A devastating earthquake hit Nepal on April 25, 2015 claiming over 5 500 lives and affecting millions of people. China Earthquake Networks Center, the closest advanced seismic network, recorded it as M8.1 at a shallow depth of 20 km while USGS recorded it as M7.8 at a depth of 15 km.
The epicenter was located in Gorkha district, between the capital city of Kathmandu with estimated population of 700 000 and Pokhara, the second largest city in Nepal with estimated population of 255 465.
At the location of this earthquake, the India plate is converging with Eurasia at a rate of 45 mm per year towards the north-northeast, driving the uplift of the Himalayan mountain range. The preliminary location, size and focal mechanism are consistent with its occurrence on the main subduction thrust interface between the India and Eurasia plates.
A number of factors made this quake a recipe for catastrophe, as reported in The Conversation:
It was shallow: an estimated 15 km below the surface at the quake’s epicenter. It saw a large movement of the earth (a maximum of 3 m). And the ruptured part of the fault plane extended under a densely populated area in Kathmandu.
From the preliminary analysis of the seismic records we already know that the rupture initiated in an area about 70 km north west of Kathmandu, with slip on a shallow dipping fault that gets deeper as you move further north.
Over about a minute, the rupture propagated east by some 130 km and south by around 60 km, breaking a fault segment some 15 000 square kilometres in area, with as much as 3 m slip in places.
The relief workers are on site, but the provision of aid is proving to be complicated as a result of heavy damage to transport routes, power supplies and communication channels, as well as the continuous aftershocks.
The full scale of the effect of the earthquake is not yet known and rescuers are struggling to reach mountain villages, some of which were completely destroyed.
While geo-scientists are using satellite measurements to analyze the effects of the earthquake on the land, space agencies are using satellite imagery to support emergency aid organizations.
Radar imagery from the Sentinel-1A satellite shows that the maximum land deformation is only 17 km from Nepal’s capital, Kathmandu, which explains the extremely high damage experienced in this area.
By combining Sentinel-1A imagery acquired before and after the quake, changes on the ground that occurred between the two acquisition dates lead to rainbow-coloured interference patterns in the combined image, known as an ‘interferogram’, enabling scientists to quantify the ground movement. (ESA)
Combining two Sentinel-1A radar scans from 17 and 29 April 2015, this interferogram shows changes on the ground that occurred during the 25 April earthquake that struck Nepal. An overall area of 120x100 km has moved – half of that uplifted and the other half, north of Kathmandu subsided. Vertical accuracy is a few cm. Copyright : ESA / Contains Copernicus data (2015)/ESA/Norut/PPO.labs/COMET–ESA SEOM INSARAP study.
Sentinel-1A’s swath width of 250 km over land surfaces has allowed for an unprecedented area size to be analyzed from a single scan. The entire area will be covered under the same geometry every 12 days, allowing for the wider region to be regularly monitored and fully analyzed for land deformation with the powerful ‘interferometry’ technique.
Interferogram over Kathmandu, Nepal, generated from two Sentinel-1A scans on 17 and 29 April 2015 – before and after the 25 April earthquake. Each ‘fringe’ of colour represents about 3 cm of deformation. The large amount of fringes indicates a large deformation pattern with ground motions of 1 m or more. Copyright: ESA / Contains Copernicus data (2015)/ESA/DLR Microwaves and Radar Institute/GFZ/e-GEOS/INGV–ESA SEOM INSARAP study
During the earthquake, tensions between the Indian and Eurasian plates were relieved and generated sudden ground movements of several meters.
The following image shows how and where the land uplifted and sank.
Near the boundary of the Indian and Eurasian tectonic plates, blue shows areas of uplift of up to 0.8 m towards the satellite (called ‘line of sight’) which could be caused by a vertical uplift of 1 m. The yellow area depicts areas of subsidence, a movement that often occurs as a counter movement to the uplift in subduction zones (where one plate dips below the other) during earthquakes. Additionally, a horizontal north–south shift of up to 2 m was detected.
This image was generated using data acquired by Sentinel-1A before and after the earthquake event. Copyright: DLR/EOC
Although a major plate boundary with a history of large-to-great sized earthquakes, large earthquakes on the Himalayan thrust are rare in the documented historical era.
Just four events of M6 or larger have occurred within 250 km of the April 25, 2015 earthquake over the past century. One, a M6.9 earthquake in August 1988, 240 km to the southeast of the April 25 event, caused close to 1 500 fatalities.
The largest, an M8.0 event known as the 1934 Nepal-Bihar earthquake, occurred in a similar location to the 1988 event. It severely damaged Kathmandu, and is thought to have caused around 10 600 fatalities.
For full report on April 25, 2015 Nepal earthquake and latest updates please visit this article.
Featured image credit: DLR/EOC.