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Minutes after the 1964 magnitude-9.2 Great Alaska Earthquake (see “The Great Alaska Earthquake and Tsunami of March 27, 1964”), a series of tsunami waves swept through the village of Chenega in Prince William Sound, destroying all but two of the buildings and killing 23 of the 75 inhabitants. Fifty years later, detailed seafloor images revealed the likely cause of the tsunami: a large set of underwater landslides. U.S. Geological Survey (USGS) scientists and their colleagues from Boise State University and the Alaska Department of Fish and Game collected the images while mapping the seafloor in May 2014 (see inset, below). Their findings, published in January 2016 in the journal Earth and Planetary Science Letters (see “A Submarine Landslide Source for the Devastating 1964 Chenega Tsunami, Southern Alaska”), underscore the tsunami hazard that submarine landslides can pose in fjords around the world where communities and ports are commonly located.

USGS geologists who investigated Alaska’s south coast shortly after the 1964 earthquake speculated that an underwater landslide might have triggered the Chenega tsunami, just as landslides triggered tsunamis that devastated the Alaskan towns of Valdez, Seward, and Whittier. But an undersea survey just after the earthquake “didn’t show evidence of a landslide in nearby Dangerous Passage or the other waterways around Chenega,” explained Daniel Brothers, USGS geophysicist and lead author of the study. “Alternate explanations involving seafloor movement during the earthquake didn’t fit the timing and severity of the Chenega tsunami as described by eyewitnesses.” The tsunami origin remained uncertain until a team led by Brothers mapped a large underwater landslide complex in Dangerous Passage, mostly in water deeper than that studied in 1964.

“What makes this slide unusual is that much of the material that slid was at 250 to 350 meters [820 to 1,150 feet] water depth,” said Peter Haeussler, USGS geologist and a coauthor of the report. “The depth made it particularly good at generating a tsunami.”

The scientists used multibeam sonar technology to collect high-resolution bathymetric (seafloor depth) data, and a single-channel seismic-reflection system to collect sub-bottom profiles (cross-sectional views of sediment layers and other features beneath the seafloor).

The researchers calculated the time it would take for a tsunami produced by a large landslide in the mapped areas to reach the village of Chenega and found a good fit with eyewitness reports. A tsunami triggered in the areas where the scientists found landslide evidence would take three to four minutes to reach the village, consistent with the arrival time of the most destructive waves.

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“It’s exciting to see the technology evolve so we can now get high-resolution images of the seafloor [and] pinpoint the most likely source for the waves,” said USGS geologist emeritus George Plafker. “After 50 years, this new work confirms our original inference that it was probably landslide-generated waves that devastated Chenega so many years ago.” Plafker and colleague Larry Mayo were among the first scientists on the scene and wrote some of the early geological field reports on surface effects of the Chenega waves (see “Effects of the Earthquake of March 27, 1964, on Various Communities”).

The newly collected bathymetric data reveal three sedimentary basins at progressively deeper levels toward the open waters of Prince William Sound. Glaciers carved these basins when sea level was lower. After the last ice age, as sea level rose and the glaciers retreated, the basins filled with sediment washed off the land. The basins are separated by ridges, likely terminal glacial moraines, and bounded by steep, rugged slopes. The new images reveal other features typical of landslides, including scarps, blocks, and lumpy surfaces. Similarly, the sub-bottom profiles show that fine-grained sediment fills the basins, with distorted layers that indicate multiple landslides.

When the researchers compared their 2014 bathymetric data with data collected in 1957 by the U.S. Coast and Geodetic Survey, they discovered dramatic differences in the seafloor in Dangerous Passage. In 2014, the lowermost basin was shallower by an average of about 11 meters (33 feet), yet the basin just above it was deeper by an average of about 21 meters (63 feet). The differences suggest that sediment moved from the higher basin into the lower basin between 1957 and 2014. The new study’s authors concluded that the sediment movement took place during the 1964 earthquake. “Once mobilized,” they wrote, “landslide debris poured over the steep, 130-m [430-foot] face of a deeper moraine and then blanketed the [lowermost] basin.”

Most known landslides in fjords occur in shallow water along the fronts of submerged deltas formed by sediment washing out of upstream glaciers—areas known to be prone to sliding during earthquakes. Such landslides triggered the tsunamis that struck Seward, Valdez, and Whittier during the 1964 earthquake. The Chenega landslides, in contrast, occurred in much deeper water, where unstable deposits of glacial sediment were hidden. Their discovery implies that similar deep landslides were a likely source for unexplained local tsunamis throughout southern Alaska during the 1964 earthquake and may be an overlooked hazard for fjords globally.

Acknowledgments

The paper’s authors wish to thank USGS ocean engineer Gerry Hatcher for his dedication and skill in maintaining and operating the mapping equipment, and the crew of the R/V Solstice for their hard work and exceptional support.

Weather Detour Leads to Landslide Discovery

by Helen Gibbons

Bad weather can take some credit for the discovery of underwater landslides that triggered the 1964 Chenega tsunami. Scientists aboard the R/V Solstice were actually focused on Cape Cleare, a rocky shoal southwest of Montague Island. Geophysicists Danny Brothers (USGS) and Lee Liberty (Boise State University) wanted to learn more about the Patton Bay fault, which cuts through the shoal. Rupture of the Patton Bay fault during the 1964 Great Alaska Earthquake lifted parts of Montague Island by as much as 10 meters. Many scientists believe the fault’s movement on the seafloor was a major contributor to a tsunami that struck Seward, Kodiak Island, and the U.S. west coast after the quake. Its offshore extent was poorly known, and the scientists on the Solstice wanted to change that. The team collected some stunning data during eight days of clear, calm weather. But on the ninth day, the wind picked up. “It happened quickly,” said Brothers. “The data quality was severely compromised, and we were really getting knocked around. We had to head in behind Montague Island for cover and think about our options.” The researchers had charted backup locations to survey in case the weather turned nasty. One of those was Dangerous Passage, near the abandoned Chenega village site. “We knew it had never been mapped with the type of tools we had on board,” Brothers said, “and it was only a two-hour detour.” Outer islands sheltered Dangerous Passage from the wind, enabling the scientists to work for about eight hours before they had to return to port. That was just long enough to collect the landslide evidence. “It took 50 years and crummy weather to finally uncover what happened,” said Brothers.