89-2 Quantifying Off-Fault Deformation in Analog Models of Strike-Slip Systems

Session: Advancing Earthquake Geology and Surficial Deformation from Geologic Provinces to Political Entities through Multidisciplinary High-Resolution Data

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Abstract:

Understanding how fault geometry, subsurface slip, and material properties control on-fault vs. off-fault deformation partitioning is key for constraining fault slip rates and improving seismic hazard assessments. Here we present initial results from a suite of analog experiments designed to quantify the evolution of off-fault deformation during strike-slip faulting. Preliminary models consist of granular media deformed above a basal velocity discontinuity simulating a simple strike-slip fault. Deformation is monitored using time-series digital photogrammetry and digital image correlation, which yield sub-millimeter measurements of displacement, strain, and topographic evolution across model domains.

Preliminary results show a relatively consistent deformation sequence comprising: (1) formation of a broad, approximately linear zone of distributed deformation (uplift) producing positive relief above the basal fault during early displacement; (2) nucleation and growth of en-echelon, synthetic minor faults (Riedel shears) with strikes oriented 10–20° to the basal fault strike; and (3) progressive but incomplete linkage of minor faults into an irregular, segmented principal fault zone. Notably, occasional minor faulting and secondary linkage structures continue to develop within the broader damage zone after formation of the principal fault zone, indicating that fault maturity and deformation localization may not be directly correlated.

Progressive slip along the basal fault and surface fault zone evolution produce complex topographic patterns, including: (1) a diffuse, broad zone of positive relief during early deformation; (2) localized uplifts bounded by minor faults oblique to the basal fault in early to mid stages; (3) dissection and offset of uplifts as minor faults propagate and link; (4) formation of pull-apart basins and topographic lows (sags) where early minor faults define the principal fault zone orientation; and (5) juxtaposition of highs and lows across the main fault trace, producing abrupt topographic gradients. As slip localizes along a dominant strand, the broader damage zone – including abandoned minor faults and early-formed relief – remains laterally extensive and structurally complex.

These experiments demonstrate that the early architecture of fault-related deformation strongly controls subsequent fault growth and deformation localization. Persistent off-fault activity within the broader damage zone highlights the need to account for distributed strain when estimating fault slip and surface rupture hazard. These findings provide experimentally derived constraints applicable to improving fault displacement models and seismic hazard assessments.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8190

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