The 25 April 2015 Mw 7.8 Nepal originated about 80 km to the northwest of the Nepalese capital of Kathmandu (the epicentre is the blue star in Figure 1) and ruptured eastward under Kathmandu and further east, as shown by the blue zone with green and yellow interior in Figure 1.
A second major earthquake occurred on 12 May 2015 with a magnitude of Mw 7.3, about 80 km east-northeast of Kathmandu, near the Chinese border between the capital of Kathmandu and Mt Everest.
Both earthquakes initiated at a depth of 15 km and occurred on the same fault plane. The aftershocks, shown by dots, were mostly located around the edges of the mainshock rupture zone, and after about five days, they migrated eastward to the epicentre of the 12 May event, which was an unusually large aftershock.
Both earthquakes had shallow dip angles of about 10 degrees, and originated at depths of about 15 km. The 25 April earthquake had a rupture length of about 160 km and a rupture width of about 80 km. The 12 May earthquake had a rupture length of about 50 km and a rupture width of about 30 km.
Himalayan Plate collision and subduction
The Nepal earthquakes were caused by thrust faulting on the Main Himalayan Thrust, which is the interface between the subducting India plate and the overriding Eurasia plate to the north, as shown in Figure 2.
The India-Australia plate is converging with Eurasia at a rate of 45 mm/yr towards the north-northeast, a fraction of which (~18 mm/yr) is driving the uplift of the Himalayan mountain range.
The boundary region of the India and Eurasia plates has a history of large and great earthquakes (Figures 3 and 4). Four events of M 6 or larger have occurred within 250 km of the 25 April, 2015 earthquake over the past century.
One, a M 6.9 earthquake in August 1988, 240 km to the southeast of the 25 April event, caused close to 1500 fatalities. The largest, an M 8.0 event known as the 1934 Nepal-Bihar earthquake, ruptured a large section of the fault to the east of the 2015 event, in a location close to the 1988 earthquake. It severely damaged Kathmandu, and is thought to have caused about 10,600 fatalities. Prior to the 20th century, a large earthquake in 1833 is thought to have ruptured a fault area similar to that of the 2015 event.
Comparison of Figure 1 with Figure 4 (see 1833 event) shows that the 2015 events were relatively small in magnitude and rupture length compared with earlier historical events, and that much larger earthquakes are possible, both to the west in the zone that is thought to have generated a Mw 8.6 earthquake in 1505, and to the east in the zone that is thought to have generated a large earthquake in about 1100.
Nepal has a small but experienced community of earthquake professionals, and a national network of seismic and geodetic monitoring stations. Several organisations that focus on risk reduction have been working actively in Kathmandu in recent years.
On 12 April, just weeks before the earthquake, two of them, the National Society for Earthquake Technology-Nepal in Sainbu, and GeoHazards International of Menlo Park, California, updated their earthquake scenarios for the Kathmandu Valley. Those scenarios were for an earthquake similar to the 1934 event. Geoscientists have long warned that crustal stresses are building up in Nepal. The 25 April earthquake was a little smaller and occurred further east than had been expected.
Strong ground motions
On the estimated ground motion intensities caused by the 25 April event, the peak horizontal acceleration recorded in Kathmandu was about 0.2g, which is considerably less than the expected value of about 0.4g. Kathmandu is located on lakebed sediments that are as much as 700 metres thick, and there has been concern that these sediments would tend to amplify the strong ground motions.
The main feature of the ground accelerations is their long durations of about one minute, which is expected from the large size of the earthquake. The earthquake generated large amplitude waves having a period of about 4.5 seconds, which are clearly seen in the acceleration, velocity and displacement records, and which may be attributable to rupture directivity and basin effects.
These waves had peak horizontal ground velocities of about 1 m/sec and peak vertical velocities of nearly 2/3 m/sec. Similarly, the peak horizontal ground displacements were over 1 m and the peak vertical displacements were nearly 2/3 m. They must have felt like a ship’s motion in rough seas.
However, very few buildings would have been vulnerable to these large long period motions because their resonant periods are mostly less than 4.5 seconds. Most buildings would have been more sensitive to the peak acceleration, which was relatively low, and which caused limited damage in Kathmandu. Many of the damaged buildings that can be seen in the media are unreinforced masonry, and others may be non-ductile concrete frame buildings, which are very vulnerable to earthquakes. It appears that most buildings in Kathmandu were able to withstand the ground shaking without collapsing.
The estimated casualties caused by the earthquakes are described by Daniell et al. (2015). The death toll as of the 12 May earthquake was 8,151, with another 377 missing. In addition, there were about 100 fatalities in Tibet, India and Bangladesh. Deaths due to landslides have been reported throughout Nepal totalling around 650 so far, but most coverage has been centred on the deaths in the Langtang landslide where approximately 300-350 people perished.
Early reports estimate that up to 1,000 people were injured, and at least 68 were killed in both Nepal and India in the 12 May earthquake. Four key factors contributed to the relatively low death toll:
• Time of day – The earthquake occurred at 11.56am local time on a Saturday, when many people were outside of their houses and working in fields, or travelling around.
• Building types – Although almost 300,000 houses were destroyed, the death toll appears to have been reduced in part by the fact many were rural masonry buildings with tile, sheet or non-heavy roof structures. It is estimated that there was one death per 350-400 destroyed houses. In Kathmandu, it appears that additional reinforcing steel bars in concrete buildings and other sound building practices saved many catastrophic collapses, thus reducing the death toll.
• Evacuation – In many cases in rural towns, there was enough time for people to leave their houses given the warning given by the early shaking. A few reports from towns indicate that only the elderly or pregnant women were unable to run out in time.
• Communication and rapid response – The mobile phone networks did not go down in Kathmandu, with data response being available. Thus, ambulances and other medical staff were able to be mobilised quickly. The sense of community in Nepal is such that where structures collapsed, people immediately responded to help pull injured people out of the rubble.
The economic impacts of the earthquake are described by Daniell et al. (2015). The capital stock of Nepal is very low, and the country has a combined building and infrastructure net capital stock of US$38.8 billion.
The gross capital stock of all structures, contents, equipment and materials is equal to about $59.1 billion. A loss value of about $3-3.5 billion is estimated for the net capital stock and production losses. Replacement costs and production losses are estimated in the order of $5-5.5 billion with a large proportion coming from Kathmandu.
The economic losses for the 12 May event are estimated at $250-1,200 million with $550 million coming from additional damage. Damage in India could be an additional $800 million. Total economic impacts in Nepal are estimated to be in the order of $10 billion, with direct losses of about $5 billion.
Nepal’s insurers collected premiums of about $277 million in 2013, with most of those funds for life coverage. Spending on property-casualty coverage is less than $4 per capita annually in Nepal, for a total of about $120 million. It is thought that less than 1% of the $800 million in estimated losses in India are covered by insurance.
Professor Paul Somerville is Chief Geoscientist at Risk Frontiers at Macquarie University in Sydney, Australia.