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Abstract Megathrusts at convergent plate boundaries generate the largest and some of the most hazardous earthquakes on Earth. However, their physical properties, including those influencing fault slip accumulation and release and earthquake‐related surface displacements, are still poorly constrained at critical depths. Here, we combine seismic imaging and geodetic modeling to investigate the structure and mechanical behavior of the Main Himalayan Thrust fault (MHT) in the center of the 2015 Mw 7.8 Gorkha rupture in Nepal. Our results from two independent observations consistently suggest the presence of a channel associated with the MHT with high compliance (shear modulus as low as ∼4 GPa) and strain anisotropy (stiffer in the vertical orientation than in the horizontal), likely arising from a weak subducting layer with north‐dipping foliation. Such mechanical heterogeneity significantly influences the quantification of short‐term fault kinematics and associated earthquake potential, with implications on across‐scale dynamics of plate boundaries in Himalaya and elsewhere. 

SUMMARY Plate-coupling estimates and previous seismicity indicate that portions of the Makran megathrust of southern Pakistan and Iran are partially coupled and have the potential to produce future magnitude 7+ earthquakes. However, the GPS observations needed to constrain coupling models are sparse and lead to an incomplete understanding of regional earthquake and tsunami hazard. In this study, we assess GPS velocities for plate coupling of the Makran subduction zone with specific attention to model resolution and the accretionary prism rheology. We use finite element model-derived Green's functions to invert for the interseismic slip deficit under both elastic and viscoelastic Earth assumptions. We use the model resolution matrix to characterize plate-coupling scenarios that are consistent with the limited spatial resolution afforded by GPS observations. We then forward model the corresponding tsunami responses at major coastal cities within the western Indian Ocean basin. Our plate-coupling results show potential segmentation of the megathrust with varying coupling from west to east, but do not rule out a scenario where the entire length of the megathrust could rupture in a single earthquake. The full subduction zone rupture scenarios suggest that the Makran may be able to produce earthquakes up to Mw 9.2. The corresponding tsunami model from the largest earthquake event (Mw 9.2) estimates maximum wave heights reaching 2–5 m at major port cities in the northern Arabian Sea region. Cities on the west coast of India are less affected (1–2 m). Coastlines bounding eastern Africa, and the Strait of Hormuz, are the least affected (<1 m). 

Abstract On 5 April 2024, 10:23 a.m. local time, a moment magnitude 4.8 earthquake struck Tewksbury Township, New Jersey, about 65 km west of New York City. Millions of people from Virginia to Maine and beyond felt the ground shaking, resulting in the largest number (>180,000) of U.S. Geological Survey (USGS) “Did You Feel It?” reports of any earthquake. A team deployed by the Geotechnical Extreme Events Reconnaissance Association and the National Institute of Standards and Technology documented structural and nonstructural damage, including substantial damage to a historic masonry building in Lebanon, New Jersey. The USGS National Earthquake Information Center reported a focal depth of about 5 km, consistent with a lack of signal in Interferometric Synthetic Aperture Radar data. The focal mechanism solution is strike slip with a substantial thrust component. Neither mechanism’s nodal plane is parallel to the primary northeast trend of geologic discontinuities and mapped faults in the region, including the Ramapo fault. However, many of the relocated aftershocks, for which locations were augmented by temporary seismic deployments, form a cluster that parallels the general northeast trend of the faults. The aftershocks lie near the Tewksbury fault, north of the Ramapo fault. 

Seafloor geodesy reveals rapid up-dip afterslip following the M w 8.2 Chignik subduction zone earthquake. 

Abstract Subduction zone accretionary prisms are commonly modeled as elastic structures where permanent deformation is accommodated by faulting and folding of otherwise elastic materials, yet accretionary prisms may exhibit other deformation styles over relatively short time scales. In this study, we use 6.5‐year (2014–2021) Sentinel‐1 interferometric synthetic aperture radar (InSAR) time‐series of post‐seismic deformation in the Makran accretionary prism of southeast Pakistan to characterize non‐linear viscoelastic deformation within an active accretionary prism on short timescales (months to years). We constructed a series of 3‐D finite‐element models of the Makran subduction zone, including an accretionary prism, and constrained the elastic thickness of the upper wedge and the flow‐law parameters (power‐law exponent, activation enthalpy, and pre‐exponential constant) of the lower wedge through forward model fits to the InSAR time‐series. Our results show that the prism is elastically thin (8–12 km) and the non‐linear viscoelastic relaxation of the deep portions of the prism alone can sufficiently explain the post‐seismic surface deformation. Our best fitting flow‐law parameters (n = 3.76 ± 0.39,Q = 82.2 ± 37.73 kJ mol⁻¹, andA = 10^{−3.36±4.69}) are consistent with triggering of low temperature dislocation creep within fluid‐saturated siliciclastic rocks. We believe that the fluids necessary for this weakening originate from sedimentary underplating and/or the presence the hydrocarbons. The presence of power‐law rheology within the lower wedge impacts the estimated plate coupling and the stress state in the subduction system, with respect to the conventional elastic wedge model, and hence should to be considered in future earthquake cycle models. 

Abstract Repeated earthquake cycles produce topography, fault damage zones, and other geologic structures along faults. These geomorphic and structural features indicate the presence of co‐seismic permanent (inelastic) surface deformation, yet a long‐standing question in earthquake research is how much of the co‐seismic deformation field is elastic versus inelastic. These questions arise in part because it is unclear what measurable co‐seismic characteristics, such as off‐fault or distributed surface deformation and cracking, represent true unrecoverable deformation. One emerging descriptor of permanent co‐seismic deformation is surface strain magnitudes inferred from imaging geodesy observations. In this study, we present the surface strain field of the 2013 Mw7.7 Baluchistan strike‐slip earthquake in southern Pakistan. We invert co‐seismic displacement fields generated from pixel‐tracking of SPOT‐5 and WorldView optical imagery for co‐seismic surface horizontal strain tensors. We observe that co‐seismic strain field is dominated by negative dilatation strains, indicating that the co‐seismic fault zone contracted during the earthquake. We show that co‐seismic inelastic failure exhibits a relatively consistent width along the rupture that is localized to a zone 100–200 m wide on the hanging wall side. The width of co‐seismic permanent deformation does not correlate with variations in off‐fault deformation or surface geology. Based on comparisons to other recent earthquakes, we posit that the permanent surface strains reflect inelastic deformation of the faults inner damage zone, and that the width of this zone reflects fault maturity.