Coseismic fold scarps and their kinematic behavior in the 1999 Chi‐Chi earthquake Taiwan

Discrete scarps that are created or reactivated during large earthquakes are a locus of concentrated hazard. A number of the coseismic scarps activated in the 1999 Chi‐Chi earthquake are actually fold scarps, which display several types of ground deformation characterized by localized folding and are distinct from classic fault scarps, which form by a fault cutting the surface. This paper documents and analyzes fold scarps that formed or reactivated in the 1999 Chi‐Chi Taiwan earthquake. Our results show the Chi‐Chi fold scarps can be generally divided into two types: (1) those associated with folding ahead of the tip of a blind thrust fault at shallow depths and (2) those associated with folding by kink band migration above fault bends at substantial depths ranging from ∼0.8 to 5 km). The previously published trishear model can be applied to model the former type, while a new curved hinge kink band migration model is provided to describe the behavior of the latter type. A key feature of fold scarps of the second type is that hinge zones are typically wide (25–100 m) relative to the displacement in a single earthquake (1–10 m), which exerts a significant control on fold scarp morphology and evolution. Because the coseismic strains of both types of fold scarps display relatively wide deformation zones (10–100 m) relative to fault scarps, wider set‐back zones might be appropriate from a public policy point of view to alleviate the risk to structural damage and collapse resulting from permanent ground deformation.