Kaj M. Johnson

Interseismic locking on the Main Himalayan Thrust (MHT) governs where crustal strain and seismic moment accumulate, yet published locking inversions vary substantially with assumptions about fault boundary and coupling kinematics. We combine Himalayan Global Navigation Satellite System (GNSS) measurements with a viscoelastic earthquake-cycle model that includes interseismic viscous flow in the lower crust and mantle and an approximation for gradual migration of the locked−creeping boundary on the MHT. We invert baseline length-change rates and evaluate 20 model configurations that vary fault-block kinematics and lower-crustal and mantle viscosity structure. Across the ensemble, strong locking is concentrated above mid-crustal ramp segments, whereas uncertainty peaks near ramp transitions and along the downdip edge of locking. Increasing lower-crustal and mantle viscosity shifts the inferred locked zone farther downdip. Inferred strongly negative process zone stressing rates along the bottom boundary of locking across much of the margin suggest up-dip propagation of creep into the locked zone. A stationary end-member with zero process zone stressing rate fits the data more poorly and distributes moment accumulation broadly across the interface from the surface to ∼45 km depth, producing a gradual transition from locking to full creep. In contrast, the nonstationary solutions concentrate nearly all moment accumulation within the locked zone and exhibit a rapid transition to full creep outside it, yielding locked-zone moment accumulation rates roughly twice those inferred from the stationary model. These results refine Himalayan moment-budget estimates and highlight how rheology and boundary dynamics influence geodetic coupling inferences.

Geodetic and seismic observations before the 2011 M _w 9.0 Tohoku‐oki earthquake indicate decadal‐scale acceleration of aseismic slip along the northern Japan Trench, but the mechanism remains debated. We show that this transient can be explained by creep‐front propagation that progressively erodes heterogeneously locked asperities. We invert GNSS velocity fields from 1998 to 2009 using a mechanically motivated model in which locked regions are confined to historical rupture areas while their effective sizes and process‐zone stressing rates evolve. The models fit both velocity fields and predict slip accelerations consistent with repeating‐earthquake estimates. Inferred asperities occupied about 66% of their corresponding rupture areas in 1998 and about 37% in 2009, indicating substantial expansion of aseismic creep over the decade before rupture. The inferred shrinkage rates are consistent with fracture‐mechanics predictions for plausible coseismic stress drops. These results suggest that megathrust locking can evolve substantially over decadal timescales.

Interseismic coupling maps are important tools for estimating future earthquake hazard at subduction zones. Often interseismic coupling is estimated from surface velocities using a homogeneous elastic Earth model. However, inversions that incorporate viscoelastic mantle flow or elastic heterogeneity show that each can change the spatial distribution and magnitude of estimated coupling. We present here a Bayesian inversion of surface deformation for the boundaries of coupling zones at the Cascadia subduction zone that incorporates both elastic heterogeneity and a viscoelastic earthquake cycle model. We find that elastic heterogeneity deepens inferred coupling and worsens fit to surface deformation compared to homogeneous elastic models, but viscoelasticity partly offsets these effects and yields similar coupling and a better fit to the vertical data than homogeneous elastic models. We also find that, of all geometries and viscosities tested, downgoing plate thickness and oceanic mantle viscosity have the greatest impact on modeled displacements and coupling.

Geodetic and seismic observations before the 2011 Mw 9.0 Tohoku-oki earthquake indicate decadal-scale acceleration of aseismic slip along the northern Japan Trench, but the mechanism remains debated. We show that this transient can be explained by creep-front propagation that progressively erodes heterogeneously locked asperities. We invert GNSS velocity fields from 1998 and 2009 using a mechanically motivated model in which locked regions are confined to historical rupture areas while their effective sizes and process-zone stressing rates evolve. The models fit both velocity fields and predict slip accelerations consistent with repeating-earthquake estimates. Inferred asperities occupied about 66% of their corresponding rupture areas in 1998 and about 37% in 2009, indicating substantial expansion of aseismic creep over the decade before rupture. The inferred shrinkage rates are consistent with fracture-mechanics predictions for plausible coseismic stress drops. These results suggest that megathrust locking can evolve substantially over decadal timescales.

Reconciling geodetic and geologic deformation remains a fundamental challenge in tectonic studies of active orogens. Elastic rebound theory posits that present‐day deformation differs from deformation recorded over time scales longer than the earthquake cycle because locked faults store and release strain. We reconcile geodetic and millennial time‐scale deformation in Taiwan with a three‐dimensional elastic plate‐based kinematic model that integrates millennial time scale geologic uplift records with geodetic measurements to resolve fault slip rates and interseismic locking. We incorporate slip on upper‐crustal faults and a basal detachment in an elastic crust overlying an inviscid mantle. We jointly estimate slip and locking across faults, basal detachment, and subduction interfaces. The results show that slip on the detachment drives the majority of orogen‐wide uplift in the mountain belt west of the Eurasia‐Philippines Sea plate suture, producing consistent geologic and geodetic vertical rates of up to 10–12 mm/yr in the Central Range. Interseismic locking concentrates primarily on upper‐crustal faults and is nearly absent on the detachment. Millennial‐time scale uplift in the Coastal Range arises largely from the Longitudinal Valley Fault, which accommodates both high dip‐slip and strike‐slip motion. Present‐day widespread subsidence of the Coastal Range is attributed to interseismic locking of the Longitudinal Valley Fault. Our model resolves creeping and locked sections of faults including widespread low coupled faults in the southwest fold and thrust belt. Equivalent moment magnitudes computed by combining slip rates and locking probabilities highlights the substantial contribution of strike‐slip motion on faults, which geology alone cannot resolve.

