Seismic Evidence of the Conrad Discontinuity on the Northeast Margin of the Tibetan Plateau and a Tentative Discussion on Its Tectonic Implications

Conrad discontinuity, a boundary between the upper and the lower crusts and also a boundary between granite and basalt, plays an important role in the study of the stability of the earth's crust, the regional tectonic activity and the evolution of the block. The northeast margin of the Tibetan Plateau is located at the junction of several blocks and characterized by the development of large faults and strong historical earthquakes. The previous results hold that the Conrad discontinuity might be missing beneath the area with strong tectonic activity. In this study, several phases caused by the Conrad discontinuity, whose amplitudes are 2~3 times larger than those of the direct waves, were observed in southeast Gansu Province on the northeastern margin of the Tibetan Plateau. These phases were conformed to be derived from the Conrad discontinuity through wave form modeling and travel time fitting. The results not only provide direct evidence of seismology for the existence of the Conrad discontinuity on the northeastern margin of the Tibetan Plateau but also indicates that this discontinuity may also exist in the region of active tectonic activity. Moreover, this study is helpful to the accurate identification of the seismic phases of the regional earthquakes and the accurate calculation of the three elements of earthquake.