Incomplete décollement rupture in the 25 April 2015 Gurkha earthquake; Implications for past and future Himalayan earthquakes

Abstract:

The failure of the 2015 Gurkha earthquake to completely rupture the Himalayan décollement from the southern edge of Tibet to the faults fronting the Himalayan foothills (MFT), raises several issues concerning the future evolution of stresses now imposed in the lesser Himalaya at the latitude of Kathmandu. InSAR imagery and coseismic GPS measurements indicate that slip in the main rupture has imposed a tapered slip distribution near the rupture’s southern edge at approximately 10 km depth, 30 km north of the MFT. Shortly after the earthquake new GPS instruments were installed, bringing to 15 the number of instruments monitoring the western and southern edge of the Gurkha rupture. In the first two months following the earthquake we observed slow continued southward displacements above the southern edge of the rupture at rates of 10-20 mm/yr, an order of magnitude larger than interseismic rates at these latitudes, which would dissipate the current slip deficit in ≈50 years. High uncertainties due to the monsoon onset and short duration of the time series prevent resolution of whether these rates are changing with time, and the budget of afterslip on the main rupture and creep outside of the coseismic rupture area. However, current rates are significantly slower than immediate afterslip rates observed on shallow California faults responding to similar coseismic slip. The similarity of the 2015 earthquake to Himalayan 7.5<Mw<7.8 earthquakes in 1803, 1833 and 1905 suggests that incomplete rupture of the Main Himalayan Thrust may be common. Occasional great earthquakes have ruptured the MFT with inferred slip approaching or exceeding 20 m and requiring renewal times of more than 1000 years at current GPS convergence rates. It appears possible that surface slip of the MFT in these great earthquakes may be supplemented by relict slip deficits from previous incomplete ruptures. One possibility, for example, is that a relatively modest rupture nucleating from the Himalayan locking line could exploit the stress associated with this slip deficit resulting in unexpectedly large slip on the southern décollement. This would add considerable complexity to the reliable estimation of moment magnitude from paleoseismic trench investigations.