| Based on the extensive collection of basic geological materials, regarding the front-range fault zone of the Longmen Mountain as research object, the characteristics of the basic structure and activity are carried out with the indoor test and previous research results by the field geological investigation, embarked on the regional geological back-ground where it is. On this basis, the characteristics of surface rupture induced by the front-range fault are found out after the Wenchuan Earthquake; control action of the earthquake and geohazards by active faults is revealed; geohazard effect of active faul-ts and formation mechanism of geohazard chain are discussed; engineering geological stability evaluation is done by the Beichuan County resite as a typical site. The main knowledge which is obtained by means of research above is as follows:(1) The front-range fault zone of the Longmen Mountain is generally NE-SW tren-ding, strike of the fault zone is N35°～45 E°, inclination NW, dip angle at 50°～70°, it is composed of three segments (Northeastern Jiangyou-Guangyuan Fault, Middle Guanxian-Jiangyou Fault and Southwestern Dachuan-Shuangshi Fault). Different bran-ch faults are roughly parallel arranged in the plane; they form imbricate structure in the section; the characteristics of segmentation is taking on in the strike.(2) The characteristics of inhomogeneity and segmentation is obviously showed al-ong the front-range fault of the Longmen Mountain. Overall performance of the activi-ty:the southwest segment is the strongest, the middle segment is secondary, the northe-ast segment is the weakest. There had been strong activity since the Late Quaternary along the front-range fault. Especially, its activity is far more significant in the 5.12 Wenchuan Earthquake, it is orderly showed that the Luding-shuangshi segment is relat-ively obvious; the Shuangshi-guanxian segment is not obvious; the Guanxian-jiangyou segment is obvious; the Jiangyou-guangyuan segment is less obvious.(3) The Ms 8.0 Wenchuan Earthquake resulted in the unilateral multi-point type surface rupture zone of about 72 km in length. The field investigation showed that the performance patterns are dominated by the two types of the surface rupture and fold-deflection. Analysis on surface rupture of the typical districts indicates that the NW plate is relatively thrusting up, with the brittle fracture characteristics of typical overthrust and dextral strike-slip movement. (4) The statistics and analysis show that in terms of the displacement of earthquake surface rupture along the front-range fault zone, the average vertical displacement is 1.25 m, up to 3.6 m; the average horizontal displacement is 1.05 m, the maximum is 1.7 m. The ratio between the vertical displacement and the horizontal displacement is from 20:17 to 17:4, and the average is about 2.7:1, it is indicated that the existence of thrust and dextral strike-slip movement in the coseismic surface rupture zone, and the fomier is more than the latter, so thrust is the key role, dextral strike-slip supplemented, with the rupture nature of thrust and tectonic shortening.(5) The regional spatial distribution of the geohazards triggered by the Wenchuan Earthquake presents the zonal or linear characteristics along the fault zone of the Longmen Mountain and river system, and they also demonstrate the upper plate/lower plate effect and the disaster chain effect. Studies on the typical geohazards show that active fault is the main controlling factor of the geohazards formation, earthquake is the trigger factor, geoenvironmental conditions is the key factor.(6) The regional stability is relatively good in the new site of Beichuan County, belonging to ClassⅡconstruction site. The unstable rock and goaf areas belong to the area of poor stability, with an area of 0.3 km2, accounting for 4.93%; the covered karst area is less stable, with an area of 0.16 km2, accounting for 2.60%; whereas others are stable, with an area of 5.69 km2, accounting for 92.47%. Overall, it is stable.Figure [56] table [10] reference [106]... |
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