A PRELIMINARY SURVEY OF THE CRUSTAL VELOCITY STRUCTURE BENEATH THE BEIJING-TIANJIN REGION AND ITS ENVIRONS

In recent years, in the region around the cities of Beijing and Tianjin, work of seismic deep sounding has been carried out. The purpose is principally for studying the structure of the crust and upper mantle which involves the earthquake source regions of this seismically active portion of the North China Plain. By observing wide angle reflected waves, the reflection PM from the Mohorovicic Discontinuity and PM from the intermediate layer within the crust are recorded. Eefracted waves from the Mohorovicic Discontinuity have been observed. Assuming a horizontally homogeneous layered structural model, computations of the velocity structure by the damped least square method of inversion were done. Eesults show that apparently, in this entire region velocity varies with depth, as a layered earth. On the other hand, horizontal inhomogenuity actually exists.