Land Subsidence in the Kathmandu Basin, Nepal before and after 2015 Mw 7.8 Gorkha Earthquake revealed from Time-series InSAR Analysis

摘要

The Kathmandu Basin located in the Lesser Himalayas is composed of thick Quaternary sediments overlaying bedrocks; these sediments have experienced severe groundwater drawdown in recent years, which leads to land subsidence. In addition, the basin lies in a very tectonically active zone that was devastated by large crustal deformation from the Mw 7.8 Gorkha earthquake on April 25, 2015, measured as ~ 1 m uplift. We acquired 16 scenes of Advanced Land Observation Satellite - Phased Array L-band Synthetic Aperture Radar 1 (ALOS-PALSAR) and 20 scenes of SENTINEL-1 SAR datasets during pre- and post-seismic periods, respectively. We developed spatial and temporal velocity profiles of land subsidence in the Kathmandu Basin before and after the earthquake by applying the Small BAseline Subset - Differential Interferometric Synthetic Aperture Radar (SBAS-DInSAR) technique. The mean land subsidence rate during 2007-2010 was ~7 cm/yr in the central part of the basin, however, this rate of subsidence significantly increased to ~12 cm/yr during 2015-2016, after the Gorkha earthquake. The distribution of subsidence areas observed before and after Mw 7.8 mainshock are almost identical, indeed the subsidence rate has increased after the mainshock. These results are useful for assessing the spatiotemporal distribution of land subsidence in the Kathmandu Basin and the influence of Earthquake Environmental Effects such as large co-seismic deformation on compressible sediment layers.