Cycles of Earthquake Deformation on the Patton Bay Splay-Fault System Implied by Late Holocene Shoreline Evolution on Montague Island, Alaska

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The coastal geomorphology of Montague Island, Prince William Sound (PWS), Alaska betrays a history of relative sea-level change recorded by emergent marine terraces, orphaned beach ridges and drained lagoons along the active Patton Bay Splay-Fault System (PBFS). The PBFS, including the Hanning Bay, Patton Bay and Cape Cleare faults, ruptured with the subduction megathrust in the 1964 Great Alaska Earthquake, contributed to tsunami generation and vertically displaced shorelines by as much as 10 m. However, few data exist to reconstruct the slip history of the PBFS or assess how often splay faults rupture with the megathrust. Here we investigate a sequence of drained lagoons barred by constructional beach ridges that chronicle 4200 yrs of prograding shorelines on the island’s western coast. Accounts of British explorers, tidal observations and satellite geodesy imply gradual, vertical subsidence of Montague Island before and after the 1964 earthquake—measured presently at -2.0 mm/yr. We test the hypothesis that the island’s coastal geology records repeated cycles of coseismic uplift on the PBFS followed by interseismic subsidence. Interpretations of lidar maps and historical aerial imagery show a series of 3-4 beach ridges that delineate coastal terraces, including lagoons that drained after the 1964 earthquake. Some ridges define older lagoons that drained prior to the 1964 earthquake. Underlying the lagoons, sharp contacts between inorganic sand/silt and overlying peat mark sudden changes in depositional environment. Fossil bivalves in the sand/silt and fossil diatoms in peat indicate changes from brackish-marine to freshwater environments. These observations suggest that earthquake uplift prior to 1964 also drained lagoons and shifted shorelines seaward. If sharp stratigraphic contacts mark episodes of coseismic uplift, then ¹⁴C ages suggest that the PBFS ruptured in 4.2 ka, 2.6 ka, 2.0 ka and 0.8 ka—times, within error, of megathrust earthquakes in PWS.

Presenting Author: Robert Witter

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