MONITORING OF SURFACE DEFORMATION DUE TO EARTHQUAKE USING DINSAR TECHNIQUE AND PALSAR-2 DATA: A CASE STUDY OF THE GORKHA EARTHQUAKE IN NEPAL, 2015

摘要

Once again Nepal was shocked by another earthquake with magnitude 7.8 Mw known as Gorkha Earthquake on April 25, 2015, after 81 years of the worst natural disaster that struck in 1934. Continued aftershocks occurred throughout Nepal within 15-20 minutes intervals, with one shock reaching a magnitude of 6.7 Mw on April 26, 2015. A major aftershock occurred on 12 May 2015, with 7.3 Mw. Geophysicists and other experts are warning for decades that Nepal is vulnerable to a deadly earthquake, particularly because of its geology as Nepal lies towards the southern limit of the diffuse collisional boundary where the Indian Plate under thrusts the Eurasian Plate, occupying the central sector of the Himalayan arc, nearly one-third of the 2,400 km (1,500 mi) long the Himalayas. Estimation of damaged especially surface deformation is an important part of the rebuilding planning process for natural disaster. Although field investigation is one of the best means for determination of surface deformation, this process is unable to explore a large area, especially in remote and inaccessible area. Remote Sensing technique, especially interferometric synthetic aperture radar (InSAR) is invaluable technique for assessment of surface deformation due to the natural disasters particularly earthquake. InSAR has the potential to provide important subsidence information over large areas because of its two-dimensional spatial coverage, competitive cost, data availability, and high vertical accuracy for remotely measuring the deformation of the ground and man-made structures from space. This technique can even achieve millimeter level accuracy using a technique called differential InSAR technique (DInSAR). However, the accuracy of this process depends on several factors such as availability of satellite data with optimal temporal and perpendicular baseline along with necessary ancillary data especially a high-resolution DEM (Digital Elevation Model). Therefore, this study is going to estimate the surface deformation in Nepal due to the Gorkha Earthquake using data from new L-band SAR sensor ALOS/PALSAR-2 which has been significantly improved in all aspects including resolution, observation band, and time lag compared to ALOS/PALSAR. Fine beam dual polarization (HH and HV) with 10 m resolution data that cover the earthquake areas were used to measure the surface deformation using the DInSAR technique. Results indicate that there was a large displacement of land surface due to the massive Gorkha Earthquake and subsequent aftershocks. However, the level of subsidence was varied from one place to another place probably due to the geological structure of the affected areas although most of the deformation was observed around the epicenters of the earthquake.