Exploration and Research of Deep Crustal Structures in the Zhangzhou Basin and Its Vicinity

The Zhangzhou basin is located at the middle section of the southeast coast seismic zone of the mainland of China. Using high-resolution refraction and wide-angle reflection/refraction seismic profiling of Zhangzhou basin and its vicinity, we have obtained the crustal geometric structure and velocity structure as well as the geometric configuration and structural relationship between the deep and shallow fractures. The results show that the crust in the region is divided into the upper crust and lower crust. The thickness of the upper crust is 16.5km- 18.8km, and that of the lower crust is 12.0km- 13.0km. The upper crust is further divided into an upper and lower section. In the lower section of the upper crust, there is a low-velocity layer with a velocity of about 6.00km/s; the depth of the top surface of the low-velocity layer is about 12.0km, and the thickness is about 5.0km. The lower crust is also divided into an upper and lower section. The depth of Moho is 29.0km- 31 .8km There are 6 normal faults in the shallow crust in this region, and most of them extend downwards to a depth of less than 4kin, the maximum depth is about 5km. Below the shallow normal faults, there is a conjectural high-dip angle deep fault zone. The fault zone extends downwards till the Moho and upwards into the low-velocity layer in lower section of the upper crust. The deep and shallow faults are not tectonically connected. The combination character of deep and shallow structures in the Zhangzhou basin indicates that the Jiulongjiang fault zone is a deep fault zone with distinct characteristics and a complex deep and shallow structure background. The acquisition of deep seismic exploration results obviously enhanced the reliability of explanation of deep-structural data and the exploration precision of the region. The combination of deep and shallow structures resulted in uniform explanation results. The delamination of the crust and the characteristic of the structures are more precise and explicit. We discovered for the first time the combination characteristics of extensional structures and listric faults in the upper crust. This is not only helpful to the integrative judgment of earthquake risk in Zhangzhou and its vicinity, but also of importance for deepening the knowledge of deep dynamic processes in the southeast coast seismic zone.

Deep Seismic Exploration in the Taiwan Straits: The HX9 Survey Line Experiment and Preliminary Results

By using an offshore large volume air-gun seismic source, onshore seismic stations( including mobile stations and permanent stations) and ocean bottom seismometers,a deep seismic exploration experiment was carried out for the first time in the Taiwan Straits. Results show that seismic stations can receive seismic signals from the air-gun arrays of the "YANPING Ⅱ"scientific investigation ship from as far as 280 km away.Tens of thousands of high quality seismic data items were obtained successfully and different types of P-wave seismic phases were identified. A one-dimensional crustal structure model of the survey profile HX9 shows that the crustal structure,which is reflected by Pc and Pm P reflection waves from two velocity discontinuities and basement refraction wave( Pg) constitutes the basic characteristic of the crustal structure in this region. The depths of Conrad discontinuity and Moho discontinuity are respectively16. 0km- 17. 5km and 28. 0km- 29. 5km.

Crustal thickness and Poisson's ratio beneath the Yunnan region

A total of 1939 receiver functions were obtained from 732 teleseismic events （M〉5.0） recorded at 21 broadband portable seis- mic stations on the Tengchong, Baoshan and Simao blocks and Yangtze platform. These stations were installed by the Institute of Crustal Dynamics, China Earthquake Administration during 2010 and 2011. Using the H-x stacking and searching method, crustal thickness and velocity ratio beneath the stations are obtained. Results show that crustal thickness and Poisson＇s ratio in- ferred from the velocity ratio clearly vary, and they illustrate block features in deep structures. Except for the Tengchong block crustal thickness increases from south to north along the same block and from west to east across different blocks. In the Yangtze platform, Poisson＇s ratio and crustal thickness show a consistent and significant increasing trend from south to north, possibly indicating that crustal thickening is caused mainly by lower crustal variations. In contrast, Poisson＇s ratio has no sig- nificant change within the Baoshan and Simao blocks. Such differences demonstrate that the Jinshajiang-Red River fault is a southern boundary of the South China block. The H-κ results inferred from three portable stations on the Tengchong block show high Poisson＇s ratios, but they vary clearly with back-azimuth, implying the existence of strong anisotropy in the crustal medium beneath the stations.