Tuesday, November 27, 2012

Cascadia's Fault: The Coming Earthquake and Tsunami That Could Devastate North America - Jerry Thompson

First Nations peoples of the Pacific Northwest have passed down stories for generations about a winter’s night when the ground shook terribly and an enormous wave hit the coast and wiped out entire communities. The Europeans who began to populate this region in the 1800s did not pay attention to these stories, nor did few others until the mid to late 20th century when it turned out that these First Nations stories were rooted in fact, and that this part of North America was not a special aseismic region of the Ring of Fire. “Cascadia’s Fault” is the story of the decades-long multi-disciplinary search that led to the scientific evidence and conclusion that one day in the future the Pacific Northwest will be struck by another devastating earthquake.

Journalist Jerry Thompson begins the story of the search for the Cascadia Fault by explaining where it started. For a long time earth scientists and residents believed that because the region from the north end of Vancouver Island to the Oregon-California border had been essentially tremblor-free in all of recorded history (since European contact), because there was no obvious fault line or trench, it was assumed that the two tectonic plates were moving smoothly past each other and there was no danger to the region. It was the 1985 earthquake in Mexico City that got West Coast geologists questioning whether this theory was correct as the fault off the west coast of Mexico was also deemed aseismic…and was very similar to Cascadia, except shorter.

This questioning led researchers back to 1964 when a tsunami hit the small Vancouver Island town of Port Alberni after the 9.2 megathrust quake in Prince William Sound off the Alaskan coast. There were incredible changes to the region: an area the size of the states of Washington and Oregon combined had heaved up or dropped down, the sea floor appeared to have lifted more than 15m, and the land also stretched horizontally up to 20m between the city of Anchorage and the edge or Prince William Sound. This quake was surprising because after it happened no one was able to say which fault had ruptured and why this fault had gone undetected for so long. However, because the scientists studying this earthquake were doing so just at the time that the theory of plate tectonics was becoming more popular, it took many years for scientific conclusions about the geology of the region to be accepted: that off the West Coast of North America were subduction zones (horizontal cracks in the earth’s crust).

All of this led scientists back to searching for clues along the Washington and Oregon coast that might shed light on the seismic history of the Cascadia fault. The remainder of the book concentrates on this research. Thompson describes how surveying engineers had set up geodetic markers on mountaintops and were using lasers and GPS co-ordinates to determine whether or not there was any movement in the peaks. There was – the mountains of Vancouver Island were being squeezed landward, towards the mainland, about 20cm in less than 40years. Hundreds of kilometres worth of mountain rock were being pushed horizontally yet even then selling the idea that a subduction zone was locked and building up energy for a future earthquake was difficult. More evidence was needed to show that there was plate tectonic activity here and that it had led to past earthquakes.

Thompson then introduces two Washington scientists: Brian Atwater and David Yamaguchi. Atwater, in 1986, discovered what he calls the ghost forest not long after his discovery of another region northwest of Seattle that showed layers of sea sediment and plants above layers of land sediments and plants:

This was no gentle or gradual transition zone from one geologic era to another. The peat had a sharp upper boundary caused by an almost instantaneous and probably cataclysmic change in the level of the land and sea. Was it physical proof that the ground had slumped during an earthquake, that the plants of a marsh or forest meadow had been drowned quite suddenly by the incoming tides and possibly buried under the sands of a huge tsunami?

Good question, and the ghost forest raised another that tree scientist David Yamaguchi attempted to answer: when did this forest end up swamped by salt water, killing the spruce trees? By studying the tree rings, Yamaguchi managed to narrow the timeframe for the last earthquake along the Cascadia Fault to sometime between 1680 and 1720. Despite cutting and counting rings on both dead and witness trees (those living at the time the others died), Yamaguchi could get no closer. However, in 1994 a Japanese geologist who specializes in subduction zone and tsunami research, Kenji Satake, learned of these dates and their association with an earthquake and was immediately intrigued. While North America did not have written records at that time, Japan did, and it was in those records that Satake made an amazing discovery. In historical documents found in four separate town there was mention of a 5m tall tsunami in 1700, and the only place it could have originated was from the Cascadia fault. By calculating backward, the quake was estimated to be at least magnitude 9. They were also able to calculate even more. As Satake wrote in Nature:

The earliest documented tsunami arrival time was around midnight on 27 January, Japan time. Because tsunami travel time from Cascadia to Japan is about 10 hours, the earthquake origin time is estimated at around 5:00 on 27 January GMT or 21:00 on 26 January local time in Cascadia. This time is consistent with Native American legends that an earthquake occurred on a winter night.

Finally, by 1997, it became a known fact that Cascadia is an active fault whose next rupture would have major consequences for at least five North American cities. This is why Thompson’s book is so important. This information is barely 15 years old but has already been taken seriously, but perhaps not yet seriously enough because there just seems to be no reliable way to predict when an earthquake will hit. There have been some efforts to make BC, Washington and Oregon earthquake disaster ready, but there is still a long way to go to both raise public awareness and emergency response standards to a level that would be able to deal with events on the scale of the 1995 Kobe quake and the 2004 Sumatra tsunami.

Every person in the Pacific Northwest should pick up this book and both enjoy the history and explanations written in an interesting and accessible manner for even those with the most minimal knowledge of geology, as well as the implicit and explicit warnings that it has happened before and will happen again, so we had better be prepared.

Emergency Management BC website with information about earthquake preparedness: http://embc.gov.bc.ca/em/hazard_preparedness/earthquake_preparedness.html

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