Your Survival Blog
The Last Subduction
Hold your hands out in front of you, palms down, elbows out, fingertips toward each other and touching. Now fold the fingers on your dominant hand down slightly, sliding it beneath the other until the non-dominant hand rests on top.
This is subduction, the process of one geological plate at a plate boundary slipping gradually beneath another. Unfortunately, when rocks are involved, the slippage is not nearly as smooth as when your hands do it (assuming your skin is not covered in rock): subduction is responsible for the deadliest megatsunami in history (the 2004 Indian Ocean event which killed a quarter-million people), and the largest earthquakes ever recorded by human beings (including Chile 1960 and Alaska 1964, and Lisbon 1755). These “megathrust” earthquakes can exceed 9.0 in magnitude. (Remember that quake measurements are logarithmic, so that a nine-point earthquake packs a thousand times the punch of the seven-pointer many Californians worry about.)
When you have a major metropolitan area sitting on top of a subduction zone, things really get interesting. In 1755, the thriving city of Lisbon — then only about 275,000 souls — experienced a megathrust earthquake, tsunami, and fire; these reshaped European history, philosophy, science and architecture. (Lisbon now has ten times as many people, so it’s fortunate that seismic building codes have been in effect for two and a half centuries.)
Yet we in North America also have populous metropolitan areas on top of subducted plates. The Seattle/Tacoma metro area in Washington Sate has 3.3 million people. Metro Vancouver, BC, on the other side of the US-Canadian border, has 2.2 million. Greater Portland, Oregon, to the south, has over two million. All sit firmly atop the Cascadia Subduction Zone (CSZ), source of the greatest recorded North American earthquake outside Alaska.
On the evening of January 26, 1700, a megathrust quake struck somewhere off the Northwest coast. (The fault ruptured over six hundred miles (1000 km), from Vancouver to northern California.) No Westerners had then settled in the area, but records of tsunami damage resulting in Japan help to fix the date.
Parts of the Washington coast fell five feet (1.5 m). Red cedar and Sitka spruce forests were drowned by the tsunami, and carbon dating shows their final rings were formed in the growing season of 1699. On Vancouver Island, the houses of the Cowichan people collapsed, and a village of the Pachena Bay people was wiped out. The Hoh and Makah people of the Olympic Peninsulas preserve legends of “trembling of the earth,” “rolling up of the great waters”, and starvation.
Geologists have found evidence of dozens of giant earthquakes in the prehistory of the region. Some use the figure of thirteen over six millennia; another says twenty 9-point-plus and eighteen 8-point-plus earthquakes over ten millennia, with a “return time” of three or four centuries. Many also agree on at least a ten percent chance of the CSZ rupturing within fifty years.
An Oregon state geologist announced in April 2009 that when the next M9 quake hits Cascadia, “The amount of devastation is going to be unbelievable.” Coastal towns might be struck by a tsunami 80-100 feet (24-30 m) high.
In August 2009, seismologists studying “deep tremors” in the CSZ suggested that the next megathrust might happen further inland, beneath the Olympic Peninsula, producing even stronger shaking on land. Major cities, full of highways and bridges and brick buildings, will suffer massive damage. Due to the scale of the devastation, it may take many days or weeks for rescue workers to reach homeless and hungry survivors.
So: if you live in the Pacific Northwest, what can you do?
- Start reading up about quake and tsunami hazards in your community and any you frequently visit. If your town conducts drills or education sessions, attend!
- Put together emergency kits for home, cars, and office. Assume that you might not be seeing emergency workers or other authorities for a week or more.
- Make plans. If you work in an office, ask about emergency drills. If you have kids, talk to their schools about emergency procedures. Make sure everyone in your family knows your game plan for reuniting and/or letting each other know you’re safe.
And good luck. Let’s all hope the CSZ will hold until the science of earthquake prediction becomes more advanced.
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