Relocations of earthquakes, recorded by a local network of stations in Afghanistan and Tadjikistan in 1966 and 1967, indicate a narrow seismic zone (width ≲30 km) dipping steeply into the mantle to a depth of 300 km beneath the Pamir and Hindu Kush ranges. Very low seismicity was observed at depths less than about 70 km, the approximate depth of the Moho. Clear gaps in activity exist also within the zone of intermediate depth seismicity. One gap, about 50 km wide near 37°N and at depths greater than 100 km, separates a steeply northward dipping zone to the southwest from a steeply southeastward dipping zone to the northeast. This gap probably marks either a tear in the downgoing slab or a gap between two oppositely dipping slabs. Fault plane solutions, determined by Soboleva for events between 1960 and 1967, generally show steeply plunging T axes approximately within the planar seismic zone. They therefore are grossly similar to those at island arcs where no deep earthquakes occur and presumably result from gravitational body forces acting on a relatively dense slab of lithosphere. At the same time there is a very large variation in the fault plane solutions, much larger than is common at island arcs. Appendix is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Ave., N.W., Washington, DC 20009. Document J80‐003; $1.00. Payment must accompany order.

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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.

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We used teleseismic travel time residuals to determine lateral velocity variations of the crust and upper mantle in the Pamir‐Hindu Kush region in Tadjikistan and Afghanistan. Data from 29 analog seismic stations in Tadjikistan and northern Afghanistan were used to determine travel time residuals for 210 teleseismic events ranging in distance from 28° to 87° and covering a broad range of azimuths. We inverted for velocity perturbations over a rectangular grid with a block size of 99 × 99 km. The model extended to a depth of 350 km with a 50‐km‐thick first layer and two 150‐km‐thick deeper layers. The results show a strong and well‐resolved zone of high velocities in the upper mantle at depths greater than 200 km, coincident with the location of the Hindu Kush seismic zone. No clear velocity perturbations are associated with the Pamir seismic zone. Above 200 km little correlation is observed with the seismic zone, but indications of thicker crust under the Pamir and thinner crust under the Tadjik Depression are seen. The high velocities are most likely caused by the presence of oceanic lithosphere at depth.