應力場變化對於三維裂隙系統演化的影響:以台灣東北部為例

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2020

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When nucleation of fractures and reactivation of pre-existing fractures was supposedly triggered by earthquake events, the stress state characterized by the focal mechanism of earthquake events can manipulate the 3D fracture patterns. That 3D fracture pattern might present the distribution, density variation, and connectivity of the fracture system in spatial always received special attention from geologists for the considerations regarding the mining and exploration development, disaster forecasting, and even the evolution of mountain building processes. Northeast Taiwan provided a special opportunity to understand the evolution of stress state from compressive subduction at the south to back-arc extension at north in the northeastern part of this active orogenic belt. The stress state in cross-section generated by inverted the focal mechanism data, which was collected from 1991 to 2016. The certain stress regime of Northeast Taiwan primarily changes from reverse faulting at the south to strike-slip faulting at the north, which approximately conducts the transformation of the orientation of the nucleated fractures network system. Even the presence of different in-situ stress regimes can cause reactivation of pre-existing in later. This region basically provides to the important situation to evaluate the evolution of fracture patterns with the stress field, contemporaneously. Therefore, our study aims to seeking, identifying, investigating the geometric characteristics of fracture planes of NE Taiwan by analyzing Digital Elevation Models (DEM). From DEM, the recent development of GIS and remote sensing technology can support to access and collect data of the fracture system in the inaccessible region. Our results show that in three domains, we recognized 187 surface fractures by applied the cross-cutting relationship between foliation and fracture planes and also calculated the fracture attitudes. In general, the spatial distribution of mean striking of fracture plane changed from N-S in the Domain III via NE-SW in the Domain II to ENE-WSW orientation in the Domain I. However, to get better results, only 120 fractures planes with R2 ≥ 0.85 and length ≥ 1000 meters are compatible to predicted fractures generated from focal mechanism analysis. After evaluating the fracture instabilities, we realized that nucleated surface fractures in DEM could be correlated with the applied stress field in the evolution of the tectonic setting from the south to the north along the coastline area at NE Taiwan. The number of nucleated surface fractures of Domain I is highest (64%) in comparison with other domains. Additionally, we examine the slip tendency analysis to measure the reactivation of nucleated surface fracture. The results confirmed the important roles of the extensional force of back-arc Okinawa Trough opening to the reactivated fracture in NE Taiwan. The examination result of slip tendency presented 54% nucleated fracture planes of Domain I could be reactivated under the stress state. Therefore, mapping and classifying fracture planes provided valuable information to the 3D fracture map, which should be able to further implication in terms of the landslide, groundwater, and scientific research issues.

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裂隙不安定性分析, 三維裂隙製圖, 應力逆推分析, Fractures Instability Analysis, 3D Fracture Mapping, Stress Inversion Analysis

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