Dendrochronological dating of landslides in western Oregon: Searching for signals of the Cascadia A.D. 1700 earthquake

Abstract

Large-magnitude earthquakes and hydrologic events in mountainous settings commonly trigger thousands of landslides, and slope failures typically constitute a significant proportion of the damage associated with these events. Large, dormant deep-seated landslides are ubiquitous in the Oregon Coast Range, western United States, yet a method for calculating landslide ages with the precision required to diagnose a specific triggering event, including the A.D. 1700 Cascadia earthquake, has remained elusive. Establishing a compelling connection between prehistoric slope instability and specific triggers requires landslide ages with precision greater than that provided by 14C dating of detrital materials. Tree-ring analysis is the only known method capable of determining landslide age with this precision. Dozens of landslide-dammed lakes in western Oregon present an opportunity to use tree rings from drowned snags, or “ghost forests,” to establish the year of death, and thus landsliding. We cross-dated tree-ring indices from drowned Douglas fir trees with live tree-ring records from the Oregon Coast Range that exhibit synchronous, time-specific patterns due to regional climate variations. Our analyses determined that the landslides responsible for creating Wasson and Klickitat Lakes occurred in A.D. 1819 and 1751, respectively. The 14C dates from selected tree rings and landslide deposit detritus are consistent with our tree-ring analysis, although the ages exhibit high variability, revealing the limitations of using 14C dating alone. Because dendrochronology provides annual precision for landsliding, sampling of tree rings at additional landslide-dammed lakes throughout the Oregon Coast Range can be used to constrain the potential effects of ground motion and major storms on Cascadia landscapes.

Rainfall triggers more deep-seated landslides than Cascadia earthquakes in the Oregon Coast Range, USA

“…This attributes 40 to 90% of the total landslide erosional budget to rainfall-induced landslides, a similar conclusion to what we draw from the OCR, despite the substantial differences in tectonic setting, lithology, and climate. Our conclusions are bolstered by recent efforts to pinpoint the timing of individual landslide deposits in the OCR using dendrochronology, often yielding ages with uncertainties of less than 1 year ( 13 ). This ongoing work has revealed multiple deep-seated landslides that date to 1890 CE, when historical records indicate severe flooding in the region ( 41 ), but has not identified any 1700 CE coseismic landslides.…”

The Preservation of Climate‐Driven Landslide Dams in Western Oregon

“…We note, however, that Bayesian constraint of 14 C ages using "wiggle matching," which utilizes either tree rings or stratigraphy to constrain the sample age probability density function (PDFs), is an enhanced 14 C dating technique that can provide comparatively robust ages (e.g., Hogg et al, 2019;Ramsey, 2009;Scharer et al, 2011;Streig et al, 2020;Struble et al, 2020). When possible, dendrochronology, or tree ring analysis, provides the only known tool to explicitly link the timing of slope failure with individual triggering events (e.g., Pringle, 2014;Struble et al, 2020;Šilhán, 2020). Although this approach is limited by the timescale of existing tree ring records, the Pacific Northwest is well-suited due to the longevity of trees and their relative abundance in the landscape.…”

“…Similarly, "ghost forests" are common at numerous landslide-dammed lakes and marshes in the OCR, where still-standing trees drowned as water rose behind the landslide dam (Figure 3). Struble et al (2020) dated landslide dams in the OCR with subannual accuracy by coupling dendrochronology and 14 C dating at Klickitat and Wasson Lakes to determine that they formed in the winters of 1751/1752 and 1819/1820 AD, respectively.…”

Assessing local impacts of the A.D. 1700 Cascadia earthquake and tsunami using tree ring growth histories: A case study in South Beach, Oregon, U.S.A.

“…The other was a chronology generated from deadsampled trees in lakes in the western and central Oregon Coast Range. These lakes had formed when landslides impounded streams, and the preserved drowned trees were then used to establish the date of lake formation, and thus the landslide event [Struble et al 2020]. Eight lakes had a combined number of 15 trees (and 31 sets of measurements) that pre-dated 1700 and were used to generate a second "control" chronology.…”