Structures and fluid flows inferred from the microseismic events around a low-resistivity anomaly in the Kakkonda geothermal field, Northeast Japan

Abstract

A low-resistivity anomaly around granitic intrusive rocks, which could represent a possible supercritical geothermal system, was identified in the Kakkonda geothermal field in Northeast (NE) Japan. We aimed to understand the nature of the possible supercritical geothermal system from the perspective of seismicity in this study. We employed a microseismic monitoring network in this field in 2017 to reveal seismic structures in and around the low-resistivity anomaly. More than 10,000 seismic events were detected until May 2019; we determined precise hypocenter locations using a double-difference algorithm. The main feature that we revealed for the low-resistivity anomaly was the lack of seismic events within its core, which is indicative of ductile conditions. The corresponding temperature is > 370 °C for quartz-dominant rocks in the tectonically active region. Conversely, seismic events were located intensely along the periphery of the low-resistivity anomaly, suggesting the possibility of lateral fluid flow along margins of the low-resistive anomaly. Some of those seismic events had the non-double couple components often associated with a compressive fracturing, which were likely derived from fracture compaction. The lateral fluid flow could be partially derived from the water squeezed by compaction. Additionally, the seismic wave tomography method indicated a region featuring a low ratio of P- and S-wave velocities (VP/VS) in the upper part of the low-resistivity anomaly, indicating a conventional hot water dominant reservoir, although the core of the anomaly was not imaged by the tomography owing to a lack of ray-paths. Pre-stack depth migration imaging using P-coda waves suggested the presence of some reflectors between the upper part and the core of the low-resistivity anomaly, which is possibly a boundary between them. As a result, we concluded that the core of the low resistivity anomaly satisfied the supercritical condition for water, and the structure of the core differed from the structures of the top and periphery of the low-resistivity anomaly. A supercritical geothermal system could be formed in the core of the low-resistivity anomaly.