We aim to better understand the overriding plate deformation during the megathrust earthquake cycle. We estimate the spatial patterns of interseismic GNSS velocities in South America, Southeast Asia, and northern Japan and the associated uncertainties due to data gaps and velocity uncertainties. The interseismic velocities with respect to the overriding plate generally decrease with distance from the trench with a steep gradient up to a “hurdle”, beyond which the gradient is distinctly lower and velocities are small. The hurdle is located 500–1000 km away from the trench, for the trench-perpendicular velocity component, and either at the same distance or closer for the trench-parallel component. Significant coseismic displacements were observed beyond these hurdles during the 2010 Maule, 2004 Sumatra-Andaman, and 2011 Tohoku earthquakes. We hypothesize that both the interseismic hurdle and the coseismic response result from a mechanical contrast in the overriding plate. We test our hypothesis using physically consistent, generic, three-dimensional finite element models of the earthquake cycle. Our models show a response similar to the interseismic and coseismic observations for a compliant near-trench overriding plate and an at least 5 times stiffer overriding plate beyond the contrast. The model results suggest that hurdles are more prominently expressed in observations near strongly locked megathrusts. Previous studies inferred major tectonic or geological boundaries and seismological contrasts located close to the observed hurdles in the studied overriding plates. The compliance contrast probably results from thermal, compositional and thickness contrasts and might cause the observed focusing of smaller-scale deformation like backthrusting.

A devastating tsunami struck Palu Bay in the wake of the 28 September 2018 M$_{\mathrm{w}}=7.5$ Palu earthquake (Sulawesi, Indonesia). With a predominantly strike-slip mechanism, the question remains whether this unexpected tsunami was generated by the earthquake itself, or rather by earthquake-induced landslides. In this study we examine the tsunami potential of the co-seismic deformation. To this end, we present a novel geodetic dataset of GPS and multiple SAR-derived displacement fields to estimate a 3D co-seismic surface deformation field. The data reveal a number of fault bends, conforming to our interpretation of the tectonic setting as a transtensional basin. Using a Bayesian framework, we provide robust finite fault solutions of the co-seismic slip distribution, incorporating several scenarios of tectonically feasible fault orientations below the bay. These finite fault scenarios involve large co-seismic uplift (~2 m) below the bay due to thrusting on a restraining fault bend that connects the offshore continuation of two parallel onshore fault segments. With the co-seismic displacement estimates as input we simulate a number of tsunami cases. For most locations for which video-derived tsunami waveforms are available our models provide a qualitative fit to leading wave arrival times and polarity. The modeled tsunamis explain most of the observed runup. We conclude that co-seismic deformation was the main driver behind the tsunami that followed the Palu earthquake. Our unique geodetic dataset constrains vertical motions of the sea floor, and sheds new light on the tsunamigenesis of strike-slip faults in transtensional basins.

Most of the seismic moment release of the complex earthquake sequence beneath the South Sandwich Islands occurred on the central part of the SS megathrust. Significant aftershock activity indicates that the central and southern megathrust was subsequently activated, i.e., where young South America lithosphere is subducted. Seismic activity thus seems to have been restricted by the lateral termination in the south of the SS Trench.   Relatively little energy release occurred on the northern part of the megathrust. It was hypothesized by Govers and Wortel (2005) that here the South America slab breaks away from the surface part of the plate at the active STEP. Geochemical observations and earthquake P-axes orientations do not seem to agree with the hypothesis and we investigate the cause.   We show results of new physical analog lab models that aim to elucidate what controls the geometry of the lithospheric STEP Fault. We study lithospheric tearing in the process of STEP evolution, which is dynamically driven by the buoyancy of the subducting slab. In our experiments, the lithosphere as well as asthenosphere are viscoelastic media in a free subduction setup. A stress-dependent rheology plays a major role in localization of strain in tearing processes of lithosphere such as slab break-off. The results show that the highly curved northern plate boundary is a STEP Fault following from lithospheric tearing at a depth of ~100km. This is a modification of the original STEP model of Govers and Wortel (2005). This is consistent with available observations along the northern Sandwich plate boundary, and likely exists in other STEP regions. The region’s largest recorded event, the 1929 Mw 8.3 earthquake, may reflect horizontal extension perpendicular to the STEP fault, which is also expected based on our experiments.

We aim to better understand the spatial distribution of interseismic overriding plate deformation at and near subduction zones. To this end, we analyze horizontal GNSS velocities in South America, southeast Asia, and northern Japan, computing and interpolating local trench-normal and -parallel velocity components. Velocities generally decrease with distance from the trench with a steep gradient up to a “hurdle”, beyond which the gradient is distinctly lower and velocities are near-zero. The hurdle is located 500–1000 km away from the trench for the trench-perpendicular component and either at the same distance or closer for the trench-parallel. In contrast, significant displacements during large megathrust earthquakes are generally observed beyond the hurdle. To test our hypothesis that the hurdle results from a lateral contrast in overriding plate compliance, we use cyclic three-dimensional finite element models . Our results are consistent with the observed interseismic velocity gradients and far-field coseismic displacement. The gradient in modeled trench-perpendicular velocities depends on the location of the contrast and on the plate compliance on both sides. Trench-parallel velocities have a progressively shallower gradient with distance from the trench and only depend on the near-trench modulus. The inferred contrast probably results from thermal, compositional and thickness contrasts. This interpretation is consistent with the presence, close to the observed hurdle, of major tectonic or geological boundaries separating the plate margin from a distinct, and likely less compliant, plate interior. Stress accumulation on the model’s locked megathrust patches is hardly affected by the distance to the contrast.