Seismic and anthropological research over the last thirty years has revealed the deep history of the Pacific Northwest’s earthquake and tsunami activity. As mentioned on previous blog posts, the Pacific Northwest is bordered to the west by the Cascadia Subduction Zone, a point where two tectonic plates, the Juan de Fuca plate, and the North American plate meet – about fifty to seventy five miles off the coast, running from southern British Columbia, roughly seven hundred miles south, to northern California. (The Juan de Fuca plate is subducting under the North American plate at a rate of ~ / =1.6″ per year to the east / northeast. The North American plate is moving to the south / southwest at a rate of ~ / = 1″ per year. Ref link: http://pnsn.org/outreach/about-earthquakes/plate-tectonics).
According to James Roddey (formerly of Oregon Geology Department) in a video published on January 9, 2014, using information from Dr. Chris Goldfinger of Oceanic and Atmospheric Science at OSU (https://www.youtube.com/watch?v=0amLbhxCiqc&spfreload=10), over the last 10,000 years, there have been twenty magnitude 9+ earthquakes with epicenters varying along the length of the Cascadia Subduction Zone, with another twenty 8.5+ magnitude earthquakes epicentered in the southern 300 miles of the subduction zone. Seventy five percent of those forty earthquakes took place within three hundred and ten years of each other. The last one being in the year 1700.
So, how do scientists discern when earthquakes occurred, their magnitude, and resulting damage? They play in the dirt…. (http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/22/1b.EarthquakesVolcanoesInThePacificNW.pdf)… see page six. Okay, so they do more than play in the dirt. But ‘playing in the dirt’, is a huge part of learning about the history of earthquakes and tsunamis. Earthquakes can cause moderate to severe elevation changes, alter the course of creeks, streams, and rivers, and change cultures and societies. These are known as Seismoarcheaological Indicators. Unlike evidence from volcanic eruptions and floods, which can be recognized in excavation sites by changes in stratigraphy or soil composition, earthquake evidence can be more difficult to ascertain or interpret. Often times, archeaological dig sites are concentrating more on the anthropological history than on the geological history of the dig site; seismic evidence may be overlooked or destroyed during the excavation. Unless the excavation is intent on seismic discovery, evidence of past earthquakes such as damage to buildings or other structures, and ecological changes, may be misinterpreted, or as previously mentioned, overlooked or destroyed.
How structures, anthropological evidence (human remains, etc…), and other organic evidence (trees, fossils, etc…) are buried may help answer questions about magnitude, duration, damage, and other time related questions that scientists are looking to answer. Separation and striation of strata may also offer evidence of past earthquakes. Returning to page 6 referenced above at IRIS.edu, evidence of a major geological catastrophe (tsunami) may be indicated by the layers of sand deposits over previous topsoil, followed by tidal mud and finally current topsoil. This as a result of the major earthquake and the following tsunami in 1700. Layers of sediment found off the coast of BC, OR, and WA show widespread, simultaneous shaking region-wide was highly likely.
Written accounts, folklore, and other cultural evidence may also help ascertain earthquake history. The last megathrust earthquake to hit this region occurred on January 26th, 1700… scientists even have it down to a time – 9PM. They were able to ascertain the date and time by collaboration with Japanese scientists and historians who identify the Cascadia Subduction Zone as the origin of a deadly tsunami that hit the coast of Japan the following day. Accounts of the tsunami were documented by Samurai in the regions affected by the tsunami (see previously referenced IRIS.edu). Native American’s in the region handed down tales and legends of strong shaking throughout the region, followed by severe flooding. Much of this can be cross-checked with physical evidence, such as the Ghost Forests (http://www.burkemuseum.org/static/earthquakes/bigone/detective.html), an area decimated by the flood of 1700, which is now all dead cedar stands, swamp land, and sea marshland.
Evidence of other past earthquakes are found in turbidites, layers of sand, silt, and muck, found at the bottom of the sea where two plates meet. The sheer drop-off from one plate to another provides excellent sources for turbidites and their secrets. According to Chris Goldfinger, turbidites provide “very, very obvious ” evidence of earthquake deposits (http://www.livescience.com/22248-earthquake-records-pacific-northwest-cascadia.html). Only the strongest earthquakes send sand, silt, and other seabottom particulates tumbling to the oceanic plate below, which over time, create these turbidites. Unlocking their secrets with carbon dating shows evidence that correlates with land based time frames for seismic events. Turbidites have proven to be highly accurate, not only for large earthquakes, but for much smaller ones, too, providing more information on smaller earthquakes than can be garnered on land.
Given the depth and breadth of discussions on ‘The Big One’, it is hard to point to any one study and say that is the right one. Based on evidence provided by turbidtes, Goldfinger says he’s not taking and chances, and has purchased earthquake insurance and made necessary modifications to his house. Probably not a bad idea in this tumultuously seismic environment we live in.