Stress regimes in the Himalaya-Karakoram-Tibet, the western part of India-Eurasia collision: stress field implications based on focal mechanism solution data

Summary The stress regime patterns of high seismically active regions within the western part of the India-Eurasia collision, spanning from 67° E to 83° E and 27° N to 39° N, are elucidated through analysis of 684 Focal Mechanism Solutions from 1962 to 2021. Eighteen seismically active zones used for the stress tensor inversion, are defined based on the spatial extent of the seismicity, the depth distribution of seismic events, focal mechanism studies and seismotectonics of the region. The defined regimes are: (1) Sulaiman Ranges and Lobe Region, (2) Hindukush, (3) Pamir, (4) Nanga Parbat Syntaxis, (5) Hazara Syntaxis, (6) Kashmir-Zanskar region, (7) Kangra-Chamba, (8) Kinnaur and Kaurik-Chango Fault Zone (KCFZ), (9) Garhwal, (10) Kumaon, (11) Karakoram Fault Zone, and (12) Gozha-Ashikule Fault Zone. Seismicity is reported only in the crust or up to mid-crust in most of the regions, except for the Pamir and Hindukush, where the seismicity can be observed down to 160 km and 280 km, respectively. We report a clockwise rotation of the maximum horizontal stress (SHmax) of about 42° and 21° in the Hindukush and Pamir regions, respectively with increasing focal depths from NW to north. The region where major and strong earthquakes occur indicates pure compressive regimes. Most of the zones support transpressive and transtensional tectonics with a few zones by normal and strike-slip fault regimes. Regions like Nanga Parbat syntaxis, Kinnaur, KCFZ, and Zanskar are exceptions, where extensional and transformational tectonic features dominate. Plate convergence force has less effect on defining the stress regime in the KFZ and Gozha-Ashikule regions, which display transtensional and pure extensional regimes, respectively. Under-thrusting of the Indian plate through complex tectonics is indicated by dominant compression stresses with evidences of normal, strike-slip, and oblique fault mechanisms.