Waveform modeling of the slab beneath Japan
The tomographic P wave model for the Japan subduction zone derived by Zhao et al. (1994) has two very striking features: a slab about 90 km thick with P wave velocities 3–6% higher than the surrounding mantle and a mantle wedge with −6% low-velocity anomalies. We study three-component seismograms from more than 600 Hi-net stations produced by two earthquakes which occurred in the downgoing Pacific Plate at depths greater than 400 km. We simulate body wave propagation in the three-dimensional (3-D) P wave model using 2-D finite difference (FDM) and 3-D spectral element (SEM) methods. As measured by cross correlation between synthetics and data, the P wave model typically explains about half of the traveltime anomaly and some of the waveform complexity but fails to predict the extended SH wave train. In this study we take advantage of the densely distributed Hi-net stations and use 2-D FDM modeling to simulate the P-SV and SH waveforms. Our 2-D model suggests that a thin, elongated low-velocity zone exists atop the slab, extending down to a depth of 300 km with an S wave velocity reduction of 14% if a thickness of 20 km is assumed. Further, 3-D SEM simulations confirm that this model explains a strong secondary arrival which cannot easily be imaged with standard tomographic techniques. The low-velocity layer could explain the relatively weak coupling associated with most subduction zones at shallow depths (<50 km), generally involving abundant volcanic activity and silent earthquakes, and it may also help to further our understanding of the water-related phase transition of ultramafic rocks, and the nature of seismicity at intermediate depths (~70–300 km).
Additional Information© 2007 American Geophysical Union. Received 16 March 2006; revised 28 August 2006; accepted 10 October 2006; published 20 February 2007. We thank the Hi-net Data Center, especially Mizuho Ishida and Masako Sakanashi, for their help in providing the waveform data. We also thank John Eiler for constructive discussions and helpful comments on mineral physics. We also thank two reviewers and the Associate Editor Jeroen Ritsema for help in improving the manuscript. This research was supported by the National Science Foundation under grant EAR-0309576. This is contribution 9137 of the Division of Geological and Planetary Sciences (GPS), California Institute of Technology. This is contribution 36 of Caltech's Tectonic Observatory. The numerical simulations for this research were performed on the GPS Dell cluster.
Published - ChenJGR2007.pdf