In-situ stress measurements over the Eastern Himalayan syntaxis and implications for seismicity

Abstract

Located in a special tectonic position, the eastern Himalayan syntaxis (EHS) has fully preserved the stress and tectonic evolution since the Indian-Eurasian plate collision. The EHS is a representative region for understanding crustal deformation, faulting and seismicity, all of which relate to the in-situ stress regime driven by the plate collision. Yet the stress field in the EHS mainly depends on indirect indicators, leaving the magnitude and heterogeneity unclear. We define this stress state at fine resolution from 331 recent hydraulic fracturing tests within depths of 1205 m. The results reveal that the maximum (σ_H) and minimum (σ_h) horizontal principal stresses increase with depth at 27.5 ± 0.8 and 20.2 ± 0.5 MPa/km, respectively. The vertical stress (σ_v) is driven by overburden weight as 26.7 MPa/km. A thrust faulting stress regime (σ_H > σ_h > σ_v) is favored at depths less than 300 m, evolving to strike-slip (σ_H > σ_v > σ_h) accompanied by thrust component, beneath it. σ_H is uniformly oriented to the NEE with a mean azimuth of N67.6 ± 13.1°E and exhibiting a slight rotation from the prevailing NNE–NE regional stress field. The northern and western sub-regions of the EHS exhibit different patterns of horizontal differential stress with depth, inferred to arise from variations in the interaction of tectonic stresses and gravity. The stress state is consistent to a subcritically stressed crust with a friction coefficient of ∼0.26 - much lower than the expected limit of Byerlee's law. The reactivation of pre-existing faults and shear fracturing is currently low, according to low shear stresses and fracturing potential values. This study also highlights that the HF-resolved stress may not have sufficient capacity to fully unravel the seismic potential, and reconcile the stress pattern with frequent seismicity in the EHS, due to lack of stress constraints on seismogenic depth.