Low-velocity accretionary wedges and sedimentary layers overlaying continental plates are widely observed in the subduction zones where historical large earthquakes (Mw>8.5) have occurred. It was observed that rupture of the 2011 Mw 9.0 Tohoku-Oki earthquake propagated to the trench with large coseismic slip on the shallow fault, but what caused the huge shallow slip remains a prominent problem. Here we explore how the two low-velocity structures, accretionary wedge and sedimentary layer, affect coseismic slip and near-fault ground motions during the 2011 Tohoku-Oki earthquake. Constrained by the observed seafloor deformation, we present a 2-D dynamic rupture model of the 2011 Tohoku-Oki earthquake with an accretionary wedge and a sedimentary layer. Compared to a homogeneous model with the same friction and stress parameters on the fault, 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. We then investigate a plausible scenario of a smaller Tohoku-Oki earthquake when its rupture does not reach the accretionary wedge. The sedimentary layer slightly enhances the coseismic slip while the accretionary wedge has almost no influence for the smaller earthquake scenario, but both structures significantly amplify ground accelerations on the overriding plate. We suggest that a co-existence of an accretionary wedge and a sedimentary layer tends to enhance coseismic slip, but the enhancement effect decreases as the up-dip limit of rupture zones terminates at a larger depth.