Numerical Investigation of the Roles of Gravel Fabric on Composite Strata Deformation Induced by Thrust Faulting

In historical earthquakes, fault ruptures have caused significant ground deformation and infrastructure damage. Because the occurrences of earthquakes are hard to predict, it is critical to assess the area influenced by fault propagation for disaster resistance. Based on trench excavations in active fault zones, earthquake faults propagated through the composite strata with gravel deposits are common. However, the stratigraphy change of the overburden strata and the non-spherical granular particles of the gravel deposit are rarely considered in most case studies. This study proposes a procedure that utilizes digital image analysis and discrete element modeling to estimate the composite strata deformation induced by thrust faulting. The procedure is demonstrated through a series of small-scale sandbox modeling and a case study of the Chushan excavation site in Taiwan. The gravel fabric, including gravel volumetric content, aspect ratio and initial long-axis orientation, are considered in the parametric study as well as the configuration of the strata composition. Simulations show that fault propagation restricts in the overburden because the faulting-induced kinematic energy is mostly dissipated by clast rotation, resulting in a lower fault extended distance and a wider tri-shear zone. The rotation of the gravels shows good agreement with the range of the tri-shear zone, indicating that the long-axis orientations of gravels can be an indicator to interpret the faulting history. Overall, it is feasible to construct a reasonable numerical model for assessing fault rupture-composite strata interaction (FR-CSI) once the outcrop photo data are available. The proposed procedure is useful for efficient and reliable scenario modeling regarding earthquake hazard mitigation.