An earthquake of magnitude 7.8 (M7.8) which struck close to Kathmandu, capital of Nepal, on 25 April, looks set to be among the largest of the year and possibly also its most deadly.
At the time of writing, news reports suggested that almost 900 people had been killed across the region and many more injured.
Later updates suggest the figure is in excess of 2,000. The city of Kathmandu reportedly sustained “massive damage” and the earthquake is believed to have triggered an avalanche on Mt Everest, with associated fatalities.
Reports on the BBC indicate that there is “damage to Kathmandu airport which could hamper relief operations.”
Alarmingly, the United States Geological Survey’s PAGER rating (which indicates the likely human and economic losses) suggests a probability of over 80% that deaths would exceed 1000 and almost a 50% chance that the toll of fatalities will be above 10,000.
April 25 Nepal Earthquake: Tectonic Setting
The earthquake occurred almost midway between the cities of Kathmandu and Pokhara, at a depth of around 15km. We know its cause, which is set out by the USGS in its initial earthquake summary (subject to updating). They state that the Nepal quake “occurred as the result of thrust faulting on or near the main frontal thrust between the subducting India plate and the overriding Eurasia plate to the north.”
This is no surprise, given the tectonics of the region. The Himalayas are the result of two continents colliding with India, which has moved thousands of miles northwards in the last 65 million years or so, colliding with the Eurasian continent. Convergence is still continuing at the relatively rapid speed of 45mm per year.
The Eurasian plate, overriding the Indian. has been uplifted along a zone consisting of major thrust faults which include the Main Central Thrust and Main Boundary Thrust, resulting in the raising of the Himalayas and the Tibetan plateau to the north. There are many smaller thrust faults in the Himalayas but as the USGS summary notes, the evidence available “[is] consistent with occurrence on the main subduction thrust interface between the India and Eurasia plates.”
Nepal and the Himalayas: Seismic History
With such significant forces involved, it’s unsurprising that the Himalayan region experiences regular and sometimes very large earth tremors, although the USGS notes that “large earthquakes on the Himalayan thrust are rare in the documented historical era.”
A map of the region drawn from the USGS earthquakes database shows that the past century has seen five earthquakes of at least M7.0 along the Himalayan range, with a further three on the Tibetan plateau; while in the past half century, Nepal has experienced several tremors in excess of M6.0, indicating that major earthquakes along this front are by no means unexpected.
The most recent is the second largest, exceeded only by an M8.0 which occurred in 1934 to the east of Kathmandu (although Yeats refers to an earthquake in 1897 with an estimated magnitude of 8.1). Full details of damage and devastation caused by the 1934 event are unclear, although the NOAA reports a death toll of 10,600.
Nepal Tremor: Death Toll likely to Rise
High energy environments such as the Himalayas, with their steep and often unstable slopes, are subject to landslides as a result of earthquakes, increasing any loss of life resulting from building collapse and by the difficulties of accessing more remote reasons. For these reasons, it’s likely that the death toll will increase in the aftermath of the largest earthquake so far in 2015.