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Offset channels are incomplete records of strike-slip fault displacement

Nadine G. Reitman, Karl J. Mueller, Greg Tucker, Ryan D. Gold, Rich Briggs, & Katherine Barnhart

Published August 13, 2019, SCEC Contribution #9531, 2019 SCEC Annual Meeting Poster #110

Slip rates for historic and paleoseismic strike-slip faults are commonly determined by correlating channels offset across a fault, measuring the offset distance, and dating the incised deposit. This process is complicated by large uncertainties in pre-faulted channel morphology, few datable offsets, and large variations in apparent offset within short (kilometer-scale) distances. In this study, we use landscape evolution modeling to reduce uncertainties of the natural world caused by poorly defined earthquake timing and slip-per-event in the paleoseismic record. Analysis of >3000 automatically measured offsets from 135 model runs with known earthquake timing and slip history suggests that offset channels record true displacement only when the zone of faulting is narrow, total slip is less than channel spacing, and offsets are measured soon after the previous earthquake (though exact timing depends on lithology and climate in the zone of surface rupture). Despite constant slip along strike in our models, we find ~30% variance in offset marker distances, suggesting that natural variation in apparent slip along fault segments can arise from pre-existing geomorphic irregularity rather than differences in tectonic displacement attributed to seismic processes alone. Unlike individual offsets, average offset distance accurately records true slip in most models. The numerical model predicts that post-earthquake landscape evolution should widen the geomorphic expression of the fault zone and modify apparent channel offsets, leading to systematic underestimation of channel offset distance that increases with time since the earthquake. This process may lead to underestimation of seismic slip and associated hazard. The results highlight the ability of landscape evolution models to explore conditions not easily defined in the natural world, as well as the need for immediate post-earthquake investigation and data collection, averaging of offset measurements, and consideration of how post-earthquake landscape evolution has modified offset markers in paleoseismic studies.

Reitman, N. G., Mueller, K. J., Tucker, G., Gold, R. D., Briggs, R., & Barnhart, K. (2019, 08). Offset channels are incomplete records of strike-slip fault displacement. Poster Presentation at 2019 SCEC Annual Meeting.

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Earthquake Geology