Greater landward velocities were recorded after 6 megathrust earthquakes in subduction zone regions adjacent to the ruptured portion. Previous explanations invoked either increased slip deficit accumulation or plate bending during postseismic relaxation, with different implications for seismic hazard. We investigate whether bending can be expected to reproduce this observed enhanced landward motion (ELM). We use 3D quasi-dynamic finite element models with periodic earthquakes. We find that afterslip downdip of the brittle megathrust exclusively produces enhanced trenchward surface motion in the overriding plate. Viscous relaxation produces ELM when a depth limit is imposed on afterslip. This landward motion results primarily from in-plane elastic bending of the overriding plate due to trenchward viscous flow in the mantle wedge near the rupture. Modeled ELM is, however, incompatible with the observations, which are an order of magnitude greater and last longer after the earthquake. Varying mantle viscosity, plate elasticity, maximum afterslip depth, earthquake size, and megathrust locking outside of the rupture does not significantly change this conclusion. The observed ELM consequently appears to reflect faster slip deficit accumulation, implying a greater seismic hazard in lateral segments of the subduction zone.@en

Plate boundary deformation zones represent a challenge in terms of understanding their underlying geodynamic drivers. Active deformation is well constrained by GNSS observations in the SW Balkans, Greece and W Turkey, and is characterized by variable extension and strike slip in an overall context of slow convergence of the Nubia plate relative to stable Eurasia. Diverse, and all potentially viable, forces and models have been proposed as the cause of the observed surface deformation, e.g., asthenospheric flow, horizontal gravitational stresses (HGSs) from lateral variations in gravitational potential energy, and rollback of the Hellenic slab. We use Bayesian inference to constrain the relative contribution of the proposed driving and resistive regional forces. Our models are spherical 2D finite element models representing vertical lithospheric averages. In addition to regional plate boundaries, the models include well-constrained fault zones like north and south branches of the North Anatolian Fault, Gulf of Corinth and faults bounding the Menderes Massif. Boundary conditions represent geodynamic processes: (1) far-field relative plate motions; (2) resistive fault tractions; (3) HGSs from lateral density variations; (4) slab pull and trench suction at subduction zones; and (5) active asthenospheric convection. The magnitude of each of these is a parameter in a Bayesian analysis of ~100,000 models and horizontal GNSS velocities. The search yields a probability distribution of all parameter values including model error, allowing us to determine mean/median parameter values, robustly estimate parameter uncertainties, and identify tradeoffs (i.e., parameter covariances). The average viscosity of the overriding plate is well resolved 3-4 10^22 Pa.s, which is higher than from published models without faults. Significant trench suction forces from the Hellenic slab act on the overriding Aegean Sea, including along the Pliny-Strabo STEP Fault. Slab pull and convective tractions have a small imprint on the observed deformation of the overriding plate. HGSs are necessary to explain local features in the velocity field, particularly in the Aegean Sea, but are less important for fitting the regional pattern of velocities. Resistive tractions on most plate boundaries and faults are low.@en

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.@en

The Gibraltar Arc (GA; Western Mediterranean) is located in between the slowly converging African and Eurasian continents and runs from the Rif in northern Morocco to the Betics in southern Spain. Geodetic velocities in this wide plate boundary region are different from the motions of both stable Eurasia and Africa. Various lithospheric and asthenospheric drivers have been called upon to explain the observed surface motions. We constrain the magnitude of these driving forces and the effective mechanical properties of the lithosphere and fault zones using regional geodynamic Finite Element models (FEMs). All of our FEMs have lithospheric forces that arise from lateral variations of gravitational potential energy, mainly from lateral variations in topography and Moho topology. They also all include far-field Africa-Eurasia convergence. The well-imaged Gibraltar slab suggests that slab pull and/or slab suction may contribute to the surface motions. Also, tractions that arise from lateral transport of the slab through the upper mantle are potentially relevant. We adopt parameterized versions of these slab forces in the FEMs. In our FEMs, we impose shear tractions on observationally constrained fault zones. We quantify the model parameter resolution and tradeoffs in light of the available kinematic surface observations with a Markov Chain Monte Carlo (MCMC) approach, implementing the Metropolis-Hastings algorithm. We compare the model results to the geodetic velocity observations, which have the smallest error margins of all the kinematic observations, and to the sense of shear along (potentially) active fault zones. Preliminary results based on a search of 60,000 models indicate that the observed GNSS velocity field and sense of slip on regional faults in the Gibraltar Arc appear to result mainly from Africa-Europe plate convergence and lateral GPE variations. Slab pull from the Gibraltar slab is very likely transmitted poorly into the overriding plate and probability distributions for the trench suction force do not display any favorable value to affect the kinematics in the Gibraltar Arc region. The best models have rms-misfits of 0.23 mm/yr, which is mostly due to a systematic SW motion of up to 4 mm/yr in the SW Rif.@en