HR: 11:20h
AN: T12C-05 INVITED    [Abstracts]
TI: Occurrence Of Deep Low-frequency Tremors Synchronized To Earth Tides
AU: * Nakata, R
EM: nakata@geol.sci.hiroshima-u.ac.jp
AF: Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi- Hiroshima, 739-8526, Japan
AU: Suda, N
EM: nsuda@hiroshima-u.ac.jp
AF: Graduate School of Science, Hiroshima University, Kagamiyama 1-3-1, Higashi- Hiroshima, 739-8526, Japan
AU: Tsuruoka, H
EM: tsuru@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-Ku, Tokyo, 113- 0032, Japan
AB: Swarms of deep low-frequency tremors occur with slow slip events along the subduction zone of the Philippine Sea plate in southwest Japan. These episodic events are considered to be linked in a stress relaxation process in the transition zone of the subducting plate interface. Tremor swarms often exhibit occurrence with a period of about 12 or 24 hours. Although it is easy to surmise that such periodicity arises from effects of the Earth tides caused by gravitational perturbations from the Moon and Sun, it has not yet been given a quantitative explanation. Here we show that the observed periodic tremor occurrence can be reproduced by seismicity rate change calculated from the periodic stress due to the Earth tides combined with the transient background stress due to a slow slip event. The result shows that although tremor source regions are always being perturbed by the periodic tidal stress, tremor swarms are not triggered until the additional transient stress is imposed. The seismicity rate theory used here is based on the rate- and state-dependent friction law. The value of Aσ, the fault constitutive parameter times the effective normal stress, is on the order of 1 kPa. This is an order of magnitude smaller than that for ordinary earthquakes, indicating a quick and sensitive response of tremor occurrence to stress changes. Observing deep low-frequency tremors is therefore effective for monitoring the stress relaxation process in the transition zone.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8168 Stresses: general
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting