Coseismic deformation due to the 2011 Tohoku, Japan, earthquake: influence of 3-D elastic heterogeneity

The 2011 M9 Tohoku earthquake ruptured a fault area with length and width of ~500 km and ~200 km, respectively, and elastically deformed broad lithospheric and mantle regions. In order to evaluate the influence of the Tohoku earthquake on lithospheric stresses throughout Japan, an accurate accounting of coseismic slip is very important as the initial condition. We investigate the effects of heterogeneous elastic moduli under Japan on the inversion for coseismic slip, utilizing the land-based Japan GPS network as well as seafloor geodetic constraints near the trench. For this purpose, we construct a 3-D finite element model (FEM) to generate Green’s functions that allows considering the influence of complex slab geometry as well as heterogeneities in elastic structure. Our FEM incorporates the Pacific and Philippine sea slabs by interpolating seismicity for the Tohoku region and the Nankai trough, as well as the Kuril, Ryukyu and Izu-Bonin arcs. As fault source geometry, we consider not only the Pacific but also the Phillipine sea slab, with initial results suggesting that slip occurring beyond the triple junction is ~1 m. As for elastic heterogeneity, we investigated the effect of the crust-mantle stratification, the contrast of oceanic and continental crust, and the effects of slabs. Results indicate that crust-mantle stratification has to be considered to obtain an appropriate coseismic slip distribution; heterogeneous models show smaller maximum slip and a wider distribution compared to homogeneous structure. This confirms some earlier work, but is in contrast to some recent suggestions. The effect of the ocean-continent contrast and deep slab heterogeneity appear negligible for co-seismic inversions.