Research On The Activity Of Red River Fault In China And Vietnam

Based on the geological and geomorphological characteristics, the quantitative data of fault activity from field investigation, and the geometric structure of the Red River fault zone in Vietnam, as well as the collection and comprehensive analyses of deep geophysical fields, cross-fault deformation measurements, earthquakes determined by instruments and historical records, and other related data, combined with the systematic integration and analyses of related research work of the Red River fault zone in Yunnan of China, we realize the overall grasp of the tempo-spatial characteristics of right-lateral strike-slip fault activity for the Red River fault since Cenozoic. The completed major tasks and understandings include:1. For the first time, we obtained a large number of data and evidence for the Red River fault zone up to 370 km range in Vietnam, including geological, geophysical, geodetic, topographic, historical, modern seismic activity, and many other aspects. In particular, the spatial distribution, geometric structure, the characteristics of Quaternary geology and geomorphology, and the quantitative data for the latest active times and the cumulative right-lateral strike-slip dislocations of the Red River fault zone in Vietnam, etc., obtained through two times of field investigation in a total of about 50 days for the fault, filled in a number of gaps in the past studies on the Red River fault zone. In addition, we also obtained digital seismic observation, cross-fault deformation measurement, GPS observation, and other relevant data of the Red River fault in Vietnam from international meetings in Asia Pacific Natural Disasters and Geodynamics initiated by UNASCO, and IGCP430 Geodynamics, which provided bases to carry out cross-border and comprehensive studies on the fault.2. The Red River fault zone in Yunnan demonstrated unusually prominent tempo-spatial heterogeneity in the fault geometric structure, historical evolution processes, the geological and geomorphological performance and active style of the fault activities, times of the latest activities, the cumulative amount of dislocation, the seismic frequency and intensity, and many other aspects. In particular, the overall level of activities, including the active times and active intensity, and etc, demonstrates a very clear trend of gradually weakening from north to south since Quaternary. It is proved through our work that the activity of the Red River fault zone in Vietnam continues to maintain this general trend, namely, the times for strong activities are older, the cumulative right-lateral strike-slip dislocations in the amount are smaller, and the level of seismic activity are lower. This is not only practically significant for planning and utilization of land in northern Vietnam, but also scientifically significant for us to reconsider the impact of the Qinghai-Tibet plateau lateral extrusion to Yunnan, so as to further explore the future earthquake risk in Sichuan-Yunnan region. 3. The analysis on seismic focal mechanism and macro-characters of strong earthquake demonstrate that, there is a major horizontal compressive stress field setting in SSE-SN direction along Red River Fault, but meanwhile, the other major compressive stress fields in different direction and with higher vertical angle exist locally. The focal mechanisms with prominent dip-slip component mainly appeared in northern segment of the fault, and the dominant direction of major compressive stress field deflects to SE-SSE in middle segment. Moreover, the tendency that the complicate turns into the simple both of the direction of major compressive stress field and the manner of action is very clear from northern to southern segment. The state of modern tectonic stress field along Red River Fault accords the result of GPS observation, and also in agreement with the movement state described by geological data.4. In order to improve the reliability of the related researches, including the spatial relation of faults and earthquakes, seismic focal parameters and the environmental characteristics of the deep fault medium, we relocated 1,233 earthquakes along the Red River fault zone with 24,168 digital seismic records using Double-Difference Hypocenter Location technique, and obtained results showing the discrete distribution of epicenters and a concentrated trend of focal depth distribution, provided evidence from one aspect for our inference that the role of the Red River fault as the southwest boundary of the Sichuan-Yunnan active block is weakening, and the Red River fault has the conditions for shallow-deep decoupling and detachment.5. Digital seismic data from Yunnan Regional Seismic Network and a part from Vietnam were used to determine and analyze the focal parameters of 1142 earthquakes (M>=2.0) based on Brune model. Results show that there are relatively obvious differences between blocks on both sides of the Red River fault, and between various sub-blocks on eastern side of the fault. In particular, the difference between deep media of the blocks on eastern side of the fault is one of the import conditions for their uneven tempo-spatial activities. In addition, in the time period represented by the selected data, the coordination between the seismic dislocation and slip observed by GPS also support to a certain extent the two correlations between shallow-deep movements and deformation.6. By making use of the historical and modern earthquake records