Even as Alaskans mark the 50th anniversary of the most powerful earthquake ever recorded in North America, scientists are uncovering signs that there may have been four much-earlier quakes of similar magnitude in the past millennium in the Kodiak region. The findings are detailed in a study accepted for publication in the journal Geophysical Research Letters and available for online viewing Thursday.
Soil samples examined by scientists from the U.S. Geological Survey and other institutions found evidence that the Magnitude 9.2 quake of 1964 was one of five subduction events that shook the Kodiak area in the last 1,000 years. Subduction quakes, such as the Great Alaska earthquake, the 2004 Sumatra earthquake in the Indian Ocean and the 2011 quake in Japan, occur when one of the earth's tectonic plates slip under another.
Evidence for the five quakes came in soil cores taken from Sitkinak Island in the Kodiak archipelago. The cores had layers of peat and silt that revealed times when the land was above and below water. The five apparent quakes occurred along the Alaska-Aleutian megathrust, the point where the North American and Pacific tectonic plates meet -- and, on occasion, overlap in earth-shaking events.
The study did not pin down the exact dates of the past quakes but said the soil evidence correlated to 1964 and four earlier events -- 1,050 to 790 years ago, 640 to 510 years ago, 520 to 300 years ago and up to 290 years ago.
Fitting that last date range would be 1788, a time when a large earthquake and tsunami struck the Kodiak area, killing some local residents, according to contemporary reports by Russian settlers.
"The signs point to, at least, 1788 being a similar size to 1964, but that remains to be seen," said Rich Briggs, a Colorado-based USGS research geologist and lead author of the study. "These other events look to be in the ballpark."
Determining the timing of the past subduction events will take additional studies, he said. The study released Thursday is just the "first cut."
"It will take quite a bit more work to narrow these ranges," Briggs said
A key result of the study is information showing that these big Kodiak-area earthquakes are more complicated than previously believed -- lifting land up in 1964 and in one other event, but pushing the same land down in the other three events.
Sitkinak Island, off the southern tip of Kodiak Island, is considered an important landmark. It was the end point of the underground rupture that caused the 1964 quake; the rupture was, apparently, stopped by some sort of geologic barrier, according to the study authors.
That barrier did not stop some of the other earlier ruptures, which possibly moved in different directions, the study said. The results were varying impacts on the island's land, according to the soil evidence analyzed in the study.
"Rather than the island always moving up or always moving down it had a mixed behavior," Briggs said.
Study of the prehistoric seismic events, including more detailed research into potential earthquake magnitudes, is ongoing, Briggs said. Soil cores from other locations in the general region are being analyzed, he said.
Contact Yereth Rosen at yereth(at)alaskadispatch.com.