Distributed Fiber-Optic Monitoring of Free-Field Soil Response under Normal Faulting: Full-Scale Experiment and Analytical Solution

Reasonable assessment of free-field soil strain is fundamental to quantifying the hazards induced by normal faulting. Previous studies provided insights into the propagation mechanisms of fault rupture within the overlying soil. However, experimental and theoretical analysis of free-field soil response is particularly limited. In this study, four distributed optical fiber types were used to monitor the soil strains under normal faulting through full-scale experiments. A trapezoidal local shear zone was formed on the hanging wall side, within which multiple longitudinal tension cracks occurred. The required ratios of fault offset over bedding depth for the occurrence of local shear zone, primary cracks, and secondary cracks were 1.7%, 2.6%, and 5.2%, respectively. The measured soil strains exhibited a V-shaped pattern, with an approximately linearly increasing longitudinal propagation range with fault offset. The Φ2 mm optical fiber was identified as an appropriate fiber type to represent the free-field soil response, because it could capture the decreasing pattern of soil strains when the fault rupture propagated through the soil. An error function–based analytical method was proposed based on a linear fitting between length scale and burial depth, which demonstrated its effectiveness in reproducing the ground deformation profile and estimating the peak soil strain.