Seismotectonics and focal mechanisms of earthquakes from Pamir-Hindukush regions

Abstract

The Hindukush, Pamir and Karakorum region lies to the north of the northern apex of the Indian subcontinent, approximately between latitude 34.5°–40°N and longitude 68°–80°E. From the point of view of seismicity, this region is one of the most active in the world, the seismicity being attributed to the collision of the northward-moving Indian plate with Eurasia. In this region lie the junctions of several major mountains such as the Himalayas, Karakorum, Pamirs, Hindukush and Kun-Lun. The northern part of this region is characterized by highly folded structures. Major faults in the region follow the trend of these structures. While a number of faults has a strike-slip nature, several are thrusts or overthrusts. The present paper discusses the seismicity of the Hindukush-Pamir region, its relationship to tectonics, the nature of focal mechanism solutions and their significance in terms of plate tectonics.Seismicity maps have been prepared for 0–50 km, 50–200 km depths. In the northwestern part of the area, the seismicity for shallow depths (0–50 km) follows a convex trend from the Hindukush mountains towards the north and northwest along the Kunduz foredeep, Kuliab foredeep, Pamir-Alai zone and the Pamir foredeep. The Badakshan massif is relatively less active, while the southern Kashmir zone in the southeast and Gardez-Chaman fault zones in the southwest are quite active. The seismicity is dispersed in the rest of the area.For intermediate depths (50–200 km), the seismicity has an east-west trend in the northern part of the Hindukush mountains and changes sharply to a NNE direction following the trend of the Badakshan massif and central Pamirs. A similar trend is observed in the Pamir-Alai zone. A number of seismic sections have been prepared through the Pamir-Alai fracture zone, the northern and central Pamirs, Badakshan massif, Kunduz foredeep and the Hindukush regions to show the nature of faults and of focal mechanisms occurring at different depths. In most cases it is found that the fault/fracture zones are steeply dipping and thrust as well as strike-slip faulting is taking place up to considerable depths. The present-day seismic activity appears to be a result of stresses generated by the northward collision of the Indian and Eurasian plates and Alpine orogenic movements which have possibly reactivated major faults of Caledonian and Hercynian age.