HR: 14:25h
AN: S43D-04    [Abstracts]
TI: High-Resolution Subduction Zone Seismicity and Velocity Structure in Ibaraki, Japan
AU: * Shelly, D R
EM: dshelly@pangea.stanford.edu
AF: Stanford University, Dept. of Geophysics 397 Panama Mall, Stanford, CA 94305-2215 United States
AU: Beroza, G C
EM: beroza@pangea.stanford.edu
AF: Stanford University, Dept. of Geophysics 397 Panama Mall, Stanford, CA 94305-2215 United States
AU: Zhang, H
EM: hjzhang@ice.geology.wisc.edu
AF: University of Wisconsin-Madison, Dept. of Geology and Geophysics 1215 W Dayton St., Madison, WI 53706 United States
AU: Thurber, C H
EM: clifft@geology.wisc.edu
AF: University of Wisconsin-Madison, Dept. of Geology and Geophysics 1215 W Dayton St., Madison, WI 53706 United States
AU: Ide, S
EM: ide@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Dept. of Earth and Planetary Science University of Tokyo 7-3-1, Tokyo, 113-0033 Japan
AB: We use double-difference tomography (tomoDD) [Zhang and Thurber, 2003] and waveform-derived cross-correlation differential arrival times to invert for the earthquake locations and P- and S-wave velocity distributions in the subduction zone under Ibaraki Prefecture of north-central Honshu, Japan. The Ibaraki region is attractive for its high rate of slab seismicity and for the presence of an intermediate-depth double seismic zone. We relocate ~8000 events occurring in this region between June 2002 and June 2004. We use a combination of ~200,000 absolute travel times, ~5 million catalog-derived differential times, and ~5 million cross-correlation differential times derived from more than 150,000 waveforms, with roughly equal numbers of P- and S-wave data. Many of the waveforms are from HiNet borehole stations that provide particularly high-quality data. We also use data from JMA, the University of Tokyo, and Tohoku University. Since it is natural to expect sharp velocity contrasts in a subduction zone, we regularize the inversion using the total variation (TV) approach implemented through iteratively reweighted least squares. Because TV is an L1-norm regularization, sharp changes in velocity are penalized no more than gradual ones, but undulations in the velocity model remain damped. We will compare the TV results with those determined by standard least-squares, L2-norm regularization. Our results show increasingly organized seismicity including narrowing by up to 50% of the upper and lower limbs of the double seismic zone as viewed in cross-section. We find a zone of interplate events extending as deep as 60 km, forming a very distinct lineation in cross-section. Focal mechanisms support the interpretation that these are low angle, subduction interface events. These earthquakes are accompanied by a zone of very high Vp/Vs ratio within the downgoing plate, just beneath the seismicity, suggesting that high pore-pressures may enable seismic slip on the subduction interface at depths where aseismic slip would otherwise predominate. These events represent significantly deeper seismic coupling than the 37-43 km maximum depth observed in this area previously by Tichelaar and Ruff [1993], but are consistent with the maximum depth of 50-70 km for low-angle thrust events found by Igarashi et al. [2001] farther to the north.
DE: 8180 Tomography
DE: 7230 Seismicity and seismotectonics
DE: 7209 Earthquake dynamics and mechanics
DE: 3260 Inverse theory
SC: Seismology [S]
MN: 2004 AGU Fall Meeting