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.