Lithospheric Deformation Due To the 2015 M7.2 Sarez (Pamir) Earthquake Constrained by 5 years of Space Geodetic Observations

The 2015 M7.2 Sarez (Pamir) earthquake occurred at the north-west margin of the Tibetan Plateau. We use Sentinel-1 and ALOS-2 Synthetic Aperture Radar and Global Navigation Satellite System data to investigate coseismic and postseismic deformation due to the Sarez earthquake. Kinematic inversions show that the earthquake ruptured a ∼80 km long, sub-vertical fault producing the maximum surface offset of 3–4 m on the south-west and central fault segments. In contrast, the largest postseismic displacements are observed at the north-east end of the earthquake rupture, predominantly on the west (hanging wall) side of the fault with an average rate of 20–30 mm/yr in the satellite line of sight. We use the derived coseismic and postseismic slip models to investigate mechanisms of time-dependent relaxation, stress transfer and possible triggering relationships between the Sarez earthquake and a sequence of strong M6+ events that occurred within ∼100 km of the 2015 earthquake. We find that the near-field postseismic displacements are best explained by shallow afterslip driven by the coseismic stress changes. The data also allow some contribution from poroelastic rebound, but do not show a clear signature of viscoelastic relaxation in the lower crust and upper mantle during the observation period, suggesting a lower bound on the effective viscosity of ∼10 19 Pa s. A pair of M6+ events that occurred within 100 km and several months of the 2015 mainshock have experienced near-zero and in some cases negative static Coulomb stress changes, suggesting either delayed dynamic triggering, or no relation to the mainshock. Plain Language Summary Large earthquakes are often followed by slow deformation that results from redistribution and relaxation of coseismic stress changes in the host rocks. The patterns and rates of postseismic deformation can be used to learn about the properties of rocks at depth. We analyzed surface deformation that occurred during and after a major (magnitude 7.2) earthquake that occurred in the Pamir orogen at the north-west margin of Tibet. We used radar imagery from satellites of the European and Japanese space agencies, as well as data from the Global Navigational Satellite System to measure subtle (centimeterscale) displacements of the Earth's surface that occurred within 100 km from the earthquake epicenter. A combination of coseismic and postseismic displacement data reveals that the earthquake rupture split into two branches as it was propagating from south-west to north-east. We found evidence of slow aseismic creep (“afterslip”) at the north-east end of the earthquake rupture. In contrast, we do not observe a broad pattern of displacements expected from enhanced viscous flow in the lower crust and/or upper mantle. The lack of the respective signal argues for a relatively strong “ductile” portion of the Tibetan lithosphere.