The 2015-2017 Pamir earthquake sequence: foreshocks, main shocks and aftershocks, seismotectonics, fault interaction and fluid processes

A sequence of three strong (M(W)7.2, 6.4, 6.6) earthquakes struck the Pamir of Central Asia in 2015-2017. With a local seismic network, we recorded the succession of the foreshock, main shock and aftershock sequences at local distances with good azimuthal coverage. We located 11 784 seismic events and determined 33 earthquake moment tensors. The seismicity delineates the tectonic structures of the Pamir in unprecedented detail, that is the thrusts that absorb shortening along the Pamir's thrust front, and the strike-slip and normal faults that dissect the Pamir Plateau into a westward extruding block and a northward advancing block. Ruptures on the kinematically dissimilar faults were activated subsequently from the initial M-W 7.2 Sarez event at times and distances that follow a diffusion equation. All main shock areas but the initial one exhibited foreshock activity, which was not modulated by the occurrence of the earlier earthquakes. Modelling of the static Coulomb stress changes indicates that aftershock triggering occurred over distances of <= 90 km on favourably oriented faults. The third event in the sequence, the M-W 6.6 Muji earthquake, ruptured despite its repeated stabilization through stress transfer in the order of -10 kPa. To explain the accumulation of M-W > 6 earthquakes, we reason that the initial main shock may have increased nearby fault permeability, and facilitated fluid migration into the mature fault zones, eventually triggering the later large earthquakes.