Characteristics of Fault Locking and Fault Slip Deficit in the Main Himalaya Thrust Fault

An Mw7.8 earthquake occurred in Nepal on April 25, 2015. The epicenter is (28.147°N, 84.708°E). It occurred as a result of fault thrusting on the main thrust interface between the subducting India plate and the overriding Eurasia plate to the north. The movement and locking of the main Himalaya thrust (MHT)fault and its seismic potential need us to do some further study. We collected 6 sets of GPS horizontal velocity field covering Indian plate, Qinghai-Tibet plateau and adjacent areas, and transformed them to an approximately unified reference frame. Then we used the negative dislocation model of DEFNODE to invert the spatial fault locking and fault slip deficit in the MHT, and analyzed the far-near field movement of the fault combining with the GPS velocity profiles.The results show that the locking depth of the MHT is about 18-24 km, and the locking width of the fault plane reaches 102-136 km. The locking depth is deeper and the locking width of the plane is wider in the fault between two major historical earthquakes rupture area and the fault of no historical earthquakes. The Mw7.8 Nepal Earthquake is located in the area between two major historical earthquakes rupture area where the fault is relatively deeply and tightly locked. The total slip deficit and vertical compressional slip deficit of the MHT gradually reduce from east to west. The vertical compressional slip deficit of the easternmost segment of the fault is about 28.0 mm/a, which gradually reduces in the tightly locked area, and decreases to about 6.4 mm/a at the westernmost segment. The dextral strike-slip of parallel slip deficit changes from 0.4-7.0 mm/a from east to west in Nepal. The three profile model fitting results of the MHT are consistent with the slip deficit of corresponding fault plane, and it is very like the profile results across the middle segment of the Longmenshan fault before the 2008 Wenchuan earthquake, which reflects that the MHT is tightly locked. We estimated that the Mw7.8 earthquake recurrence is about 230 years according to the peak slip and the vertical compressional slip deficit at the epicenter. Because only the 1905 India Kangra 8.0 earthquake, the 1934 Nepal Bihar 8.1 earthquake and this Nepal Mw7.8 earthquake happened for recent 500 years, it is possible that M8 earthquake happens on this fault especially on the border of Nepal and India and possibly unruptured Bhutan.