The April 2015 Nepal earthquake (also known as the Gorkha earthquake) killed 8,962 people and injured 21,952 across the countries of Nepal, India, China and Bangladesh. It occurred at 11:56 Nepal Standard Time on Saturday 25 April 2015, with a magnitude of Mw 7.8–7.9 or Ms 8.1 and a maximum Mercalli Intensity of X (Extreme). Its epicenter was east of Gorkha District at Barpak, Gorkha, roughly 85 km (53 mi) northwest of central Kathmandu, and its hypocenter was at a depth of approximately 8.2 km (5.1 mi). It was the worst natural disaster to strike Nepal since the 1934 Nepal–India earthquake. The ground motion recorded in Kathmandu, the capital of Nepal, was of low frequency, which, along with its occurrence at an hour when many people in rural areas were working outdoors, decreased the loss of human lives.
The earthquake triggered an avalanche on Mount Everest, killing 22 people, the deadliest incident on the mountain on record. The earthquake triggered another huge avalanche in the Langtang valley, where 250 people were reported missing.
Hundreds of thousands of Nepalese were made homeless with entire villages flattened across many districts of the country. Centuries-old buildings were destroyed at UNESCO World Heritage Sites in the Kathmandu Valley, including some at the Kathmandu Durbar Square, the Patan Durbar Square, the Bhaktapur Durbar Square, the Changu Narayan Temple, the Boudhanath stupa, and the Swayambhunath stupa. Geophysicists and other experts had warned for decades that Nepal was vulnerable to a deadly earthquake, particularly because of its geology, urbanization, and architecture. Dharahara, also called Bhimsen Tower, a nine-storey 61.88-metre (203.0 ft) tall tower, was destroyed. It was a part of the architecture of Kathmandu recognized by UNESCO.
Continued aftershocks occurred throughout Nepal at intervals of 15–20 minutes, with one shock reaching a magnitude of 6.7 on 26 April at 12:54:08 NST. The country also had a continued risk of landslides.
A major aftershock occurred on 12 May 2015 at 12:50 NST with a moment magnitude (Mw) of 7.3. The epicenter was near the Chinese border between the capital, Kathmandu, and Mount Everest. More than 200 people were killed and over 2,500 were injured by this aftershock, and many were left homeless.
The earthquake occurred on 25 April 2015 at 11:56 am NPT (06:11:25 UTC) at a depth of approximately 8.2 km (5.1 mi) (which is considered shallow and therefore more damaging than quakes that originate deeper in the ground), with its epicentre approximately 34 km (21 mi) southeast of Lamjung, Nepal, lasting approximately 50 seconds. The earthquake was initially reported as Mww 7.5 by the United States Geological Survey (USGS) before it was quickly upgraded to Mww 7.8, while the Global Centroid Moment Tensor (GCMT) reported a magnitude of Mw 7.9. The China Earthquake Networks Center (CENC) reported the earthquake's magnitude to be 8.1 Ms. The India Meteorological Department (IMD) said two powerful quakes were registered in Nepal at 06:11 UTC and 06:45 UTC. The first quake had its epicenter was identified at a distance of 80 km to the northwest of Kathmandu, the capital of Nepal. Bharatpur was the nearest major city to the main earthquake, 53 km (33 mi) as the crow flies from the epicenter. The second earthquake was somewhat less powerful at 6.6 Mw. It occurred 65 km (40 mi) east of Kathmandu and its seismic focus lay at a depth of 10 km (6.2 mi) below the earth's surface. Over 38 aftershocks of magnitude 4.5 Mw or greater occurred in the day following the initial earthquake, including one of 6.8 Mw.
According to the USGS, the earthquake was caused by a sudden short sharp upthrust along the convergent boundary where the Indian plate, carrying India, is slowly diving underneath the Eurasian plate, carrying much of Europe and Asia. Kathmandu, situated on a block of crust approximately 120 km (74 miles) wide and 60 km (37 miles) long, shifted 3 m (10 ft) to the south in just 30 seconds.
