The last great megathrust earthquake in the Cascadia subduction zone occurred in AD 1700, predating the instrumental record. While modern geodetic techniques capture the ongoing interseismic deformation, paleoseismic records provide estimates of the coastal subsidence over the AD 1700 event. Here, we connect these geodetic and paleoseismic observations using a 3D quasi-dynamic earthquake cycle model. The model includes a realistic slab geometry and simulates the earthquake cycle under the assumption that the coseismic rupture is consistent with the interseismic locking pattern. The model predictions agree well with both paleoseismic estimates of coastal subsidence during and after the earthquake, as well as geodetic data on the current interseismic surface velocity field. This suggests that our rupture-locking consistency assumption is realistic for the central Cascadia subduction zone, and the rupture patches in the AD 1700 event may be persistent in the next megathrust event.

Relaxation following large subduction earthquakes produces landward changes in surface velocities. Near-trench landward velocity changes in the vicinity of the rupture zone have been attributed to rapid viscoelastic relaxation in the sub-slab asthenosphere. Lateral landward velocity changes, hundreds of km along-trench from the rupture, have been variously explained invoking interplate coupling changes, transient slab acceleration, or overriding plate bending, with different implications for seismic hazard. We investigate whether the lateral landward velocity changes can result from postseismic relaxation with constant interplate coupling and convergence rate. We use a finite element model with periodic megathrust earthquakes resulting from unlocking of asperities on the megathrust, with instantaneous, dynamically driven coseismic slip and afterslip and with bulk visco-elastic relaxation. A mechanical contrast in the overriding plate is required to reproduce the observed near-trench focusing of interseismic deformation. A maximum depth limit to afterslip of around 100 km is consistent with inverted afterslip distributions and the hypothesized depth extent of the megathrust. The presence of both the contrast and depth limit causes viscous relaxation in the mantle wedge to result in lateral landward velocity changes. We discuss the responsible mechanism, which amounts to elastic deformation of the overriding plate and is due to the finite compressibility of the plate and its in-plane bending. The spatial pattern of landward velocity changes is consistent with observations for the Maule and Tohoku earthquakes. Velocity change magnitudes are comparable with observations, scale with viscosity and seismic moment, are only partly counteracted by the effect of primary afterslip, and are little affected by interplate coupling pattern. In the years following the largest earthquakes with rapidly decaying afterslip, this mechanism is expected to produce detectable landward velocity changes. The models also shows near-trench landward velocity changes close to the rupture, consistently with previous research. However, we find that the extent and timing of shallow interface (re)locking is critical for reproducing near-trench observations on the overriding plate.

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.

In Southeast Alaska, extreme uplift rates are primarily caused by glacial isostatic adjustment (GIA), as a result of ice thickness changes from the Little Ice Age to the present combined with a low-viscosity asthenosphere. Previous GIA models adopted a 1-D Earth structure. However, the actual Earth structure is likely more complex due to the long history of subduction and tectonism and the transition from a continental to an oceanic plate. Seismic evidence shows a laterally heterogenous Earth structure. In this study a numeral model is constructed for Southeast Alaska, which allows for the inclusion of lateral viscosity variations. The viscosity follows from scaling relationships between seismic velocity anomalies and viscosity variations. We use this scaling relationship to constrain the thermal effect on seismic variations and investigate the importance of lateral viscosity variations. We find that a thermal contribution to seismic anomalies of 10% is required to explain the GIA observations. This implies that non-thermal effects control seismic anomaly variations in the shallow upper mantle. Due to the regional geologic history, it is likely that hydration of the mantle impact both viscosity and seismic velocity. The best-fit model has a background viscosity of 5.0×10^19 Pa-s, and viscosities at ~80 km depth range from 1.8×10^19 to 4.5×10^19 Pa-s. A 1-D averaged version of the 3-D model performed slightly better, however, the two models were statistically equivalent within a 2σ measurement uncertainty. Thus, lateral viscosity variations do not contribute significantly to the uplift rates measured with the current accuracy and distribution of sites.