The Nepal Himalaya had experienced the last great earthquake of Mw8.0 in 1934. However, the recent GPS studies have reported a high shortening rate of 18-20 mm/year in the Nepal Himalaya. Further, modelling of GPS data also suggested a slip deficit of 6.6E+19 N-m/year due to locking of main Himalayan thrust in the central Himalaya (Ader et al., 2012). Thus, it was predicted that a massive earthquake of M8.5+ is due for the central Himalaya. Following this prediction, a large earthquake of Mw7.8 took place on 25 April 2015 in the Nepal Himalaya, and it was followed by the occurrence of an earthquake of Mw7.3 on 12 May 2015. We performed a DC (double couple) constrained multiple point source moment-tensor inversion on the band-passed (0.08–0.10 Hz) displacement data of the 25 April 2015 Nepal mainshock of Mw7.8, from 17 broadband stations in India, which suggests a north-dipping thrust faulting (strike = 324o, dip = 14o, rake = 88o) at 16 km depth. The modelled fault plane is assumed to be coinciding with the MHT in the region. Coulomb failure stress changes (DCFS) have been modelled using the slip distribution on the fault plane of the 25 April mainshock. Modelling results suggest a strong correlation with occurrences of aftershocks and regions of increased positive DCFS below the aftershock zone of the 2015 Nepal mainshock (Fig.1a-d). A positive DCFS of 0.06 MPa is modelled at the focal depth (~16 km) of the 12 May 2015 Nepal event of Mw7.3. Based on modelling results, we infer that the 25 April event increased the Coulomb stress changes by 0.06 MPa at 16 km depth (Fig.1d) below the site of the 12 May event, and thus, this event can be termed as triggered (Fig.1e). Moreover, these small modelled stress changes can lead to trigger events if the crust is already near to failure, but these small stresses can also advance the occurrence of future earthquakes. Hence, we propose that the mainshock slip on the MHT not only cause the seismic hazard in the Himalaya; instead, the occurrence of a large triggered event on the MHT can also enhance our understanding of the seismic hazard in the Nepal Himalaya.
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Figure 1: Coulomb failure stress changes due to the slip distribution with a maximum slip of 5.8 m on the rupture plane of the 25 April 2015 Nepal earthquake of Mw7.8: (a) at 16 km depth (using geometry of the causative fault of 25 April event as obtained from USGS’s MT inversion), (b) at 12 km depth (using geometry of the causative fault of 25 April event as obtained from Harvard’s MT inversion), (c) at 17 km depth [using geometry of the causative fault of 25 April event as obtained from Mitra et al.’s (2015) MT inversion], and (d) at 16 km depth (using geometry of the causative fault of 25 April event as obtained from our DC-constrained MT inversion). Large solid light blue circles mark locations of the 25 April 2015 (Mw7.8) and the 12 May 2015 (Mw7.3) earthquakes. Medium-sized solid blue circles mark the M6 aftershocks, while small sized white solid circles mark M5 aftershocks. (e) Hypothetical Geodynamic model is showing the occurrences of 2015 Nepal earthquakes on the main Himalayan Thrust (MHT) of Nepal Himalaya. Large solid red circles mark the locations of the 25 April 2015(Mw7.8) and the 12 May 2015(Mw7.3) earthquakes. MFT, MBT, PT2 and MCT represent the main frontal thrust, main boundary thrust, Patu thrust and main central thrust (after Sapkota et al., 2013).