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Repeated Coseismic Uplift of Coastal Lagoons Above the Patton Bay Splay Fault System, Montague Island, Alaska, USA

Coseismic slip on the Patton Bay Splay Fault System during the 1964 Mw 9.2 Great Alaska Earthquake contributed to local tsunami generation and vertically uplifted shorelines on Montague Island, Prince William Sound (PWS) by as much as 11 m. Sudden uplift of the island caused coastal lagoons along the northwestern coast to gradually drain leading to changes in depositional environment from marine lagoon to freshwater muskeg which created a sharp, laterally continuous stratigraphic contact between silt and overlying peat. Here, we characterize the geomorphology, sedimentology, and diatom ecology signatures across the 1964 earthquake stratigraphic contact and three similar, widespread prehistoric contacts within the stratigraphy of the Hidden Lagoons locality. We find that all four contacts signal instances of abrupt coastal uplift with radiocarbon, 137Cs, and 210Pb dating methods constraining them to ages that, within error, overlap the timing of independently constrained megathrust earthquakes in PWS – 1964 CE, 760-870 yr BP, 2500-2700 yr BP, and 4120-4500 yr BP. Changes in fossil diatom assemblages across the inferred prehistoric earthquake contacts reflect ecological shifts consistent with repeated draining of a lagoon system caused by >3 m of coseismic uplift, similar to the environmental changes across the 1964 earthquake contact. Our observations provide evidence for instances of splay fault ruptures that have occurred close in time, within less than a century, of four of the last eight megathrust earthquakes over the past ~4,200 years in PWS. These results provide new information about potential rupture dynamics and tsunami risks associated with future combined megathrust-splay fault system ruptures which should be considered in hazards assessments.

Session: From Faults to Fjords: Earthquake Evidence in Terrestrial and Subaqueous Environments - III

Type: Oral

Room: K’enakatnu 6

Date: 5/1/2024

Presentation Time: 02:00 PM (local time)

Presenting Author: Jessica DePaolis

Student Presenter: No

Additional Authors