| Summary: | Great earthquakes have repeatedly occurred on the plate interface in a few shallow-dipping subduction zones where the subducting and overriding plates are strongly locked. Silent earthquakes (or slow slip events) were recently discovered at the down-dip extension of the locked zone and interact with the earthquake cycle. Here, we show that locally observed converted SP arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern Mexico reveal that the ultra-slow velocity layer (USL) varies spatially (3 to 5 kilometers, with an S-wave velocity of ~2.0 to 2.7 kilometers per second). Most slow slip patches coincide with the presence of the USL, and they are bounded by the absence of the USL. The extent of the USL delineates the zone of transitional frictional behavior. © 2009 American Association for the Advancement of Science. 23 October 2008; accepted 20 February 2009. We thank S. Sacks, P. Silver, S. Shirey, M. Bostock, J.-P. Avouav, and V. Manea for helpful discussions; R. Engdahl provided the global relocated seismic catalog used in fig. S10. We thank CENS at UCLA for support and the entire MASE team for making the MASE data available. We thank H. Hinojosa-Prieto, E. Cabral-Cano, A. Arciniega, O. Diaz-Molina, and C. DeMets for sharing their work on episodic tremor and slip in Oaxaca. Incorporated Research Institutions for Seismology Data Management Center and Canadian National Data Centre provided the GEOSCOPE data and Yellowknife short-period data used in this analysis, respectively. Station UNM is operated and maintained by Servicio Sismológical Nacional. This work is partially supported by NSF grant EAR-0636012. This is Tectonic Observatory contribution 10015. T.-R.A.S. is supported by a Carnegie fellowship at the Department of Terrestrial Magnetism. Published - Song2009p2048Science.pdf Supplemental Material - SUPPSong2009p2048Science.pdf
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