P receiver functions of many seismograph stations in the Tien Shan in central Asia show a Ps phase converted from the bottom of a fractured surficial layer with a reduction in S velocity of around 30%. In the northern Tien Shan the depth of the bottom of this layer increases sharply from a few kilometers beneath the foreland to 15 km beneath the Kyrgyz range. This low S velocity region extending to mid-crustal depths is associated with a high level of ambient seismicity and is likely caused by fluids at high pore pressure that prevent closure of cracks. The low velocity layer is absent beneath some stations in the central Tien Shan, including a region of anomalously thin crust beneath the Naryn basin. The thin crust and reduced fracturing suggest a strong lithosphere that transfers compressive stress from the Aksay plateau in the south to the Kyrgyz and associated ranges in the north.