obtained in Vietnam, and integrating with the relative data in Yunnan, we have studied and analyzed the seismicity of Red River Fault during history and recent period respectively. The results demonstrate that the general character of the seismicity along Red River Fault is gradually weakened from northern to southern through comprehensive analysis, though there are some non-integrity of the data in Vietnam.Based on the above-mentioned studies and understandings, in view that the previous studies on the Quaternary activities of the Red River fault were more concentrated in the last century, the 80s to mid-90s, and from then on to present, many important data and results have not yet been systematically introduced into the seismic researches for the Red River fault, in particular the research on the dynamic mechanism for the uneven activities of the Red River fault, we focused on the three aspects of work:First, based on the understanding of the related tempo-spatial parameters for right-lateral strike-slip activities of the Red River fault and research results by the previous investigators, we systematically collect, collate, summarize and analyze the latest data and research results for the above-mentioned two aspects obtained in recent years.Second, from a point of view of classification for tectonics and active blocks, using seismic wave tomography inversion technique, transfer function, we calculated the Earth's crust average velocity, Poisson's ratio, and etc. for the active blocks on both sides of the Red River fault, an attempt to more clearly describe the deep medium structure and dynamic environment of the Red River fault.Third, combining the latest results based on geodetic deformation GPS observation and analysis, deep medium structure, and research results from dynamic environment, as the purpose, we explored the shallow-deep coupling relations, and systematically studied in-depth the dynamic mechanism for the significant heterogeneity of the Red River fault in Quaternary.New understandings are obtained based on the researches mentioned-above:1. Through analyses of the different driving mechanism for the early left-lateral strike-slip and late right-lateral strike-slip of the Red River fault in Cenozoic, we proposed that the area on the eastern active wall of the Red River fault, during right-lateral strike-slip period, was composed of several sub-blocks or units with different evolution history of geological development, different structure and medium characteristics of the crust and upper-mantle, and even significant differences in some aspects, which is one of the most important reasons for the significant tempo-spatial heterogeneity of the activities of the Red River fault.2. With a point of view that the crustal thickening in the large Sichuan-Yunnan area, southeast edge of the Qinghai-Tibet Plateau, is mainly the thickening of the lower crust with the rheological and ductile properties, combined with the significant difference between the crustal structures on both sides of the Xiaojinhe fault with strike in northeast direction and located inside the Sichuan-Yunnan block, we proposed that the rift extensional deformation in the area to northern of Mizu may be related with the "climbing" and creep of the lower crust crossing the Xiaojinhe fault, and effects of the going up and penetration of mantle heat flow along the Red River-Xiaojinhe deep faults.3. Based on the comprehensive analyses of the alternative exchanges for the vertical differential movements and horizontal strike-slip movements, and the vertical brittle-ductile coupling conversion in shallow and deep depth of some structure units or blocks, we pointed out that there may be some problems in segmentation of a fault activities based on the dislocation and age data from investigation of the shallow geometric structure and geological geomorphology, in particular, for the small segmentations. We put forward that we should pay much attention to the deep evidence, and the deformation mechanism of the transverse transition, shallow-deep coupling relation, and "conversion between segments" and "inter-segment transition" with a rheological sense.4. By comparing the research results obtained from geodetic GPS observation of space-to-ground and seismic tomography for crust and upper mantle, combining the findings that the characteristics for the whole Red River fault zone in China and Vietnam, from north to south, the active times of dextral strike-slip become older and older, the cumulative quantity of dislocation smaller and smaller, and the seismicity weaker and weaker, as well as the deep inversion results that the incorporation occurred in the upper crust for the Chuxiong-Jianshui fault and Wuliangshan fault which are almost parallel to the Red River fault zone on the shallow surface, it is comprehensively analyzed that there are partial connection and link up for the low-velocity layers on both sides of the Red River fault in the lower crust, even in the upper and middle crust, so as to provide conditions for clockwise rotation for the Sichuan-Yunnan block, when which is blocked by the central Yunnan block and south China block. We reasonably explained the clockwise rotation of the Sichuan-Yunnan block and the dynamic mechanism for the continuous deformation on both sides of the Red River fault revealed by the GPS observation; meanwhile, we put forward the inference that the role of the Red River zone as the southwest boundary of the so-called Sichuan-Yunnan block is weakening.