Oral Presentation

What governs whether a fault creeps, produces slow slip events, or ruptures catastrophically? Modern geodetic and geophysical methods now capture on- and off-fault deformation throughout the earthquake cycle in unprecedented spatiotemporal detail. Fault geometry and stress state may influence slip behavior, but growing evidence highlights the critical role of fault material properties and rheology. These observations come from integrating interdisciplinary observations from the rock record, deformation experiments at near in situ conditions, and geophysical datasets, as well as leveraging geochemical and chronometric tools in innovative ways.

The San Andreas fault system (SAFS) provides a laboratory to explore this idea. Its diverse lithologies reflect a complex subduction margin history that seeded the transform boundary with a spectrum of rock types and inherited fabrics. As these rocks deformed and exhumed, they were also progressively altered. Ultimately, shallowly exhumed fault rocks offer access to materials that deformed at depth in the past and now reside at the surface with limited overprinting. Similarly, rocks from the interface of the 6 February 2023 Mw7.6 Elbistan, Türkiye, earthquake rupture, sampled in the aftermath of the event, provide the opportunity to connect lithology, fabric, and rupture characteristics.

Here I highlight examples of shallow fault material properties and rheology influencing fault slip behavior. Along the southern San Andreas fault, a distinct “red clay” gouge defines the fault. Field and experimental data, compared with geodetic observations, show this material can host geodetically observed shallow slow slip events and facilitate rupture propagation from depth. Along the Çardak and Yeşilyurt faults in Türkiye, the Mw 7.6 rupture propagated through variable lithologies. Single-strand segments with high-offset and supershear rupture velocities correspond to smectite-rich fault rocks with mature fabrics. Where rupture strands are distributed and displacement is lower, fault materials are less evolved. Two-years of CO₂ flux surveys along the surface rupture characterize how fault properties evolve post-seismically. Elevated flux restricted only to the surface rupture has declined dramatically, recording rapid post-seismic permeability changes. The rate and spatial pattern of this permeability change correlate with rupture characteristics, offering a real-time proxy for fault healing and material property evolution.