Very-low-frequency earthquakes accompanied with episodic tremors and slips on the plate boundary in the transition zone from locked to aseismic interface

Anomalous very-low-frequency (VLF) signals in the frequency range 0.02--0.05Hz have been recorded by a dense broadband seismic network in southwestern Japan. One of signals is generated from shallow VLF earthquakes that occur in the accretionary wedge along the Nankai subduction zone. Another signal originates from the region of deep VLF earthquakes in the region of transition from a locked zone to an aseismic slip region. Here we provide a detailed seismicity of VLF earthquakes in the transition zone in the down-dip part of the Nankai subduction zone, including the calculation of their moment tensor solutions, for the period from April 2003 to May 2008. We also consider the processes responsible for the seismic moment release during VLF earthquakes in the transition zone on the plate boundary. Using continuously recorded seismograms for a five-year period obtained from a very dense broadband seismic network, we performed grid moment tensor inversion and centroid moment tensor inversion to calculate 242 moment tensor solutions with moment magnitudes ranging between 3.1 and 3.8. There were at least 5--10 sequences of repetitive activity during the five years. The VLF events formed clustered distributions along the 35-km isodepth contour of the subducting plate surface. The nodal planes, which dip landward, determined from the 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 corresponding to the slip vectors clearly reflected the configuration of the upper surface of the subducting plate. These observations suggested that VLF seismic signals are radiated from shear slips on the upper surface of the subducting Philippine Sea Plate. We also 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 release from detectable VLF sources was 0.1 percents of the rates for short-term slow-slip events, suggesting that the source areas occupied only 0.1 percents of the fault segment on which the short-term slow slip occurred.