The Alpine‐Himalayan belt is one of Earth's most dynamic and complex regions, characterized by intense tectonic deformation and seismicity. Comprehensive analyses of continental‐scale crustal deformation and seismic hazards along this extensive orogenic belt require the compilation of large geodetic data sets. In this study, we integrate 42 published Global Navigation Satellite System (GNSS) velocity fields, building an internally consistent data set for the entire belt, spanning from Iberia to Southeast Asia and comprising 11,177 horizontal and 3,940 vertical velocities. We use this unified GNSS velocity field to estimate surface strain rates and their posterior uncertainties in the eastern Mediterranean region and the India‐Asia collision zone. Our results show large‐scale agreement between the orientation and style of geodetic and seismic strain rate tensors across the belt. Additionally, our analyses substantiate previously documented azimuthal alignments between principal strain rate directions and seismic anisotropy orientations, often used as a proxy for finite strain in the convecting mantle. These correlations are particularly apparent in the Aegean, North Anatolia, Tibet, Tian Shan, Altai, Sayan, and Baikal regions, underscoring the need for future research on the relationship between mantle flow and lithospheric deformation.

The Alpine-Himalayan belt is one of Earth's most dynamic and complex regions, characterized by intense tectonic deformation and seismicity. Comprehensive analyses of continental-scale crustal deformation and seismic hazards along this extensive orogenic belt require the compilation of large geodetic datasets. In this study, we integrate 42 published Global Navigation Satellite System (GNSS) velocity fields, building an internally consistent dataset for the entire belt, spanning from Iberia to Southeast Asia and comprising 11,177 horizontal and 3,940 vertical velocities. We use this unified GNSS velocity field to estimate surface strain rates and their posterior uncertainties in the eastern Mediterranean region and the India-Asia collision zone. Our results show large-scale agreement between the orientation and style of geodetic and seismic strain rate tensors across the belt. Additionally, our analyses substantiate previously documented azimuthal alignments between principal strain rate directions and seismic anisotropy orientations, often used as a proxy for finite strain in the convecting mantle. These correlations are particularly apparent in the Aegean, North Anatolia, Tibet, Tian Shan, Altai, Sayan, and Baikal regions, underscoring the need for future research on the relationship between mantle flow and lithospheric deformation.

Interseismic coupling maps and, especially, estimates of the location of the fully coupled (locked) zone relative to the trench, coastline, and slow slip events are crucial for determining megathrust earthquake hazard at subduction zones. We present an interseismic coupling inversion that estimates the locations of the upper and lower boundaries of the locked zone, the lower boundary of the deep transition zone, and downdip gradient of creep rate in the transition from locked to freely creeping in the downdip transition zone. We show that the locked zone at Cascadia is west of the coastline and 10 km updip of the slow slip zone along much of the margin, widest (25–125 km, extending to ∼19 km depth) in northern Cascadia, narrowest (0–70 km) in central Cascadia, with moment accumulation rate equivalent to a M_w 8.71 and M_w 8.85 earthquake for 300‐ and 500‐year earthquake cycles. We find a steep gradient in creep immediately below the locked zone, indicative of propagating creep, along the entire margin. At Nankai, we find three distinct zones of locking (offshore Shikoku, offshore southeast Kii peninsula, and offshore Shima peninsula) with a total moment accumulation rate equivalent to a M_w 8.70 earthquake for a 150‐year earthquake cycle. The bottom of the locked zone is nearly under the coastline for all three locked regions at Nankai and is positioned 0–5 km updip of the slow slip zone. In contrast with Cascadia, creep rate gradients below the locked zone at Nankai are generally gradual, consistent with stationary locking.

Fault slip rates estimated from geodetic data are being integrated into seismic hazard models. The standard approach requires modeling velocities and relative (micro‐)plate motions, which is challenging for fault‐based models. We present a new approach to directly invert strain rates to solve for slip rates and distributed strain simultaneously. We generate velocity and strain rate fields over the southeastern Tibetan Plateau, utilizing Sentinel‐1 Interferometric Synthetic Aperture Radar data spanning 2014–2023. We derive slip rates using block modeling and by inverting strain rates. Our results show a partitioning between localized strain on faults and distributed deformation. The direct inversion of strain rates matches the geodetic data best when incorporating distributed moment sources, accounting for a similar proportion to on‐fault sources. The direct strain methodology also aligns best with the independent geological slip rates, especially near fault tips. As high‐resolution strain rate fields become increasingly available, we recommend direct inversion as the preferred practice.

Interseismic coupling maps and, especially, estimates of the location of the fully coupled (locked) zone relative to the trench, coastline, and slow slip events are crucial for determining megathrust earthquake hazard at subduction zones. We present a physically motivated interseismic coupling inversion that explicitly incorporates locked zones with boundaries bordering an updip transition zone creeping at constant stress and a downdip transition zone with creep rate distributions consistent with updip propagation of the creep front into the locked zone. We show that the locked zone at Cascadia is west of the coastline and 10 km updip of the slow slip zone along much of the margin, widest (25-125 km, extending to ~22 km depth) in northern Cascadia, narrowest (0-70 km) in central Cascadia, with moment accumulation rate equivalent to a Mw 8.78 and Mw 8.89 earthquake for 300- and 500-year earthquake cycles. We find a steep gradient in creep immediately below the locked zone, indicative of propagating creep, along the entire margin. At Nankai, we find three distinct zones of locking (offshore Shikoku, offshore southeast Kii peninsula, and offshore Shima peninsula) with a total moment accumulation rate equivalent to a Mw 8.73 earthquake for a 150-year earthquake cycle. The bottom of the locked zone is nearly under the coastline for all three locked regions at Nankai and is positioned 0-5 km updip of the the slow slip zone. In contrast with Cascadia, creep rate gradients below the locked zone at Nankai are generally gradual, consistent with stationary locking.