GORKHA EARTHQUAKE 2015, NEPAL: PRESENT KNOWLEDGE AND FUTURE RESEARCH DIRECTION

Although Nepal has been anticipating a large earthquake at any time, the Magnitude 7.8 Gorkha Earthquake of 25th April, 2015 was somewhat surprising. Instead of rupturing the entire western Nepal seismic gap as expected, it started along the eastern side of the seismic gap and ruptured eastward--even partly overlapping the rupture zone of 1934 Bihar-Nepal earthquake, but not achieving the Magnitude 8.0-8.6 that was expected. The N-S array of GPS stations along the Marsyangdi River section in western Nepal nicely defined a rupture boundary which surprisingly also acted as a damping zone for westward travel of energy from the quake. Lower than expected shaking level of Kathmandu valley was another surprise. Knowledge of Himalayan seismotectonics and its earthquake behaviour is presently at best considered only imperfectly known and a concerted national and international effort is needed in order better understand and mitigate earthquake hazard in Nepal. The failure of the earthquake to completely rupture the Himalayan décollement and consequent stress build up in the frontal part of the rupture zone is a concern and needs urgent multidimensional research initiatives.

Gorkha Earthquake is the first major earthquake in the central Himalaya providing a rare opportunity for instrumental recordings of the main shock, aftershocks, and post-earthquake deformation. Other possible research directions include CO2, radon emissions, and magnetotelluric. Although few faults have been trenched in Nepal, those that have greatly refined our understanding of the earthquake history in the region, but did not get through yet into the mainstream research. Mapping and additional trenching of the Main Frontal Thrust (MFT) region will help bridge the gap between geology and seismology.

Earthquake-induced landslides killed hundreds of people. Steep sloped valleys just south of the MCT where the GPS recordings show the highest uplift rates during the interseismic periods seem to be the worst hit areas.

In the last few years, Nepal did work on some training and capacity building on disaster management including simulation exercises. This immensely helped in search and rescue operation. National capacity building, both in scientific research as well as in disaster management, needs to be strengthened to prepare for future disasters.