Seismic Deformations due to 2023 Nepal Earthquake Sequence using Satellite Remote Sensing Techniques

The seismic activity in the Himalayan region results from the ongoing collision between the Indian and Eurasian tectonic plates. The Himalayan thrust zone, comprising critical fault zones like the Main Central Thrust (MCT), Main Boundary Thrust (MBT), and Main Frontal Fault (MFF), is highly seismically active, leading to numerous moderate-to-high magnitude earthquakes annually. Nepal, situated in one of the world's most seismically active continental collision orogenic belts, has experience numerous devastating earthquakes in its history. Earthquakes can result in wide-ranging devastation that includes other, cascading natural disasters, including avalanches, landslides and glacial lake outburst floods (GLOFs). The present study aims to identify ground surface deformations associated with the 2023 Nepal earthquake sequence using C-band radar interferometry from the ESA Sentinel-1A/B synthetic aperture radar (SAR) datasets. The earthquake sequence includes a mainshock (M_w 5.7 triggered at a hypocentre depth of 32.6 km) on November 3, 2023, followed by an aftershock (M_w 5.3 triggered at a hypocentre depth of 10 km) on November 6, 2023. The mainshock's influence radius, determined using shake maps from the USGS earthquake catalog, is 57 km. Because the aftershock's influence radius of 51 km which is smaller than that of the mainshock, we use that as the study radius. Differential interferometric SAR (DInSAR) is employed for this region, utilizing co-seismic single-look complex (SLC) datasets acquired on October 25 and November 6, 2023, respectively. The DInSAR analysis reveals a maximum reliable (regions with coherence ≥ 0.4) and atmospherically-corrected ground deformation of -79 mm. The most significant ground deformations are observed around the Sisne Himal glacial region and the mountain slopes of the Ragda region. Other areas with ground deformations are identified over the mountain slopes of the Guthi Chaur region. The topographic slope of these regions is ≥35° except for Sisne Himal glacial region as observed through ALOS PALSAR high-resolution terrain corrected digital elevation model (DEM) at 12.5m ground resolution. Analysis of pre- and co-seismic coherence images revealed decreased co-seismic coherence in certain locations within the influence radius. These areas are further investigated for soil liquefaction/cyclic mobility using the Temporal Difference Liquefaction Index (TDLI) using Landsat-8 and -9 datasets of October 27 and November 4, 2023 respectively. TDLI detects changes in soil moisture content after an earthquake event. The observed ground deformations indicate potential earthquake-induced slope failures including some of the locations with liquefaction/cyclic mobility susceptibility. This emphasises the importance of monitoring such vulnerable areas for enhanced seismic risk assessment and disaster preparedness.