Effects of the accretionary wedge and sedimentary layers on subduction zone earthquake ruptures and ground motion: 2-D numerical simulations

Abstract

SUMMARYLow-velocity accretionary wedges and sedimentary layers overlying continental plates are common in subduction zones. These low-velocity sedimentary structures should be considered to accurately model ground motions and estimate coseismic slip on subsurface faults. First, we simulated the rupture process of the 2011 Mw 9.0 Tohoku-Oki earthquake in 2-D dynamic rupture models and found that the co-existence of the accretionary wedge and sedimentary layer can cause an approximately 60 per cent increase in the shallow coseismic slip. Considering the inelastic attenuation, wave reverberations in the accretionary wedge and sedimentary layer significantly amplified offshore ground acceleration at 0.1–0.5 Hz by factors of 1–5 and prolonged ground motion durations. Additionally, high-frequency acceleration at 0.5–2.0 Hz is also amplified on the accretionary wedge with a maximum factor of 5. Our Tohoku-Oki earthquake model reproduces the observed coseismic slip qualitatively, but the model also results in higher scaled energy, due to the overestimation of radiated energy in 2-D models and the inadequate plastic properties in the accretionary wedge and sedimentary layer. We further simulated a suite of earthquake scenarios where the updip rupture terminates at different depths. Our results show that a sedimentary layer enhances coseismic slip in all cases, while an accretionary wedge can decrease the slip when the shallow fault has a strengthening frictional behaviour. Additionally, the effects on slip diminished when the extent of updip rupture becomes deeper. However, offshore ground acceleration at 0.1–0.5 Hz is still amplified due to the dynamic wave effects in the two structures. Furthermore, in the scenarios when earthquake rupture reaches the shallow fault, the existence of an accretionary wedge can cause amplified high-frequency acceleration (0.5–2.0 Hz) near the trench. Our results indicate that compared to those subduction zones with neither an accretionary wedge nor sedimentary layers, subduction zones featuring a co-existence between an accretionary wedge and sedimentary layers are susceptible to host earthquakes with larger shallow slip and amplified offshore ground motions.