Deep Structure Using Teleseismic Receiver Functions Western Yunnan

The West Yunnan area is a hotspot for geodynamical study, due to its peculiar geotectonic, both of genesis, monitoring and prediction of volcanic activity form the major objectivity in The International Disaster Reduction Program, broadband seismic data will inverse high-resolution information for the underneath structure in the West Yunnan and volcanic area.Based on the previous study, teleseismic waveform data, observed in the West Yunnan and volcanic area, is gathered to study their S wave velocity structures by the receiver function. Underneath structure features are shown in those areas, and their geodynamical implication is also discussed.The receiver function approach and nonlinear inversion proceedings are reviewed in this paper; the basic theory of the neighborhood algorithm is also analyzed. The receiver functions with different-level noise backgrounds are inverted using the neighborhood algorithm. The results show that the neighborhood algorithm is nearly noise free; even under large noise background, the neighborhood algorithm can also distinguish main velocity structures and reflect the whole character of S wave velocity. The neighborhood algorithm possesses global search ability and can recover robust models. The neighborhood algorithm adapts to the receiver function inversion, a kind of non-linear problem. The neighborhood algorithm is then used to infer underneath structures of Western Yunnan area and Wudalianchi and Tengchong volcanic areas, and some important results are obtained.The main conclusions from the receiver function inversion about the Wudalianchi volcanic area are summarized as: Low velocity structure is widely distributed in Wudalianchi volcanic area, with thickness between 15 and 22km and S velocity between 2.5km and 3.0km/s. The lower velocity structure spreads in linearity and consistent to the tectonic pattern. Possibly, some lava was blocked during the latest eruption due to the original vent being choked; lower lava was forced to find new channels and formed shallow lava. Seismic activity was correlated with lower velocity structure. It is shown by the receiver function inversion in Tengchong volcanic area: tectonic environments influence the volcanic activity, especially, influenced by the Dayingjiang fault. Low velocity zone is between 10 and 20km to the south of this fault, and velocity is about 2.5km/s or so. As a whole the low velocity zone are within same depth at Dayingjiang fault south. The low velocity zone isn't evident to the north of the Dayingjiang fault. It shows that low velocity structure is obstructed by the fault.Due to the different tectonic backgrounds, the underneath structure share some common characters as well as certain differences, mainly shown by: (1) The S wave velocity structure is imbedded at shallow position within 10~20km in Tengchong volcanic area, in contrast to underneath position within 15~25km in Wudalianchi volcanic area. (2) Moho in Tengchong volcanic area is deeper than that of Wudalianchi volcanic area. (3) Shallow lower velocity structure is within 4-12km in Tengchong volcanic area, contrast to 5~25km in Wudalianchi volcanic area. (4) The lateral variation in Tengchong volcanic area is larger than that in Wudalianchi volcanic area. (5) The lower velocity structure in two volcanic areas quite correlates with seismicity. (6) The low velocity structure located at certain depth implies that the reoccurrence possibility of Tengchong and Wudalianchi volcanic activity.This receiver function study in Western Yunnan area supports the previous results that the S wave velocity is lower globally and the crustal thickness becomes thin from north to south. It shows seismic evidences for the upper mantle upwelling and crust thinner in Panxi tectonic zone. It is shown that the crustal thickness significantly varies across the Nujiang River fault, Honghe fault and Xiaojiang fault. As a whole it is thinner in the west of Nujiang River than that in the east of Nujiang River. It is shown that Nujiang River is an important suture. Crustal t...