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ABSTRACT A major earthquake ruptured the Cascadia subduction zone (CSZ) on 26 January 1700. Key paleoseismic evidence associated with this event include tsunami deposits, stratigraphic evidence of coastal coseismic subsidence, written Japanese records of a tsunami unaccompanied by earthquake shaking, and margin-wide turbidites found offshore and in lacustrine environments. Despite this wealth of independent clues, important details about this event remain unresolved. Dating uncertainties do not conclusively establish whether the proxies are from one earthquake or a sequence of them, and we have limited knowledge of the likely slip distributions of the event or events. Here, we use a catalog of 37,500 candidate synthetic ruptures between Mw 7.8 and 9.2 and simulate their resulting coseismic deformation and tsunami inundation. Each model is then compared against estimated Japan tsunami arrivals, regional coastal subsidence records, and local paleotsunami deposits mapped at six different coastal marshes and one coastal lake along the CSZ. We find that seven full-margin ruptures with a median magnitude of Mw 9.1 satisfy all three constraints. We favor one Mw 9.11 model that best matches all site paleoseismic observations and suggests that the Cascadia megathrust slipped up to ∼30 m and must have shallow geodetic coupling. We also find that some sequences composed of three or four ruptures can still satisfy the observations, yet no sequences of two ruptures can. Sequences are differentiated into three groups based on whether they contain a mainshock rupture located in the south (>44° N) or further north. All sequences contain unruptured portions of the megathrust and most contain mainshocks with peak slip above 40 m. The fit of the geologic evidence from sequences is poor in comparison to single-event models. Therefore, sequences are generally less favored compared to full-margin events.
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Compiled onshore and offshore paleoseismic data along the Cascadia Subduction zone
The USGS Powell Center Cascadia earthquake hazards working group compiled published onshore and offshore paleoseismic data along the Cascadia subduction zone, spanning sites from Vancouver Island to the Mendocino triple junction. Evidence for megathrust rupture includes coastal land-level change, tsunami inundation, onshore shaking proxies such as landslides or liquefaction, and offshore shaking proxies such as marine turbidites. The quality of paleoseismic data for megathrust rupture along the Cascadia subduction zone collected over the past three decades varies because analytical capabilities and data collection methodologies have evolved. Thus, as part of the compilation, we also present a ranking scheme to assess the quality of age estimates and evidence for great megathrust rupture. With the age ranking scheme, we ask: "How well is a proposed paleoseismic event dated?" based on the materials and methods used. With the evidence ranking scheme, we ask: "How confident are we that a proposed event is, in fact, the result of a Cascadia megathrust rupture?" based on the sedimentological characteristics, correlation, and mapping. The evidence ranking scheme also helps to evaluate possible alternative mechanisms for creating paleoseismic evidence such as crustal fault, intraslab, or distant tsunamigenic earthquake."
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- Award ID(s):
- 2225286
- PAR ID:
- 10689502
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- U.S. Geological Survey
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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A major earthquake ruptured the Cascadia subduction zone (CSZ) on 26 January 1700. Key paleoseismic evidence associated with this event include tsunami deposits, stratigraphic evidence of coastal coseismic subsidence, written Japanese records of a tsunami unaccompanied by earthquake shaking, and margin‐wide turbidites found offshore and in lacustrine environments. Despite this wealth of independent clues, important details about this event remain unresolved. Dating uncertainties do not conclusively establish whether the proxies are from one earthquake or a sequence of them, and we have limited knowledge of the likely slip distributions of the event or events. Here, we use a catalog of 37,500 candidate synthetic ruptures between 7.8 and 9.2 and simulate their resulting coseismic deformation and tsunami inundation. Each model is then compared against estimated Japan tsunami arrivals, regional coastal subsidence records, and local paleotsunami deposits mapped at six different coastal marshes and one coastal lake along the CSZ. We find that seven full‐margin ruptures with a median magnitude of 9.1 satisfy all three constraints. We favor one 9.11 model that best matches all site paleoseismic observations and suggests that the Cascadia megathrust slipped up to ∼30 m and must have shallow geodetic coupling. We also find that some sequences composed of three or four ruptures can still satisfy the observations, yet no sequences of two ruptures can. Sequences are differentiated into three groups based on whether they contain a mainshock rupture located in the south (>44° N) or further north. All sequences contain unruptured portions of the megathrust and most contain mainshocks with peak slip above 40 m. The fit of the geologic evidence from sequences is poor in comparison to single‐event models. Therefore, sequences are generally less favored compared to full‐margin events.more » « less
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