Seismic Hazard Analysis On The Wide Spaced Anticlines In Chongqing

Chongqing is located in the southeast margin of sichuan basin, Where the Yangtze meet the Jialing river. The main structural forms are wide spaced anticlines, with complex geological structure and development fault, at the same time,it is such a area with weak but highly frequency earthquake. We always had a vague understanding of the moderate earthquakes seismogenic structure in the region In tln the past. We made a summary of the tectonic pattern of the moderately strong earthquakes, analysised the seismic risk based on the research on the formation and evolution of the fold and fauit in this area, tectonic characteristics, the latest activities and their relationships, which can help the government may adopt feasible earthquake disaster prevention measures and be more reasonable in urban planning and social scientific development.Chongqing, as a bedrock exposed region city, the quaternary cover rare occurrence and the attitude of the bedrock are ordinarily steep, geological geomorphic character was different from cities in Plain and Basin. Considering these characteristics, break through the limitation of present tectonic activity study methods, Taking anticline-fracture zone as the research object, determined a set of comprehensive analysis methods combination of shallow surface and deep structure as the features. Firstly, the large scale geological geomorphic Surveying and several geological cross section lines measurement cross major anticline show us the structure characteristic of the anticline-fracture combination; Based on combination measurement cross major anticline and quaternary dating we got the tectonic activity; By deep seismic reflection, deep oil geophysical data re-interpretion, more accurate locating small medium Earthquakes, regional crustal dynamics and deformation measurement data analysis, we got the seismic risk.Research area includes two big rivers Yangtze river and Jialing river valley area, formed the geomorphic surfaces by fluvial erosion and accumulation to distinguish the fault activity which go across the valley area; Therefore, on the basis of widely collecting drilling data, we did systematically detailed investigation and survey on the river terraces, planation surface, corrosion surface, combinating drilling work to get the terrace classification and the terrace age, to achieve the goal of identitying the Quaternary stratum and dividing geomorphic surface. Based on the river terraces phase diagram map across the anticline, planation surface comparison diagram and standard drills on different terrace, we designed multi-group late quaternary integrated profiles, Based on these we confirmed the anticline and its related surface rupture is inactive since late quaternary.In order to analyse the seismic risk, we accurately located small medium nature Earthquake recorded by existing seismic station to corroborate the potential position of the seismic source; based on the deep seismic reflection, deep oil geophysical data re-interpretion in Tongjing and Rongchang seismic sources, We got the fine Geometric Features from up crustal to near furface and information which can reflect the deep-shallow tectonics coupling relationship, further revealed the seismic preparation structure conditions. Among them, find out the deep extension of main active fault, especially the presence or absence of the "abdominal fault", and and its relationship with deep structure and shallow fault, all these is the key to study the crustal structure in study area, explore the physical properties of the medium and analysised the seismic risk. We made two deep seismic reflection profiles across the typical moderately strong seismic source area Rongchang and Hechuang-Changshou, as well as the five middle and shallow geophysical prospecting exploration CSAMT line, adding continuation analysed the gravity anomaly field of different deep, based on this we get the structural characteristics of main anticline from up crustal to near surface. Especially the 2 deep seismic reflection profiles clearly reveals the fine crustal structure and the deep-shallow tectonics in Tongjing and Rongchang seismic sources for the first time.Based on different detection methods from deep to shallow (including deep seismic reflection, deep oil geophysical data re-interpretion, middle-shallow electromagnetic exploration, shallow geological profile measurement, etc.), with the support of a large number of actual detection results, combined with more accurate locating small medium Earthquakes, deep geophysical field continuation interpretation, regional crustal dynamics and deformation measurement data analysis, we comprehensively analysised the deep-shallow tectonics relationship of the two typical seismic sources. On this basis, we put forward that as a separate seismic tectonic area the wide spaced anticlines area located east of HuaYingShan base faults in eastern Sichuan own such a special moderately strong earthquakes seismogenic structure model as mutiperiodic movments of folding structure as the background, shallow focal depth and long re-occurence period. Besides, the basement fault, surface fold morphology and dynamic Field location also influence the earthquake activity that there will be some risk of 5-6 magnitude earthquake risk. On these basis, combining with spatial smoothing seismic method of probability analysis, comprehensively evaluatied the maximum magnitude of probable future earthquake and the seismic hazard probability in the target area. Then we had a further discussion on the Earthquake preparation conditions and dynamic background, that this area located in the residual strain field formed between the thrust front of Xuefengshan in the Mesozoic and uplift of Longmenshan in Cenozoic which finally formed face to face vergencing structure preparation conditions, then the seismic tectonic modelis established.All these combination and analysis can be useful reference to the seismic risk analyze of other moderately strong earthquakes area especially in South China. The related achievements have been used in urban planning and engineering construction, which has obtained good social and economic benefits.