Field Reconnaissance Subsequent to the M7.8 April 25, 2015 Gorkha Earthquake and Implications to Past and Current Paleoseismic Investigations along the Himalayan Frontal Thrust

Thursday, 17 December 2015: 11:20
305 (Moscone South)
Steven G Wesnousky1,2, Stephen Angster1,2, Ian Kerens Driscoll Pierce3, Eric Jameson Fielding4, Deepak Chamlagain5, Dipendra Gautam5, Bishal Nath Upreti5, Yasuhiro Kumahara6, Takashi Nakata7, R. Jayangondaperumal8 and Pradeep Srivastava8, (1)Nevada Seismological Lab, University of Nevada Reno, Reno, NV, United States, (2)Center for Neotectonic Studies, University of Nevada Reno, Reno, NV, United States, (3)Center for Neotectonic Studies and Nevada Seismological Laboratory, University of Nevada, Reno, NV, United States, (4)Jet Propulsion Lab Caltech, Pasadena, CA, United States, (5)Tribhuvan University, Department of Geology, Tri-Chandra College, Kathmandu, Nepal, (6)Hiroshima University, Graduate School of Education, Hiroshima, Japan, (7)Hiroshima University, Department of Geography, Hiroshima, Japan, (8)Wadia Institute of Himalayan Geology, Dehradun, India
The northward dipping Himalayan Frontal Thrust (HFT) is commonly marked by the occurrence of young earthquake fault scarps and uplifted terraces along its trace and it was generally expected that the April 25 M7.8 Gorkha earthquake would produce new displacements along the fault trace. Instead, Insar and aftershock studies showed soon after the the quake that 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 conducted a brief field survey along the active Himalayan Frontal Thrust and along faults and lineaments within the Kathmandu Valley in the fortnight after the earthquake. Our field survey likewise illustrates the lack of surface rupture along the HFT as well as mapped faults and lineaments in Kathmandu Valley. Significant disturbance of the ground surface was limited to a ~1 km long NE trending fracture set in the district of Kausaltar within Kathmandu and interpreted to be the result of localized extension along a ridge, and not the result of deep-seated tectonic displacement. The observations allow that slip on the remaining shallow and southward 50 km section of the HFT is 1) ultimately recovered by post seismic creep, 2) will be accommodated by an earthquake limited to the shallowest southern section of the fault, or 3) that rupture of the southern section will await the occurrence of a yet larger earthquake that slips along the entire decollement. The conundrum serves here as context to present and discuss the interpretation of past and ongoing paleoseismic investigations along the Himalayan arc.