224-6 Seismic Signatures in the Landscape: A Landslide-Based Record of Holocene Fault Ruptures in the Puget Lowland

Session: From the Cosmos and Back: Quantifying Processes and Rates of Landscape Change (Posters)

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Abstract:

Earthquake-triggered landslides pose a major hazard in tectonically active regions and may leave lasting imprints in the landscape that reflect past seismic activity. In the Puget Lowland of Washington State, an urban corridor transected by multiple Holocene-active crustal faults, we investigate whether deep-seated landslides retain a record of prehistoric earthquakes. Using over 2100 deep-seated landslides mapped using high-resolution lidar, we applied a surface roughness-age model, calibrated with 16 radiocarbon-dated deposits to reconstruct a 4000-year landslide chronology. Temporal clustering of landslides was compared against a steady-state landslide production model and earthquake-based scenarios incorporating known ruptures on the Tacoma Fault Zone (TFZ), Seattle Fault Zone (SFZ), Southern Whidbey Island Fault Zone (SWIFZ), and Darrington-Devil’s Mountain Fault Zone (DDMFZ). Our results show significantly elevated landslide frequencies near ~1100, ~2000, and ~3000 ybp that broadly align with known Holocene earthquakes. Single-fault models for the SWIFZ and DDMFZ with earthquakes at 3000 and 2000 ybp outperformed the steady-state model (rmsle = 0.0426 and 0.0432 vs. 0.0440), while a multi-fault model incorporating ruptures of all four faults reproduced the overall observed frequency pattern with a modestly higher rmsle of 0.0480. Frequency ratio (FR) methods show largely age-independent fault-proximal landslide clustering for the DDMFZ (e.g., FR > 2.0 within 5 km distance), and significant clustering near the SFZ during from 600–900 ybp (R² = 0.97; p = 0.003) and 1200–1500 ybp (R² = 0.92; p = 0.010). Clustering near the TFZ is strongest from 900–1500 ybp, bracketing its most recent rupture. Our results demonstrate that earthquake-triggered landslides have left detectable temporal and spatial signals in the landscape and highlight the value of roughness-based chronologies for paleoseismic reconstruction in glaciated terrains

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

doi: 10.1130/abs/2025AM-9350

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