Earthquake cycles in fault-bend folds

Subduction zones and fold-and-thrust belts host both the biggest earthquakes on Earth, as well as a variety of events that break different segments of the megathrust. Numerical modeling has been used to try to understand the diversity of earthquakes at these margins. However, these models typically do not incorporate the active fault-related folding processes that are accommodating much of the upper plate deformation. The aim of this study is to investigate the impact of fault bends and associated folding on earthquake cycles and associated behavior, including earthquake segmentation, off-fault plasticity, and the propagation of shallow ruptures. We perform sensitivity tests on geometric elements, namely the magnitude of the fault bend and angularity of the hanging wall cut-off, that influence different earthquake processes.

We incorporate fault-bend fold theory into a fault dynamic modeling framework, and develop two-dimensional, quasi-dynamic numerical simulations of slip evolution under rate-and-state friction. This helps us understand the ways in which fault geometry and fault friction properties interact to shape the seismic cycle. We numerically incorporate axial surfaces in our models to simulate the kinematics of off-fault deformation associated with folding. We find that earthquake behavior is sensitive to fault friction parameters, the magnitude of fault bending, and the cut-off angle between incoming sediments and the fault. We find that fault bends can produce earthquake segmentation as a result of nonlinear fault dynamics, for both coseismic and interseismic slip. Shallow earthquakes that initiate, propagate, and terminate near the surface are facilitated when the stratigraphy within incoming thrust sheets is not parallel to the underlying fault, as this changes the loading rate across the fault bend. This study provides a first step towards creating more realistic rupture models that account for realistic geological structures and off-fault deformation.