A large magnitude earthquake (Mw 7.8) struck the center of Nepal on April 25th 2015. This destructive earthquake caused extensive damage in the valley of Kathmandu and the city itself, as well as very many human losses.
UPDATE on May 12th : The earthquake of April 25th was followed by many aftershocks, one of which was of magnitude 6.7 Mw one day after the main shock. Most of them were located on the edges of the fracture zone, which can be roughly identified from the seismological and geodetic data. They are particularly concentrated on its most Eastern segment. On May 12th, a high magnitude earthquake (Mw 7.3) occurred in the same location and with the same mechanism as the main shock of April 25th (i.e., a flat thrust). In spite of the large magnitude of this last earthquake, its characteristics and its localization show that it belongs to the same sequence as the principal earthquake and its aftershocks. Therefore, it can be regarded as a very large aftershock.
We can already conclude that the earthquake broke a part of the Main Himalayan Thrust (MHT) on the basis of our knowledge of the Himalayan structure and of seismological data, which were available a few days after the main shock. The seismic rupture started at the epicenter which is located at approximately 80 km at the NW of Kathmandu. Then, it propagated over more than a hundred kilometers towards the SE, i.e. under the valley of Kathmandu. Moreover, the first seismological modelings suggest that the zone of stronger coseismic slip might be localized under the city itself. The fault-plane solution of the earthquake, deduced from the seismological analysis, indicates a very flat overlapping plane (weak dip of 5 to 7°) to the North, with a direction of N104°E.
Many aftershocks (orange and yellow dots on the map) followed the major earthquake, some of them exceeding magnitude 6. Two earthquakes of Mw 6.6-6.7 occurred, one in the epicentral zone and the other one at the NE of Kathmandu, as well as many Mw 5-6 and Mw 4-5 aftershocks. The aftershocks are spread over a length of 150 km and a width of 50 km, towards the East of the epicenter in the direction of Kathmandu. The zone affected by the aftershocks provides a first order indication of the size of the fault which broke (red zone on the map). This spatial organization, as well as the mechanism indicating very shallow dip, suggests that the fault which broke on April 25th is likely the flat part of the MHT (Main Himalayan Thrust), primarily in its relatively deep part. The hypothetical extension of the rupture area is illustrated in red on the cross-section of the Himalayas, at crustal-scale (see below). The distribution of the aftershocks under the valley of Kathmandu and further N, NE and NW suggests that the earthquake would not have been propagated until the front. Nevertheless, the question of whether the earthquake broke the surface at the level of the MFT has not yet been answered.
The Himalayan front has experienced many earthquakes which are more or less well-documented. Recent work in paleoseismology has made it possible to specify the image of the seismicity of the east of the Himalayan arc, and in particular the extension of the rupture associated with the earthquake in 1934 Mw 8.2, which had already devastated Kathmandu (see also additional information compiled by F. Perrier here). This same work also has made it possible to determine the time of occurence of the previous earthquake, in 1255, as well as the former earthquakes in order to establish a recurrence period of around 800 years for the great earthquakes of the Himalayan front (Bollinger et al., 2014). The most recent work of the CEA-IPGP team (L. Bollinger and Y. Klinger) for France, the EOS team (P. Tapponnier) for Singapore and the DMG team (S. Sapkota) for Nepal focuses on the quantification of seismic momentum in the area of Kathmandu based on paleoseismology, historical information and the rate of regional deformation. This work indicates that the region directly localized between the earthquakes of 1505 and 1934, which broke before only in 1344 with a magnitude Mw ≥ 7.5 earthquake, was close to its breaking point. This conclusion was confirmed by the earthquake of the April 25th, 2015.
Interferogram calculated from the radar images acquired by the Sentinel1 satellite between April 17th 2015 and April 29th 2015. The town of Kathmandu corresponds to the shiny zone in the center of the picture. The high chain of the Himalayas is to the north and India to the south of the map. Each fringe corresponds to a displacement of 2.8 cm in the ground-to-satellite direction. The signal indicates a maximum rise to the north of the city of about 1 meter. Because of geometry of acquisition, measurement is primarily sensitive to the vertical movements.
Interferogram calculated by R. Grandin, IPGP
MORE : : inSAR study of the earthquake (Sentinel 1 interferograms by R. Grandin, tectonics IPGP)
- Grandin, R., Doin, M. P., Bollinger, L., Pinel-Puysségur, B., Ducret, G., Jolivet, R., & Sapkota, S. N. (2012). Long-term growth of the Himalaya inferred from interseismic InSAR measurement. Geology, 40(12), 1059-1062
- Avouac, J. P., Bollinger, L., Lavé, J., Cattin, R., & Flouzat, M. (2001). Le cycle sismique en Himalaya. Comptes Rendus de l'Académie des Sciences-Series IIA-Earth and Planetary Science, 333(9), 513-529
- Sapkota, S. N., Bollinger, L., Klinger, Y., Tapponnier, P., Gaudemer, Y., & Tiwari, D. (2013). Primary surface ruptures of the great Himalayan earthquakes in 1934 and 1255. Nature Geoscience, 6(1), 71-76.
- Bollinger L., Sapkota S., Tapponier P., Klinger Y., Rizza M., Van der Woerd J., Tiwari D., Pandey R., Bitri A., Bes de Berc S., Estimating the return times of great Himalayan earthquakes in eastern Nepal : Evidence from the Patu and Bardibas strands of the Main Frontal Thrust. J. Geophys. Res. Solid Earth, 119, doi : 10.1002/2014JB010970, 2014.
Translation / revision carried out by the Masters 2 ILTS team