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The Past and Future Great Cascadian Earthquakes


The sheer power of large-magnitude earthquakes has recently been demonstrated in the most terrible way, through the three deadliest earthquakes of the past 100 years: the 2004 Indian Ocean earthquake, 2008 Sichuan earthquake, and 2011 Tohoku earthquake, each with death tolls in the tens of thousands of people.

With the memory of these events fresh in our collective minds, we need to look ahead to the future. Between the network of subduction zones underlying Japan, the maze of Eastern Mediterranean fault systems affecting Turkey and the Middle East, and the tectonic complex of Western North America, massively deadly earthquakes will continue to happen.

The latter is the focus of an in-depth article by Kathryn Schulz in The New Yorker. Entitled “The Really Big One”, Schulz discusses the aftermath of an earthquake triggered by the Cascadia subduction zone, which lies north of the San Andreas fault that caused the 1906 San Francisco earthquake.

This Really Big Earthquake will be accompanied by a Really Big Tsunami that can go up to 30 meters in height, it will happen at some point, probably in the near future, and it will destroy the Pacific Northwest region, including such cities as Seattle, Portland, and Vancouver. Besides the thousands of projected deaths and injuries, millions of people will be displaced and deprived of basic necessities.

In short, it will be a disaster of unprecedented proportions, and its consequences will be worsened by the unpreparedness detailed in Schultz’s article. This blanket statement must be qualified though – it was only in the 1980s that Cascadia was even implicated in earthquakes.

The history of its discovery is a fascinating detective story involving indigenous tales, palaeoseismology, and international collaborations. Brian Atwater, a geologist from the USGS, first made waves in a 1987 Science paper recording Holocene subsidences of up to 2 meters in soil layers from western Washington bays. These events resulted in the formation of salt marshes, now inhabited by dead trees that can’t grow in salt water. Atwater and dendrochronologist David Yamaguchi then looked at when the trees in one of the salt marshes died using growth rings and found that they stopped growing after the summer of 1699, so an earthquake must have happened sometime in late 1699/early 1700 to drop the level of the land, form the salt marsh, and kill the trees.

Atwater and colleagues continued gathering evidence of more subsidence along Pacific Northwest salt marshes, resulting in the 1995 Summary of coastal geologic evidence for past great earthquakes at the Cascadia Subduction Zone. So far, the true magnitude of these earthquakes was still a mystery.

Indigenous tribes of the region had oral variations of a story in which whole tribes disappeared or were drowned one night after a a large earthquake. These stories were collected and analysed by Ruth Ludwin (University of Washington) and her colleagues in a 2005 paper, who found that these stories all point to this catastrophic event happening more or less around 1701, coincident with the earthquake discovered by Atwater in 1987.

That this earthquake caused 2 meter elevational shift and the extinguishing of several tribes is ominous enough without knowing that its timing also corresponds to a tsunami recorded at the same time in Japan, as discovered by Kenji Satake and colleagues and published in a 1996 Nature article. Given its history, Japan has kept excellent seismic records since at least 599AD, and this one tsunami stood out as not being associated with any earthquake. The inference is thus that it was caused by the Cascadian earthquake of 1700 and travelled all the way across the Pacific.

This is a great history of discovery, showing how geology can intersect with ethnology and history to provide new information. A very detailed look at this story can be found in Atwater et al.‘s 2005 poetically-titled report, The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, available for download at the link, and Schultz handily summarises it in the article too.

Given that Atwater & Hemphill-Haley (1997) calculated an recurrence interval of 500 ± 300 years, we ought to start getting worried about the next big earthquake. As Schultz’s informative and frankly terrifying article makes clear, this next quake will be the really big one and we’re woefully unprepared for it, since we only found out about this a few years ago. I highly-recommend reading it, since it touches on the most socially important aspects of geology, from earthquake prediction to disaster mitigation.


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