A three-dimensional (3D) P-wave seismic velocity (Vp) model of the crust at the northern South China Sea margin drilled by IODP Expeditions 367/368/368X has been obtained with first-arrival travel-time tomography using wide-angle seismic data from a network of 49 OBSs and 11 air-gun shot lines. The 3D Vp distribution constrains the extent, structure and nature of the continental, continent to ocean transition (COT), and oceanic domains. Continental crust laterally ranges in thickness from ~8 to 20 km, a ~20 km-width COT contains no evidence of exhumed mantle, and crust with clear oceanic seismic structure ranges in thickness from ~4.5 to 9 km. A high-velocity (7.0-7.5 km/s) lower crust (HVLC) ranges in thickness from ~1 to 9 km across the continental and COT domains, which is interpreted as a proxy of syn-rift and syn-breakup magma associated to underplating and/or intrusions. Continental crust thinning style is abrupter in the NE segment and gradual in the SW segment. Abrupter continental thinning exhibits thicker HVLC at stretching factor (β) <~3, whereas gentler thinning associates to thinner HVLC at β>~4. Opening of the NE segment thus occurred by comparatively increased magmatism, whereas tectonic extension was more important in the SW segment. The Vp distribution shows the changes in deformation and magmatism are abrupt along the strike of the margin, with the segments possibly bounded by a transfer fault system. No conventional model explains the structure and segmentation of tectonic and magmatic processes. Local inherited lithospheric heterogeneities during rifting may have modulated the contrasting opening styles.

Megathrust earthquakes are strongly influenced by the elastic properties of rocks surrounding the fault. However, these properties are often overestimated in numerical simulations, particularly in the shallow megathrust. Here we explore the influence that realistic depth-varying upper-plate elastic properties along the megathrust have on earthquake rupture dynamics and tsunamigenesis using 3D dynamic rupture and tsunami simulations. We compare results from three subduction zone scenarios with homogeneous and heterogeneous elastic media, and bimaterial fault. Elastic properties in the heterogeneous model follow a realistic depth-distribution derived from controlled-source tomography models of subduction zones. We assume the same friction properties for all scenarios. Simulations in the heterogeneous and homogeneous models show that rigidity variation of the country rock determines the depth-varying behavior of slip, slip rate, frequency content, and rupture time. Fault friction may provide additional constraints, but to a lesser extent. The depth-varying behavior of slip, frequency content, and rupture duration quantitatively agree with previous predictions based on worldwide data compilations, explaining the main depth-dependent traits of tsunami earthquakes and large shallow megathrust earthquakes. Large slip, slow rupture and slip rate amplification in bimaterial simulations are largely controlled by the elastic rock properties of the most compliant side of the fault, which in subduction zones is the upper plate. Large shallow slip and trenchward increasing upper-plate compliance of the heterogeneous model lead to the largest co-seismic seafloor deformation and tsunami amplitude. This highlights the importance of considering realistic variations in upper-plate rigidity to properly assess the tsunamigenic potential of megathrust earthquakes.