HR: 1340h
AN: S33A-0293    [Abstracts]
TI: The frequency structure and characteristics of the deep low frequency tremor in Western Shikoku region, Southwest Japan
AU: * Nakamula, S
EM: show@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Takeo, M
EM: takeo@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AB: The continuous seismic tremor called the deep low-frequency tremor (LFT) was found very recently in the Southwest Japan fore-arc, along the the subducting Philippine Sea Plate (Obara, 2002). The source depths were about 30 - 40km, at the lower part of the crust. The same kinds of tremor events are also found in the Cascadia region, in which the subducting plate is very young, as well as the Southwest Japan (e.g. Rogers and Dragert, 2003). In the Cascadia region, tremor events are accompanied with deep slow slip, which is detected by the observation of GPS. So in this region, the tremor events are called as episodic tremor and slip (ETS). The small amplitude of the LFT makes the analysis difficult and onsets of the tremor can hardly be detected. Previous studies about the LFT mainly dealt with the temporal and spatial distribution of LFT events, but this study attends to the properties of the individual event. The causes of the LFT and the slow slip which is associated with the LFT activities are thought to be related with fluids from the subducting plate (Obara, 2002). However, these properties discussed only from the qualitative side and the physical mechanism of the tremor associated with fluids is not clear. Extremely small amplitudes of the tremor make the onsets detection very difficult and even the hypocenter locations become inaccurate. Due to the low signal/noise ratio of the LFT, it is difficult to obtain useful information by applying the method for normal earthquakes. Since little information is available about the physical process of LFT, we should not suppose a priori assumption before analysis as long as possible. In this sense, we use the theory of KM_2O-Langevin equations developed by Okabe et al.. This theory is based on the fluctuation-dissipation theorem which is one of the principles of the statistical physics. The advantage of using this theory is that we require no prior information about the data and do not have to define any parametric models like AR-model or ARMA-model before analysis. The matrix functions which characterize the time series are extracted directly from data itself, checking the stationary property of the data. We developed the automatic phase detecting and picking algorithm using this theory and built it in the method of automatic hypocenter determination of the LFT. We applied this to the LFT in the Shikoku region, Japan, and obtained the precise location of the tremor events with lower signal/noise ratio data than before. The obtained hypocenter distribution was in the valid range within about 30 to 40km depth. We also obtained the discrete spectrum in complex frequency space which characterize the LFT signal by the method of ``Average Dissipation Spectrum (ADS)'' based on the thory of KM_2O-Langevin equations. Compared to the Sompi method which is also the method to obtain the characteristic frequencies and decay rates(e.g. Kumazawa et al., 1990), our method can get more precise frequencies when we deal with the highly noised data. Several peaks existed from 1.3Hz, 1.75Hz, 2.3Hz, 2.8Hz, 3.2Hz, 3.8Hz, 4.3Hz in the dominant frequency range 1-5Hz of the LFT, at intervals of about 0.5Hz. The decay rate of the LFT is about 0.20 for the data of 2003 and 0.15 to 0.18 for the data of 2002. This characteristic frequency structure will be the help to understand the physical properties of the LFT events.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7280 Volcano seismology (8419)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Seismology [S]
MN: Fall Meeting 2005