What Controls Depth of Hypocentres In Kakkonda Geothermal Field, Japan

Abstract

Hypocentres for more than 6,000 microearthquakes which occurred in 1988 at Kakkonda geothermal field are located. Errors calculated by a computer program for determining hypocentre are typically 50 m horizontally but more than 100 m vertically if hypocentres are located about 1 km below sea-level. We have to take the errors into account even if the hypocentre solution is represented as a point in a hypocentre distribution map.

The number of microearthquakes decreases rapidly 2 km below sea-level. A conceptual model with probability density and seismic energy distribution is presented to indicate active seismic regions. High seismic energy density region is present in the geothermal field and is concordaut with highly fractured region suggested by geological and petrological studies based on well data. The model also shows that a contour map of bottomboundary of high seismic energy region is accord to that of top level of neo-granitic pluton body, inferring that occurrence of microearthquakes in Kakkonda geothermal field is controlled by the neo-granitic rocks in depth.