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.

Identification of potentially dangerous glacial lakes in the northern Tien Shan

“…Several major earthquakes have occurred since the end of the 19 th century and thousands of smaller seismic events have been recorded (Lukk et al 1995). The major earthquake of the year 1887 (M s = 7.3) affected the large town Almaty (called Vernyi at that time) situated at the : Identification and monitoring of potentially dangerous glacial lakes in the northern Tien Shan.…”

Geodynamic Implications for the 8 October 2005 North Pakistan Earthquake

“…This surface-wave CMT methodology, which yields compatible and very similar results as in standard CMT, allows study of the complex processes of stress transfer to different fault segments that marked the unusual time evolution of the sequence (Arvidsson and Ekström 1998;Ekström et al 1998;Morelli et al 2000). Several workers (Menke and Jacob 1976;Armbruster et al 1978;Chatelain et al 1980;Seeber et al 1981;Ni and Ibenbrahim 1991;Fan et al 1994;Mellors et al 1995;Lukk et al 1995;Zhu et al 1997;Searle et al 2001;Khan 2003) have studied the seismicity and the geological processes of the northwestern part of the Indian plate, and have suggested that the seismicity in this region is apparently caused by the mutual interaction between the Kohistan Arc and the Indian plate (Fig. 1).…”

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“…Due to the northward indenting of the Pamir block into the Eurasian Plate (Lukk, et al, 1995;Neil, Houseman, 1997), the Tianshan Mountain from north and the Kunlun Mountain from south meet each other, squeezing the Tarim block and pressing its northwestern corner bending downward. To the north of the Jiashi earthquake region the Tianshan overthrust southward, overlying on the Tarim block and bending it downward.…”