The Mechanics of Subduction in Japan from High-Precision Earthquake Location and Tomography

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THE MECHANICS OF SUBDUCTION IN JAPAN FROM HIGH-PRECISION EARTHQUAKE LOCATION AND TOMOGRAPHYP.I. Gregory C. BerozaSubduction zone plate boundaries are host the largest known earthquakes. They are also the sites of damaging upper crustal and intermediate-depth earthquakes that in themselves pose substantial seismic hazard. Under this grant we are developing an improved understanding of the mechanics of shallow and intermediate-depth earthquakes in the subduction zone in Japan. Japan is the ideal place for our study for several reasons. Subduction zones in Japan exhibit a diversity of styles including: rapid subduction of old oceanic crust in Hokkaido and North Honshu, nearly flat subduction of much younger Philippine Sea plate in South Honshu and Shikoku, and much steeper subduction of relatively young oceanic crust under the southern island of Kyushu. Japan is highly seismic. Since 1994 there have been over half a million earthquakes recorded by seismic networks in Japan that appear in the JMA catalog. What really sets the subduction zones in Japan apart from those elsewhere on Earth is the level of seismic instrumentation. With over 1500 high-gain short period instruments available for earthquake monitoring, the subduction zones in Japan are by far the best instrumented in the world. Moreover, the new Hi-Net seismic network, which now numbers nearly 700 3-component borehole stations, offers a unique capability for earthquake monitoring found nowhere else. In our research we are applying and improving recently developed high-precision, multiple-event earthquake relocation and high-precision tomography techniques to earthquakes in Japan. These methods have revealed remarkable details of the faulting process in other seismically active regions of the world that have lead to new insights into how earthquakes work and their application to Japan holds similar promise.Our research has two main goals. First are trying to understand the geometry and mechanics of the plate-boundary interface, where the very largest earthquakes occur, and the factors that limit the depth extent of their occurrence. The combination of precise earthquake locations and how they are situated relative to the velocity structure at depth is the key to this analysis. Second, we are trying to relate precisely located earthquakes to the detailed seismic velocity structure of the downgoing plate. We are particularly interested in constraining the possible role that dehydration reactions due to metamorphism in the downgoing plate play in enabling intermediate-depth earthquakes.