Abstract

Low-velocity accretionary wedges and sedimentary layers overlying continental plates widely exist in subduction zones. However, the two structures are commonly neglected in velocity models used in slip inversion, ground motion estimation, and dynamic rupture simulation, which may cause a biased estimation of coseismic slip and near-fault ground motions during subduction zone earthquakes. We use the 2011 Mw 9.0 Tohoku-Oki earthquake as an example and reproduce the observed seafloor deformation using 2-D dynamic rupture models with or without an accretionary wedge and a sedimentary layer. We find that the co-existence of the accretionary wedge and sedimentary layer significantly enhances the shallow coseismic slip and amplifies ground accelerations near the accretionary wedge. Hence, stress drop on the shallow fault estimated from the coseismic slip or surface deformation is overestimated when the two structures are neglected. We further simulate a suite of earthquakes where the up-dip rupture terminates at different depths. Results show that a sedimentary layer enhances coseismic slip in all cases, while an accretionary wedge can lead to a sharper decline in slip when negative dynamic stress drop exists on the shallow fault. However, a combination of the two structures tends to enhance fault slip, especially when rupture breaks through a trench. Thus, their combined effects are nonlinear and can be larger than the respective contribution of each structure. Our results emphasize that subduction zones featuring a co-existence of an accretionary wedge and a sedimentary layer may have inherently higher earthquake and tsunami hazards.