THE ESTIMATION OF STRESS DROP OF MAJOR EARTHQUAKES IN NORTHWEST HIMALAYAN FOLD AND THRUST BELT AND MAKRAN SUBDUCTION ZONE

Abstract

Northwest Himalayan Fold and Thrust Belt and Makran Subduction Zone are the most active seismo-tectonic features in South Asia. Significant seismic events of magnitude M_w≥6.0 with focal depth upto 300 km were considered. The magnitude of both static (Ds₁) and dynamic (Ds₂) stress drops were obtained using in-situ source parameters and empirically ground motion derived relationships. The analysis reveals that in strike-slip faulting Ds₁ is an increasing function of magnitude whereas Ds₂ is a decreasing function of magnitude. These results are in accordance with the previous published studies. The results indicate that Ds₂ values decrease nonlinearly with increase in rupture width and surface rupture length. This decrease is very sharp in shallow depth up to 15 km depth after which an increasing trend in the dynamic stress drop is observed. This depth perhaps indicates to the brittle-ductile boundary in the lithosphere. This trend is more pronounced in reverse and strike-slip faults. The mean Ds₂ / Ds₁ is 18.4, 10.4 and 3.1 for strike-slip, reverse, and normal fault mechanisms, respectively. The rate of drop in Ds₂ in the strike-slip faulting is higher than other fault types which inferred the presence of transform boundaries. These results may lead to designed a forecast model on empirical ground motion parameters