Focal mechanisms of deep low-frequency earthquakes in Eastern Shimane in Western Japan

Deep low-frequency earthquakes (LFEs) are small earthquakes (M < 2) that occur at depths of similar to 10-45 km with seismic wave radiation mainly in the frequency range of 2-8 Hz. LFEs in western Japan are categorized into tectonic LFEs in the Nankai subduction zone, volcanic LFEs beneath active volcanoes, and semi-volcanic LFEs that occur far from active volcanoes but are otherwise similar to volcanic LFEs. While tectonic LFEs are considered to reflect shear slip on a plate boundary, the mechanisms of volcanic and semi-volcanic LFEs remain unclear. We have determined the mechanisms of 38 semi-volcanic LFEs in eastern Shimane, which is the site of the most frequent semi-volcanic LFE activity in Japan. For each event, velocity seismograms at five stations were inverted into a focal mechanism and moment rate function by a combined grid search and linear inversion analysis. Synthetic waveforms were calculated for a one-dimensional structure and the local site amplification effects were corrected using body waves from deep earthquakes. The estimated moment rate function was found to often oscillate between positive and negative values, which are unlike those of regular earthquakes. The focal mechanisms for many LFEs are dominated by a compensated linear vector dipole with symmetry axes parallel to the lineation formed by the hypocenter distribution and the direction of the minimum principal axis of regional stress.