The role of permanent upper-plate deformation in coseismic deformation and megathrust earthquakes dynamics

In subduction zones, seismic imaging reveals increasing permanent active deformation toward the trench, particularly within accretionary systems. Yet, how these rock bodies deform coseismically and influence megathrust rupture behavior is elusive. Here we combine geophysical observations from seismic imaging with visco-plastic dynamic rupture simulations to investigate how realistic upper-plate rock bodies influence megathrust earthquake dynamics and off-fault deformation. Our models reproduce the elastic structure of three subduction systems that differ primarily in the width of the accretionary prism, a key parameter for comparison. These configurations, referred to as Models I, II, and III, include a narrow compliant prism of approximately 20 km, an intermediate prism of about 60 km, and a wide prism exceeding 100 km, respectively. Elastic rock properties for each upper-plate model are derived from 2D P-wave velocity models obtained from controlled-source seismic data. Upper-plate bulk cohesion and bulk friction define visco-plastic strength and are set to depend on rigidity distribution and empirical observations. Based on laboratory measurements from JFAST drilling samples, we use rate-and-state friction law with strong velocity weakening in the shallow portion of the fault.

Results show that coseismic upper-plate plastic deformation in Model I is confined to the ~20-km-wide wedge, whereas in Models II and III it extends 40–60 km landward from the trench. This is consistent with seismic reflection profiles that reveal increasing active internal deformation of the prism at similar distances from the trench in regions such as the Japan Trench, Chile, and Sumatra. Such contrasting upper-plate deformation patterns lead to distinct uplift scenarios, particularly in their high-frequency response. In particular, Model I produces shorter-wavelength uplift near the trench, likely generating a tsunami with higher-frequency content than Model II and III, where uplift exhibits a longer wavelength. Although we do not explicitly simulate independent faults within the prism, the bulk plastic strain can be considered a proxy for the amount of deformation that is accommodated by these structures. Our results suggest that permanent deformation within accretionary prisms is active during trench-breaching megathrust earthquakes, indicating that substantial prism deformation occurs coseismically. Plastic deformation leads to a reduction in slip toward the trench, implying that coseismic energy is absorbed by the overlying rock body. This effect explains the low-radiated-energy of near-trench earthquakes, including tsunami earthquakes. Depending on the plastic strength of upper-plate material and the available energy along the fault, this effect may even prevent the rupture from reaching the trench, while still producing substantial coseismic uplift and horizontal seafloor displacement. Overall, this study indicates that identifying permanently deformed, low-rigidity regions near the trench can serve as a proxy for locating areas where coseismic deformation is strongly accommodated and tsunamigenic uplift is likely amplified.