Abstract
Abstract Anomalous seismic events were observed after the occurrence of the foreshock (Mw=7.2) and the main shock (Mw=7.5) of the 2004 off the Kii peninsula earthquakes. These anomalous events are characterized by very low-frequency energy of around 10 seconds with almost no higher-frequency energy and are considered the same as the very low-frequency (VLF) earthquakes discovered by Ishihara (2003) in some places along the Nankai trough, southwest Japan. The VLF seismic activity is mainly coincident with the aftershock area of the 2004 off the Kii peninsula earthquakes; however a minor activity was also excited in the southern Kii channel area. The VLF seismograms sometimes include higher-frequency wave trains with amplitudes much smaller than that of regular aftershocks. This indicates that VLF earthquakes have different source properties from the regular earthquakes. The centroid moment tensor analysis for one of the larger events suggests that the source depth is very shallow and the focal mechanism is the reverse faulting. These features suggest that the event occurs on the well-developed reverse fault system in the large accretionary prism near the Nankai trough. The swarm activity of VLF earthquakes might be considered as a chain-like occurrence of slips on the reverse fault system and thus the signature of a dynamic deformation process in the accretionary prism.
Highlights
On the northwestern margin of the Philippine Sea, a young oceanic plate subducts beneath southwest Japan from the Nankai trough and mega-thrust earthquakes occur at intervals of about 100 years
The stress drop is expected to be very low for slip on a reverse fault in the accretionary prism because of the enriched existence fluids and unconsolidated materials on the fault plane surrounded by sedimentary material. Such very low-frequency (VLF) earthquakes have only been detected in southwest Japan in the region of Philippine Sea plate subduction (Ishihara, 2003; Obara and Ito, 2004) and there have been no reports in the northern Japan
A sequence of VLF earthquakes which are characterized by a frequency content of about 10 seconds occurred just after the 2004 off the Kii peninsula earthquakes
Summary
On the northwestern margin of the Philippine Sea, a young oceanic plate subducts beneath southwest Japan from the Nankai trough and mega-thrust earthquakes occur at intervals of about 100 years. Many slow earthquakes have been detected by the GPS and the broadband seismograph data monitoring system in some regions along the Nankai trough (Kawasaki, 2004) These are distributed in a stableunstable transition zone on the plate boundary separately with rupture areas of huge mega-thrust earthquakes. Ishihara (2003) has discovered an anomalous type of seismic event near the trench axis along the Nankai trough by using the continuous broadband seismograph data of the National Research Institute for Earth Science and Disaster Prevention (NIED) F-net (Okada et al, 2004). This anomalous event has only very low-frequency (VLF) energy of. ITO: VERY LOW-FREQUENCY EARTHQUAKES EXCITED BY OFF THE KII PENINSULA EARTHQUAKES
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