the nepal earthquake of mw 7.9 of 25 april...
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The Nepal earthquake of Mw 7.9 of 25 April 2015 (J. van der Woerd, 15/05/2015) An earthquake of magnitude Mw 7.9 hit the region of Kathmandu in central Nepal Saturday 25 April 2015 at 11:46am local time (6 :11 UTC). The epicenter is located mid-‐way between Pokhara and Kathmandu (N28.147 ; E84.708 after USGS) at a depth of about 15 km. The magnitude of Mw 7.9 (Duputel et al., 2015 ; USGS) indicates it is one of the largest earthquakes to hit Nepal since 1934 (Bihar Nepal earthquake of Mw 8.4; Sapkota et al., 2012 ; Bollinger et al., 2014). To this day, the death toll reaches 8000 and 15000 injuries. The damage to houses and roads are numerous, buildings are destroyed in Kathmandu. Numerous landslides and rock avalanches are reported up to the high Himalaya. As a reminder, the 1934 earthquake had a death toll of about 15000, the more recent earthquake of Wenquan in China in 2008 in a similar setting (in eastern Tibet) reached 80,000 deaths. The earthquake has been followed by numerous aftershocks. The aftershocks encompasses an area of 150 km by 50 km in an east-west direction, towards the East of the epicenter in the direction of Kathmandu. Two aftershocks of magnitude 6.1 and 6.6 occurred the same day, followed by a Mw 6.7 event on 26 April 2015 about 150 km towards east of the epicenter near the eastern extremity of the main rupture. A larger aftershock of magnitude Mw 7.2 (Duputel et al., 2015 ; USGS) occurred exactly in this area on 12 Mai 2015 at 12:50pm local time followed rapidly by an event of magnitude 6.3. This earthquake occurred along the collision zone between the Indian and Eurasian tectonic plates that converge at a rate of about 20 mm/yr along the Himalaya. The stress accumulated by this convergence is periodically released during large earthquake events that occur on the fault system of the Himalayan front. Several events occurred in recent time, such as the Kashmir earthquake (Mw 7.6 in 2005), the Kangra earthquake (Ms 7.8 in 1905), the Assam earthquake (M 8.6 in 1950) or the Bihar Nepal earthquake (Mw 8.4 in 1934). The earthquake focal mechanism of the main shock (W-phase solution, Z. Duputel and L. Rivera, IPG Strasbourg) like the one of the magnitude 7.2 aftershock, suggests the rupture occurred on a shallow dipping (5-10° to North) reverse fault aligned with the main range front (N104°). This mechanism, together with the depth of the hypocenter at about 15-20 km, indicates that the event ruptured the Himalayan frontal décollement (Main Frontal Thrust or Main Himalayan Thrust), the major fault that accommodates deformation at the front of the Himalaya. The location of epicenter and radar interferometry (see INSU-CNRS) show that the fault that ruptured did not reached to the surface. The observations of the rupture location (surface damage, aftershock distribution, interferometry) suggest that this event is located in a seismic gap identified between the 1934 rupture to the East and the 1505 event to the West, segment of the MFT that has not ruptured since the historical event of 1344. Thus, similarly to the seismic sequence of 1255 and 1344, the earthquake of 2015 follows the one of 1934 (by 81 years instead of 89 years)(Bollinger et al. submitted to Nature communication). However, the earthquake of 2015 is located in the area of the 1833 earthquake, which may have occurred at a greater depth, and it did not ruptured to the surface, i.e. the shallowest part of the MFT that crops out 50 km south of Kathmandu. It is thus possible that the event of 25 April 2015, even added to the 1833 event, did not release the whole stress accumulated since 1344 and that one or several large events are still due in this part of the Nepalese Himalaya.
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Links : Duputel, Z., L. Rivera, H. Kanamori, W-Phase source inversion results 2015/04/25 (Mw 7.9), Nepal, weblink : http://wphase.unistra.fr/events/nepal_2015/ and http://wphase.unistra.fr/events/nepal_2015_aftershock/ INSU - CNRS : http://www.insu.cnrs.fr/node/5291 and http://www.insu.cnrs.fr/node/5299 IPGP : http://www.ipgp.fr/fr/seisme-‐nepal-‐25-‐avril-‐2015 ISTERRE : http://isterre.fr/annuaire/pages-‐web-‐du-‐personnel/jean-‐louis-‐mugnier/le-‐grand-‐seisme-‐du-‐25-‐avril-‐2015-‐au-‐nepal/ Geoscope : http://geoscope.ipgp.fr/index.php/en/catalog/earthquake-‐description?seis=us20002926 USGS : http://earthquake.usgs.gov/earthquakes/eventpage/us20002926#scientific_tensor EMSC : http://www.emsc-‐csem.org/Earthquake/earthquake.php?id=438530 References : Bollinger, L., S.N. Sapkota, P. Tapponnier, Y. Klinger, M. Rizza, J. van der Woerd, D.R. Tiwari, R. Pandey,
Estimating the return times of great Himalayan earthquakes in Eastern Nepal: evidence from the Patu and Bardibas strands of the Main Frontal Thrust, Journal of Geophysical Research, doi: 10.1002/2014JB010970, 2014.
Duputel, Z., L. Rivera, H. Kanamori, W-Phase source inversion results 2015/04/25 (Mw 7.9), Nepal, weblink : http://wphase.unistra.fr/events/nepal_2015/ and http://wphase.unistra.fr/events/nepal_2015_aftershock/ Grandin, R. et al. Long-term growth of the Himalaya inferred from interseismic InSAR measurement, Geology 40,
1059-1062 (2012).
Mugnier, J. L. et al. Structural interpretation of the great earthquakes of the last millennium in the central Himalaya. Earth-Science Reviews127, 30-47 (2013).
Sapkota, S.N., et al. Primary surface ruptures of the great Himalayan earthquakes in 1934 and 1255, Nature Geoscience, 71-76 (2013).
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Figure 2: Location of 2015 event along Himalayan front (A) and schematic cross-section of the rupture plane (in black) (B). (After Bollinger et al. 2014).
After Bollinger et al. 2014
1344
2015
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Figure 3: Earthquake sequence along the Himalayan thrust front. The 25 April 2015 event fills a gap between the earthquakes of 1505 and 1934. It follows the earth-quake of 1934 (by 81 years) like probably the 1344 event followed the 1255 event (by 89 years)(after Bollinger et al., 2014).