NASA and its partners are contributing important observations and expertise to the ongoing response to the April 25, 2015, magnitude 7.8 Gorkha earthquake in Nepal. The quake was the strongest to occur in that area since the 1934 Nepal-Bihar magnitude 8.0 event and caused significant regional damage and a humanitarian crisis.
Scientists with the Advanced Rapid Imaging and Analysis project (ARIA), a collaboration between NASA's Jet Propulsion Laboratory, Pasadena, California, and the California Institute of Technology in Pasadena, analyzed interferometric synthetic aperture radar images from the European Union's Copernicus Sentinel-1A satellite, operated by the European Space Agency and also available from the Alaska Satellite Facility (https://www.asf.alaska.edu), to calculate a map of the deformation of Earth's surface caused by the quake. This false-color map shows the amount of permanent surface movement caused almost entirely by the earthquake, as viewed by the satellite, during a 12-day interval between two Sentinel-1 images acquired on April 17 and April 29, 2015.
In the map, surface displacements are seen as color contours (or "fringes"), where each color cycle represents 8 inches (20 centimeters) of surface motion. The contours show the land around Kathmandu has moved upward by more than 40 inches (1 meter). Areas without the color contours have snow or heavy vegetation that affects the radar measurements. Scientists use these maps to build detailed models of the fault and associated land movements to better understand the impact on future earthquake activity. The background image is from Google Earth. The map contains Copernicus data (2015).
The magnitude 7.8 Gorkha earthquake that struck Nepal on April 25, 2015, caused extensive damage in Kathmandu Valley and severely affected Nepal's rural areas. The Langtang Valley in the Rasuwa district was particularly hard hit, as became apparent from pictures taken by a rescue helicopter mission on April 26. Numerous tourists and Nepali were, or are, still trapped in the valley as access is completely blocked by avalanches and landslides. The valley’s main village, Langtang , was completely destroyed by the earthquake and the large, wet, debris- and ice-rich avalanche and likely pressure wave from dust avalanche that it triggered, resulting in an unknown number of casualties. Other avalanches also struck elsewhere in the valley.
Space agencies around the world are providing extensive resources in a huge international effort. They are tasking their satellites to observe the areas hit by the earthquake. This effort began immediately after the disaster. Imaging initially focused on Kathmandu. Following the first social media reporting of the helicopter pilot’s comments, an emergency NASA-USGS-interagency Earthquake Response Team alerted satellite mission operations teams about the likely serious plight of Langtang and other Himalayan valleys. Advisories were also delivered to Nepal officials. The first relief missions arrived in the Langtang Valley about April 28.
United States Geological Survey/NASA Landsat-8 satellite observations were first obtained over Nepal after the earthquake on April 30. Landsat 8 acquired the first largely cloud-free image of the Langtang Valley (post-event Landsat image ID: LS081410412015043000000000MS00_GO006005004). Scientists analyzed the imagery and compared it with pre-earthquake imagery from a year earlier.
The analyses revealed the true extent of the disaster that took place in the Langtang Valley. Part of Langtang village was completely buried by a very large rock and ice avalanche that originated on the northwestern slopes above the village. The eastern part of Langtang village appears to have been destroyed by the pressure wave from the related dust avalanche. Large landslides or avalanches also affected the villages of Thyangshyup, Tsarding, Chyamki, Gumba, Mundu, Sindum and Kyangjing. The extent of the damage around these small settlements will require further investigation using higher-resolution imagery to be obtained from satellites, and word from relief crews on the ground. The river at the Langtang village avalanche appears to be blocked, but there is no evidence yet of a lake forming behind the blockage. This may indicate that the water has found its way through the debris, snow and ice. The valley is vulnerable to secondary events such as mudslides and debris and ice avalanches, and this situation could continue into the coming monsoon. Researchers will continue to monitor this situation closely using satellite data.
These scientists are part of a 35-member international volunteer group led by University of Arizona scientists Jeffrey Kargel and Gregory Leonard, who launched the group soon after the earthquake occurred. This effort has been incorporated into the NASA-USGS-interagency Earthquake Response Team. Their goal is to systematically investigate the entire quake-affected area using remote sensing. Their results will support relief operations and identify secondary hazards, such as glacier lake outbursts, rivers blocked by landslides and other unstable areas. This is the first volunteer report of the project.
Nepal's Imja Lake, Khumbu Region, Appears Resilient Against Gorkha Quake
Nepal's Imja Lake is a typical large glacial lake created by a moraine dam (formed by accumulations of dirt and rocks -- commonly cemented by ice -- that have been transported and deposited by a glacier). Located near Mount Everest, the local topographic relief is extremely high. Imja Lake was recently the site of an intensive scientific survey supported by the United Nations' Development Program to re-assess the outburst flood potential of the lake and to design and, this spring, implement a project to lower the lake level and reduce the flood hazard. The project had not been completed when the earthquake struck. The lake is also the site of a planned micro-hydroelectric power plant.
The lake began forming as a series of ponds on the glacier surface around 1960. By the mid 1970’s, the ponds had combined into a single large lake, which has continued to grow since then as the glaciers feeding it have retreated and thinned. The lake’s growth effectively cut off what had been the debris-covered terminus of the glacier, which now stands as an ice-cored end moraine. The moraine’s unconsolidated rock debris and the ice have been unstable, both gravitationally and thermally, for many years. The high lateral moraines, the high bedrock ridges that almost surround the lake, and the hanging glaciers perched above it could be de-stabilized, such as from ground shaking, and trigger an outburst flood. Fortunately, the present lake level is only about 100 feet (30 meters) higher than the area immediately downstream from the end moraine, thereby limiting the volume of water that could be released. The wideness of the end moraine previously had reduced the chances of a sudden outburst, but the growth in the past few years of ponds on the end moraine has weakened the dam, making it less able to contain the lake.
A bathymetric (lake depth) map from the recent survey is shown above, superposed on an EO-1 ALI image dating from before the earthquake. The survey found the lake to be up to 492 feet (149.8 meters) deep and to contain about 75 million cubic meters of water, of which about one-third is readily drainable in the event of a glacier lake outburst flood. The aforementioned lake lowering will reduce the level by only 10 feet (3 meters), which will reduce the outburst flood problem but not eliminate it.
The lake remains very hazardous, so it is with much relief that Imja Lake survived the 2015 Gorkha quake without huge effects visible in the first acquired post-quake satellite image from NASA’s EO-1 satellite (black and white image below). However, further satellite image analysis and boots and eyes on the ground are needed to confirm this.
This work is supported by NASA’s SERVIR Applied Science Team and is in support of the emergency NASA-USGS-Interagency Earthquake Response Team.