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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. Plain Language Summary: The Main Himalayan Thrust fault marks the boundary where the Indian continent slides beneath the Eurasian plate, causing earthquakes like the 2015 magnitude 7.8 event in Nepal. Subsurface images constructed using seismic waves suggest a weak layer surrounding the fault. However, we show that the seismic signature of this layer changes depending on the direction in which the seismic waves travel through it. We compare this information on the subsurface structure to insights from static surface motions during the earthquake. We find that the fit to the motion is poor when we assume the subsurface rock around the fault has the same strength in both horizontal and vertical orientations. The fit improves when we assume the near‐fault rock is stronger under vertical compression than under horizontal compression. This assumption also helps explain the images constructed using seismic waves. We suggest that a strong oriented rock fabric develops in a channel around the plate boundary. The presence of this fabric may have influenced our estimates of fault slip before, during and after great earthquakes. Accurately describing this behavior is crucial for understanding the earthquake potential of plate boundary faults. Key Points: Seismic imaging suggests a Main Himalayan Thrust‐associated low‐velocity channel with north‐dipping anisotropic foliationModeling of InSAR and GNSS data together suggests a weak channel with anisotropic rigidity whose orientation matches seismic constraintsThe weak anisotropic plate boundary may be related to S‐C fabrics and influence the margin geodynamics on different time scales [ABSTRACT FROM AUTHOR]

Sets of marine terraces, sediments, and paleoshorelines are commonly found in forearc regions worldwide. A common assumption holds that crustal uplift prevents these features from littoral erosion. Here, we study the vertical deformation of Karpathos, a forearc island in the eastern Mediterranean, whose long axis extends at a high angle to the strike of the Hellenic Subduction System (HSS). We target three key coastal localities along the island to discuss spatial and temporal variability of vertical motion. We mapped sets of up to 19 marine terraces per locality, with elevations ranging from 1.5 to ∼350 masl. Ages for terraces and sediments are constrained by radiocarbon (<31 masl) and Sr‐isotope (2–310 masl) dating, and range from 2.4 ka to ∼4.3 Ma. Data analysis shows that average uplift rates are up to two orders of magnitude faster over shorter (⪅100 ka) than longer (⪆100 ka) timescales, in agreement with other local and global data sets. Further, we find evidence for multiple marine reoccupations of late Pleistocene terraces, indicating that carbonate beachrock is often resistant to multiple interactions with sea‐level. Neogene marine sequences that witness longer periods (∼4 Ma) show signs of alternating vertical motion. Using this novel data set, we explore the effects of various mechanisms (i.e., upper‐plate normal faulting, splay‐thrust faulting, basal underplating) on the spatial and temporal patterns of vertical deformation. Although the contribution of each mechanism to the net vertical deformation cannot be isolated with certainty, our results show that none alone could account for the observations. Plain Language Summary: In this study, we focus on understanding how the landmass of Karpathos Island, in the Hellenic Subduction System in the Mediterranean Sea, moved vertically over time. We focus mainly on marine terraces, which are wave‐cut platforms near the sea forming staircase‐like topography. The age of these platforms has been constrained by dating fossil shells, shell fragments, and soils that mantle the terraces: for young (closer to the sea‐level) markers we used the isotope Carbon‐14, while for older terraces (further inland) Strontium‐87. We found that the island of Karpathos subsided prior to ∼4.3 Ma, but since then it experiences uplift. From our analysis, we find that the vertical motion of the island is the result of, at least, three interrelated processes: (a) sediment underplating, that scrapes sediments from the down‐going plate during subduction and attaches them to the base of the upper plate, pushing it upwards; (b) large earthquakes that occur either at the interface of two converging plates or on large faults that splay from this interface; and (c) smaller earthquakes produced by the horizontal stretching of the Aegean crust. Key Points: Karpathos provides a natural transect across the Hellenic forearc for exploring vertical deformationNo single mechanism of vertical movement alone can explain the full range of observations and dataUnderplating, splay faulting, and normal faulting interact in space and time to produce overall uplift on Karpathos since ∼4.3 Ma [ABSTRACT FROM AUTHOR]

The shear force along convergent plate boundary faults (megathrusts) determines the height of mountain ranges that can be mechanically sustained.sup.1-4. However, whether the true height of mountain ranges corresponds to this tectonically supported elevation is debated.sup.4-7. In particular, climate-dependent erosional processes are often assumed to exert a first-order control on mountain height.sup.5-12, although this assumption has remained difficult to validate.sup.12. Here we constrain the shear force along active megathrusts using their rheological properties and then determine the tectonically supported elevation using a force balance model. We show that the height of mountain ranges around the globe matches this elevation, irrespective of climatic conditions and the rate of erosion. This finding indicates that mountain ranges are close to force equilibrium and that their height is primarily controlled by the megathrust shear force. We conclude that temporal variations in mountain height reflect long-term changes in the force balance but are not indicative of a direct climate control on mountain elevation. Simulations using a force balance model match mountain heights observed around the globe, suggesting that mountain elevation is almost completely controlled by tectonic forces rather than erosion., Author(s): Armin Dielforder [sup.1] , Ralf Hetzel [sup.2] , Onno Oncken [sup.1] Author Affiliations: (1) Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany (2) Institute of Geology [...]

