Over the past 2,300 years Sitkinak Island in the Kodiak Archipelago has repeatedly risen above the waves and been plunged into the ocean as the Alaska-Aleutian megathrust fault underlying it ruptured. Using a variety of observational techniques, Briggs et al. tracked the changing elevation of Sitkinak Island, in the process uncovering a previously unstudied history of the fault.
The Alaska-Aleutian subduction zone has a record of producing powerful earthquakes, including three magnitude 8.6 or greater events since 1957. These earthquakes, and the tsunamis they can generate, pose a threat to people living along the Pacific coasts. However, despite the seismic potential of the subduction zone, little research has been done on the western extent of the megathrust fault, near and beyond Sitkinak Island. The eastern extent, nearer south central Alaska, has been relatively well studied.
Using radiocarbon, radiocesium, lead isotope, lithostratigraphic, and microfossil observations, the authors found indications of five periods of uplift and subsidence. Changes in sediment layers and species of foraminifera and diatom fossils indicate which way the island moved because some species live in salt water and others in freshwater. Radioisotope measurements indicate when these changes took place.
Using their observations, the authors found three sudden periods of uplift and two of subsidence over the past 2300 years. The most recent subsidence marks the edge of the magnitude 9.2 earthquake in 1964, an event that caused the island to tilt above the western end point of the megathrust rupture zone. An earlier earthquake in 1788 C.E. had the opposite effect: Uplift records rupture propagation beneath and beyond Sitkinak.
Previous research into the Alaska-Aleutian fault suggested that fault ruptures stalled out near Sitkinak Island, propagating no farther west, as was observed in 1964. Following their observations of mixed abrupt vertical motions, the authors suggest that this was not the case. The identification of this boundary as nonpersistent throughout history is important for seismic hazard analyses of Alaska. (Geophysical Research Letters, doi:10.1002/2014GL059380, 2014)
—Colin Schultz, Writer
Citation: Schultz, C. (2014), Ancient earthquakes made an island rise and fall, Eos, 95, doi:10.1029/2014EO021391. Published on 31 December 2014.