NOUVEAU (communiqué) : Le satellite SENTINEL-1 mesure les déplacements provoqués par le séisme du Népal (en français)
The large earthquake of magnitude Mw7.8 that occurred in central Nepal on April 25th, 2015 is responsible for more than 5000 deaths and massive destructions in the city of Kathmandu and the surrounding areas.
Space geodesy, in particular SAR interferometry, plays a key role in monitoring strain accumulation that preceded the earthquake. In a recent study using ENVISAT and GPS data, the French-Nepalese team has shown that the area struck by the 25 April 2015 earthquake was experiencing as much as 20 mm/yr of north-south contraction and a maximum uplift rate of 6 mm/yr (Grandin et al., 2012, see ref below). In the Sentinel-1 interferogram spanning the 25 April 2015 earthquake, intense coseismic deformation is observed in the area of Kathmandu. This deformation corresponds to strain that was suddenly released by the earthquake as the causative fault slipped by several meters. The city itself was uplifted by nearly one meter during the earthquake, as can be deduced by counting the fringes that span the city. Shaking led to widespread damage in the old parts of the city, as attested by interferometric coherence loss matching areas of intense destruction. In the coming weeks, a careful analysis of forthcoming Sentinel-1 acquisitions will allow scientists to precisely quantify the extent of the rupture and constrain the amount of slip accommodated by the earthquake. This will provide key information for improving seismic hazard scenarios in the future.
The map below shows an unwrapped Sentinel-1 interferogram for the interval 17 April - 29 April. The figure is shown in two versions : a red-to-blue color palette, with red for uplift and blue for subsidence; a cyclic color palette wrapped every 20 cm.
Stack profiles across the interferogram are plotted in the third figure. Displacement in the line-of-sight reaches a maximum of approx. 1.25 m (in red on the map), which is the approximate value of peak uplift located just north of Kathmandu. Localized subsidence also occurs further to the north (in blue on the map), with a minimum of approx. -50 cm. These values are minimum estimates, as displacement seems to increase further to the East, where the acquisition is clipped. The measurement does not include the horizontal component of displacement yet.