Spatio-temporal characteristics and focal mechanisms of deep low-frequency earthquakes beneath the Zao volcano, northeastern Japan

Abstract

Deep low-frequency earthquakes (DLFs) beneath volcanoes are possible evidence for deep-seated magmatic activity in the mid-to-lower crust and uppermost mantle. After the 2011 Tohoku-oki earthquake (Mw 9.0), the number of DLFs beneath the Zao volcano in northeastern Japan began to increase. The hypocenters of the DLFs formed two clusters at shallower (20–28 km) and deeper (28–38 km) depths located near the side and lower parts of the high Vp/Vs zone, respectively. To understand the distribution and behavior of deep volcanic fluids, we examined the spatio-temporal characteristics and focal mechanisms of the DLFs beneath the Zao volcano. Systematic event detection and hypocenter determination based on waveform correlations were performed, which indicated that the activity of the deeper cluster intensified prior to the activation of DLFs in the shallower cluster. In addition, the focal mechanisms based on the averaged S/P wave power spectral ratios of the DLFs grouped by correlation were estimated. We found that the focal mechanisms of all DLF groups had a dominant isotropic component and that the maximum principal axis direction differed slightly from the regional stress field. The source process suggests a chain of tensile-shear cracks with different orientations, which were affected by the local stress field. Our results indicate that deep magma at different depths interacted on the time scale of the magmatic processes other than pressure diffusion.