The risk of a large earthquake was well known beforehand. In 2013, in an interview with seismologist Vinod Kumar Gaur, The Hindu quoted him as saying, "Calculations show that there is sufficient accumulated energy [in the Main Frontal Thrust], now to produce an 8-magnitude earthquake. I cannot say when. It may not happen tomorrow, but it could possibly happen sometime this century, or wait longer to produce a much larger one." According to Brian Tucker, founder of a nonprofit organization devoted to reducing casualties from natural disasters, some government officials had expressed confidence that such an earthquake would not occur again. Tucker recounted a conversation he had with a government official in the 1990s who said, "We don't have to worry about earthquakes anymore, because we already had an earthquake." The previous earthquake to which he referred occurred in 1934. In fact, the 600-km-long Central Himalayan Gap hadn't ruptured since 1505.
Nepal lies towards the southern limit of the diffuse collisional boundary where the Indian plate underthrusts the Eurasian plate, occupying the central sector of the Himalayan arc, nearly one-third of the 2,400 km (1,500 mi) long Himalayas. Geologically, the Nepal Himalayas are subdivided into five tectonic zones from north to south, east to west, and almost parallel to sub-parallel. These five distinct morpho-geotectonic zones are: (1) Terai Plain; (2) Sub Himalaya (Shivalik Range); (3) Lesser Himalaya (Mahabharat Range and mid valleys); (4) Higher Himalaya; and (5) Inner Himalaya (Tibetan Tethys). Each of these zones is clearly identified by its morphological, geological, and tectonic features.
The convergence rate between the plates in central Nepal is about 45 mm (1.8 in) per year. The location, magnitude, and focal mechanism of the earthquake suggest that it was caused by a slip along the Main Frontal Thrust.
The earthquake's effects were amplified in Kathmandu as it sits on the Kathmandu Basin, which contains up to 600 m (2,000 ft) of sedimentary rocks, representing the infilling of a lake.
Based on a study published in 2014, of the Main Frontal Thrust, on average a great earthquake occurs every 750 ± 140 and 870 ± 350 years in the east Nepal region. A study from 2015 found a 700-year delay between earthquakes in the region. The study also suggests that because of tectonic stress buildup, the 1934 earthquake and the 2015 quake are connected, following a historic earthquake pattern. A 2016 study on historical great (M ≥ 8) earthquake pairs and cycles found that associated great earthquakes are likely to occur in the West China region through the 2020s.
According to the USGS website, the maximum intensity was VIII (Severe), however certain reports state an intensity of X (Extreme) or higher. In most of Kathmandu the intensity was VIII-IX, as evidenced by the numerous undamaged water towers installed on top of undamaged multi story buildings. Tremors were felt in the neighboring Indian states of Bihar, Uttar Pradesh, Assam, West Bengal, Odisha, Sikkim, Jharkhand, Uttarakhand, Gujarat, in the National Capital Region around New Delhi and as far south as Karnataka. Damage was extensive in northern Bihar and minor damage was also reported from parts of Odisha. Shaking was felt in high-rise buildings as far as Kochi in the southern state of Kerala. The intensity in Patna was V (Moderate). The intensity was IV (Light) in Dhaka, Bangladesh. The earthquake was also experienced across southwest China, ranging from the Tibet Autonomous Region to Chengdu, which is 1,900 km (1,200 mi) away from the epicenter. Tremors were felt in Pakistan and Bhutan.
A series of aftershocks began immediately after the mainshock, at intervals of 15–30 minutes, with one aftershock reaching 6.6Mw within 34 minutes of the initial quake. A major aftershock of magnitude 6.9 Mw occurred on 26 April 2015 in the same region at 12:54 NST (07:08 UTC), with an epicenter located about 17 km (11 mi) south of Kodari, Nepal. The aftershock caused fresh avalanches on Mount Everest and was felt in many places in northern India including Kolkata, Siliguri, Jalpaiguri, and Assam. The aftershock caused a landslide on the Koshi Highway, which blocked the section of the road between Bhedetar and Mulghat.