169-7 How Do We Decrease the Uncertainty in Fault Slip Rate Estimates, in Slow-Deforming Regions?

Session: Advances and Challenges in Seismotectonic Studies in Slow-Deforming Regions

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In slow-deforming regions Quaternary faults are typically dip-slip, with slip rates from 0.1 to 0.3 mm/yr. Using traditional geologic methods for calculating fault slip rate (scarp heights or trenching), the uncertainty range can be a large percentage of the rate (e.g., 0.2±0.1 mm/yr), which induces major uncertainty in seismic source characterization. Much uncertainty can arise from using fault scarp height (easily measured from lidar) as a proxy for vertical fault displacement. This assumption is invalid if the downthrown geomorphic surface is younger than the upthrown surface, which is the case on most fault scarps older than ca. 30-50 ka, and sometimes even younger. The downthrown block becomes progressively buried by inter-faulting and post-faulting alluvium very similar to that on the upthrown block surface. This burial (Hanging-Wall aggradation, or HWa, on normal faults) creates apparent scarp heights smaller than the true vertical displacement, and slip rates that appear to decrease with age on older scarps/surfaces. This was first noticed in the late early 1980s in the Rio Grande Rift, late 1980s on the Wasatch fault, and later on most other Quaternary faults in the Basin and Range Province. But is that trend a true region-wide tectonic decrease in slip rate, or an artifact of deriving slip rates from topographic profiles affected by HWa? These slip rates are now embedded in the USGS Quaternary Fault and Fold Database and in the National Seismic Hazard Model (Hatem et al, 2023, USGS Data Release, doi.org/10.5066/P9AWINWZ). We show case histories where trenching exposed a more complex sequence of faulting and deposition than would be interpreted from lidar scarp profiles (Southern Lost River fault, ID, Malde, 1971; Major Creek, CO, McCalpin, 1987; Greys River and Rock Creek faults, WY, Jones, 1995; East Franklin Mountains fault, TX, McCalpin et al., 2021). HWa decreased the scarp heights by 50% in many cases, over the true vertical displacement. This means the scarp-profile-derived slip rate was an underestimate of tectonic slip rate by 100%. Conversely, tectonic slip rates from trenches can be overestimated if their time span includes incomplete (‘open’) seismic cycles rather than only complete (‘closed’) seismic cycles.

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

doi: 10.1130/abs/2025AM-8938

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