Shallow Very-Low-Frequency Earthquakes off the Kii Peninsula Observed by Broadband Ocean Bottom Seismographs

Anomalous very-low-frequency (VLF) earthquakes have been observed near the trough axis along the Nankai trough by land seismic networks (Ishihara, 2003, Obara and Ito, 2005). The focal mechanisms of the VLF earthquakes indicate an activity of thrust faults in the accretionary prism (Ito and Obara, 2006). After the 2004 Off Kii Peninsula earthquake (Mw=7.5), which occurred in the subducting Philippine Sea plate, many VLF earthquakes were observed in the aftershock area by the land seismic network (Obara and Ito, 2005). In addition to this, low-frequency tremors with dominant frequency range of 2-8Hz were observed by short-period ocean bottom seismographs (OBS) deployed off the Kii peninsula along the Nankai trough before the 2004 earthquake (Obana and Kodaira, 2009). We have conducted broadband ocean bottom seismographs (BBOBS) experiments off the Kii peninsula from July 2008 until October 2009. Aims of this BBOBS experiment are to obtain broadband records in source region and investigate source locations and focal mechanisms of VLF earthquakes. In March 2009, the VLF earthquake activity near the Nankai trough off the Kii peninsula was detected by land-based seismic network (NIED, 2009). Three BBOBSs observed these events in the source region and provided near-field broadband seismograms. The BBOBS records indicate that the VLF earthquakes contain the energy not only in the lower frequency range (~0.1 Hz) as observed by the land network but also in higher frequency range (1-10Hz). However, the VLF earthquakes do not contain the energy in the frequency range above 10 Hz. Source locations of the VLF earthquakes were estimated by using the seismograms in the higher frequency range. Onset times of the VLF signals were picked manually from the 1-8 Hz band-pass filtered seismograms. In addition to the onsets of the signals, relative arrival times were estimated from cross-correlation of the envelope seismograms. The VLF earthquakes were mainly located in the shallow accretionary prism near the trough. The locations of the VLF earthquakes suggest that the VLF earthquakes are related to activity of reverse faults in the shallow accretionary prism near the trough.