Radially anisotropic structure beneath the Shikoku Basin from broadband surface wave analysis of ocean bottom seismometer records

Abstract

We have analyzed broadband surface wave data from ocean bottom seismometers deployed in the Shikoku Basin in the northeastern Philippine Sea to determine the radially anisotropic uppermost mantle structure beneath this oceanic basin. We first applied noise correlation method to continuous microseismic records to obtain phase velocities for fundamental-mode and first higher-mode Rayleigh waves and fundamental-mode Love waves at periods of 7-29 s. At longer periods, we applied an array analysis method to teleseismic surface waves to obtain phase velocities of fundamental-mode Rayleigh and Love waves at periods of 29-117 s. Using these broadband phase velocity measurements, we have determined the one-dimensional radially anisotropic structure from the crust to the low velocity zone (LVZ) beneath the Shikoku Basin without assuming a priori structure in the uppermost mantle. The final structural model (SB-RA10) has a high-velocity lid from the Moho to a depth of ∼40 km, with an LVZ at greater depths. S wave velocities decrease by 6%-10% at a depth range of ∼40-70 km. This large decrease in velocity suggests that there is either a large difference in grain size between these layers or indicates the presence of partial melt or water in the LVZ. Furthermore, strong radial anisotropy of 4%-5% (VSH>VSV) is observed in the uppermost mantle, which may be stronger in the LVZ. © 2013. American Geophysical Union. All Rights Reserved.