The Fault Activity And Seismogenic Analysis Of The Southwestern Segment Of Xinyi-lianjiang Fault Zone In Guangdong Province

The Xinyi- Lianjiang fault zone(XLF) is a major NE-striking tectonic belt in the Yunkai Dashan uplift of western Guangdong province. This thesis studied its southwestern segment(SWXLF), which is about 40 km long and can be divided into east and west branches, extends southwestward from Guanqiao Town by Xin'an Town, Shajing and Xintang to near Houtangzai. It is of normal or reverse nature, and strikes 30° ~ 60°, dipping NW or SE at angles 60° ~ 75°. The new Catalogue of Chinese Historical Strong Earthquakes(from the 23 rd B.C. to 1911 A.D.) reports that two earthquakes M61/2 and M6.0 struck Lianjiang nearby this fault on 19 July and 17 August of 1605 AD, respectivy. That what is the seismogenic structure for these two moderately-sized earthquakes and its maximum potential earthquake in the future is a key problem to assessment of seismic safety for important engineering sites nearby. So it is necessary to study the fault activity of the SWXLF. The study is not only meaningful to site selections, seismic zoning and seismic safety evaluation of major projects, but also significant for better understanding of seismogenic structure of medium-sized earthquakes in southern China.In this work, firstly 14 boreholes and logging of two rows drilling were used to reveal the development characteristics of Quaternary strata in the JiuzhouJiang plain. Secondly, in terms of geological surveys and drilling joint geological sections, the activity segment characteristics of SWXLF since Quaternary were analyzed. Finally, the magnitude of the maximum potential earthquake was evaluated as M 6.5 through synthesizing verification information of historical earthquakes and the magnitude evaluation concept model of the SWXLF, which was based on the focal depths of small earthquakes and segmentation of relative activity. The content and primary results of this thesis are summarized below.1. Subdivision of Quaternary strata and sedimentary environment in the Jiuzhoujiang alluvial plain(1) Based on dating data of sixteen samples by electron spin resonance(ESR) and three samples by 14 C, this work made stratigraphic time divisions of strata since the late Early Pleistocene. A sporopollen analysis of 27 samples permitted to characterize development of the Quaternary strata, which provides data for a further research on stratigraphic comparison of the Quaternary strata in the JiuzhouJiang plain and the southeastern coastland basins.(2) Analyses of sporo-pollen assemblage from silt clay in each row of boreholes on the south and the north shows that the silt clay indicate a marsh or lacustrine origin and a product of the relatively warm Wurm interglacial epoch(30 ~ 50 ka), similar to climatic variation of the northern Leizhou Peninsula, providing a reference for paleoenvironmental contrast of southeast coastal areas.(3) Obvious sedimentary discontinuity was observed in the boreholes, which was from the middle and late periods of the Middle Pleistocene to the middle and late periods of Late Pleistocene, consistent with a relatively short-term quasi-denudation planation during the middle and late periods of Middle Pleistocene in the coastal areas of Leizhou Peninsula and the border between Guangxi and Guangdong.2. Fault activity of the SWXLF(1) The fault gouge developed in the geological sections of the fault at Lianjiang site. Combined with dating data, this evidence suggests there was a surface activity of the west branch characterized by dominant right-lateral strike-slip in early Middle Pleistocene(576 ka). The fault gouge developed in the geological sections of the fault at Shajing site, showing that there was a surface activity of the east branch characterized by dominant right-lateral strike-slip in middle period of Middle Pleistocene(348 ka). The fault zone between Hengshan and Tiantouzai is buried in Quaternary strata, in which the latest active time is from late Early Pleistocene(Tiantouzai village joint sections) to early Middle Pleistocene(Hengshan town joint sections). Based on the observsional data and relative activity segmentation and geometric segmentation, the lengths of the each segment was estimated to be 21 km, 20 km and 25 km, respectively.(2) Drilling joint sections confirm the buried segment of the SWXLF obviously offsets the lower part of the Quaternary strata. The down-slip rate obtained from the joint profiles in the Hengshan town during early Middle Pleistocene is 0.1 mm/a. And the down-slip rate obtained from the joint profiles in the Tiantouzai village during the middle and late periods of Early Pleistocene is 0.013 mm/a.3. Judgement of seismogenic structure for the SWXLF.(1) Based on the focal depths of small earthquakes and relative activity segmentation and geometric segmentation, the magnitude evaluation concept model of the SWXLF was established from the perspective of tectonic evolution.(2) The magnitude of the maximum potential earthquake was evaluated as M 6.5, synthesizing the magnitude evaluation concept model of the SWXLF and verification data of the historical earthquakes.