A Study On Identification Of Seismogenic Structure Of Moderate-strong Earthquakes In South China And Its Application

Seismogenic structure is geological structure which has or is likely to generate destructive earthquakes. Regardless of in determination of potential seismic regions in probabilistic seismic hazard analysis (PSHA), or determination of potential seismogenic structure in the determining method of diffuse seismicity regions, determination of seismogenic structure plays an important and basic role. Therefore, identifying research on seismogenic structure is always an extremely important subject in seismic zonation and engineering seismology research. Scientific and reasonable earthquake safety evaluation depends on the research level of seismogenic structure to a greater extent. As a result, seismogenic structure research is always an important content in seismiczonation and engineering seismological research.Because of fault activity marks of seismogenic structure identification of moderate-strong earthquakes are not obviously, there is a greater difficulty in its identification process. How to identify seismogenic structure of moderate-strong earthquaks is always a difficult and key point in research fields of engineering earthquake. At present, identification of seismogenic structure for moderate-strong earthquakes is in the exploring stage. There are not relatively mature and widely accepted tectonic indicators generalized to identify seismogenic structure of moderate-strong earthquakes.This thesis briefly introduces seismogenic structure concept and some concepts associated with seismogenic structure, reviews the role of seismogenic structure in engineering seismological research and research status of seimogenic structure of moderate-strong earthquakes. I selected South China where seismic activity is relative weaker as the study area, analyzed spatial and temporal distribution characteristic of earthquakes, and discussed same place recurrence characteristic of moderate-strong earthquakes in the study area. For the Changde earthquake with M6(3/4) in Hunan province and the Ruichang earthquake with Ms5.7 in Jiangxi province, I analysed in detail its seismogenic structure, then selected seven earthquakes of M6.0-6^3/4 to analyse their seismotectonic environments in the study area. According to results of statistic analysis, I pointed out some identifying marks of seismogenic structure of moderate-strong earthquakes, and discussed application of seismogenic structure discrimination in engineering seismology. The main research contents and results are summarized as follows:1 The spatial and temporal distribution characteristic of moderate- strong earthquakes in South China.(1) Seismic activity of South China is overall weaker than western China and North China. Earthquakes with M (?) 7.0 rarely occur in the continental margins and coastal areas, Most of earthquakes occurred in continental interiors are of moderate-strong size. Temporal and spatial distribution of earthquakes is complex. The distribution characteristics of earthquakes are obvious of cluster and zone patterns in the study area. There are two obvious seismic activity cycles about 300 years since 1400 A.D.. The first seismic activity cycle was during 1400-1700 A.D.; the second seismic activity cycle was from 1700 A.D. to now. In the two seismic activity cycles, the quiescence periods and active periods appeared in turn, In addition there are many moderate-strong earthquakes which occurred at same place repeatedly in South China.(2)The thesis analysed the characteristic of moderate-strong earthquakes occurred at same place repeatedly in South China. It is found that the distribution characteristic of these moderate-strong earthquakes is non-uniform in Space. Seismic region with same place recurrence mainly concentrates in the southeast coast, Hehuai areas-upper reach areas of Yangtze River. The statistics result shows that at same place earthquake recurrence rate is 61.0% in the study area. Through the comparative analysis of maximum magnitude and secondary maximum of earthquakes in the same place recurrence scope in the study area, it is found that exist the some regularities in magnitude. There are about 76.6% magnitude variations concentrating in -0.6 to 0.6 range. The statistical analysis on recurrence times of earthquakse with same place repetition shows that there are 90.4% of all earthquakes with same place have concentrating on 320 years in the study area. During the 320 years there are three remarkable periods, it also has three remarkable repeated intervals, respectively less than 20 years, 70-120 years and 280-290 years.2 The analysis of seismogenic structure of the Changde M6(3/4) earthquake in 1631.The thesis analysed the characteristic of geologic structure background, development characteristic of the river system and the landform surface. By investigation of main fault activity in the seismic region named the Taiyang area, combined with intensity distribution of this earthquake, this work analysed the seismogenic structure of Changde M6(3/4) earthquakes in Hunan province in 1631. The results show that the Taiyangshan tilting and salient block, Fenhuangshan tilted and salient block and the Dalongzhan valley composed the seismogenic structure of the Changde earthquake.Indicators of geological structure are summarized as follows:(1)The changde earthquake occurred in the Taiyangshan uplift area where exist five grades of layered