Very low frequency earthquakes excited by the 2004 off the Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism

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.

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“…Land-based geodetic studies suggest that the plate boundary thrust here is strongly locked (Miyazaki and Heki, 2001). Similarly, the relatively low level of microseismicity near the updip limits of the 1940s earthquakes (Obana et al, 2001) implies significant interseismic strain accumulation on the megathrust; however, recent observations of very low frequency earthquake event swarms apparently taking place within the accretionary prism in the drilling area (Obara and Ito, 2005) demonstrate that interseismic strain is not confined to slow elastic strain accumulation.…”

Seismic reflection and bathymetric evidences for the Nankai earthquake rupture across a stable segment-boundary

“…The activity of VLF earthquakes is considered to be the result of slips on the reverse fault system and, thus, the signature of a dynamic deformation process in the accretionary prism. The VLF earthquakes occurred around the splay fault in segment B in 2005 (Obara and Ito, 2005), as shown in Figs. 1 and 5.…”

“…The episodic activity of the low-frequency tremors suggests that the splay faults are conditionally stable and thus can become unstable under suf ciently strong dynamic loading. It is also well known that very low-frequency (VLF) earthquakes occur on the well-developed reverse fault system in the accretionary prism along the Nankai Trough (e.g., Obara and Ito, 2005). The activity of VLF earthquakes is considered to be the result of slips on the reverse fault system and, thus, the signature of a dynamic deformation process in the accretionary prism.…”

“…At the Nankai Trough margin, the Philippine Sea plate (PSP) is being subducted beneath the Eurasian Plate to the northwest at a convergence rate of ∼4 cm/yr (Seno et al, 1993). The Shikoku Basin, the northern part of the PSP, is estimated to have opened between 25 and 15 Ma by backarc spreading of the Izu-Bonin arc (Okino et al, 1994).…”

“…However, it has been controversial whether, or not, there has been an earthquake rupture Inset: Regional map showing the location of the study area (red box). The Nankai subduction zone may be divided into five discrete segments (A through E) (Ando, 1975 (Obara and Ito, 2005). Green circles are cold seeps (Ashi et al, 2009).…”