Mapping the subducting Pacific slab beneath southwest Japan with Hi-net receiver functions

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Abstract We have used 4th root receiver function stacks, and pre-stack receiver function depth migrations to study the transition zone discontinuity structure beneath southwestern Japan. Receiver functions were calculated from the quiet short-period seismograms recorded by a recently deployed borehole network, Hi-net. We found that a relatively broad frequency band can be retrieved from a short-period seismogram by a deconvolution of the instrument response. The quality of the receiver functions formed from large earthquake recordings is comparable to those from broadband instruments. We applied common-conversion-point gathering to the receiver-function data to image the P to S conversion events beneath the network by stacking with a 4th root technique to improve lateral coherence. We found that the topographic anomalies of the 410- and 660-km discontinuities beneath southwest Japan have very different length scales. The former is characterized by a narrow, ∼150–200 km wide, topographic high, while the latter exhibits a broad, > 400 km wide, moderate topographic low together with a small-scale, larger-amplitude depression. A 2.5D pre-stack depth migration of the receiver functions shows the transition zone features clearly, as well as images of a change of slope in the subducting slab at the 410 discontinuity and flattening of the slab onto the 660-km. These observations show that the subducted Pacific slab is deflected when it encounters the upper and lower boundaries of the transition zone, and is flat lying either above or across the 660-km discontinuity. The flat lying slab is, however, restricted to the bottom of the transition zone, and probably experiences much less thickening than is suggested by some global tomographic images in which subhorizontal high velocity anomalies are seen throughout the transition zone between the two discontinuities.