Near-field and far field effects of elastic structure of Japan on the slip distribution of the 2011 Tohoku earthquake

Abstract:

Broad coseismic deformation due to the 2011 Tohoku earthquake, Japan, was detected by over 1200 land based GPS stations and several seafloor stations. Using these observations as constraints, we investigate how elastic structure under Japan influences coseismic deformation using finite element models incorporating geometry of the regional plate boundaries and elastic structures. Effects of elastic structures are examined by comparing analyses of following three structure models: (a) homogeneous half-space model, (b) two-layered model considering crust-mantle structure (rigidity of 35 and 65 GPa, respectively), (c) crust-mantle model with cold slab (85 GPa). The patterns of inverted slip distribution are basically similar for all three models, but taking a closer look, we can see the amount of maximum slip is not simply related to average rigidity of structure models: it increases from 37 m in homogeneous model to 40 m in two-layered model and then falls to 38 m in slab model. These characteristics can be understood by separately observing the behavior of the onshore (far field) and offshore (near field) displacements. We found the following two contradicting effects: (1) In the far field, relatively lower rigidity in the shallow part decreases the amount of displacement for the same slip. This can be explained by the decrease in seismic moment. (2) In the near field or just above the thrust, relatively lower rigidity in the shallow part increases the amount of displacement for the same slip amount. This is because the stiffer (less deformable) footwall requires more movement of the hanging wall to accommodate the slip. Comparing the computed displacements with three structure models, we found that crust-mantle layering is more effective on far field while slab effect is more prominent in the near field.