HR: 09:45h
AN: T41F-08    [Abstracts]
TI: Low-Frequency Earthquakes Associated With Nonvolcanic Tremor in Southwest Japan
AU: * Shelly, D R
EM: dshelly@pangea.stanford.edu
AF: Stanford University, Department of Geophysics, 397 Panama Mall, Stanford, CA 94305 United States
AU: Beroza, G C
EM: beroza@pangea.stanford.edu
AF: Stanford University, Department of Geophysics, 397 Panama Mall, Stanford, CA 94305 United States
AU: Ide, S
EM: ide@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Department of Earth and Planetary Science, 7-3-1, Hongo, Bunkyo, Tokyo, 113-0033 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
AB: We relocate low-frequency earthquakes that occur as part of nonvolcanic tremor sequences in the western Shikoku region of the Nankai trough subduction zone in southwest Japan. We use a combination of double-difference tomography and waveform cross-correlation to determine precise hypocenters and high-resolution velocity structure in this area. In total, we relocate 6700 events, including nearly 1200 low frequency events, using waveforms and arrival time picks from the Japan Meteorological Agency catalog. This catalog includes data from the NIED Hi-net high-sensitivity borehole network. The dataset used in this study spans the time period from June 2002 to June 2005 and covers the western 2/3 of Shikoku. Despite low signal to noise ratios and sometimes-indistinct arrivals for recordings of the low-frequency events, their waveform similarity is high enough that they can be successfully relocated. Starting with only a rough estimate of the P- or S-wave arrival time, we are able to obtain very precise differential travel time measurements by cross-correlating similar waveforms from two events at a given station. Among the low-frequency earthquakes, S-wave correlations are particularly successful, due to the greater amplitude, and thus greater signal to noise ratio, of the S-wave recordings compared with those of P-waves. We are also able to obtain successful P-wave correlations between some low-frequency events and normal earthquakes, thereby helping to locate the low-frequency earthquakes relative to the normal Wadati-Benioff zone seismicity. The larger S-wave amplitudes and the ability to correlate the low frequency events with normal seismicity, suggests that they represent shear failure. We find that many of these events locate along a distinct lineation at 35-40 km depth when viewed in cross-section. This lineation is 5-8 km above, and approximately parallel to, the seismicity within the subducting Philippine Sea slab. In places, the lineation extends for roughly 20 km in the dip direction of the slab. In the immediate vicinity of the low-frequency events, we find a zone of high Vp/Vs (1.9-1.95), likely indicating high pore-fluid pressures in this region, which is consistent with the hypothesis that these low frequency events, and the tremor they occur within, are enabled by the release of fluids in the subduction zone. We believe these low-frequency earthquakes are probably associated with slip on the plate interface, an idea consistent with plate location models [Baba et al., 2002] as well with models of fault slip associated with the tremor sequences [Obara et al., 2004].
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8123 Dynamics: seismotectonics
DE: 8180 Tomography (6982, 7270)
SC: Tectonophysics [T]
MN: Fall Meeting 2005