[S14-2] Upper mantle and transition zone dynamics and structure II

[S14-2-02] Transition-zone imaging below Japan with ScS reverberations

^○Elmer Ruigrok^{1, 2}, Kiwamu Nishida³, Katsuhiko Shiomi⁴ (1.Royal Netherlands Meteorological Institute, De Bilt, Netherlands, 2.Utrecht University, Utrecht, Netherlands, 3.Earthquake Research Institute at the University of Tokyo, Tokyo, Japan, 4.National Research Institute for Earth Science and Disaster Resilience, Tsukuba-City, Japan)

The first-order structure of the mantle transition zone (MTZ) is well known below Japan. The 410 and 660 discontinuities are clearly seen on receiver functions. The 660 shows a marked depression where the Pacific Slab is thought to interact with this boundary. The more precise shape of the MTZ, however, is unknown. Possible smaller discontinuities like the 520, are hard to interpret with clarity. Moreover, receiver functions show clear interfaces below some parts of Japan, but are hard to interpret at other parts, possibly due to multiples from the slab. To obtain a consistent image below the entire Island arc we apply seismic interferometry together with reflection imaging. Large magnitude earthquakes nearby Japan generate a clear ScS phase that is observed over the complete Hi-net array, operated by the NIED. Also, ScS reverberations of the MTZ can directly be identified on the high-sensitivity accelerometers (tiltmeters). We apply seismic interferometry to a multitude of these ScS arrivals and reverberations. Doing so, the responses are retrieved as if there were shear-wave sources at all Hi-net tiltmeter stations and zero-offset (co-located source and receiver) reflection responses were measured. We further migrate the reflection responses to a reflectivity image. This results in a pseudo-3D image of the MTZ below Japan. The main features are the topography of the 410 and 660 discontinuities, but also more subtle impedance contrasts can be followed. We discuss how our image was obtained and show an interpretation of the thus-far identified structures.