Revised hypocenter solutions for microearthquakes in the kakkonda geothermal field, japan

Abstract

The hypocenters of microearthquakes in the Kakkonda geothermal field have been relocated along the Kakkonda River using a new velocity structure model. Compared to the solution used by the previous velocity model, the depth of the hypocenters is shallower in the relocation. The microearthquakes occurred in a highly fractured region, as suggested by geological and petrological studies based on well data, but did not occur along major tectonic folds and faults. An alternative hydraulic condition might be necessary to trigger the earthquakes. Seismicity in 1995 was lower than in 1988 in Kakkonda. The decrease in the number of events is possibly due to the decrease in the amount of reinjection fluid or the change in the characteristics of the geothermal reservoir. The number of microearthquakes decreases rapidly at 1–2 km below sea-level. Probability density of seismic energy distribution is utilized to indicate the active seismic regions. The model also shows that a contour map of the lower boundary of the high seismic energy region corresponds to the occurrence of cordierite, which was produced by heat from the neo-granitic pluton body, implying that the occurrence of microearthquakes in the Kakkonda geothermal field is controlled by the neo-granitic rocks at depth. The top of the granite can be imaged, using the probability density of seismic energy distribution.