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

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 a regional inventory of more than 2,000 deep-seated landslides mapped from high-resolution lidar, we reconstructed a 4,000-year landslide chronology by relating deposit surface roughness to age, calibrated with16 radiocarbon-dated landslides. Temporal clustering of landslides was assessed by identifying peaks in landslide frequency that exceeded a steady-state landslide production model. Those peaks were then compared to 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 reconstructed landslide history reveals clustering of landslides 1000, 1250, 1900, and 2800–3200 years before present (ybp), coinciding with the timing of major Holocene earthquakes around 1000, 2000, and 3000 ybp. A multi-fault earthquake model reproduces these elevated periods more closely than a steady-state scenario. Frequency Ratio (FR) analyses show persistent fault-proximal landslide clustering, particularly along the SFZ and TFZ, where FR decreases with distance from the fault during time intervals containing well-constrained surface rupturing earthquakes. Interpretation of landslide clustering on the DDMFZ is complicated by geomorphic predisposition; on the other hand, weaker signals near SWIFZ likely reflect preservation bias within the landslide record. Nonetheless, temporal and spatial patterns show that Holocene crustal earthquakes generally leave detectable signals in the landslide record of the Puget Lowland. These results show the usefulness of roughness-calibrated landslide chronology as an independent paleoseismic indicator. Overall, this study underscores the value of integrating paleolandslides in reconstructing past earthquake activity and refining hazard assessment in landslide-prone, seismically active landscapes.