| This research integrates regional geological and seismological observations in the Tien Shan, one of the largest and most active intracontinental mountain systems of the world. It is composed of three major studies. In the first study, source parameters of 32 moderate-sized (M{dollar}sb{lcub}rm w{rcub}{dollar} = 3.5-6.0) earthquakes, distributed throughout the Tien Shan, were determined by regional-distance moment tensor inversion. The P-axes of the fault plane solutions are near horizontal and trend approximately 350-170{dollar}spcirc ,{dollar} indicating roughly north-south horizontal shortening in the Tien Shan. This compression, presumably a result of the continued northward penetration of India into Eurasia, is manifested in reverse faulting along a series of roughly east-west trending fault planes, which are relatively gently dipping at the range boundaries but are moderate to steeply dipping within the interior of the Tien Shan. The seismicity is shallow and mostly concentrated in the depth range of 5-25 km.; The second study focuses on determination of the three-dimensional P- and S-wave velocity models of the upper crust and relocating earthquakes beneath an actively deforming mountain front and its associated foreland in the northern Tien Shan by local earthquake tomography. At shallower depths ({dollar}<{dollar}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 contrast, at mid-crustal depths, the mountains are underlain by relatively lower velocity materials, caused by thickening of the mid-crustal layers due to basement-involved reverse faulting. The transition from the mountains to the foreland is marked by a sharp lateral gradient in seismic velocities. A pronounced low-velocity zone at mid-crustal depths was identified, which might be associated with rheological variations in the crust that could act to decouple the brittle upper crust from the ductile lower crust. The seismicity follow the traces of major active faults, and is mostly shallower than 20 km.; In the third study, the surface deformation and the aftershocks of the 1992 M{dollar}sb{lcub}rm s{rcub}{dollar} = 7.3 Suusamyr earthquake were investigated. The thrust-type earthquake produced reverse fault scarps on the surface, and left a wide variety of secondary surface effects throughout the Suusamyr Valley. The minimal surface faulting ({dollar}<{dollar}4 km in length) from such a large event can be explained by a combination of (1) redistribution of the net slip at depth among several secondary splays branching out from the main fault near the surface, and (2) variable surface slip over the fault length. The active deformation associated with this event documents a structural style that involves steeply dipping reverse faults, extending from the surface down to mid-crustal depths, accommodating crustal shortening in the interior of the Tien Shan. |
