Tree rings from old-growth Douglas firs on the Oregon Coast show evidence of 1700 tsunami

An old growth Douglas-fir stand near South Beach's Mike Miller Park. Credit: Bob Dziak
Core samples from an old-growth Douglas Fir tree stand in South Beach, just south of Newport, showed reduced growth after the 9.0 earthquake in 1997 and the subsequent tsunami in 2000.

Robert Dziak, Hatfield Marine Science Center-based scientist at the National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory, stated that the physical evidence of the Douglas Fir tree rings supports modeling that shows the magnitude of the January 1700 earthquake, which was the last major earthquake in the Cascadia subduction Zone.

Dziak said that the tsunami was the most significant event to have affected tree growth in that year. His work includes ocean acoustic research, signal analysis, and tsunami modeling. Dziak also holds a courtesy position at Oregon State University's College of Earth, Ocean, and Atmospheric Sciences. These little pieces of the picture help us to understand what might happen when the next "big one" hits.

These findings were recently published in the journal Natural Hazards and Earth System Sciences.

Dziak had the idea for this study for more than a decade. He was aware of previous research that had demonstrated evidence of the 1700 earthquake in trees in Washington and thought it would be worthwhile to see if there is similar evidence in Oregon.

Finding an old-growth Douglas Fir stand in the tsunami zone was the first challenge. Before locating the stand in Mike Miller Park, South Beach, two kilometers south from Yaquina Bay but 1.2 kilometers east the current-day ocean shoreline, the researchers had to look at several places.

Bryan Black, coauthor at the Laboratory of Tree-Ring Research, University of Arizona, Tucson, said, "We don't know why this tree stand wasn't logged over time, but we are very fortunate to have had a site that's survived so close to the coast."

Dziak said that a new tsunami model was created by researchers for the study. It shows that the area could be inundated with up to 10 metres of water in the 1700 tsunami.

After identifying the stand of old-growth trees, researchers took core samples from 38 trees. This allowed them to examine the tree rings and not harm the overall health of their trees. Dziak stated that the majority of the trees were dated between 1670 and 1650. One tree was dated 1650.

They examined the growth rates of the rings and compared them to other old-growth Douglas Firs not located in the tsunami zone. The growth rates of trees in the tsunami zone had a significantly lower rate than those in other locations.

Because the trees are not directly at the coast, researchers are still trying to determine why the tsunami may have affected their growth. It could be due to a combination of ground shaking and inundation by seawater, according to researchers.

Dziak stated that the salty seawater from a tsunami drains quickly. However, there is a pond in Mike Miller Park where it likely settled and remained longer.

Black stated that researchers will next conduct an isotopic analysis of the wood starting at 1700.

He said that signatures similar to those found in Japan's 2011 Tohoku tsunami will be searched for. We could create a powerful new method to map the Pacific Northwest coast's tsunami run-up if we are successful.

More information: Robert P. Dziak and colleagues, Assessing the local impacts of 1700 CE Cascadia earthquake and tsunami by tree-ring growth histories: A case study in South Beach (Oregon, USA), Natural Hazards and Earth System Sciences (2021). Robert P. Dziak and colleagues, Assessing the local effects of the 1700 CE Cascadia earthquake, tsunami, using tree-ring history: A case study in South Beach (Oregon, USA),(2021). DOI: 10.5194/nhess-21-1971-2021