Analysis of Near Surface Deformation by Discrete Element Modeling and Inversion for Crustal Stress and Strength Using Stress Perturbations from Large Earthquakes
Date Issued
2013
Date
2013
Author(s)
Yang, Yi-Rong
Abstract
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. 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.
The magnitude of stress in the crust and the shear strength of faults are poorly known, yet fundamental quantities, in lithospheric dynamics. While stress magnitude cannot be measured directly, deviatoric stress state can be inferred indirectly from focal mechanism solutions collected before and after an earthquake. We extend a standard stress inversion method to invert for the 3D spatial distribution of absolute deviatoric stress and variation of fault strength with depth using focal mechanism solutions and coseismic stress changes produced by large earthquakes. We apply the method to the 1999 M7.6 Chi-Chi, Taiwan earthquake and the 2011 M9 Tohoku-oki, Japan earthquake. The estimated fault strength in Central Taiwan is constrained between 60 and 140 MPa in the upper 5 km of the crust and exceeds 80 MPa at greater depths. The shallow Taiwan crust above 10 km depth is relatively strong with coefficient of friction of at least 0.5 with a favored value of 0.7, assuming hydrostatic pore pressures. The inversion favors a somewhat weaker middle crust between depths of 10-20 km with coefficient of friction less than 0.5. The northern Japan forearc crust between 5 and 15 km depth appears to be weak with fault strength of 40-90 MPa, consistent with a coefficient of friction of 0.2-0.5. The Tohoku-oki coseismic stress change was large enough, relative to the ambient stress, to rotate the principal stress directions typically ~20° in the upper 20 km of the crust. The data from Japan require a heterogeneous ambient deviatoric stress field with short wavelength (~20-50 km) fluctions in primicpal stress orientations. In contrast, the ambient field of the stronger Taiwan crust is more homogeneous.
We use the focal mechanisms and stress change of the Chi-Chi earthquake to invert the 3D absolute stress field in Central Taiwan and discuss the relationship between background stress and the tectonic framework. The crust at a depth of 12–20 km shows a low friction coefficient, μ≈0.4-0.5, and is weaker within a depth interval of 0–30 km. The estimated background stress state explains the primary tectonic and regional structures of Taiwan: the plate motion of the Eurasian plate and the Philippine Sea plate, the orogenic collapse of the Central Range, the tectonic escape. The compressional stress state results from the action and reaction of compressional forces of the plate convergence located individually in the Longitudinal Valley and the Western Foothills around the hard Peikang High, causing current active faults and frequent earthquakes. Between the reaction force zone and the Central Range, the tectonic unit escapes toward the northwest, resulting in shear stress. For regional stress, we conjecture the Puli basins are a series of the pull-apart basins developed by the NE-SW extension in Hsuehshan Range adjacent to Puli basins and the local strike-slip faults on the sides. High left-slip of left-lateral faults and strong extension in the northern part of Puli basins cause these basins increase in size from the southwest to the northeast. In the south of Taichung basin, stress presents east-west extension resulted from tectonic loading.
Subjects
斷層擴展褶皺
離散元素法
應力反衍
地殼強度
集集地震
日本東北地震
埔里盆地群
Type
thesis
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