ALBERT

Notes: We have examined arrival-time data from two seismic networks operating within the Pamir-Tien Shan region of Central Asia in order to evaluate crustal structure and earthquake distribution in one of the most seismically active zones in the region. The data are from two networks surrounding the Soviet earthquake-prediction laboratory at Garm, Tadjikistan: a 15-station local network operated by the Soviet Complex Seismological Expedition (CSE) since the mid-1950s, and a dense telemetered network, nested within the larger CSE network, operated jointly by the US Geological Survey (USGS) and the CSE since 1975. A simultaneous inversion procedure was used to calculate 1-D (layered) velocity models and station corrections independently for each of the two networks. Using a variety of data subsets and starting models, we obtained stable estimates of the velocity structure, independent of the initial velocity models. For both the CSE and the USGS network data the velocities of two upper-crustal layers proved to be indistinguishable from a model with a single layer over a half-space. For the CSE network, the best P-wave velocity model comprises an 8 km thick upper crustal layer of 5.2 km s-1 overlying a 5.9 km s-1 half-space. Using data from the USGS network alone, a similar velocity model was obtained, with the exception of a significantly higher velocity (Vp= 6.3km s-1) for the mid-crustal half-space. These results contradict previously published velocity models featuring anomalously low or high seismic velocities in the upper to mid-crust. Hypocentral parameters were redetermined for approximately 26 000 events occurring during the period 1975-82 using the newly obtained velocity models and station corrections. Most of the seismicity in the study area is associated with active deformation of the sedimentary rocks of the Peter the First Range. The distribution of hypocentres within the range suggests the presence of a southeast-dipping zone of seismicity that can be traced updip to the Petrovsky thrust fault. Beneath this upper crustal seismicity, at a depth of 12-17 km, is an aseismic zone that may be related to the presence of low-strength evaporites at the base of the sedimentary section. This aseismic zone is in turn underlain by anomalously deep crustal seismicity, at a depth of 17-30 km. The deeper seismicity appears to be associated with a southeast-dipping feature approximately 10 km in thickness. This dipping seismic zone is interpreted as a zone of intracontinental subduction, whose surface expression is the Vakhsh thrust fault, at the frontal edge of the Peter the First Range.