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Geodetic observations after large subduction earthquakes reflect multiple postseismic processes, including megathrust relocking. The timing of relocking and the observational constraints on it are unclear. Relocking was inferred to explain some observed landward motion that occurs within months. It was also considered unable to explain other, greater landward motion, including that off the coast of Japan beginning weeks after the 2011 Tohoku earthquake, attributed to postseismic relaxation. We use generic, 3D numerical models to show that relocking, particularly of the shallow interface, is needed for postseismic relaxation to produce landward motion on the tip of the overriding plate. We argue that this finding is consistent with previous simulations that implicitly relock the megathrust where afterslip is not included. We conclude that the Tohoku megathrust relocked within less than 2 months of the earthquake. This suggests that the shallow megathrust probably behaves as a true, unstably sliding asperity. Plain Language Summary: In the largest earthquakes, a tectonic plate suddenly slides under another, where previously the interface between them was largely locked. The deformation of the Earth's surface reflects multiple deep processes. One process is the restoration of interface locking. This means that elastic energy starts to accumulate again, building up toward the next earthquake. How long after an earthquake does the interface relock? Relocking is suggested to have caused some deformation observed months after some earthquakes, and so to have occurred by then. It is also thought to not explain the rapid landward motion of the seafloor on the upper plate, observed beginning less than 2 months after the 2011 Japan earthquake. We use numerical simulations to show that locking the shallow portion of the plate interface is needed for landward motion to transmit from the lower plate to the upper plate, as happened in Japan. Landward motion is understood to occur below and in the lower plate because of mantle flow and deep sliding taking place after the earthquake and caused by it. We conclude that this result agrees with simulations in previous studies, even though they were not interpreted in terms of locking by their authors. Key Points: Landward motion of the overriding plate, attributed to postseismic relaxation, was observed offshore 2 months after the Tohoku earthquakeModels show that landward motion only transmits from the slab to the overriding plate, reaching the surface, if the megathrust is lockedObservations imply shallow megathrust relocking within 2 months of the Tohoku earthquake, consistent with unstably sliding behavior [ABSTRACT FROM AUTHOR]

A devastating tsunami struck Palu Bay in the wake of the 28 September 2018 Mw = 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 data set of Global Positioning System and multiple Synthetic Aperture Radar‐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 data set constrains vertical motions of the sea floor, and sheds new light on the tsunamigenesis of strike‐slip faults in transtensional basins. Plain Language Summary: The 28 September Palu earthquake ruptured the Palu‐Koro fault in NW Sulawesi, Indonesia, and was followed by a devastating tsunami in Palu Bay. As the Palu‐Koro fault accommodates mostly horizontal motion, many studies proposed that sub‐marine landslides, rather than the earthquake itself, triggered the tsunami. This study focuses on the contribution of the earthquake to sea floor displacements. We present a unique geodetic data set and estimate a high‐resolution 3D displacement field. The rupture is not a straight feature in the landscape, but rather contains bends. It is near those bends that significant vertical displacements occurred. From the onshore geodetic data we infer another fault bend below Palu Bay. Estimations of fault slip for several scenarios of offshore fault geometries point to a few meters of sea floor uplift. We use these slip models as input for tsunami models, and can qualitatively explain the observations of tsunami runup heights and video‐based tsunami arrival times around Palu Bay. Only at a few locations our models cannot explain tsunami observations, which leaves open the contribution of other possible sources to the tsunami locally. The Palu case underlines the potential importance of fault bends to tsunami generation for similar tectonic settings around the world. Key Points: New Global Positioning System observations as part of a geodetic solution for the full 3D onshore co‐seismic displacements of the 2018 Palu earthquakeBelow Palu Bay we find a strong indication of dip‐slip on a fault bend, agreeing with the notion of Palu Valley being a transtensional basinA large part of the tsunami, both in terms of arrival times and runup heights, can be explained by co‐seismic slip [ABSTRACT FROM AUTHOR]

Strait uplift due to isostasy played a pivotal role in the development and maintenance of a desiccated Mediterranean basin during the Late Miocene Messinian salinity crisis. New threedimensional flexure models for the western Mediterranean suggest that most of the giant evaporite body was deposited before sea-level lowering. Any mechanism that subsequently caused a Mediterranean sea-level lowering by a few hundred meters during several thousand years sufficed to cause isostatic uplift and closure of the gateways, thus choking the Mediterranean to desiccation. Due to this isostatic feedback, multiple desiccation and reflooding phases driven by Milankovitch cycles are unlikely. When the Atlantic connection had been lost and sea level had dropped, a sizeable regional uplift of the Strait of Gibraltar developed that presented a formidable obstacle to reflooding. Rollback and steepening of the Gibraltar slab may have played a key role at the end of the Miocene by dynamically lowering the sill region, thus reinstating the oceanic connection.