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The April 25, 2015 Nepal Earthquake

26 Apr 2015

The magnitude (Mw) 7.9 April 25th, 2015 earthquake in central Nepal occurred where the Indian and Eurasian plates collide. The earthquake ruptured the large, gently dipping Main Himalayan Thrust fault, which represents the boundary between the plates, at shallow depth (between 10-15 km based on preliminary moment tensor solutions).

Location of the April 25, 2015 earthquake (red star) and previous major earthquakes on the Himalayan frontal system in and near Nepal (colored bands).

 India and southern Tibet are converging at a rate of ~17-21 mm/yr (Ader et al., 2012), and the resulting strain is periodically released by earthquakes along the faults in the frontal Himalaya. The last large earthquake in Nepal occurred in 1934 and killed 16,000 people. That earthquake has recently been shown through trenching to have produced a surface rupture with ~5-7 m of displacement, lifting up ground on one side of the fault by as much as 4-5 m along a minimum width of ~150 km along the range front (Sapkota et al., 2012; Bollinger et al., 2014). Several other large and damaging earthquake have happened in the last century along faults in the frontal Himalaya: (from west to east) 2005 Mw7.6 Kashmir, 1905 Ms7.8 Kangra, 1934 Mw8.2 Bihar-Nepal, and 1950 M8.6 Assam.

Location of the April 25, 2015 earthquake (red star) and previous major earthquakes on the Himalayan frontal system (white stars).

 The April 25, 2015 earthquake occurred about 200 km west of the 1934 earthquake. It ruptured a segment of the fault that likely slipped in 1344 AD (Bollinger et al., 2015 submitted to Nature Communications). This is similar to the time interval between the last two earthquakes on the fault system to the east, which ruptured in both 1934 and 1255. We expect that this 2015 earthquake produced surface ruptures in the frontal Himalaya, as the 1934 earthquake did.

The distribution of aftershocks, which extend up to 130 km to the east of the epicenter, suggests that the rupture may have propagated from west to east, potentially leading to more severe destruction in Kathmandu.

Location of main shock (red star) and aftershocks (orange circles) for the April 25, 2015 earthquake.

 

As figure 3, with USGS ShakeMap contours overlain.

Although it is not possible to predict when an earthquake will occur, scientists have been warning of the possibility of an earthquake like this for years. The extent of loss of life and property in an earthquake depends greatly on whether buildings are constructed to withstand earthquakes.

The most recent comparable event occurred in 2008 on the eastern boundary of the Tibetan plateau (2008 Mw7.9 Wenchuan). That earthquake caused over 80,000 fatalities, and massive landsliding within the steep gorges blocked roads and made access to villages within the region slow and difficult. In addition, these landslides created temporary dams in the rivers and led to a risk of catastrophic dam break and flooding downstream. We also expect that this earthquake will have produced significant landsliding, hampering rescue efforts. Damage will likely be widespread, throughout central Nepal and northern India. Strong shaking has been felt as far as Calcutta, 750 km away.