EVALUATING SLIP RATES ALONG THE MAIN FRONTAL THRUST IN WESTERN NEPAL FROM THE GEOMORPHOLOGY OF ACTIVE FOLDS IN THE SIWALIK HILLS

In western Nepal, over 500 years have elapsed since the last great earthquake along the Main Frontal Thrust (MFT), the surface expression of the primary plate interface accommodating convergence across the Himalayan orogen. Geodetic measurements of arc-perpendicular shortening imply ~19-21 mm/yr of convergence, and most estimates of seismic hazard presume that all of this strain is released in great earthquakes (e.g., Stevens and Avouac, 2016). However, direct determinations of slip rates along the MFT are restricted to a handful of localities along the fault system. Here we employ an approach to estimate fault slip rates that exploits the dynamic adjustment of topography and erosion to uplift above the MFT.

The Siwalik Hills form the fault-bounded topographic ranges in the Sub-Himalaya. Due to rapid uplift rates, easily erodible lithologies, and high monsoonal precipitation, the Siwalik Hills represent a relatively unique example of a steady-state landscape in which rock uplift is thought to be in balance with erosion. In the Siwaliks, south of Kathmandu, where rock uplift rates have been quantified by Holocene river terraces (Lavé & Avouac, 2000), we confirm prior results that find a linear correspondence between rock uplift rate and fluvial channel steepness. We apply the relationship between uplift rate and channel steepness to the topography in the hanging wall of three segments of the MFT in western Nepal, where uplift rates are largely unknown. Our results indicate that rock uplift rates vary systematically along the Siwalik Hills in western Nepal, with the highest uplift rates being above the westernmost, and most steeply dipping, fault segment. Accounting for variations in the fault ramp geometry along strike, and the geometric relationship between fault dip, rock uplift rate and fault slip rate for a fault-bend fold, we determine a slip rate of ~18-22 mm/yr for the MFT in western Nepal. Given a full slip deficit rate of ~18-22 mm/yr since the last great earthquake in 1505 CE, we infer a high slip potential of 9-11 m for the MFT in western Nepal, consistent with geodetically inferred estimates.