Very low frequency earthquakes related to small asperities on the plate boundary interface at the locked to aseismic transition

Very low frequency (VLF) seismic signals observed in southwestern Japan are evidently radiated from shear slips on the upper surface of the subducting Philippine Sea Plate. We used grid moment tensor inversion and centroid moment tensor inversion to calculate 242 moment tensor solutions with moment magnitudes between 3.1 and 3.8 from continuous seismograms recorded over a 5 year period by using a very dense broadband seismic network. At least 5-10 sequences of repetitive activity were observed during the 5 years. The VLF events formed clustered distributions along the 35 km isodepth contour of the subducting plate surface. The nodal planes (which dip landward) of moment tensor solutions of the VLF events reflected the configuration of the subducting plate interface. The slip vectors were consistent with the direction of movement of the subducting plate; the dip and strike of the slip vectors clearly reflected the configuration of the upper surface of the subducting plate. We found that the rates of seismic moment release per unit area associated with five major VLF clusters were very similar, although both the seismic magnitudes and sizes of the clusters varied considerably. The rate of seismic moment released from detectable VLF sources was 0.1% of the rates of short-term slow slip events, suggesting that the source areas occupied only 0.1% of the fault segment on which the short-term slow slip events occurred.