Estimation of Stress State at Yanaizu-Nishiyama Geothermal Field Using Microseismic Multiplets

Abstract

A 65,000kW geothermal power plant is under operation at Yanaizu-Nishiyama, Fukushima, Japan since 1995. An earthquake with a magnitude of 4.9 occurred in November 2009, and considerable damage was brought to the power plant and residential areas around the site. The fault plane solution (FPS) of the felt earthquake and those of previously occurred felt earthquakes in this field were nearly identical. Meanwhile the seismic structure of the hypocenters of the microseismic multiplets showed variation in their orientation. We attempt to interpret parts of the physics behind the felt earthquakes in this field through estimating the stress state by integrating FPSs, multiplet seismic structure, and geological/rock mechanical information. We estimated the vertical stress from the density of the overburden and orientation of the horizontal components of the stress field from the FPS of the felt earthquakes. The magnitude of each component of the horizontal stress was inferred as to most of the seismically active fractures can have shear slip by the change in the pore pressure. Our result shows that the maximum and minimum horizontal stress is homogeneous in this field. We also find that events with larger magnitude tend to have lower critical pore pressure for shear slip, confirming what we have previously found for other EGS sites. This shows that there is a tendency for “slip-able” fractures to release large strain energy in the form of seismic events.