Preliminary results of fault material properties and structural distribution in northern segment of the Chelungpu fault at Dakeng area
Date Issued
2007
Date
2007
Author(s)
Lin, Yi-Kai
DOI
zh-TW
Abstract
On Sep 21 of 1999, a destructive earthquake (Mw=7.6) took place around the Chi-Chi Town of central western Taiwan and its hypocenter is located at 120.81°E, 23.86N°. The surface rupture during the Chi-Chi earthquake is parallel and closed to the N-trend Chelungpu fault of about 100km in length. The displacement between north and south segments of Chi-Chi rupture is dramatically different. The northern rupture reaches the largest displacement of ~8.5 m. The Chelungpu fault is one of the major thrusts at the fold-and-thrust belt of western Taiwan and slips within the Chinshui Shale.
The study area is along the Dali riverbed closed to the drilling site of the Taiwan Chelungpu-fault Drilling Project (TCDP) in Dakeng area of the Taichung city. Most of stratigraphy in the study area is the Chinshui Shale. According to the results of TCDP, the 1999 Chi-Chi slip zone of TCDP Hole_A is located at the fault gouge zone of 1111m depth. In order to complement the limitation of geological information and fault gouge sample on the retrieved core, the detail mapping of representative region and collection of the fault gouge sample on the field were conducted. Furthermore, the analysis of anisotropy of magnetic susceptibility and examination of microfabric on fault gouge samples was performed for understanding the variation of physical properties across the fault zone, such as the change of paleostress direction and grain size variation.
Based on field mapping, most of strike-slip faults is sinistral faults. Especially, crosscutting of sinistral fault on the bedding is distinct at the hanging-wall locations of 30 m, 160 m, and 200 m away from the surface rupture. Also, at these locations, bedding was bended and the direction of the maximum stress cloclwisely rotated about 80°-100° from the regional direction of N130° to local N030°, suggesting the possibility of local block rotation. From the distribution of fracture density, a strong shear zone between the Chi-Chi surface rupture and the hanging wall of 50m away from surface rupture is identified. The result of microstructural examination indicates that the reduction of grain size within the fault gouge. Also, the long-axis orientation on slip plane rotates clockwisely in the foliated gouge with respective to the foot wall. The Flinn diagram of magnetic susceptibility anisotropy and mineral grain ellipsoid illustrates the oblate shape within the fault gouge. Furthermore, compared with the wall rock, the degree of foliation and anisotropy increase but the degree of lineation decrease in the fault gouge. The result of anisotropy of magnetic susceptibility displays that maximum stress orientation in the wall rock is parallel to regional stress direction (130°). However, maximum stress orientation in the foliated gouge is aligned to 150°. It also shows that maximum stress direction rotates clockwisely in the fault gouge with respective to the foot wall. The block rotation might be the reason of developing the foliated gouge. The evaluation of results between magnetic susceptibility anisotropy and mineral grain ellipsoid suggests that deformation of the magnetic susceptibility ellipsoid is the output of the matrix deformation, and the change of grain ellipsoid is mainly the reflection of rigid body deformation.
Subjects
葉理狀斷層泥
集集地震
磁感率異向性
礦物顆粒優勢排列
塊體旋轉
Foliated gouge
Chi-Chi earthquake
anisotropy of magnetic susceptibility
shape preferred orientations
block rotation
Type
thesis
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