Earthquake processes of the Himalayan collision zone in eastern Nepal and the southern Tibetan Plateau

Abstract

SUMMARY Focal mechanisms determined from moment tensor inversion and first motion polarities of the Himalayan Nepal Tibet Seismic Experiment (HIMNT) coupled with previously published solutions show the Himalayan continental collision zone near eastern Nepal is deforming by a variety of styles of deformation. These styles include strike-slip, thrust and normal faulting in the upper and lower crust, but mostly strike-slip faulting near or below the crust‐mantle boundary (Moho). One normal faulting earthquake from this experiment accommodates east‐ west extension beneath the Main Himalayan Thrust of the Lesser Himalaya while three upper crustal normal events on the southern Tibetan Plateau are consistent with east‐west extension of the Tibetan crust. Strike-slip earthquakes near the Himalayan Moho at depths >60 km also absorb this continental collision. Shallow plunging P-axes and shallow plunging EW trending T-axes, proxies for the predominant strain orientations, show active shearing at focal depths ∼60‐90 km beneath the High Himalaya and southern Tibetan Plateau. Beneath the southern Tibetan Plateau the plunge of the P-axes shift from vertical in the upper crust to mostly horizontal near the crust‐mantle boundary, indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate.