Shear-wave splitting observed above small earthquakes in a geothermal area of Japan

Abstract

The phenomenon of shear-wave splitting has been observed at two seismic stations with three-component seismographs above shallow microearthquakes which occurred at depths of 1–2 km in the Takinoue geothermal area of northern Honshu. An almost uniform alignment of the polarizations of the faster-split shear wave in the direction of ENE-WSW may result from the presence of non-randomly distributed liquid-filled microcracks. Two models of crack distribution reproduce the observed polarization pattern. In the first model vertical microcracks are aligned in the ENE-WSW direction, while the second supposes a distribution of microcracks striking in the ENE-WSW direction but dipping randomly. The ENE-WSW strike of the cracks in both models is consistent with the direction of the ENE-WSW maximum horizontal compressive stress inferred from the analysis of earthquake mechanisms in the study area. It differs slightly from the strike of vertical fractures (WNW-ESE) obtained by hydraulic fracturing experiments at sites about 10 km west of the study area. The tectonic stress regime obtained from an analysis of focal mechanisms of microearthquakes occurring in this area seems to be consistent with the latter model of the crack distribution from the shear-wave analysis, at least at depths less than about 700 m. An analysis of the delay times between the faster- and the slower-split shear waves has revealed a region with anomalously large delay times normalized to ray path lengths. A 3-D P-wave velocity inversion indicates that this region is a low P-wave velocity zone. The low P-wave velocities and high shear-wave birefringence is consistent with a relatively dense distribution of liquid-filled microcracks.