landform surface. The layered landform surface formed in late Early Pleistocene, Its average rising rate is about 0.05mm/a;(2)The earthquake occurred in a taphrogenic valley with NNE direction. The valley is about 20km in length and 2km in width. The valley and bodies beside the valley express hinge tilting since the neofectonic period;(3)The investigation results of main fault activity show that the seismogenic fault of this earthquake dislocated the stratum of late Q₂.3 The analysis of seismogenic structure of the Ruichang M5.7 earthquake in 2005.The thesis analyzed the characteristics of macroscopic destructions, distribution of isoseismic lines and focal mechanism solution of the Ruichang M5.7 earthquake in Jiangxi province in 2005. Combining the characteristics of geological structure background, landform, the Cenozoic stratigraphy and fault activity, this thesis makes a focused analysis of seismogenic structure of this event. The results show that the causative structure of the Ruichang earthquakes is Ruichang-Wuning NE-trending fault. The seismogenic structure of this earthquake is the fault of east edge of the Ruichang basins. Indicators of geological structure are summarized up primarily, signs of Geological Structure as follows:(1)The Quaternary basins with assemblage characters of a string of beads, theRuichang basin with 25km long axis and Fanzheng basin with 12km long axis;(2)The evident asymmetric distribution of river systems and Cenozoic strata in the basins, the tilting characteristic of the landform surface in basins;(3)According to the results of shallow seismic prospecting and exploration drilling, the fault of east edge of the Ruichang basin possibly dislocated strata in early Pleistocene.4 Integrated analysis on seismogenic structure of moderate-strong earthquakes in the research areas.Through analysis on seismogenic structure of typical moderate-strong earthquakes in the research area, the main seimogeology signs for moderate-strong earthquakes are summarized as:(1)There are active faults, active basins since Early Quaternary, earthquakes often occurred in Quaternary rift - subsidence basin edge, area with evident difference of activity since neotectonic time, part with the evident difference of landform;(2) Regions of small-moderate earthquake clustering.(3) The faults and basins which actived since Early Quaternary are the mainly kinds of seismogenic structure;(4) The dominant orientation of seismogenic structure is north north-east, north-east;(5) When using the method of seismotectonic analogy to identify seismogenic structure of moderate-strong event, it needs to pay attention to the meaning distinguished age of faulting activity. For example the faults of early and middle Pleistocene whose ages are dated by measuring the substance from the fault, maximum potential magnitude of its seismogenic structure can be M (?) 6.0, and it needs to associate concrete marks of seismic activity in comprehensive analysis; but for the fault which dislocated the strata of early and middle Pleistocene can be identified as the seismogenic structure with maximum potential magnitude M(?)6.0, the fault which dislocated the strata of late Pleistocene can be identified as the seismogenic structure with maximum potential magnitude M(?)6.5.5 Applied analyses on determination of seismogenic structure of moderate-strong earthquakes in engineering seismology.(1) Applications of seismogenic structure identification of moderate-strong earthquakes to identifying of potential earthquake regions According to the characteristics of seismogenic structure of moderate-strong earthquakes, the geotectonic signs, the characteristics of same place recurrence of moderate-strong earthquakes in South China, the thesis discusses identifying of potential earthquake regions from determining of orientation, range, upper magnitude limit aspects of potential earthquake source regions.(2) Application of repeatedly factors of moderate-strong earthquakes at same place to determination of seismic spatial distribution function.Seismic spatial distribution function is a weight coefficient of earthquake yearly average occurrence rate in the magnitude multi-levels in the potential earthquake source region. It can reasonably reflect non-homgeneous of spatial distribution in magnitude using the seismic spatial distribution functions. Repetitive factor of moderate-strong earthquakes at same place is a relatively independent factor in determination of seismic spatial distribution function. Its introduction reflects the characteristic of moderate-strong earthquakes in situ in South China. Therefore it is more reasonable to distribute earthquake yearly average occurrence rate early magnitude intervals in the potential seismic region.(3)Applications of seismogenic structure identification of moderate-strong earthquakes to seismic safety assessment of nuclear engineerings.The distantance of seismogenic structure of moderate-strong earthquakes to the site of nuclear engineerings and maximum potential magnitude of seismogenic structure can affect the result of seismic safety assessment of nuclear engineerings. In applications of seismogenic structure identification of moderate-strong earthquake to seismic safety assessment of nuclear engineerings, differences of treatment methods of the distantance from seismogenic structure of moderate-strong events to the site of the nuclear engineerings and estimation of maximum potential magnitude are discussed in the thesis.