Studying The Crust And Upper Mantle Structure In The Northeastern Margin Of The Tibetan Plateau And Its Adjacent Area By Using Seismic Wave Characteristics

The Tibetan Plateau is the largest and highest orogenic belt in the world at present, and it is one of the most complex crust-mantle structure and the most active tectonic zones, compared with other areas in the world.The northeastern margin of the Tibeten Plateau is the intersection of the Tibetan Plateau, Ordos and Alashan. Not only is this area with the new strong tectonic activity the most important part of the Tibeten Plateau but also the front part of the entire Tibeten Plateau to the inner part of the mainland. So it is very important to study the structure of crust and mantle in the northeastern margin of the Qinghai Tibet Plateau. Regarding the northeastern margin of the Tibetan Plateau and its surrounding areas as the research areas,the main contents of this dissertation are the following three parts by using the seismic wave amplitude, phase analysis and numerical simulation.(1)Analysis of the phase characteristics of shallow area caused by the low-velocity layer:(1)We analyse at length typical seismic phase characteristics in the shadow zone through comparing original data with simulated data recorded by seismic station of Lanzhou for seismic waveform data. It shows that the simulated seismic phases is more obvious, which is helpful to the identification of seismic phase; when the magnitude is not too large, common phases are very weak and seismic phases are ambiguous. In this case, we use other particular seismic phase in the shadow area to analyse seismic wave instead of conventional methods of seismic phase analysis.(2) Collecting digital seismic waveform data recorded by seismic station of Lanzhou in the 2002~2014 years, we select seismic waveform data of relatively high noise-signal ratio as the research object, and for these earthquakes the epicentral distances are in less than 30 degrees, the magnitude is about Ms5.0, focal depth is about 15 km. We have carried out a series of processes on the waveform data, such as format conversion, writing epicentre information into sac header file and exchange longitude and latitude with stations’, removing mean, removing pinch, removing linear trend, bandpass filtering and other process. "Shadow" phenomenon was seen in the seismic wave profile in the epicentral distance about 5~18°after a series of processes.(2)Studying upper mantle low-velocity layer beneath the northeastern margin of Tibetan Plateau by using "shadow zone" seismic waveform data reported by seismic network: By analyzing the vertical seismic P waveforms recorded by 35 broadband digital seismic instruments that belong to seismic networks of Tibet and its surrounding areas for the Pu’Er earthquake in Yunnan province on October 7, 2014, we found that P phase amplitudes are relatively weak between 5~18°epicenter distance, which shows phenomenon of shadow zone. The low velocity area within lithosphere beneath the Tibetan Plateau might be the most possible reason of the shadow zone by analysis. Based on the numerical tests of trial and error method, we many times compare P wave amplitude change trend of theoretical waveform from different models with observed waveform with the change of the epicenter distance and determine a low velocity layer of 24 km thickness at about 78 km depth and the velocity gradient within the low-velocity layer is-0.05/s.(3)Seismic evidence of the Conrad discontinuity in the northeastern margin of the Tibetan Plateau: The Conrad discontinuity, a boundary between the upper and the lower crusts, and a boundary between granite and basalt, which has of great significance for the study of the stability of the earth’s crust. Several strong and typical regional phases corresponding to the waves are observed in Southeast of Gansu Province in the northeastern margin of the Tibetan Plateau. These phases show strong amplitudes of 2~3 times larger than those of the direct waves and have been confirmed to be derived from the Conrad discontinuity through numerical simulations. This explain the existence of the Conrad discontinuity in the northeastern margin of the Tibeten Plateau. The existence of the discontinuity in the active region of tectonic activity suggests that the region may have a smaller size of the secondary block and this block maintain the original structural features in the strong tectonic movement. This result has important reference value for the accurate identification of the seismic phases of the regional earthquakes and the accurate calculation of the three elements of earthquake.