EVIDENCE OF A LARGE-MAGNITUDE PALEOEARTHQUAKE AT THE LAL DHANG TRENCH SITE ON THE HIMALAYAN FRONTAL THRUST IN INDIA

The Himalayan Frontal Thrust (HFT) lies at the active, tectonic boundary between Eurasia and the Indian subcontinent and accommodates a portion of the strain associated with convergence. Recent, large-magnitude earthquakes on segments of the HFT are constrained through instrumental and historical data; while earlier, pre-instrumental earthquakes have been constrained primarily through paleoseismic investigation. Paleoearthquakes on the fault are commonly expressed as ductile thrust sheets that breach the ground surface as fault-generated folds, with fault scarps reaching heights >10 m at some locations. Assuming the entire vertical offset results from one event, retrodeformation shows very high coseismic slip measurements of >20 m at these sites. This, combined with the lengths and locked depths of the individual fault segments, suggests the magnitude of these events may have approached Mw 9.0, which has led some to question whether these scarps might actually represent more than one rupture of the fault. To test this question, we were motivated to re-evaluate the site of Lal Dhang based on a previous study at this location that found evidence for a single earthquake across a 9-m-high fault scarp (Kumar et al., 2006). In Fall 2015, a new trench was excavated (~ 30 m long, 5 m wide, and 8 m deep) across a 13-m-high fault scarp, adjacent to the trench location in the previous study. Excavation exposed a main thrust dipping at 28° and bending to a lower angle at the paleosurface where the overriding sheet pushed into ductile overbank layers, warping the units into a recumbent anticline on the hanging wall and dragging the units upward into a recumbent syncline on the footwall. Horizontal units in the footwall are bulldozed and deformed into flame structures in front of the nose of the overthrusted anticline. Not documented in the previous study were second and third thrusts, higher in the trench wall, that show the same general folding and faulting pattern of the same stratigraphic units. Based on these observations and that only one colluvial wedge exists across the three thrusts, our preliminary interpretation of the trench data corroborate the findings of the previous study while increasing coseismic slip, leading to the conclusion that the fault scarp at this location was likely formed in one great, megathrust earthquake.