89-10 2025 QG&G Kirk Bryan Award: Refined Earthquake History at the Cascadia Subduction Zone

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

Presenting Author:

Jason Padgett

Authors:

Padgett, Jason S.¹, Nelson, Alan R.², Witter, Robert C.³, Engelhart, Simon E.⁴, Mahan, Shannon A.⁵, Gray, Harrison J.⁶, DuRoss, Christopher B.⁷, Kelsey, Harvey M.⁸, Hawkes, Andrea D.⁹, Horton, Benjamin P.¹⁰

(1) Pacific Coastal and Marine Science Center, U.S. Geological Survey, Santa Cruz, California, USA, (2) Geologic Hazards Science Center, U.S. Geological Survey, Golden, Colorado, USA, (3) Alaska Science Center, U.S. Geological Survey, Anchorage, Alaska, USA, (4) Department of Geography, Durham University, Durham, United Kingdom, (5) Geosciences and Environmental Change Science Center, U.S. Geological Survey, Denver, Colorado, USA, (6) Geosciences and Environmental Change Science Center, U.S. Geological Survey, Denver, Colorado, USA, (7) Geologic Hazards Science Center, Golden, U.S. Geological Survey, Golden, Colorado, USA, (8) Department of Geology, Cal Poly Humboldt University, Arcata, California, USA, (9) Earth and Ocean Sciences, Center for Marine Science, University of North Carolina Wilmington, Wilmington, North Carolina, USA, (10) School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong SAR, China,

Abstract:

In our 2021 Kirk Bryan Award paper, we proposed a maximum earthquake rupture model for Cascadia that compared overlaps among age probability distributions based largely on early (1986-2015) ¹⁴C ages with large errors to correlate coastal earthquake evidence at sites from Humboldt Bay, CA to Willapa Bay, WA. Now, a new generation of onshore and offshore researchers is applying improved dating methods and increased numbers of ages to develop increasingly complex reconstructions of Cascadia’s great (magnitude >8) earthquake history. Over the past three decades the accuracy of ¹⁴C-based times of earthquakes have dramatically increased through (1) the routine use of AMS ¹⁴C samples that weigh 1-0.1 mg, (2) thorough pretreatment of single-genera samples, (3) a >5-fold increase in routine analytical precision, and (4) sedimentation-rate-based Bayesian statistical age models of earthquake sequences, rather than the single-age-bracketing, stratigraphic-position-based, models that we used in 2021. Continually improving methods of luminescence dating of quartz and feldspar sand grains (particularly on deposits of great-earthquake tsunamis), date sequences where ¹⁴C samples are lacking and(or) help us evaluate conflicting ¹⁴C ages, as do comparisons of climate proxy data from earthquake sequences with regional climate histories. New methods of dendrochronology applied to stumps of earthquake-killed trees may yield ages spanning only a few years for the most recent earthquakes. Although comparisons of different types of ages on the same earthquake contacts help evaluate ages, of greatest importance is the new focus on comparing age sequence models developed for estuarine sites with those being developed for lacustrine (turbidite) coastal sites and new offshore cores (marine turbidites). Lacustrine and offshore sites commonly host longer earthquake records with more consistent sedimentation rates and greater numbers of ages, which yield more precise reconstructions that better test earthquake histories through identifying age differences among sites.  The longer, more precise reconstructions being developed, along with a sustained interest of onshore and offshore researchers in collaboration, are helping us reach our goal of the past four decades of mapping and dating partial as well as full-margin ruptures of the past 14,000 years along the subduction zone. 

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

doi: 10.1130/abs/2025AM-7035

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