Seismic activity of the 2015 Nepal Ms8.1 and Ms7.5 earthquakes

The 2015 Ms8.1 Nepal earthquake occurred as the result of thrust faulting with low dip angle unilateral rupture along the Himalayan seismic belt. The aftershocks spread over a region with dimension of 170 km×60 km, striking along the direction of WNW-ESE. The mainshock and the strong aftershocks are all located at the two ends of this seismic region, while only few earthquakes occur in the middle, where the main rupture of the Ms8.1 earthquake occurs. The strongest aftershock of Ms7.5 is located at the east end and is significantly triggered by the main shock. The Ms8.1 and Ms7.5 event occurred in the seismic gap between the 1505 Mw8.2 Lo Mustang earthquake and the 1934 Mw8.1 Nepal-Bihar earthquake. Moreover no earthquakes with M ≥ 6.0 occurs in the seismic gap since 1870. In the last 13 years, there exists a seismogenic gap of 590 km long without M ≥ 5.0 earthquakes. Nineteen months before the 2015 Ms8.1 Nepal event, the seismogenic gap was broken. Noticed that the Ms8.1 earthquake sequence is located in the middle of the existing seismogenic gap, whose east and west parts are still quiescent. The calculated static Coulomb stress changes show that the Ms8.1 earthquake has apparent stress triggering effect to both sides of the faults. Taking into account the historical earthquake activity and the recurrence period, the seismic potential in the rupture zone of the 1505 Mw8.2 is higher, comparing to the rupture zone of the 1934 Mw8.4. The occurrences of the 2005 Mw7.6 Pakistan event and the 2015 Ms8.1 Nepal earthquake indicate that the Himalayan seismic belt has started a relatively active phase for Mw ≥ 7.0 earthquakes.