GEOPHYSICAL INVESTIGATION BETWEEN THE TIBETAN PLATEAU AND THE ORDOS BLOCK

The Tibet Plateau is rising rapidly and, as a consequence, exerts a lateral force on the adjacent crustal blocks. The Ordos Block, located NE of the plateau, is a rigid and stable block, as evidenced by the fact that there are no moderate or strong earthquakes within the block. Thus, an important question is what happens when the active but soft Tibet Plateau compresses the passive but very solid Ordos Block? The 1920 M=8.6 Haiyuan earthquake took place just at the junction of the two blocks, thus this question is also an important one for understanding seismic hazards. To investigate this question, a 1000-km-long geophysical profile was recorded. This profile incorporated deep seismic sounding, teleseismic observation, and magnetotelluric sounding profiles. This “tri-combintion profile” is the first profile of its kind in China.

Here we present results from the interprertation of the deep seismic sounding data. We show an example record section from this study, as well as the 2-D structure. The crust is well-stratified into an upper and lower crust. From NE to SW the crust gradually thickens, mainly due to increased thickness of the lower crust. At the same time, the average crustal velocity decreases and the number of reflecting interfaces becomes greater. There exists strong anomalies in the Haiyuan Region and west of the Zeku region, including very low average-crustal velocities, a low-velocity layer, and complexity of the Pc and PmP waveforms. These anomalies clearly indicate the complex crustal structures in these two regions due to the strong interaction between the Tibetan Plateau and the Ordos Block.