14.8 C
New York
Saturday, April 13, 2024

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.”

And trees don’t forget. In the 1990s, researchers identified a “ghost forest” of dead cedars near the Washington coast; tree-ring dating confirmed that they’d indeed died in 1700. But Black and Dziak sought out trees that experienced the tsunami—and survived. The rings of those trees could contain evidence of the stress caused by living through an enormous flood.

Finding them wasn’t easy. “It takes a little bit of sleuthing to find some old growth forests that are close enough to the coastline,” Dziak says, “and there's good reason.” Large, accessible trees near the coastline were like gold for loggers who colonized the area in the centuries after the quake. Fires have taken down others. Still, the team found trees that seemed to fit the bill: Old-growth Douglas firs congregating in a stand within Mike Miller State Park, nearly one mile from shore in South Beach, Oregon.

If you had been standing beside the then-young firs in 1700, you would likely have felt the ground rumble. Minutes later, the water would have rolled in. It wouldn’t have been a biblical wall of water, but rather “like a rapid influx of high tide,” says Dziak. (Here's a video of Japan’s 2011 tsunami for reference.) His model suggests velocities between two and 10 meters per second in this park, and depths reaching up to 10 meters. Nearby sand dunes tell Dziak that the tsunami would have probably drained quickly; a nearby pond tells him the water may have brined the roots for longer. In either case, that rush of seawater would be enough to cause some damage to trees unaccustomed to such salt.

To find proof that the trees had coped with tsunami-related damage, Black extracted cylindrical cores from trees at the site, ultimately identifying seven that were old enough to have been around during the quake. He sanded the cores, each one about as wide as a pencil, revealing the concentric patterns left by annual growth. An unusually productive year appears as a wide space between tree rings; a bad year appears narrow. Black juxtaposed each core with the rest to make sure each tree’s calendar year aligned with its neighbors who, over the past three centuries, had experienced the same climate. “It’s kind of like working a puzzle,” says Black. And it revealed a clear trend: Trees in the flood zone predicted by the model all had weak growth during 1700.

Now he and Dziak are eager to test the chemical differences in each tree ring, which could irrefutably ascribe the slowdown to seawater. Will Struble, a geomorphologist from the University of Arizona who was not involved in the work, agrees with the team’s caution. (Struble and Black have worked together, but he was not involved in this study.) Having chemical evidence will be important to prove the theory that the saltwater—not earthquake shaking or changes in climate—stymied the Mike Miller stand in 1700.

Still, Struble stresses how valuable such evidence is to support simulations of tsunami inundation, since on-the-ground data from 1700 is so hard to come by. “To actually be able to go in the field and use a dataset like tree rings to ground truth these models is really where I think the novelty lies,” says Struble.

Pockets of other old-growth trees along streams in Oregon and Washington would have been inundated, too. If the chemical analysis pans out, this tool could map out the extent of the 1700 tsunami far beyond just the Mike Miller stand.

Figuring out which of the trees survived saltwater stress might be valuable, too, suggests Pearl: “Are older trees more likely to perish?” Younger trees have more shallow roots, so they depend more on precipitation than groundwater. They may also rebound faster, or even thrive later on if the taller sun-blocking canopy dies off. “And not only future tsunamis, but also sea-level rise—what species might be the most resilient in the face of saltwater?” she asks.

Validating tsunami models this way helps scientists and planners create maps of high-risk areas, or routes for safe egress, says Pearl: “It’s what’s going to drive where we're putting the tsunami signs, where we're putting the evacuation routes.”

Corina Allen’s team at the Washington Geological Survey creates those particular maps for the state. “All these lines of evidence can help us to determine what happened in the past. And that can help us figure out where to draw the lines in the future,” says Allen, the chief hazards geologist.

Reading the trees’ internal barcodes also uncovered a surprise about their past: The quake and tsunami weren’t their biggest stressor over the past 400 years. The effects of heat, drought, wind, or cold snaps were more severe. In 1739, for example, the tree rings were even more narrow. The team suggests it may be the lasting trace of a drought. “It was actually a fairly widespread event throughout the Northwest,” says Dziak.

Such climate events are becoming more frequent. Black traveled back to the Oregon coast right after this summer’s “heat dome,” which “scorched” trees—overheating and overdrying them without literally charring them. “Having visited the coast for about 20 years and having lived there for 10,” he says, “that degree of scorch was unlike anything I'd ever seen before.”

That climate events can traumatize—or kill—coastal trees that survived a quake and tsunami is important to acknowledge. “The potential for droughts and fires is going to be much more impactful for our forests than this tsunami was at Mike Miller,” Black says. Still, for people in coastal communities, a repeat of the Cascadia quake and tsunami would be the greatest natural disaster that the US has ever seen. And the trees, once again, may keep the record, etched in wood for future sleuths to find.

Related Articles

Latest Articles