Model-Predicted Shear-Wave-Splitting Parameters in NE Japan: Comparison With Observations

Abstract

We model olivine crystal preferred orientation (CPO) and shear wave splitting (SWS) in the mantle wedge, using the results of a 3-D numerical thermomechanical model for the NE Japan subduction zone. The model predicts a quasi-2-D trench-normal (E-W) mantle wedge flow pattern beneath Tohoku, given the trench-normal subduction, but a more complex flow pattern beneath Hokkaido due to oblique subduction. The calculations of CPO in the mantle wedge indicate a predominantly E-W fast axis beneath Tohoku, aligned with the mantle flow, but beneath Hokkaido the fast axis orientation changes laterally and with depth (ranging from approximately N-S to E-W) and is generally less aligned with the flow, especially near the base of the mantle wedge. The model calculations for vertical wave propagation yield location-averaged fast directions consistent with observationally constrained station-averaged fast directions: E-W in Tohoku and N-S in Hokkaido. However, the model significantly overestimates delay times in both study areas. Furthermore, model-predicted fast directions from individual event-station pairs cannot explain the variability in the observationally constrained fast directions for the same pairs even when the incidence angle and backazimuth are incorporated into the calculation instead of assuming vertical wave propagation. These discrepancies may reflect the impacts of simplifications in the model calculations and other factors, such as the anisotropy outside of the mantle wedge and data uncertainties.