Evolution of coseismic fault-related folds induced by the Chi-Chi earthquake: A case study of the Wufeng site, Central Taiwan by using 2D distinct element modeling

摘要

Coseismic fault-propagation folds are generally associated with blind faults, which are of considerable academic interest and have recently been recognized as critical for assessing seismic hazards. The Wufeng excavation site was characterized by a major east-dipping basal thrust exhibiting a dip angle of 34° and 2 opposing vergent thrusts generated a pop-up anticlinal fold in soil cover induced by the 1999 Chi-Chi earthquake (M_w = 7.6). In order to represent the soft soil behavior, we set up a direct shear simulation test to obtain valid parameters for soil. A series of 2D distinct element models possessing different bonding types and strengths was conducted to determine the deformation pattern near the surface and the evolution of the fault tip propagation. The coseismic deformation features of soft and plastic soil cover like the mutative limb thickness, complex ruptures and the overturned forelimbs in small-scale caused by the propagation of the blind fault tip were accurately predicted by contact-bond model in which grains were allowed to rotate and slip without cements breaking. Our results show that the alternative thrusts and pop-up structure developed before the main basal thrust fault ruptured through the ground surface at the Wufeng excavation site. We evaluated the slipping distance of the main fault at approximately 6 m in Wufeng during the 1999 Chi-Chi earthquake.