The influence of accretionary wedge and sedimentary layer on coseismic slip and ground motion of the 2011 Tohoku-Oki earthquake

The low-velocity accretionary wedge and sedimentary layer overlying the continental plate are widely observed in subduction zones, playing important roles in affecting dynamic rupture process and ground motion of subduction zone earthquakes. It was observed that the 2011 Tohoku-Oki earthquake rupture propagated to the trench with large fault slip in the shallow region, but what caused the huge shallow slip still remains a prominent problem. Here we aim at exploring how the accretionary wedge and sedimentary layer affect the coseismic slip and near-fault ground motion of the Tohoku-Oki earthquake. We first present a dynamic rupture model of the Tohoku-Oki earthquake with both accretionary wedge and sedimentary layer (aw-and-sed model), constrained by the observed seafloor deformation. Compared to a homogeneous model with the same friction parameters, the maximum coseismic slip and horizontal deformation of the aw-and-sed model are 25m and 20m higher, respectively. Both the accretionary wedge and sedimentary layer have positive influence on the fault slip and deformation, but the integrated effect is not a linear sum of each effect. We also observe that high-frequency (0.5-2 Hz) ground motions are greatly amplified near the accretionary wedge compared to a homogeneous medium.

We then investigate a plausible scenario of a smaller Tohoku-Oki earthquake when its rupture does not reach the accretionary wedge. We find that the sedimentary layer enhances the fault slip and deformation while the accretionary wedge has almost no influence, but both structures significantly amplify the peak ground acceleration (PGA) on the overriding plate. The PGA of the aw-and-sed model is amplified by a maximum factor of 4.1 for the small earthquake compared to a homogeneous medium, larger than the amplification factor of PGA (2.6) recorded in the Tohoku-Oki earthquake simulation.