Megathrust earthquakes are responsible for some of the most devastating natural disasters.sup.1. To better understand the physical mechanisms of earthquake generation, subduction zones worldwide are continuously monitored with geophysical instrumentation. One key strategy is to install stations that record signals from Global Navigation Satellite Systems.sup.2,3 (GNSS), enabling us to track the non-steady surface motion of the subducting and overriding plates before, during and after the largest events.sup.4-6. Here we use a recently developed trajectory modelling approach.sup.7 that is designed to isolate secular tectonic motions from the daily GNSS time series to show that the 2010 Maule, Chile (moment magnitude 8.8) and 2011 Tohoku-oki, Japan (moment magnitude 9.0) earthquakes were preceded by reversals of 4-8 millimetres in surface displacement that lasted several months and spanned thousands of kilometres. Modelling of the surface displacement reversal that occurred before the Tohoku-oki earthquake suggests an initial slow slip followed by a sudden pulldown of the Philippine Sea slab so rapid that it caused a viscoelastic rebound across the whole of Japan. Therefore, to understand better when large earthquakes are imminent, we must consider not only the evolution of plate interface frictional processes but also the dynamic boundary conditions from deeper subduction processes, such as sudden densification of metastable slab. Observed reversals in GNSS surface motions suggests greatly enhanced slab pull in the months preceding the great subduction earthquakes in Maule (Chile, 2010) and Tohoku-oki (Japan, 2011) of moment magnitudes 8.8 and 9.0., Author(s): Jonathan R. Bedford [sup.1] , Marcos Moreno [sup.2] , Zhiguo Deng [sup.1] , Onno Oncken [sup.1] [sup.3] , Bernd Schurr [sup.1] , Timm John [sup.3] , Juan Carlos Báez [...]

Triaxial deformation is a general feature of continental tectonics, but its controls and the systematics of associated fault networks remain poorly understood. We present triaxial analog experiments mimicking crustal thinning resulting from distributed longitudinal extension and lateral shortening. Contemporary longitudinal extension and lateral shortening are related by the principal horizontal strain ratio (PHSR). We investigate the effect of crustal geometry, rheology and strain rate on deformation localization, faulting regime and pattern, and PHSR in brittle and brittle‐viscous crustal‐scale models. We find that in brittle models the fault networks reflect the basal boundary condition and fault‐density scales inversely with brittle layer thickness. In brittle‐viscous models, as strain rate (ė) decreases, (a) Three fault patterns emerge: conjugate sets of strike‐slip faults (ė > 3 × 10−4 s−1, PHSR > 0.31), sets of parallel oblique normal faults (ė = 0.3–3 × 10−4 s−1, PHSR = 0.15–0.25), horst‐and‐graben system (ė < 0.3 × 10−4 s−1, PHSR < 0.1). (b) The strain localization increases systematically and gradually. We interpret the strain rate dependent of faulting regimes to be controlled by vertical coupling between the model upper mantle and model upper crust resulting in spontaneous permutation of principal stress axes. Rate‐dependency of strain localization can be related to mechanical coupling between the upper and lower crust. We identify the following parameters controlling triaxial tectonic deformation: upper crustal thickness and friction coefficient, lower crustal thickness and viscosity, as well as strain rate. We test our models and predictions against natural prototypes (Tibet, Anatolia, Apennines, and Basin and Range Province) thus providing new perspectives on triaxial deformation. Key Points: We conduct analog models where basal forces induce distributed deformation of continental tectonics under a triaxial deformation fieldStrain rate controls the strain localization and faulting regime via mechanical coupling between model upper crust, lower crust and mantleWe identify crustal geometry and rheology as factors controlling triaxial tectonics [ABSTRACT FROM AUTHOR]

The Kaikōura Earthquake uplifted Kaikōura Peninsula by ≤∼1 m. Uplift in 2016 mainly resulted from slip on an offshore thrust fault (OSTF), modelled to splay from the plate-interface, and was further influenced by slip on two newly identified faults (Armers Beach Fault, ABF; Te Taumanu Fault, TTF) mapped onshore from differential lidar (D-lidar). Forward dislocation modelling indicates that 2016 peninsula uplift can be reproduced by mean slip of ∼2.3 m on the OSTF and 0.25–0.5 m on the ABF and TTF. The variable co-seismic uplift recorded during the 2016 earthquake differs from the near-uniform (1.2 ± 0.2°) northwest tilting of MIS5c (96 ± 5 ka) and MIS5e (123 ± 5 ka) marine terraces; these ages are constrained by Optically Stimulated Luminescence (OSL) dating and correlation to sea-level curves. Tilting of Late Quaternary marine terraces can be primarily reproduced by slip rates of ∼0.8–2.7 mm/yr on the OSTF and 0.3–0.6 mm/yr on the ABF. Slip on the TTF is not required to produce tilting of the marine terraces, suggesting that it may have ruptured less frequently than the OSTF and ABF in the Late Quaternary. The OSTF links 2016 ruptures north and south of Kaikōura, with the earthquake rupturing an interconnected network of faults. [ABSTRACT FROM AUTHOR]