UC Berkeley

Rupture in the 2015 M7.8 Gorkha earthquake nucleated at the downdip edge of the Main Himalayan Thrust (MHT) near the transition from interseismic locking to aseismic creep beneath the Tibetan plateau, and propagated incompletely towards the Main Frontal Thrusts (MFT). Despite the imposition of a substantial static strain in the mid-décollement, afterslip on the MHT within a year of the earthquake had decayed to negligible levels. Earthquakes that incompletely rupture the MHT (7 < Mw < 7.9) have been relatively common in the past two centuries, and as a consequence heterogeneous patches of stored elastic strain must exist throughout the Himalaya similar to that emplaced by the Gorkha earthquake. We show that these patches of stored strain are not dissipated by creep or by subsequent updip earthquakes, with the possible exceptions of a sequence of moderate earthquakes to the west of the great 1950 Assam earthquake, and to the east of the Kangra 1905 earthquake. It is thus considered likely that mid-décollement strain newly imposed by the Gorkha earthquake, and other recent incomplete ruptures will be incorporated in the rupture of a future much larger earthquake. Incomplete ruptures (i.e. those that nucleate downdip but fail to rupture the frontal thrusts) appear to occur preferentially in parts of the central Himalaya characterized by relatively narrow transition regions of interseismic decoupling (<30 km downdip). Assuming uniform strain at failure these narrow zones are unable to store large amounts of strain energy compared to wide zones of interseismic decoupling. Since the transition from fully locked to a fully creeping rheology depends partly on temperature, to first order the width of the interseismic decoupling transition zone depends on the local dip of the MHT. Where the decoupling zone is narrow (25 km) moderate earthquakes (6 < Mw < 7) are observed to occur at intervals of a few hundred years. Where the transition zone is wide (e.g. Kashmir and Assam, 150 km) great earthquakes nucleate at long time intervals (millennia). Because the cumulative moment release of moderate earthquakes in regions of narrow seismic decoupling is insufficient to keep up with plate convergence, we conclude that megaquakes that eventually sweep through these regions are augmented by the heterogenous fossil strain of former incomplete ruptures. Because great earthquakes in the central Himalaya are inferred to nucleate from moderate earthquakes near the base of the MHT, the preparation zones of these moderate earthquakes may provide opportunities for forecasting the approach of future great earthquakes.