Earthquake focal mechanisms, deformation state, and seismotectonics of the Pamir‐Tien Shan region, Central Asia

Abstract

This paper presents a review of the seismotectonics of the Pamir‐Tien Shan collision zone in the Garm region, Tajikistan, based on geological structure, seismicity, and focal mechanism solutions. The region is dominated by horizontal compression, manifested by imbricate, low‐angle thrust faults that separate the upper crust into a series of tectonic sheets. These thrust systems verge northward from the Tajik Depression toward the southern Tien Shan and southward toward the northern Pamir. The pattern of seismicity across the region suggests that similar low‐angle thrust faults exist within the crystalline basement as well. In order to reconstruct the present‐day stress‐strain state of the region, we used data from over 15,000 earthquake focal mechanism solutions for small earthquakes (M ≥ 1.0) gathered over the 27‐year period, 1963–1989. The method of reconstruction of the seismotectonic deformation (STD) field involves dividing the data set into small spatial windows and summing the individual focal mechanism solutions to form an average earthquake mechanism tensor for each cell. The STD state of that cell can then be represented by the orientation of the principal axes of compression and tension, the intensity (or relative uniformity) of the average mechanism, and the Lode‐Nadai coefficient, which defines the relation between the magnitudes of the principal strain components. We observe throughout the study area a general predominance of subhorizontal compression, manifested in a mixture of thrust and strike‐slip deformation; normal‐fault deformation is observed in only two small portions of the study area. The orientation of principal compression varies from nearly N‐S in the northern Pamir and the southern Tien Shan to NW‐SE in the Peter the First Range. In general, the STD field has a clearly developed “mosaic” structure, defined by spatial groupings of nearly uniform orientations of compression and tension axes. The boundaries of these groups cannot in most cases be directly correlated with known geological contacts. The STD structure also shows some variation with depth, with more or less uniform STD orientation in the upper crust (0–14 km depth), changing significantly at greater depths. Examination of various subsets of the focal mechanism catalog shows the spatial structure of the STD field to be largely stable with respect to both time and magnitude. The reliability of the reconstructed STD field was verified using three approaches: (1) comparison of individual focal mechanism determinations for a large subset of the data using both Soviet and U.S. algorithms, (2) comparison of STD reconstruction results using the two independent focal mechanism catalogs, and (3) examination of the effect of spatial sampling on the results. The dominance of generally north oriented subhorizontal compression is interpreted to be primarily the result of convergence between the Pamir and Tien Shan ranges, in turn caused by the ongoing collision of India and Eurasia.