The Long Lost Story of the 18th-Century Tsunami, as Trees Say
And the trees are not forgotten. In the 1990s, researchers identified a “ghost forest” of dead cedars near the Washington coast; dating wood ring confirmed that they actually died in 1700. But Black and Dziak were looking for trees that had experienced the tsunami – and SAVED. The rings of such trees may have evidence of stress caused by living in a severe flood.
Finding them is not easy. “It takes a little bit of digging to find some old growing forests that are close enough to the coast,” Dziak said, “and with good reason.” Large, accessible trees near the coast look like gold for loggers who colonized the area hundreds of years after the earthquake. The fires took away some. Still, the team found the trees that seemed appropriate for the research: Old Douglas firs congregating in an indoor stand at Mike Miller State Park, nearly a mile off the coast of South Beach, Oregon.
If you were standing next to those young firs in the 1700s, you could feel the ground crumbling. After a few minutes, the water would have seeped. It wouldn’t have been a biblical wall of water, rather “like a rapid surge of high tide,” Dziak said. (Here is a video of Japan’s tsunami in 2011 for reference.) His model suggests speeds between two and 10 meters per second in this park, and depths that can reach up to 10 meters. The nearby sand dunes of the mountain told Dziak that the tsunami might have quickly subsided; a nearby pond tells him that the water may be prolonging the roots. In any case, that runoff of sea water is enough to inflict damage on trees that are not accustomed to such salt.
To find evidence that the trees had sustained tsunami-related damage, Black removed cylindrical cores from the trees in the area, eventually identifying seven old enough to be there. during the earthquake. He laid out the cores, each about the width of a pencil, revealing the concentric patterns left by the annual growth. An unusually fruitful year appears as a wide gap between the tree rings; a bad year looks narrow. Black picks each core with the rest to ensure that the calendar year of each tree aligns with its neighbors that, over the past three centuries, have experienced the same climate. “It’s like doing a puzzle,” Black said. And it reveals a clear trend: The flood zone trees predicted by the model all had weak growth during the 1700s.
Now he and Dziak are excited to test the chemical differences in each wood ring, which is undeniably meant to mean slowing down sea water. Will Struble, a geomorphologist from the University of Arizona who was not involved in the work, agreed to watch the team. (Struble and Black collaborated, but he was not involved in this study.) Chemical evidence is essential to substantiate the theory that saltwater-not earthquakes or climate change-suggests erection of Mike Miller in 1700.
However, Struble stressed how valuable such evidence is to support tsunami flood simulations, because groundwater data from the 1700s are very difficult to obtain. “To be able to go to the field and use a dataset like tree rings to make these models real is exactly what I think innovation is all about,” Struble said.
Pockets of other old -growing trees along the Oregon and Washington rivers may also be flooded. When the chemical analysis is completed, this tool can map the extent of the 1700 tsunami beyond what Mike Miller stood for.
Knowing which of the trees can withstand saltwater stress can be important, Pearl also suggests: Smaller trees have more shallow roots, so they are more reliant on rain than water. underground. They can also rebound faster, or even advance if the taller canopy blocking the sun dies. “And not just tsunamis in the future, but sea level rise as well-what else could be stronger in the face of salt water?” he asked.