Geophysical study of deep crustal structure in China started in 1958, with seismic sounding profiles that now total 25000 km. The north‐south earthquake belt, oriented 100°–105°E, divides the whole country into two parts. The eastern part is the Circum‐Pacific structural unit, which is affected mainly by the evolution of the Pacific, whereas the western part is the Tethys‐Himalayas structural unit, which is affected mainly by the evolution of the Tethys. In the Tethys‐Himalayas unit, isostasy has not been reached. The Tibetan plate and the Indian plate have collided successively with Eurasian old land since the Mesozoic, giving rise to a series of overthrusts converging 20–30 km deep into the low velocity ‐ low resistivity layer of the earth's crust. As a result, the lower crust has crept and thickened under the propulsive force of the Indian plate. On the top of the upper mantle in northern Tibet, an anomalous layer has formed. In the Circum‐Pacific structural unit, the isostatic anomaly approaches zero. In North China, many dustpan‐like block basins were formed due to the spreading of the continent toward the ocean. The crust became heavier with cooling and intrusion of basalt, leading to continual subsidence of the North China region since the Tertiary.
Since the first shot for deep seismic sounding resounded in Qaidam basin in 1958, a total length of 25000 km of seismic sounding profiling has been carried out in China by the State Bureau of Seismology, Academia Sinica, and the Ministry of Geology and Mineral Resources.