Ghost Forests of the Pacific N.W.—Evidence for Cascadia’s Past Earthquakes (“orphan tsunami”)


Ghost forests along the coast are part of the evidence that tsunami-generating earthquakes
have occurred. We will first show the geologic process, then the geologic evidence that scientists have uncovered.
In this simplified model the continental plate is locked to the diving oceanic plate along a vast sloping fault. Stress-and-release cycles of deformation, followed by great earthquakes occur
every few hundred to a thousand years. As the leading edge of the overlying plate is pushed back the land deforms and rises, releasing energy along the way In smaller earthquakes in the brittle plates. When the stress between plates becomes too great, friction will be overcome in a process called elastic rebound during which the overlying plate will pop out in a Great earthquake dropping coastal forests below sea level. If the seafloor pops up it can send tsunami waves in all directions away from the fault carrying sand and debris deep inland. The drowned forest dies in a salty marsh. The process begins anew and the dead tree trunks are raised above sea level creating what is
known as the ghost forest. Let’s look at close up. The forest, represented by this single tree grows in a fertile soil upslope of the shoreline. During rebound the forest drops below sea level. The tsunami carries sand and debris inland. Over time the tree roots die in the salty marsh and the mud accumulates. The land gradually rises again.How do we know this? Native American oral history, art, and
ceremonies tell of a Cascadia earthquake and great waves tearing up trees and sweeping away entire villages in the night. The account placed a great earthquake in the early
1700’s Geologists digging in coastal cut banks noticed evidence of tsunami sands far from the shore. In this cutaway we see an example of classic three layer cake geology. The lower layer reveals an organic rich topsoil with plants and charcoal from Native American fire pits that are buried by multiple layers of tsunami sands. These are used to determine the age of burial, plus or minus 20 years using carbon-14 dating methods. Pale gray sandy tsunami deposits
leave a story of on-rushing waves. The tsunami deposits are
overlain by intertidal muds and clays. Scientists looked at the tree ring history of drowned trees compared to ancient trees of known age to determine when the trees died. Initially they narrowed the time of death to between 1695 and 1720. Ultimately by dating
the final ring from bark-covered roots they discovered that the last
ring recorded a 1699 growing season. So the trees died between the Fall of 1699 and Spring of 1700. The final piece to the puzzle came from
Japan, over 4,000 miles away. Abundant written records tell of a tsunami that flooded coastal areas from north to south on January 26, 1700. They called it an orphan tsunami as it arrived without a parent earthquake. Calculating travel times down the coast of Japan places the origin of the earthquake in the Pacific Northwest at around 9:00 PM on January 26, 1700. Further evidence showing repeat cycles of tsunami deposits over thousands of years, coupled with current GPS evidence indicate that the plates are indeed moving. This has led scientists to conclude the
Pacific Northwest can expect another great earthquake in
the future.

8 thoughts on “Ghost Forests of the Pacific N.W.—Evidence for Cascadia’s Past Earthquakes (“orphan tsunami”)

  1. This is awesome. It shows a complete picture of the story of the tsunami, and pieces together the evidence in a coherent manner. The images bring the story to life. I will definitely use this in my class!

  2. The Copalis River in Gray's Harbor County, Washington State has a ghost forest just east of the highway. It can be seen from the road.

  3. Stick with what you do best avoid your amateur background sound effects.

  4. Ok so I live in Seaside and many nights going to bed in the tsunami zone, I think, "It's raining, an earthquake right now would be really bad." So that earthquake hit at 9:00 at night on January 26th, 1700, in the dead of winter, maybe during a rain storm, maybe when it was freezing, and I can't help but to think of the people in their longhouses waking from sleeping to an earthquake and then a tsunami, and how terrifying it would have been in the cold and the dark.

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