Various mechanisms have been proposed to explain the transient, enhanced surface deformation rates following earthquakes. Unfortunately, these different mechanisms can produce very similar surface deformation patterns leading to difficulty in distinguishing between them. Here we return to the observations themselves and compile near‐field postseismic velocity measurements following moderate to large continental earthquakes. We find that these velocities have a remarkably consistent pattern, with velocity inversely proportional to time since the earthquake. This suggests that postseismic velocities show an Omori‐like decay and that postseismic displacements increase logarithmically over time. These observations are inconsistent with simple, linear Maxwell or Burgers body viscoelastic relaxation mechanisms but are consistent with rate‐and‐state frictional afterslip models and power law shear zone models. The results imply that near‐field postseismic surface deformation measurements are primarily the result of fault zone processes and, therefore, that the inference of lower crustal viscosities from near‐field postseismic deformation requires care.

S U M M A R YThe extent to which aseismic deformation relaxes co-seismic stress changes on a fault zone is fundamental to assessing the future seismic hazard following any earthquake, and in understanding the mechanical behaviour of faults. Here we use models of stress-driven afterslip and viscoelastic relaxation, in conjunction with post-seismic InSAR measurements, to show that there has been minimal release of co-seismic stress changes through post-seismic deformation following the 2003 M w 6.6 Bam earthquake. Our analysis indicates the faults at Bam remain predominantly locked, suggesting that the co-plus interseismically accumulated elastic strain stored downdip of the 2003 rupture patch may be released in a future M w 6 earthquake. Our observations and models also provide an opportunity to probe the growth of topography at Bam. We find that, for our modelled afterslip distribution to be consistent with forming the sharp step in the local topography over repeated earthquake cycles, and also to be consistent with the geodetic observations, requires either (1) far-field tectonic loading equivalent to a 2-10 MPa deviatoric stress acting across the fault system, which suggests it supports stresses 60-100 times less than classical views of static fault strength, or (2) that the fault surface has some form of mechanical anisotropy, potentially related to corrugations on the fault plane, that controls the sense of slip.

The geometry and frictional properties of a fault system are key parameters required to understand its seismic behavior. The Main Himalayan Thrust in Nepal is the type example of a continental megathrust and forms part of a fault system which accommodates a significant fraction of India‐Eurasia convergence. Despite extensive study of this zone of shortening, the geometry of the fault system remains controversial. Here, we use interseismic, coseismic, and postseismic geodetic data in Nepal to investigate the proposed downdip geometries. We use interseismic and coseismic data from previous studies, acquired before and during the 2015 Mw 7.8 Gorkha earthquake. We then supplement these by processing our own postseismic deformation data, acquired following the Gorkha earthquake. We find that kinematic modeling of geodetic data alone cannot easily distinguish between the previously proposed geometries. We therefore develop a mechanical joint coseismic‐postseismic slip inversion which simultaneously solves for the distribution of coseismic slip and rate‐strengthening friction parameters. We run this inversion using the proposed geometries and find that they are all capable of explaining the majority of geodetic data. We find values for the rate parameter, a, from the rate‐and‐state friction law that are between 0.8 and 1.6×10−3, depending on the geometry used. These values are in agreement with results from laboratory studies and those inferred from other earthquakes. We suggest that the limitations of earthquake cycle geodesy partly explain the continued controversy over the geometry and role of various faults in the Nepal Himalaya.