The Fault Geometry and Co-seismic Surface DeformationAround the Northern Taichung Basin

• Main Authors: Chung Huang, 黃鐘
• Other Authors: Jyr-Ching Hu
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/36799408530638463319

碩士 === 國立臺灣大學 === 地質科學研究所 === 95 === The northern Taichung basin is a transition zone between two structural domains in central Taiwan: folds-dominated domain in the north and imbricate thrusts- dominated domain in the south. Both of the north and south domains are highly active area. Series of large earthquakes occurred in 1935 in the north domain, and the devastating Mw 7.6 Chi-Chi earthquake occurred in 1999 in the south domain. The main purposes of this study are: to construct the structure geometry in the highly active structural transition area; to reveal detailed deformation pattern in the hanging wall of Chi-Chi earthquake and to construct a kilometer scale geometry model of the earthquake fault; and to simulate the slip distribution on the proposed fault plane. A 3D geological model was established based on seven published balanced cross sections plus one cross section constructed in the first part of the thesis. The constructed cross section is based on the surface geological data and relocated earthquake hypocenters. This 3D geological model indicated the high co-seismic displacements in northern section of Chi-Chi earthquake fault is result from a fault- bend fold in its footwall. The fold is caused by a lateral ramp underneath the Daan and Dacha Rivers. The anticlinal fold is also responsible for the changing direction of the rupture trace in the northern Chi-Chi earthquake fault. The second part is to characterize the deformational behaviors of the hanging wall block due to earthquake fault slip in northern Chi-Chi earthquake fault. In the case of the 1999 Chi-Chi earthquake, hanging wall block of the earthquake fault showed complex deformation pattern at the kilometer scale. Because previous studies mainly characterize the fault at the regional scale, it is of interest and also a challenge to characterize the fault at a smaller scale with a higher resolution. In this study, the geometry of a kilometer-scale patch of the fault plane is reconstructed using the displacement data collected from the densely distributed city planning benchmarks. The study area is approximately 4 km by 8 km in size, and contains as many as 924 benchmarks. Among the benchmarks, 62 have both horizontal and vertical displacement data, and the rest of the benchmarks have the horizontal displacement data. Based on the assumption of constant volume, the earthquake fault geometry is built by using the 62 slip vectors. The derived fault geometry model is rather consistent with the borehole data from the nearby 450 m well. The last part of the thesis applies numerical model to evaluate the fault geometry and to simulate the slip distribution on the fault plane. The fault geometry can be evaluated using the concept of fault parallel flow. The method of fault parallel flow can restore the topography before the earthquake and the deformation controlled by the fault geometry. Comparing pre-earthquake and post-earthquake cross sections of topography shows very high correlation coefficient, above 0.99 in average. Half-space elastic dislocation model is then used to simulate the slip distribution on the fault plane, which is inversed from the displacement data of city planning benchmarks. The result shows significant slip variation at the kilometer scale and is interpreted to be related to the deformation zone. The thesis uses different methods to reveal the spatial distribution and 3D geometry of seismogenic structures in northern Taichung basin at different scale. The results indicate better resolution of the seismogenic can apply to the earthquake hazards mitigation in this area in the future.