We describe an 86-km-long, NS-trending deep seismic reflection profile, which passes through the Ürümqi depression (southern Junggar basin) of the northern Tien Shan piedmont. Two distinct anticlines beneath the northern margin of the Tien Shan are clearly imaged in the seismic section. In addition we have imaged two detachment surfaces at depths of ~10 km and ~20 km. The detachment surface at 20 km depth corresponds to the main detachment that converges with the steep angle fault (the Junggar Southern Marginal Fault) on which the M=7.7 1906 Manas earthquake occurred. A 12-14 km thick sedimentary basin is imaged beneath the southern Junggar basin near Shihezi. The crust beneath the northern margin of the Tien Shan is about 50 km thick, and decreases beneath the Junggar basin to about 45 km thick. The image on the deep seismic reflection profile is consistent with models of seismic refraction data and Bouguer gravity anomalies in the same region. The faulting associated with the 1906 Manas earthquake can be explained in the framework of this crustal model, which suggests that both a high-angle fault and sub-horizontal detachment surfaces moved during that event.

Present day micro-seismicity shows a hypocentral depth-distribution between 5-35 km, with a peak at 20 km. We suggest that the 1906 Manas earthquake initiated at a depth of ~20 km and propagated upward to cause northward slip on the sub-horizontal detachments beneath the southern Junggar basin.