Crusal Structure Across Sichuan And Yunnan Region From Wide-angle Seismic And Gravity Data

Sichuan-Yunnan region which locates on the southeast boundary of Tibet plateau is the transitional zone between uplifted Tibet plateau and the stable Yangtze block. This area characterized by frequent seismic activities has a strong intra-continental deformation. Because of the special tectonic environment this area becomes an important site for researches of continental dynamics, seismogenic environment and the monitoring of future strong motion.2D compression velocity model of Pu’er-Luxi profile obtained by interpretation of deep seismic sounding data shows that crustal thickness across Honghe fault has strong lateral variations, which is 35 km in the southeast and 43 km in the northwest caused by thickening of the lower crust. Velocity in upper crust is distinctively different across Honghe fault, which in the southwest is about 0.4 km/s slower than that in the northeast, and this contrast is decreasing with the increasing of depth. Average crustal velocity is 5.90km/s in the southwest side of Honghe fault and 6.05km/s in northeast side, which increases to 6.13 km/s to the southeast end of the profile. The interfaces in crust near Xiaojiang fault show significant flexure, and the variation of crustal thickness in our model is not remarkable.This paper analyzes the compositions of crust by interpretation of compressional wave velocity model from wide angle seismic data, density model from gravity data, S wave model of three broadband stations from joint inversion of receiver function and surface wave dispersion, and laboratory measurements of seismic velocity for a wide range of rocks. The analysis indicates in the upper crust there maybe Metagraywacke rich in a facies felsic composition under Simao station at southwest of Honghe fault, basalt under Tonghai and Mile station. Middle crust of both sides of Honghe fault has similar feature which characterized by felsic components. Lower crust under Simao station has uniform composition of felsic components, and under Tonghai and Mile stations there are two layers, that the up layer is the same as that under Simao station, but the lower layer may be composed of intermediate rocks.Because that inversion in frequency domain provides a quick and efficient method, the paper combining the advantages of the frequency domain and parabolic function presents a processing method for gravity anomaly inversion of underground interfaces using parabolic density function in frequency domain based on the algorithm posed by Parker and Oldenburg. Both the synthetic test and interpretation of real data prove the effectiveness and correction of the method we proposed.The paper also presents a joint method of gravity data and seismic data which deployed the seismic interpretation as constraint of gravity interface inversion to improve the accuracy of the result based on the above researches. We obtain Moho depth distributions of Sichuan-Yunnan region combining constraint interface inversion using variable density proposed in this paper with interpretation data of 22 seismic sounding profiles in the researched area. Comparison with the result of receiver function verifies the the efficient of the mehod we proposed.The paper also analyzes the isostasy of Sichuan-Yunnan region by compare the Moho variations from constraint interface inversion by variable density and isostatic Moho depth calculated from topographic data. The relations between epicenter distribution and the isostatic anomaly indicate that most medium and strong earthquakes occurs in areas where the difference between the Moho depth from constraint interface inversion by variable density and isostatic Moho depth varies dramaticly.