Shear wave anisotropy beneath the volcanic front in South Kyushu area, Japan: Development of C‐type olivine CPO under H₂O‐rich conditions

Abstract

[1] Shear wave splitting from local intermediate-depth earthquakes is investigated to detect the anisotropic structure in the mantle wedge beneath the South Kyushu area, Japan. We observed shear wave splitting with NEE-SWW to NWW-SEE polarization directions and delay times of 0.04-0.63 s. Trench-normal shear wave polarization anisotropy with a delay time greater than 0.3 s probably overlaps in the high V P /V S region at a depth of 100-150 km beneath the volcanic front. Model calculations for the cause of the anisotropy suggest that the development of "C-type" olivine crystallographic preferred orientation (CPO) with a trench-parallel b axis concentration and a trench-normal a axis concentration best reproduces observations compared to A-, B-, or E-type olivine CPOs and antigorite CPO. We conclude that a thick anisotropic layer, approximately 50 km, is formed by concentration of interstitial fluid in peridotite. We briefly discuss the mechanism carrying water to the deep mantle wedge. A possible explanation is that interstitial fluids in the mantle wedge are effectively released because of a decrease in dihedral angles of olivine-fluid interfaces at 150 km depth. Our results imply a close relation between the high density of explosive volcanoes in the South Kyushu area and the underlying water-rich mantle.

Seismic anisotropy tomography: New insight into subduction dynamics

“…Trench-parallel FVDs are revealed in the fore-arc mantle wedge under Kyushu, suggesting the existence of a B-type olivine fabric there. This FVD pattern is consistent with the SWS measurements in Kyushu (e.g., Salah et al, 2008;Terada et al, 2013). Their result for Vp radial anisotropy is similar to that of the Tohoku arc, revealing that Vpv N Vph in the low-V zones in the mantle wedge under the arc volcanoes in Kyushu, as well as in the low-V zones below the PHS slab under SW Japan, which may reflect hot upwelling flows and transitions of olivine fabrics with the presence of water due to the slab dehydration.…”

“…Recent teleseismic tomography shows that the PHS plate has subducted aseismically down to 460 km depth under the Japan Sea, Tsushima Strait and East China Sea, and a window exists within the aseismic PHS slab . SWS measurements have been made, which revealed the existence of significant anisotropy in SW Japan (e.g., Ando et al, 1983;Kaneshima, 1990;Hiramatsu et al, 1997;Salah et al, 2008;Terada et al, 2013). P-wave tomography for azimuthal anisotropy beneath SW Japan has been investigated by Ishise and Oda (2008) and Zhao (2012, 2013).…”

Subduction Zone Tomography

“…Trench-parallel FVDs also appear in the mantle wedge under the volcanic front in Tohoku, but not in Kyushu, suggesting that 3-D flow may exist in the mantle wedge under Tohoku caused by the large subduction rate of the Pacific plate (7-10 cm/year). This FVD pattern of trench-normal in the back-arc and trench-parallel in the fore-arc is very consistent with shear-wave splitting measurements in Tohoku (Okada et al 1995;Nakajima et al 2006;Huang et al 2011b, c) and in Kyushu (e.g., Salah et al 2008;Terada et al 2013). …”

Constraints on Olivine Deformation From SKS Shear‐Wave Splitting Beneath the Southern Cascadia Subduction Zone Back‐Arc

“…Terada et al. (2013) linked source‐side SWS measurements below South Kyushu to C‐type fabric. They further argued that the spatial agreement with detected slow propagation velocities and an increased V P / V S ratio (based on Matsubara & Obara, 2011; Matsubara et al., 2008) suggest an acutely hydrated mantle environment.…”

“…In this context, compared source-side SWS measurements with mantle flow simulations for subduction systems (based on and inferred that C-type might be dominant in the subslab mantle beneath Central America while for Tonga, E-type explained the measurements significantly better. Terada et al (2013) linked source-side SWS measurements below South Kyushu to C-type fabric. They further argued that the spatial agreement with detected slow propagation velocities and an increased V P /V S ratio (based on Matsubara & Obara, 2011;Matsubara et al, 2008) suggest an acutely hydrated mantle environment.…”