143-8 High Resolution Mapping of the 1700 CE Cascadia Subduction Zone Earthquake and Tsunami in Johns River, Washington, USA

Abstract:

Subduction zone great earthquakes (≥ M8) and their potential tsunamis are significant hazards for coastal regions, as evident by several catastrophic earthquakes over the past few decades, including 2004 Sumatra (M9.3), 2011 Japan (M9), and 2025 Kamchatka (M8.8) events. Unlike other subduction zones, the Cascadia Subduction Zone (CSZ) lacks instrumental records of any great earthquake and tsunami, which forces hazard assessments to be reliant upon paleoseismic records. Dendrochronology and coastal stratigraphy along the CSZ reveal evidence of the most recent great earthquake and tsunami to have occurred in 1700 CE. Even though detailed tsunami inundation maps play an important role informing hazard assessments, a lack of high-resolution paleotsunami inundation mapping exists along much of coastal Cascadia. Additionally, the limited number of sites with mapped paleotsunami deposits only represent the potential minimum reach of inundation, as recent tsunamis have shown that the tsunami sand deposition extent is only a fraction of the total inundation distance. To improve CSZ tsunami hazard maps and hazard assessments, additional high-resolution mapping of the tsunami sand and tsunami silt, is necessary.  

Along Johns River, a tributary into Grays Harbor, WA, stratigraphy of 93 sediment cores were described up to ~5.5km upriver during summer 2026 fieldwork. Forty-seven fossil core samples were taken for additional lab analysis. Initial results show 1700 CE deposits thinning and fining inland, with sandier deposits extending up to ~3km upriver. In 9 cores, there is evidence of 2-3 thin (<3mm-thick) tsunami sand to silt sequences potentially representing wave pulses. High-resolution stratigraphic mapping, radiodensity imagery of fossil sediment cores, and grain size analysis indicate that the 1700 CE tsunami likely inundated at least ~1km farther inland than previous estimates suggested, based on how far the traceable tsunami sand deposit extends. 

However, the 1700 CE coseismic subsidence contact is observable up to ~5.5km upriver. In addition, stratigraphic observations suggest main channel location changes, perhaps associated with 1700 CE coseismic subsidence. Data presented here can further constrain hypothetical earthquake and tsunami model scenarios and better inform hazard characterization.  

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