Tree rings show evidence of 1700 Cascadia earthquake and tsunami

Evidence of the immense destructive power of the Cascadia earthquake appears ashore in Nisquín, … [+] Oregon. The “ghost forest” of Sitka fir trees was likely buried by tsunami debris 2,000 years ago, partly exposed by storms in 1997.

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The last major earthquake to hit the Pacific Northwest of the United States was in January 1700. The earthquake, which was estimated to have a magnitude of 9+ on the Richter scale, resulted along the Cascadia Subduction Zone (CSZ) and was powerful enough to send a tsunami across the Pacific. Crossing the sea at a flying speed, a tsunami came ashore in Japan, as written records of floods and devastated villages attest. The original oral traditions along the Cascadia margin also seem to record the effects of the earthquake and tsunami.

But physical evidence for this event is surprisingly few. “Ghost Forest”?? They are long-dead trees that stand along the coasts of Washington and Oregon. During the earthquake, the ground suddenly fell and the water flooded the ground afterwards, killing the trees. But the history of ghost forests is not always unambiguous. Now a study of core samples taken from a stand of old-growth Douglas-fir trees in the South Shore area south of Newport, Oregon, has shown direct evidence of the earthquake and tsunami that struck the Pacific Northwest in 1700.

“It appears that the tsunami was the most impactful event on tree growth that year,” said Robert Dziak, a scientist at the Hatfield Marine Science Center at NOAA’s Pacific Marine Environmental Laboratory, who works alongside researchers at Oregon State University.

The idea of ​​the study dates back more than a decade. Dziak was aware of previous research showing evidence of a 1700 earthquake in trees in Washington, and thought it might be useful to see if similar evidence existed in Oregon.

The first challenge was to find a stand of old-growth Douglas fir in the area of ​​the tsunami inundation. The researchers looked at a few places before locating the pavilion in Mike Miller County Park in South Beach, Oregon, today about one mile east of the current ocean coast.

Stand of an old Douglas fir near a pond in Mike Miller Park in South Beach, Oregon.

Bob Dziak

“We’re not sure why this tree stand has not been recorded over the years, but we are very fortunate to have a site so close to the coast that it has survived,” said co-author Brian Black of the Tree Lab. Ring Research at the University of Arizona, Tucson.

A new and updated tsunami model that the researchers are running as part of the study shows that the area was inundated by up to 32 feet (10 meters) of water in the 1700 tsunami event.

Once the old growth position was determined, the researchers collected core samples from about 38 live trees. The majority of the trees began growing around 1670, Dziak said, and one of them dates back to 1650. They analyzed the growth rates of the rings and compared the growth rates to those of old Douglas fir at sites not in the tsunami inundation area. They found that in 1700 the tsunami did not kill the trees, but the lowland was submerged by sea water. Combined with the earth-shaking damage to the plant’s roots, this significantly reduced the growth rate, and Douglas fir formed only a few narrow rings. As the ground recovered after the event, the trees returned to normal growth.

The next step for the researchers is to conduct a chemical analysis of wood from the 1700s.

“We will look for signatures consistent with those found in trees that were inundated with the 2011 Tohoku tsunami in Japan,” Black said. “If successful, we can develop a powerful new technology to map prehistoric tsunamis along the Pacific Northwest Coast.”

Based on past events, the average recurrence time for a major earthquake along the CSZ is 300 to 500 years. A magnitude 9 earthquake in the subduction zone can generate a tsunami that reaches the coast in 20 minutes with waves as high as 40 feet.

The study was published in the journal Natural Hazards and Earth System Sciences.

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