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Temporal Clues Point to an Along-Strike Cascadia Megathrust Rupture Sequence Between 680–950 Years Ago

Description: 

Globally, subduction zones have ruptured piecewise in sequences of great earthquakes, yet conclusive evidence of along-strike ruptures of separate sections of the Cascadia megathrust, occurring within decades to centuries, is scarce. Here, we reevaluate ages for stratigraphic evidence of earthquakes over a ~270-year interval and show that the ages are consistent with three separate ruptures of the northern, central, and southern sections of the Cascadia megathrust. Most ages for coseismic coastal subsidence, tsunami inundation, and seismic shaking use radiocarbon methods with age uncertainties that span decades to centuries. Many studies use the overlap of age ranges (probability density functions, PDF) to infer maximum rupture length based on temporal correlation of dated evidence from Cascadia sites tens of kilometers apart. We take a different approach and look for differences in ages, in the interval 680–950 years ago, for great earthquakes at 10 sites that span a distance of 860 km between the Strait of Juan de Fuca, WA and Humboldt Bay, CA. Prior studies used PDF overlaps of ages in the same time interval at these sites to infer a ~610 km-long rupture between Humboldt Bay and the Columbia River. However, when grouped by site proximity and combined using OxCal (Bronk Ramsey, 2009), the PDFs fail a X² goodness-of-fit test. The statistically different combined PDFs along strike support an alternative hypothesis of sequential rupture of the southern (950–870), northern (890–790), and central (760–680 cal yr BP) sections. Stratigraphic support for piecewise rupture includes: tsunami and shaking deposits in Bradley Lake that suggest ruptures of adjoining central and southern sections; and contact W at Willapa Bay, which may mark less coastal subsidence during a northern rupture. Although statistical tests cannot rule out full-margin rupture, the data also support the hypothesis of along-strike rupture sequences. New dating techniques (dendrochronology, luminescence) and alternative age models can further test full-margin versus piecewise rupture hypotheses to improve Cascadia earthquake hazards assessments.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments [Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Robert

Student Presenter: No

Invited Presentation: 

Authors