The Study Of Deep Structure In The Middle Of Longmenshan

Longmenshan lies between Sichuan Basin and Qinghai-Xizang Plateau. It is a transitional zone of the active Songpan-Ganzi Block and stable Yangzi Block. As a part of North-South seismic zone in China, Longmenshan fault is very active with frequent seismic activity. The occurrence of Wenchuan Ms8.0 earthquake overturns our knowledge of the past, which makes it necessary to do a further study in this region. Geophysical Exploration Center of China Earthquake Administration carried out a deep seismic survey profile with a length of 500km in 2010. The profile stretches across the middle of Longmenshan, and it connects Yangzi Block and Songpan-Ganzi Block. Along the profile, a quite perfect deep seismic sounding observing system was set up.We improved trial-and-error procedure to study the crust structure by deep seismic wide-angle reflection/refraction method. A new inversion method overcomes the shortcomings of time-consuming and no-error analysis in trial-and-error method. We studied a deep seismic wide-angle reflection/refraction profile cross the middle part of Longmenshan by using the new method. Travel time inversion for one-dimensional and two-dimensional velocity models were carried out. Sum of square residues between calculated and observed travel times decreases and converges with the increase of iteration. The inversed results show that Sichuan Basin and Northwest Plateau have a stable and horizontal layer crust structure. Songpan-Ganzi structural transition zone has a stronger tectonic movement, which causes abnormal complexity of structure in this region. There are two velocity discontinuity located in the middle and lower crust in the west of Longmenshan fault. But we have not found the similar velocity discontinuity in the east of the fault. For the same layer of upper, middle and lower crust, the velocity in the west is smaller than that in the east. The difference of the velocities near the Moho in lower crust is about 0.3km/s. Thickness of upper, middle and lower crust is larger than that in the same layer, the differences of the thickness between east and west are 3-7km,6-8km,6-9km, respectively. Crustal thickening is mainly caused by the deformation of crustal medium in the middle and lower crust, which makes the difference of the depth of Moho in the east and west parts of the profile. Thickness of crust in Sichuan Basin of Yangzi Block is about 43-44km, and the thickness of crust in Northwest Plateau increases to 57-58km. In the structural transition zone, there are some lower velocity layers with different scales of thickness that increases from west to east. Some higher velocity zones are also found in the structural transition zone.