Dynamic deformation of the accretionary prism excites very-low-frequency earthquakes

The Nankai Trough, southern Japan, is the deformation front, where the Philippine Sea Plate begins to subducts. A recent seismic profile revealed that numerous reverse-fault systems with landward-dipping faults are well developed within the accretionary prism landward of the trough axis. No seismic activity has been previously documented within the accretionary prism, although the development of reverse faults indicates shortening of the prism due to the plate motion. The anomalous very-low-frequency (VLF) earthquakes are detected along the Nankai Trough. These earthquakes are not described in any earthquake catalog because they have no clear P- or S-wave contents, and their wave trains are predominated by a surface wave content. We used the National Research Institute for Earth Science and Disaster Prevention's (NIED) broadband seismic network (F-net) and high sensitive accelerometers network (Hi-net TILT) to detect and analyze VLF earthquakes. Appling centroid moment tensor inversion method to waveforms of very-low-frequency earthquakes, we found that hypocenters were distributed at shallow depths landward of the trough axis and above the subducting Philippine Sea plate, and that the focal mechanisms of VLF earthquakes indicated reverse faulting. Neither nodal plane was similar to the orientation of the plate boundary. Assuming that nodal planes dipping landward were the source faults of VLF earthquakes, we compared the orientation of the fault strikes to seafloor surface features. The strike of the nodal planes was generally subparallel to the trough axis, and varied with the strike of seafloor topography. These results indicate that VLF earthquakes occurred within the accretionary prism. Most of the VLF earthquakes occurred within the sedimentary wedge of the accretionary prism, which has a P-wave velocity of 2-4 km/sec, while few occurred within older accretionary sediments that have a P-wave velocity of more than 5 km/sec. The fault planes of VLF earthquakes were steeper in more landward area. These changes in the dip of fault planes may be attributed to increasing friction coefficient down-dip along the decollement. On seismic reflection profiles, the thrust faults within the accretionary prism generate a reverse polarity reflection that indicates elevated fluid pressures within the fault zone. The generation of VLF earthquakes may be related to fluid within the fault zone and the low seismic velocity of the accretionary prism. High fluid pressure may weaken the fault via a reduction in the normal stress on the fault plane. Therefore, stick-slip behavior may produce low stress-drop earthquakes in zone of low seismic velocity.