Orogenic segmentation across central and eastern Himalaya: Insights from source parameters derived from local earthquakes

Abstract

Orogenic segmentation across the arcuate Himalayan belt complicates deformational trends beyond central Nepal. Here, we attempt to discern the implications of spatially varying deformation on seismic hazard potential in eastern Nepal, Sikkim and Bhutan Himalayas. A comprehensive catalogue of 2,018 local earthquakes recorded during past deployments is utilized to estimate source parameters from P and S wave spectra. Results indicate stress drop (Δσ) and moment magnitude exhibit scaled dependence with linearly increasing relation. Scaling traced between seismic moment and corner frequency ($M_0$ $\propto$ $f_c$^{\protect \relax \special {t4ht=−}3}) indicates no breakdown of earthquake scaling law for small earthquakes, thus holding potential to be scaled for modelling large earthquakes in the region. Empirical relationships established ($M_w$=0.55$M_l$+1.43, $M_w$=0.68$lo{g}_{10}$ ($M_0$)-6.20; $M_w$=0.61$M_l$+1.33, $M_w$=0.66$lo{g}_{10}$ ($M_0$)-6.06; and $M_w$=0.66$M_l$+1.37, $M_w$=0.66$lo{g}_{10}$ ($M_0$)-5.94) can serve as input for seismic hazard assessment in eastern Nepal, Sikkim and Bhutan. Spatial variability of Δσ is reflective of high coupling along the Main Himalayan Thrust (MHT), high strain rate and lower seismic activity in eastern Nepal and Bhutan. Strike-slip faulting in Bhutan (along DCFZ and KFZ) releases higher Δσ ($\sim$7–15 MPa) as compared to thrusting in Nepal releasing lower Δσ ($\sim$2 MPa). Low coupling along MHT in Sikkim substantiated by frequent release of stress through high seismic activity impedes low Δσ ($\sim$2 MPa). Beneath MHT, high radiated energy estimates ($\sim$9$\times$ $10^{11}$ Nm) evidenced a high degree of heterogeneity except for eastern Nepal ($\sim$8$\times$ $10^7$ Nm). Thus, eastern Nepal and Bhutan have potential to experience great earthquake, while Sikkim is susceptible to hazard due to active deformation along a multi-fault system. Consensus of spatial variation of source parameters with interseismic coupling, strain rate and deformation mechanism indicates orogenic segmentation has crucial implications on seismic hazard potential.