What is the evidence for megathrust earthquakes in the Pacific Northwest?
This simplified animation illustrates both the subduction-zone processes that lead to a "ghost forest" as well as the evidence that scientists collected to determine that the Pacific Northwest has had many great earthquakes and tsunamis in the past, and will again in the future. This is based on the work of Brian Atwater who published his findings in the book "The Orphan Tsunami of 1700" (USGS Professional Paper 1707). On January 26, 1700 at 9:00 pm a great earthquake (M8.7?9.2) struck the Pacific Northwest shaking mountains, dropping coastal forests, and causing a tsunami that wiped away entire villages (http://walrus.wr.usgs.gov/tsunami/NAlegends.html). (This occurred before written history. Oral history of Native Americans, interviewed in 1868, told of their ancestors relating these events.) Nine hours later, in Japan, a mysterious tsunami arrived without warning flooding fields and washing away houses along the coastline from north to south. Samurai, merchants, and villagers recorded the event, but nearly three centuries would pass before scientific discoveries in North America revealed the tsunami's source. Evidence supporting the occurrence of the 1700 earthquake is detailed in the book The Orphan Tsunami of 1700 (geologist Brian Atwater and others, 2005; http://pubs.usgs.gov/pp/pp1707/). Recent findings conclude that the Cascadia Subduction zone is more complex and volatile than previously believed. Geologists predict a 37 percent chance of a M8.2+ event in the next 50 years, and a 10 to 15 percent chance that the entire Cascadia Subduction will rupture with a M9+ event within the same time frame (http://oregonstate.edu/ua/ncs/node/13426). Sumatra, Chile, and Japan provide vivid examples of what could happen.
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Evidence for past megathrust earthquakes in the Pacific Northwest
- Oral history of Native Americans,
- Radiocarbon dating Tree-ring analysis
- Geologic evidence for past tsunamis
- Samurai records of "orphan tsunami" on January 26, 1700