Fault zone deformation results from the combined effects of discrete fault slip and strain distributed within surrounding materials. Structural complexity and material heterogeneity obscure how deformation is partitioned across scales, a challenge compounded by limited direct observation at depth. Variations in lithology, friction, damage, and fluid pressure may control whether strain localizes along primary faults or spreads into surrounding crust, while fault geometry, from local roughness to large-scale stepovers, shapes stress conditions and failure patterns. Together, these factors govern the expression of off-fault damage, which remains poorly constrained despite its fundamental role in accommodating deformation.

This session examines how fault zone properties, including material characteristics and geometric complexity, govern the balance between on-fault slip and off-fault strain. We invite contributions across spatial scales from micrometers to kilometers and timescales from milliseconds to millions of years, including studies that observe, quantify, or model off-fault deformation using experimental, observational, and numerical approaches.