Three-dimensional seismic-velocity heterogeneities (to a depth of 125 km) under the Tarbela array are determined by the Aki et al. (1976a) method of inverting teleseismic travel-time residuals. Velocity anomalies are clearly present and are elongated in the northwest direction. An overall 2 to 3 per cent decrease in velocity to the northeast is observed across any horizontal layer. These features result from a 4° dip of geologic structures in the direction N41°E. This direction is similar to some observed trends of seismicity in the Tarbela area and to the trend of the Himalayan Main Central Thrust (MCT) east and north of Tarbela, but not to the trend of the fault traces nor the strike of geologic structures in the Tarbela area. Just to the southeast of the Tarbela array, these faults bend sharply westward, forming a mountainous loop. In this study it is concluded that the westerly trending fault traces and westerly striking geology are both only surficial and not representative of structures at greater depth. These deep structures within the lower crust and upper mantle preserve a strike similar to more eastern areas along the MCT. They are shown to be volumetrically and tectonically the more important features.

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