3-DIMENSIONAL VELOCITY STRUCTURE AND EARTHQUAKE LOCATIONS BENEATH THENORTHERN TIEN-SHAN OF KYRGYZSTAN, CENTRAL-ASIA

We used the arrival times of local earthquakes and quarry blasts recor ded by the Kyrgyzstan Broadband Network (KNET) to obtain three-dimensi onal (3-D) P and S wave velocity models of the upper crust beneath an actively deforming mountain front and its associated foreland in the K yrgyz Tien Shan. The continuous velocity models, described by cubic B spline interpolation of the squared slowness over a regular 3-D grid, were computed by simultaneous inversion of hypocenter and medium param eters. Exact ray tracing was done in the smooth 3-D medium by shooting rays from the sources to the stations by an analytical perturbation m ethod based on the paraxial ray theory. The deduced large, sparse, lin ear system was solved using the damped, iterative, least squares algor ithm LSQR. The stability and resolution of the result was qualitativel y tested by two synthetic tests: the spike test and the checkerboard r esolution test. We found that the models are well resolved up to a dep th of similar to 27 km for most parts of our image domain. The P and S wave velocity models are consistent with each other and provide evide nce for marked heterogeneity in the upper crustal structure beneath th e northern Tien Shan. At shallower depths (< 7 km) the sediment-filled foreland is imaged as a relatively lower velocity feature compared to the mountains, which are cored by crystalline basement rocks. In cont rast, at midcrustal depths the mountains are underlain by relatively l ower velocity materials compared to the foreland. A distinct contrast in velocity structure is also observed between the eastern and western parts of the Kyrgyz Range at midcrustal depths, with the presence of relatively higher velocities toward the east. The seismicity is concen trated near the traces of major active faults and extends deeper benea th the foreland compared to the mountains. The regional compression in the Tien Shan is accommodated along a series of high-angle reverse fa ults distributed throughout the orogenic system that extend from the s urface down to midcrustal depths. The range-bounding fault zone can be identified by a sharp lateral gradient in seismic velocities with a p ronounced southward dip combined with a zone of seismicity that also d eepens to the south and reverse fault source mechanisms from moderate- sized events. A pronounced low-velocity zone (LVZ) is imaged in the P wave field, at midcrustal depths, beneath the western part of the Kyrg yz Range. This LVZ is presumably correlatable with reported high-condu ctivity zones in this region that have been proposed to mark active fa ult zones along which fluid migration occurs. The location of the LVZ, which is closely coincident with the depth of maximum earthquake gene ration, might imply that it is a crustal decoupling zone at the brittl e-ductile transition.