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Tree Rings Provide Tsunami Evidence

In January 1700, two tectonic plates running along the Pacific Northwest coast released the tension they had accumulated during a centuries-long tête-à-tête.

The resulting earthquake created a tsunami that washed over the entire Northwest coast. Indigenous tales recount the disaster, but scientists only connected these dots later, as geological evidence of the event wasn’t found until the 1980s.

More recently, researchers have found a new piece of evidence that they believe shows traces of the tsunami triggered by the quake: It’s buried inside the old coastal Oregon Douglas firs that weathered it. Based on tree rings, Bryan Black, an associate professor at the University of Arizona, and oceanographer Robert Dziak report that tree growth slowed the year the tsunami inundated the ground with seawater. Even Black - the team’s dendrochronologist, or tree-ring date expert - wasn’t expecting to find this connection. “I was pleasantly surprised,” he says. Connecting the stunted tree growth to the geographic reach of the flood waters opens a window between present and past. “We could have a new tool for mapping tsunami inundation,” he continues.

The idea that coastal trees could be a new seismic record-keeping tool is a welcome one for geoscientists. The good news is that they could use them to better understand the aftermath of quakes and tsunamis and to validate the flooding models that policymakers use to prepare for future disasters.


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