The Signature of an Unbalanced Earthquake Cycle in Himalayan Topography?

Abstract

Fifty percent of the relative motion between the Indian and Asian plates is accommodated by active convergence at the Himalayan Range Front (HRF). Earthquake cycle processes on shallowly dipping HRF thrust faults generate large earthquakes \((M_W \geq 7)\) and contribute to the growth of HRF topography. Interseismic rock uplift rates reach a maximum north of the active Main Frontal Thrust and have been suggested to significantly influence the collocated convex bulge in HRF topography. Using geodetically constrained models of interseismic rock uplift rates and simple channel erosion rate laws, we show that convex channel profiles are predicted when interseismic deformation outpaces coseismic deformation. Applying this model to the observed elevation profiles of 20 HRF-spanning channels in Nepal yields a minimum mean residual elevation (72 m) if interseismic deformation has outpaced coseismic deformation by a factor of four. The long-term earthquake deficit required for the application of this model is consistent with some estimates of historical moment imbalance but requires temporally variable fault system activity. The spatial correlation between nominally interseismic rock uplift and the HRF topographic bulge may be explained by (1) a noncausal geometric coincidence, (2) geodetic observations of significant deformation not directly related to earthquake cycle processes, or (3) an unbalanced earthquake cycle at the HRF.