SEISMICITY AND TECTONICS OF PAKISTAN AND SURROUNDING REGIONS.

摘要

The tectonics of Pakistan and surrounding regions are examined primarily on the basis of seismicity data and focal mechanisms determined from an analysis of Rayleigh waves. The foundation of these studies is a compilation and critical review of earthquakes that are documented by noninstrumental and instrumental means. Relocated aftershocks and the distribution of intensity allow the length and orientation of the rupture zone to be determined for some larger shocks. Long recurrence intervals, and plate boundaries associated with broad zones of seismicity, combine to preclude the use of the seismic gap hypothesis for most of the region under consideration. While a few faults or fault zones can be classified as active, the limitations of teleseismic locations make it impossible to attribute most earthquakes to specific mapped faults.;Focal mechanisms and depths of 16 moderate to small earthquakes, located in central and southern Pakistan and the northern Arabian Sea, were determined from an analysis of Rayleigh waves in conjunction with limited first-motion data. The estimated focal parameters provide new constraints on the tectonics of the regions in which they occur. Of particular interest are the identification of transform-type motion near the Murray ridge, and activity that is consistent with a decollement style of tectonics in central Pakistan.;The Makran region of southern Pakistan and Iran appears to be the site of an active subduction zone. The volcanic arc is poorly developed; the dipping seismic zone extends to depths of about 80 km; focal mechanisms show down-dip tension within the slab. Teleseismic activity prior to the great Makran coast earthquake of 1945 first occurred inland between the future rupture zone and the volcanic arc. Next, in the 10 years prior to the mainshock, the activity was concentrated along the coast near the site of the future epicenter. Since 1954 the rupture zone has been relatively inactive. These observations are similar to patterns associated with other large earthquakes. Recent seismicity suggests the area immediately west of the 1945 rupture zone may be the location of the next large earthquake along the Makran coast.