Field Reconnaissance after the 25 April 2015 M 7.8 Gorkha Earthquake

Abstract

Fault scarps and uplifted terraces in young alluvium are frequent occurrences along the trace of the northerly dipping Himalayan frontal thrust (HFT). Generally, it was expected that the 25 April 2015 M 7.8 Gorkha earthquake of Nepal would produce fresh scarps along the fault trace. Contrary to expectation, Interferometric Synthetic Aperture Radar and aftershock studies soon indicated the rupture of the HFT was confined to the subsurface, terminating on the order of 50 km north of the trace of the HFT. We undertook a field survey along the trace of the HFT and along faults and lineaments within the Kathmandu Valley eight days after the earthquake. Our field survey confirmed the lack of surface rupture along the HFT and the mapped faults and lineaments in Kathmandu Valley. The only significant ground deformation we observed was limited to an ∼1-km-long northeast-trending fracture set in the district of Kausaltar within Kathmandu. This feature is interpreted not to be the result of tectonic displacement, but rather a localized extension along a ridge. Our survey also shows the ubiquitous presence of fallen chimneys of brick kilns along the HFT and within the Kathmandu Valley. Measurements of a small subset of fallen chimneys across the region suggest a degree of systematic fall direction of the chimneys when subdivided geographically.