Nepal Earthquake of 2015

This chapter mentions about need of a book on different seismological aspects of the devastating Nepal earthquake of 2015 for Nepal, India and other earthquake-prone countries of the World. It informs that how India has been affected by large earthquakes in Nepal, hence, knowledge about the Nepal 2015 earthquake is of great interest to India. It is also important for all the developing countries where buildings are mostly not designed for earthquake resistance. A new understanding has come out of amplification of low as well as high frequencies over a deep basin like that of Kathmandu. The knowledge of very high acceleration recorded for low frequencies, not hitherto considered would be useful for designing sky scrapers world-wide. This chapter introduces the contents of different chapters of the book.

This chapter gives seismic parameters of Mw 7.8 April 25, 2015 Gorkha, Nepal earthquake. The salient aspects of tectonics, slip modeling, geodetic deformation, rupture process, aftershocks, strong ground motion and damage caused are highlighted.

Nepal has varied topography from low lands (EL 60 m) in the south to high mountains in the north including the World’s highest peak Mt. Everest (Fig. 2.1). It is in the center of the concave Himalayan belt with about 870 km in WNW-ESE direction and 130–260 km in N-S direction lying between latitudes 26.5°–32° N and longitudes 80°–88° E. Nepal occupies one-third of the active Himalayan belt having high seismicity in its northern part. Nepal has numerous active faults in three major tectonic zones of Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Himalayan (Frontal) Thrust (MFT). On April 25, 2015, an earthquake occurred near the Main Central Thrust. The Global Seismic Hazard Map (GSHAP) depicts that the Nepal region has a seismic hazard level of the order of 0.25 g for 10% probability in 50 years (Bhatia et al. 1999). Maximum hazard is in and around the MCT and the entire country has a hazard range of 0.1–0.3 g. The probabilistic seismic hazard map prepared by the National Seismological Center, Nepal shows a hazard of 0.1–0.4 g for a 500-year period at bedrock (Wijeyewickrema et al. 2015). Damage is severe because of non-compliance of codes except for some modern buildings. Houses in villages and ancient towns of Kathmandu Valley are of brick or stone masonry with mud mortar, having little resistance to seismic loads.

In this chapter, geology, seismotectonics and geodetic deformation of Nepal is described in short. Geology of the Kathmandu Valley is also covered. Major thrusts and rock formations are shown in maps. Importance of the Main Himalayan Thrust (MHT) being the seat of large earthquakes is reiterated. Past Significant Earthquakes in Himalaya and especially in Nepal are listed and their effect is mentioned. The Bihar-Nepal Earthquake of 15 January 1934 and August 20, 1988 Udaipur Nepal Earthquake of Magnitude 6.8 earthquakes are described in much detail. Estimates of rupture lengths and areas of large earthquakes in Nepal and vicinity are given. Seismotectonics of Himalaya in general and Nepal in particular is described in detail including crustal structure, duplex structure, locked and active zones, and, role of transverse faults. India-Eurasia convergence rates derived from GPS measurements in different sectors derived by various investigators are given.

Geotechnical and other engineering aspects of the 2015 Nepal Earthquake have been investigated by various workers (Aydan and Ulusay 2015, Bhagat et al. 2017, Goda et al. 2015, Parameswaran et al. 2015, Pokhrel et al. 2015a, b, Wijeyewickrema et al. 2015). Some important observations are summarized here.

Information about damage caused by the large earthquake of April 25 (Mw 7.8) and its major aftershock of May 12 (Mw 7.3), 2015 that struck Nepal is compiled. The earthquakes caused about 8964 human deaths (8,857 in Nepal, 79 in India, 27 in Tibet, China, and 4 in Bangladesh). It has caused 22,300 injuries and full or partial damage to 756,000 buildings in Nepal. In India, deaths were mostly in Bihar and some in Uttar Pradesh. Damage was reported from West Bengal, Jharkhand, Rajasthan and Odisha. Lives of 8 million, i.e., one-third population of Nepal in 31 of 75 districts got affected. Eight districts were severely affected and six more were also badly affected. Both the mainshock and the major aftershock caused landslides near their epicenters. The economic loss was $ 10 billion (~ half of Nepal's GDP) and reconstruction cost nearly double of that. Intensity data at over 200 locations were collected in Nepal, India, and Tibet, and an isoseismal map was plotted. The 25th April earthquake caused the collapse of RC structures in a 75 × 15 km area trending WNW-ESE around Kathmandu where MM Intensity IX is assigned. Intensity VIII is over the rest of the rupture zone. Intensity VII extends to parts of Bihar, India. Areas in northwestern Bihar and adjoining UP have some collapsed houses where intensity VI is assigned. Earthquake has been strongly felt up to Delhi and Kolkata with intensity IV while intensity III extends to 1500 km distance up to Ahmedabad, etc. in the west and Odisha as well as Andhra in the east where it was mildly felt. Felt distance is 1800 km in India as people in Chennai felt the earthquake on the upper floors. In China, the felt distance was 1900 km. The 12th May major aftershock caused the collapse of several hundred houses and buildings including some RC structures in Sindhupalchowk-Kodari-Dolakha. The death toll due to the May earthquake was about 1000 in Nepal.

After any large earthquake, people are scared of aftershocks and wish to know that how long the aftershocks may continue and what could be the maximum magnitude. The behavior may be somewhat different in different regions; hence, it is important to study the same for the Gorkha Nepal earthquake. Generally, it has been observed that the largest aftershock may have one unit less than the mainshock. However, in the case of the Gorkha earthquake, the difference is only 0.5 units. It may be due to the fact that the largest aftershock had an independent but contiguous rupture zone. The duration of aftershocks is longer than expected for the region. Rupture of Nepal April 2015 earthquake of Mw 7.8 in Eastern Nepal occurred along the Main Himalayan thrust (MHT) and that of May 12 major earthquake was NE of it. Adhikari et al. (2015) presented the temporal and spatial distribution of aftershock activity that occurred for the first 3 months by using the data of the Nepal seismic network. Baillard et al. (2017) processed the same data with automatic onset and hypocenter determination procedures and stated that most aftershocks are in the Main Central Thrust zone and the May 12 Mw 7.3 earthquake had its own aftershock activity. Yamada et al. (2019) studied the aftershocks for an 11-month duration obtained with additional local seismograph stations. Results of Rastogi and Mittal (2016) on the investigation of aftershock behavior of the two major 2015 earthquakes using the EMSC catalog of M ≥ 4 earthquakes until September 2015 are presented here. The aftershock behavior of the two major earthquakes is compared with each other and also with the total distribution. Time and space variation as well as the “b” value (ratio of smaller to larger earthquakes) and “p” value rate of decay of aftershocks) were investigated.

The Gorkha, Nepal earthquake of April 25, 2015 (occurring on a Saturday morning) affected 8 million people of Kathmandu and nearby districts which urgently needed all kinds of relief materials like food, water, medicines, shelter, and sanitary facilities. Nearly 9000 people died and 23,000 injured. A short summary of search, rescue, and relief operations is prepared using Nepal Govt. Planning Commission report, 2015, (http://​docplayer.​net/​45013071-Post-disaster-needs-assessment.​html) Vol. A: Key Findings of 2015 Nepal Earthquake, other Reports from press and web were also used. The salient points regarding the Rehabilitation Plan for Nepal are also given. This description would be helpful in knowing the needs of and how to do disaster management.