Imaging and 2.5D Modeling of Receiver Functions from Deep Virtual Receivers in Kyushu Island, Japan

Abstract:

Receiver function analysis is very useful tool to image the seismic velocity structures. We apply it to image seismic structure beneath Kyushu island, Japan. Kyushu region is characterized by active subduction of the Philippine Sea plate (PHS) beneath the Eurasian plate and many active volcanoes. In this study, we use teleseismic records from Hi-net seismic stations (borehole stations) and F-net stations (near-surface stations) in Kyushu, which are supplied by the NIED. Because most of these seismic stations are located at the top or in the sedimentary layer, the records may include strong effect of reverberation within the sedimentary layer, and this effect makes the image of the structure unclear. To overcome this problem, we exploit the modified S-wavevector receiver functions (SWV-RFs). The SWV-RFs are derived by deconvolving the upgoing S-wave component with the upgoing P-wave component of the records. Although it was originally proposed for surface records by Reading et al. (2003, GRL), we use Takenaka and Murakoshi's (2010) method in which we virtually move the seismic sensor at the surface or in the borehole down to the top of the basement layer and calculate the SWV-RFs at that location for suppressing the sedimentary layer effect. This method needs the structure model from the surface to the virtual sensor location. Here we apply the Integrated Velocity Structure Model by the Headquarters for Earthquake Research Promotion. We take several cross sections in Kyushu Island to map the calculated SWV-RFs. We then interpret the continental Moho and low velocity zones in the SWV-RFs. The Moho depth beneath Kyushu varies from 25 km to 33 km. We found two types of exceptional occurrence of 'inverted' Moho (IM): one is the top of sill-like low velocity zones just under active volcanoes, and the other is non-sill type events beneath fore-arc regions of northern Kyushu, which may be attributed to dehydration of PHS. The latter-type IM has been observed by Abe et al. (2013, JGR), while, to our knowledge, the former type IM have been imaged by us for the first time. We further model some SWVRF sections by the 2.5D finite-difference method [Takenaka and Okamoto, 2012, InTech]: we confirm that the assumed depths of Moho and sill-like low velocity zones in the simulation model are well reproduced in the simulated RF imaging results.