HR: 11:05h
AN: S42B-04    [Abstracts]
TI: Fluid Activity Around the Downward Extension of the Seismogenic Fault of the 2000 Western Tottori Earthquake Inferred From Deep Low-Frequency Earthquakes
AU: * Ohmi, S
EM: ohmi@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Hirose, I
EM: hirose@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Mori, J
EM: mori@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AB: Low-frequency tremors were newly detected in the forearc region of the Nankai and Cascadia subduction zones recently. They are associated with the subduction of the young plates and attributed to the fluid activity around the plate boundary. On the other hand, there is another example of low-frequency events in the backarc region in southwest Japan that is associated with active faults. One example is the western Tottori area, where we had a Mw=6.7 earthquake in 2000. It is an unusual example because the seismogenic fault is outlined by an intense aftershock activity, beneath which many deep low-frequency (DLF) earthquakes were observed. DLF earthquakes were observed at depths of around 30 km beneath the aftershock activity. A fault model derived from the coseismic crustal movements (Sagiya et al., 2002) indicates that the DLF earthquakes are located around the downward extension of the fault. The DLF events are classified into three groups in features of the waveform. Type-1 are the most commonly observed ones. One of them shows a single-force type source mechanism (Ohmi and Obara, 2002). Type-2 events have larger P-wave onsets compared to those of type-1 events. Magnitudes of the type-2 events are slightly larger than those of type-1 events. They have been observed since mid 2002. Assuming that type-2 events are caused by shear faulting, we estimated the seismic moment and source dimension from the source pulse. Relation between the source dimension and moment indicates that the stress drop of the type-2 events are extremely low compared to those of ordinary earthquakes. It suggests the existence of soft materials such as fluid saturated gauge zone at the fault interface. Type-3 event is a tremor-like event observed in April 2003. We examined the tilt data in the region if the associated slip of the fault is observed. However, it was difficult to detect the tilt change more than 1.0 \times $10^{-7}$ radian, which is apparently equal to 1.3 cm slip on the fault model of Sagiya et al. (2002). As we described, observed features suggest the fluid activity in the focal region of the DLF events and is also supported by the seismic tomography analysis (e.g. Zhao et al., 2004). It shows the existence of low velocity bodies in the focal region of the DLF events, that reflects the fluid related to the dehydration process of the subducting Philippine Sea plate. Recent studies (e.g. Iio and Kobayashi, 2002) proposed that the seismogenic faults have downward extension in the lower crust, whose aseismic slip accumulate stress on the seismogenic faults in the upper crust and controls the occurrence of the earthquake. Hypocenters of the DLF earthquakes discussed in this paper are distributed around the deeper extension of the shallow aftershock distribution and probably located on the downward extension of the seismogenic fault of the Western Tottori earthquake. It is important to understand the nature of DLF events beneath active faults, in relation to the behavior of fluids in the lower crust that might affect the aseismic slip of the downward extension of the seismogenic faults and control the occurrence of the shallow crustal earthquakes.
DE: 7230 Seismicity and seismotectonics
DE: 8045 Role of fluids
SC: Seismology [S]
MN: 2004 AGU Fall Meeting