In Situ Stress Characteristics In Western Coast Of Taiwan Strait

The western coast of Taiwan Strait,facing Taiwan Island across the sea,comprises Fujian province and east coast of Guangdong province,and it is an economically advanced area,which occupies a very important and attractive geographical location.However,this region with intense crustal motion and deformation and well-developed active fault due to interaction of the Eurasian plate,the pacific plate and the Philippines plate,has complex stress field characteristics and is a relatively active earthquake-prone area.Regional tectonic stress is closely related to earthquake and other tectonic activities,but crustal stress state of the study area is still not well researched.The author conducts respectively hydraulic fracturing in situ stress measurements of boreholes with depth of 450 m in Pingtan and Raoping.The results are indicative of that the dominant directions of in situ stress in Pingtan,where maximum and minimum horizontal principal stresses within the depth between 90m and 432m range from 5.02 MPa to 22.93 MPa and 3.99 MPa to 13.14 MPa separately,is N19°W;the dominant directions of in situ stress in Raoping,where maximum and minimum horizontal principal stresses within the depth between 132m and 432m range range from 5.98 MPa to 30.58 MPa and 4.61 MPa to 16.52 MPa separately,is N37°W;and tectonic stress level of both boreholes are not high.In order to further study the in situ stress characteristics,this work discusses the tectonic stress field characteristics and dynamic mechanism of the research area based on in situ stress measurements of boreholes in Pingtan and Raoping and previous measurements combined with focal mechanism data,shear-wave splitting data and crustal deformation observation data.Then the author explores spatial distribution of tectonic stress field in this area by virtue of numerical simulation by finite element.The analysis indicates that direction of maximum horizontal principal stress in the study area has fan-like distribution originates from NNW and expands to EW;in situ stress state in horizontal direction is roughly divided by Shaxian-Nanridao fault and stress of northern area is low,while stress of southern area is high;and in situ stress level of south area gradually decreases from north to south,starting from fault zone in the downstream area of Jiulongjiang.This work selects three boreholes in Pingtan,Quanzhou and Raoping respectively and evaluated the regional crustal stability of western coast of Taiwan Strait.According to Coulomb friction criteria and the?m representing shear-stress state,it was considered that fault activity of northern areas of Western Taiwan Strait at low stress level is weak;faults in middle areas of Western Taiwan Strait,where Quanzhou is located,have a high possibility of unstable sliding;and faults in adjacent areas of Raoping is unlikely to slide in the near future.In addition,exploratory research about relationships between deep and shallow crustal stress state is carried out.Through comparison in direction,magnitude and stress structure between deep and shallow in situ stress,the author concludes that the direction and magnitude of the deep tectonic stress are consistent with those of the shallow in situ stress,but the effect of the horizontal stress in shallow part is higher than that in deep part.