Offset Channels May Not Accurately Record Strike-Slip Fault Displacement: Evidence From Landscape Evolution Models

Slip distribution, slip rate, and slip per event for strike-slip faults are commonly determined by correlating offset stream channels-under the assumption that they record seismic slip-but offset channels are formed by the interplay of tectonic and geomorphic processes. To constrain offset channel development under known tectonic and geomorphic conditions, we use numerical landscape evolution simulations along a theoretical strike-slip fault with uniform and steady uplift, erosion, and diffusion. We investigate the influence of four tectonic parameters (fault zone width, earthquake recurrence interval, variance of the recurrence interval, and total slip relative to channel spacing) on offset channel development through multiple earthquake cycles. Analysis of >3,000 automatically measured offsets from >135 simulations suggests similar to 30% variability in individual measurements, but modeled displacement is recovered by averaging multiple measurements. However, the average of multiple offset measurements systematically underestimates modeled slip except when the fault zone is less than similar to 5 m wide, total slip is less than channel spacing, and offsets are measured shortly after an earthquake. In these simulations, postearthquake landscape evolution widens the geomorphic expression of the fault zone and modifies apparent channel offsets. We distinguish this "geomorphic fault zone" from the tectonic fault zone (zone of coseismic distributed deformation). This study highlights the capability of landscape evolution models to explore a range of conditions not easily defined in natural examples and the importance of averaging multiple measurements. Our results verify that paleoseismic studies must consider how geomorphic change has modified offset markers and use caution interpreting slip histories with multiple earthquakes. Plain Language Summary We use landscape evolution simulations to investigate how geomorphic markers (e.g., stream channels) record displacement from earthquakes on strike-slip faults. Stream channels offset across a fault are commonly used to determine the amount of displacement and slip rate of a fault, critical inputs to seismic hazard calculations. For example, Wallace Creek on the San Andreas Fault was famously calculated by Sieh and Jahns (1984, https://doi.org/10.1130/0016-606(1984)95<883: HAOTSA>2.0.CO;2) to be offset similar to 128 m in similar to 3,750 years (similar to 34 mm/year slip rate). However, actual displacement is rarely known, and displaced stream channels are created and modified by both tectonic processes, such as earthquakes, and geomorphic processes, such as erosion. We run landscape evolution simulations to characterize how offset channels develop under certain tectonic and geomorphic conditions. After an earthquake, the initially discrete channel offset is gradually smeared out and diminished. The results imply that offset stream channels accurately record modeled slip only when the zone of faulting is narrow, cumulative slip is less than the distance between stream channels, and offset distances are measured soon after an earthquake. Otherwise, average channel offset distances underestimate modeled slip, which indicates that slip rates and associated seismic hazard may be underestimated on real faults.