Image the Zhefang‐Binchuan and Monglian‐Malong Wide‐Angle Seismic Profiles in Yunnan Province

Abstract

Based on the finite difference inversion and ray inversion method, the crust and upper mantle structure along the Zhefang–Binchuan and Menglian–Malong wide–angle seismic profiles, both of which are located in Yunnan province, are imaged using the geophysical data of travel‐time, amplitude ratio and Bouguer gravity anomalies. Thus some new recognitions about the geodynamics and the seismotectonic environment are derived. The crust thickness along the Zhefang–Binchuan profile is 35~46km, and that of the Menglian–Malong profile is 33~44.5km. The geometry of the Moho interface looks like that given by Hu H X et al. The P wave velocity on the top of the mantle beneath some places is relatively low and the variation range of the velocity is very large, which may indicates that the Yunnan region is a typical area of active tectonics. Moreover, there exits a prominent consistency between the shallow and deep structures, which implies that the activity of shallow material always has a deep background. The velocity structure of Zhefang–Binchuan profile indicates that there exists a large low velocity anomaly which penetrates the whole crust to the east of the Nujiang fault exposed on the ground, and this possibly associated with the upwelling of the deep material . The characteristic of the large faults which are the borders between some first order tectonic units can also be derived from the both velocity structure profiles: the Red River fault is a hyper‐fault that cuts the lithosphere, the Nujiang fault cuts deeply in the crust and even stretches downward into the top of the upper mantle, yet the Changning–Shuangjiang fault has a listric shape with a small dip angle, which may suggest that its incision depth is not very deep. The large earthquakes took place in Yunnan province always have a relationship with the large and deep faults that stretches into the upper mantle. Some earthquakes with shallow focal depths are generally located at the converge positions of different faults within the upper‐middle crust or the places where the velocity contours on the belt between the high and low velocity anomaly blocks are crooked. It is estimated that these locations are favorable to the accumulation of energy and regional stress.

Crustal P-wave velocity structure in Lower Yangtze region: Reinterpretation of Fuliji-Fengxian deep seismic sounding profile

“…However, in comparison with that of the other active domains in Chinese mainland, especially that of Yunnan [17,24,25] and Tibet [26,27] , this undulation is small, and its range approximates that of northeastern China [5] . The P n velocity of 8.06-8.29 km/s was obviously larger than that of the Yunnan and Tibet zones.…”

“…Owing to the advantages of flexibility of parameterization and inversion process, high stability of algorithm, fast computation ability, these methods have been widely adopted to image the crust-upper mantle velocity structure [13][14][15][16][17][18] . In this study, these two schemes of seismic travel time tomography are combined to reconstruct the 2D crustal P-wave velocity model along the Fuliji-Fengxian DSS profile, and a specific procedure is presented as follows: firstly taking advantage of the finite-difference inversion method, the upper crust velocity model is derived then the RayInvr technique is employed to acquire the velocity structure beneath the upper crust, thus the whole crust structure is reconstructed layer by layer.…”

Crustal thickness and average Vp/Vs ratio variations in southwest Yunnan, China, from teleseismic receiver functions

“…In the 1980s, several active source seismic projects were carried out in Yunnan [ Kan et al , 1986; Lin et al , 1993; Yan et al , 1985]. The upper lithosphere P velocity structures in Yunnan obtained from tomography analysis recently [ Bai and Wang , 2004; Zhang et al , 2006; Zhang et al , 2007], reveal the crustal structure as follows: the crust in Yunnan is composed of upper crust, middle crust and lower crust, and the upper crust includes sedimentary cover and basement. The average P wave velocity of the crust is ∼6.25 km/s in the Simao basin and ∼6.45 km/s in the Chuxiong basin.…”

3D Shear‐Wave Velocity Structure beneath the Southeastern Tibetan Plateau from Ambient Noise

“…Figure 9d,g shows the Red River fault divides lowshear-velocity and high-shear-velocity regions. According to previous geophysical studies, velocity variations across the Red River fault can also be seen at upper-mantle depth (Huang et al, 2002;Wang et al, 2003;Bai and Wang, 2004). The crust also thickens from southwest to northeast (Kan and Lin, 1986;Wang et al, 2003;Xu et al, 2006;.…”