Of all the segments along the Alaskan convergent margin, the Semidi segment has a geometry that is most likely to direct a powerful tsunami toward the United States’ West Coast. Centrally located along the Alaska Peninsula southwest of Kodiak Island, the Semidi last generated a great tsunami in 1788. Multiple factors suggest that the elastic strain stored in the region may be nearing a critical level: The absence of shallow high-magnitude earthquakes exceeding magnitude 7 since that earthquake, the 180- to 270-year repeat time suggested by paleoseismic studies, and recent observations indicating that the segment is 90% locked are signs commonly cited as indications of approaching earthquakes.
To better understand the tectonic structure of the Semidi segment and how this relates to its potential to generate tsunamis, von Huene et al. analyzed a suite of data, including legacy multibeam bathymetric and reprocessed legacy seismic data, and compared these to more recent geophysical data acquired with the R/V Marcus G. Langseth. Their results indicate that the margin is comparable to the type associated with giant tsunamis, with structures capable of generating a large wave.
The data reveal an escarpment, 200 kilometers long and more than 1 kilometer high, in the middle of the continental slope. The authors attribute this ridge to an active zone of out-of-sequence thrust faults separating older, more rigid basements rocks from sediment accreted against them. The authors argue that slip along this fault zone creates a “shortcut” that can result in greater displacement of the seafloor, which models indicate would generate a larger tsunami than traditional slip along the longer and rougher plate interface.
The similarities in the tectonic structure to other segments that have generated historic tsunamis (including the nearby Unimak segment, the source of a 1946 earthquake and tsunami that caused widespread damage and 173 fatalities, and Japan’s Tohoku segment, the origin of the 2011 earthquake and tsunami) raise the level of concern that the Semidi may also be capable of generating deadly waves. To better constrain the Semidi’s potential seismic and tsunami hazard, the team recommends deploying seafloor instruments to monitor future physical changes across the seafloor fault morphology. (Geochemistry, Geophysics, Geosystems, doi:10.1002/2015GC006147, 2015)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), Alaska’s Semidi segment could unleash a devastating tsunami, Eos, 97, doi:10.1029/2016EO046303. Published on 19 February 2016.