The Long-Lost Tale of an 18th-Century Tsunami, as Told by Trees

One night in late January 1700, two tectonic plates running along the Pacific Northwest coast released the tension they had accumulated during a centuries-long tête-à-tête. In a tectonic roar, the Juan de Fuca plate slipped past the North American plate, and a roughly 9.0-magnitude earthquake rattled the entire region. The coastline dropped and tsunamis washed over the entire Northwest coast.

Indigenous stories recount the disaster, but scientists only connected these dots later. Geological evidence of the events wasn’t found until the 1980s.

But it’s not enough to only uncover when a major quake happens. Precise details about its extent and aftermath are crucial to future preparedness. Studies have left no doubt that another Big One will come eventually. The estimated window for magnitude 8 to 9 quakes on the Cascadia subduction zone is about every 500 years, but there hasn’t been one since modern instruments began recording data in the late 19th century. “We have no observations because they're just so rare,” says Bryan Black, an associate professor at the University of Arizona. “But sooner or later they’re going to hit again.”Now, Black’s team has 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, Black 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 team’s study appeared in Natural Hazards and Earth System Sciences in late June.The idea that coastal trees could be a new seismic record-keeping tool is a welcome one for geoscientists. They could use them to better understand the aftermath of quakes and tsunamis in this highly populated yet risky zone, and to validate the flooding models that policymakers use to prepare for future disasters. “We're so dependent on the geologic record here,” says Jessie Pearl, a geologist with the US Geological Survey who was not involved in the study. “It’s one of the few places in the world where a huge diversity of types of scientists have to converge to come up with a story.”

A lot has changed on this coast in the 321 years since the last enormous temblor. Coastal communities are more populous—and there are more buildings and roads that could be damaged. So the more scientists understand what happened in 1700, the better prepared they will be.

Since the 1980s, geoscientists have scoured the Pacific Northwest for signs of the Cascadia event. Japanese historical records described an “orphan tsunami” in 1700 that flooded that country’s coast with no apparent earthquake nearby. But the local traces of this quake and ensuing tsunami have literally washed away over the years. For example, evidence of liquefaction—where the shaking ground causes sand to erupt as if from small volcanoes—is hard to find in the region, likely thanks to 300 years of rainfall.Dziak, who works with the National Oceanic and Atmospheric Administration, uses a tsunami model based on geophysical parameters, like earthquake size and topography, to simulate the depth of historical floods and where they lingered. Having a system to visualize where tsunamis have struck before helps in crafting maps for future evacuations. Of course, a simulation is just a simulation. The real information required to reimagine that event must come from the ground that actually felt it. “We need to find the so-called ‘proxies’ of the magnitude of the disturbance,” says Black, “some kind of clues in the geological or biological systems that tell us more about what these events were like.”