Movie S1Correction: In table S1, the displacement at station SNDL was reported erroneously. The correct displacement is: east, 0.047 ±0.002 m; north, -0.223 ±0.003 m; vertical, 0.003 ±0.003 m. The PDF has been corrected.

The Rakhine (Arakan)‐Bangladesh megathrust, along which the Indian and Burma plates collide, is assumed by some to be inactive/aseismic due to the lack of notable interplate earthquakes in the modern instrumental catalog. However, geological and historical evidence of the great 1762 Arakan earthquake suggest the megathrust can produce M~8 events that could adversely affect the lives of millions of people in the region. To investigate the seismogenic potential and determine the slip budget of the megathrust, we first need to solve for India‐Burma‐Sunda relative plate motions. We present a new set of 24 GPS velocities (2011–2017) from the Myanmar‐India‐Bangladesh‐Bhutan continuous GPS network. We use the new velocities and those from previously published studies to explore the geometries and slip rates of three major faults (Rakhine‐Bangladesh megathrust, Churachandpur‐Mao Fault, and Sagaing Fault) that accommodate the India‐Burma‐Sunda plate motion. Our results suggest that the three major faults we studied are likely fully coupled; the modern shortening rate across the Burma plate is 12–24 mm/year, while the total dextral shear rate is 25–32 mm/year. The possibly fully coupled shallow megathrust, and splay faults that may sole into it, are geodetically invisible while they are not slipping. However, we can identify the transition from coupling to steady creep on the deeper extension of the megathrust; we use this to show active oblique India‐Burma convergence and to map along‐strike and along‐dip variations in dip‐slip and strike‐slip motion. This implies that the megathrust is currently accumulating strain which will eventually be released in earthquakes.

We use high resolution interferometric synthetic aperture radar and GPS measurements of crustal motion across the southern San Andreas Fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths. Geodetically measured strain rates are asymmetric with respect to the mapped traces of both the southern San Andreas and San Jacinto faults. Two possibilities have been proposed to explain this observation: large contrasts in crustal rigidity across the faults, or an alternate fault geometry such as a dipping San Andreas fault or a blind segment of the San Jacinto Fault. We evaluate these possibilities using a two‐dimensional elastic model accounting for heterogeneous structure computed from the Southern California Earthquake Center crustal velocity model CVM‐H 6.3. The results demonstrate that moderate variations in elastic properties of the crust do not produce a significant strain rate asymmetry and have only a minor effect on the inferred slip rates. However, we find that small changes in the location of faults at depth can strongly impact the results. Our preferred model includes a San Andreas Fault dipping northeast at 60°, and two active branches of the San Jacinto fault zone. In this case, we infer nearly equal slip rates of 18 ± 1 and 19 ± 2 mm/yr for the San Andreas and San Jacinto fault zones, respectively. These values are in good agreement with geologic measurements representing average slip rates over the last 104–106 years, implying steady long‐term motion on these faults.

Interferometric synthetic aperture radar (InSAR) is a key tool for the analysis of displacement and stress changes caused by large crustal earthquakes, particularly in remote areas. A challenge for traditional InSAR has been its limited spatial and temporal coverage especially for very large events, whose dimensions exceed the typical swath width of 70–100 km. This problem is addressed by the ALOS‐2 satellite, whose PALSAR‐2 instrument operates in ScanSAR mode, enabling a repeat time of 2 weeks and a swath width of 350 km. Here we present InSAR line‐of‐sight displacement data from ALOS‐2/PALSAR‐2 observations covering the Mw 7.8 Gorkha, Nepal earthquake and its Mw 7.3 aftershock that were acquired within 1 week of each event. The data are made freely available and we encourage their use in models of the fault slip and associated stress changes. The Mw 7.3 aftershock not only extended the rupture area of the main shock toward the east but also left a 20 km gap where the fault has little or no coseismic slip. We estimate this unslipped fault patch has the potential to generate a Mw 6.9 event.

Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 M w 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a décollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide.

We investigate the spatial pattern of surface creep and off‐fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries and survey‐mode GPS occupations of a dense array crossing the fault. Radar observations from Envisat during the period 2003–2010 were used to separate the pattern of horizontal and vertical motion, providing a high‐resolution image of uplift and shallow creep along the fault trace. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well‐defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1–2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two‐dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long‐term slip rates.

Most destructive tsunamis are caused by seismic slip on the shallow part of offshore megathrusts. The likelihood of this behaviour is partly determined by the interseismic slip rate deficit, which is often assumed to be low based on frictional studies of shallow fault material. Here we present a new method for inferring the slip rate deficit from geodetic data that accounts for the stress shadow cast by frictionally locked patches, and show that this approach greatly improves our offshore resolution. We apply this technique to the Cascadia and Japan Trench megathrusts and find that wherever locked patches are present, the shallow fault generally has a slip rate deficit between 80 and 100% of the plate convergence rate, irrespective of its frictional properties. This finding rules out areas of low kinematic coupling at the trench considered by previous studies. If these areas of the shallow fault can slip seismically, global tsunami hazard could be higher than currently recognized. Our method identifies critical locations where seafloor observations could yield information about frictional properties of these faults in order to better understand their slip behaviour.Megathrust faults at convergent tectonic margins produce devastating great earthquakes and tsunamis. Understanding their potential rupture behavior, particularly in the shallow offshore part of the fault where most destructive tsunamis are generated 1 , is therefore a critical task for geoscientists forecasting seismic and tsunami inundation hazards 2 .

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