Three-dimensional structure of P-wave anisotropy in the presence of small-scale convection in the mantle wedge

Abstract

[1] As a possible explanation of "hot fingers" in the mantle wedge below the Tohoku region, Japan, the existence of small-scale convection has been proposed. In this study, we performed numerical calculations around the subduction zone in 3D with composite rheology (i.e., a combination of linear and non-linear rheology) and found that small-scale convection could arise when the rheology determined from laboratory experiments is considered. We also calculated 3D structure of expected P-wave anisotropy for the case with and without small-scale convection based on a theory of LPO development and an assumption that anisotropy is approximately represented as hexagonal symmetry, and found that the fast axis of the P-wave propagation projected on the horizontal cross-sections is nearly in the same direction as that of plate motion in many places. It implies that effects of large-scale mantle flow associated with subducting slab is still dominant even in the presence of small-scale convection in mantle wedge. However, in vertical crosssections, the projected fast axis of the P-wave propagation could tilt vertically while that without small-scale convection is almost horizontal. Therefore, future seismological studies that determine the fast direction of P-wave propagation in 3D would give us critical information on the possible existence of small-scale convection in the mantle wedge.

Crustal seismic structure beneath the source area of the Columbia River flood basalt: Bifurcation of the Moho driven by lithosphere delamination

“…None of our seismic stations show such simple back azimuthal patterns, indicating disturbance of multiple layers of anisotropy and/or a combination of dipping layers. For example, the segmentation of the low shear wave velocity anomalies along the Cascade back arc in the upper mantle is suggestive of the presence of small‐scale convection beneath the Wallowa area [ Gao and Shen , ], which can significantly complicate seismic anisotropy [e.g., Morishige and Honda , ; Wirth and Korenaga , ].…”

Radial and Azimuthal Anisotropy Tomography of the NE Japan Subduction Zone: Implications for the Pacific Slab and Mantle Wedge Dynamics

“…Morishige and Honda () numerically investigated the detailed 3‐D flow incorporating small‐scale convection and resulting seismic anisotropy in the mantle wedge designated for the NE Japan subduction zone (cf. Honda & Yoshida, ).…”

“…Radial anisotropy predicted by the model of small‐scale convection in the mantle wedge (case B1 of Morishige & Honda, ). The black lines show contours of temperature anomaly (i.e., deviation from horizontally averaged temperature) every 35 K. The trench is located at y = 990 km, and the direction of plate motion is in the negative y ‐direction.…”

Constraints on Subduction Geodynamics From Seismic Anisotropy

“…(In contrast, simplified models based on the finite strain distribution assume that LPO adjusts instantaneously to changes in the strain geometry.) D‐Rex has been applied by several different workers to model texture development in subduction systems [e.g., Lassak et al ., ; Conder and Wiens , ; Morishige and Honda , ; Faccenda and Capitanio , , ]. Once a scheme has been used to predict elastic constants at different points in the model, various seismologic modeling tools can be applied to predict observable quantities such as shear wave splitting.…”