fractures in a rock body. In this study, the tensile seismic events were employed as a proxy of water flows. We conducted a focal mechanism inversion considering tensile angle in addition to strike, dip and rake angles, which are used in conventional inversions. Hypocenters of seismic events were determined by a cluster analysis (double-difference method) and the focal parameters were determined by a genetic algorithm. The result showed that seismic events with positive tensile angle, which inferred inflow into fractures, are mainly appeared around the injection point and gradually evolved toward a northwest part of the injection point. It suggests that the injected water may flow toward the northwest direction. Meanwhile, our past research pointed out that a high-permeable zone may exist between the injection point and the northwest part. The result of the focal mechanism inversion supports the existence of the fracture zone. We concluded that the focal mechanism analysis considering tensile angle is one of the powerful tools to estimate water flows in subsurface.

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