INTERPRETATION OF PAST AND ONGOING PALEOSEISMIC INVESTIGATIONS ALONG THE HIMALAYAN FRONTAL THRUST IN CONTEXT OF BRIEF FIELD RECONNAISSANCE AFTER THE M7.8 APRIL 25, 2015 GORKHA EARTHQUAKE

Fault scarps and uplifted terraces in young alluvium are frequent occurrences along the trace of the northerly dipping Himalayan Frontal Thrust (HFT). It was generally expected that the M7.8 April 25, 2015 Gorkha earthquake of Nepal would produce fresh scarps along the fault trace. Contrary to expectation, InSAR and aftershock studies soon showed that the the southward rupture of the HFT was confined to the subsurface and stopped on the order of 50 km north of the trace of the HFT. We mounted a brief field survey along the trace of the HFT and along faults and lineaments within the Kathmandu Valley beginning about 1 week after the earthquake. Our field survey confirmed the lack of surface rupture along the HFT as well as mapped faults and lineaments in Kathmandu Valley. The only significant ground deformation we observed was limited to a ~1 km long NE trending fracture set in the district of Kausaltar within Kathmandu and interpreted not to be to be the result of tectonic displacement but rather localized extension along a ridge. How slip on the remaining southward 50 km section of the Himalayan Frontal Thrust will be accommodated remains a puzzle. Will it take place by 1) post seismic creep along the now unruptured portion of the HFT?, 2) by a future earthquake limited to the shallowest southern section of the HFT or 3) will rupture of the southern section await the occurrence of a yet larger earthquake that slips along the entire decollement including that portion which ruptured in 2015?. The questions are important to the interpretation of paleoseismic interpretations of the past seismic behavior of the Himalayan arc and here provide a basis to discuss the results of past and ongoing paleoseismic investigations along the Himalayan arc.