Wednesday, August 20, 2014

Rock doc: Shake, rattle and roll

From page A4 | November 25, 2013 |

“It’s 8:16 on a chilly, wet morning…You’ve just arrived at work and are pouring a cup of coffee when you become aware of a low rumbling noise. Within seconds, the rumbling becomes a roar, the floor beneath you heaves, and the building begins to pitch and shake so violently that you’re thrown to the floor. The roaring is joined by a cacophony of crashing as windows shatter and every unsecured object in the room — from the desk chair to the coffee pot — is sent flying. Shaken loose by the shuddering and jolting of the building, dust and ceiling particles drift down like snow. Then the lights flicker and go out.”
That’s the arresting start of a new report produced by several governmental agencies that describes what can happen when a magnitude 9.0 earthquake hits what’s called the Cascadia Region, an area that stretches from the coast of Northern California northward through western Oregon, Washington, and southwestern British Columbia. The quake will be triggered by movement along the faults that lie between the oceanic tectonic plates and the plate on which North America rides. When the plates move suddenly, absolutely enormous amounts of energy are released, with violent shaking of the ground and tsunamis as the result. The report that describes all this is Cascadia Subduction Zone Earthquakes: A Magnitude 9.0 Earthquake Scenario.
Cascadia isn’t the only place in danger of having major earthquakes. Most famously, the San Andreas and associated faults in California are a constant threat to local residents. And the New Madrid fault zone, centered where the states of Missouri, Kentucky, and Tennessee come together, is a threat to the lower Midwest. Finally, states as different as South Carolina and Alaska also run the risk of significant earthquakes. In short, the U.S. has a number of regions where enormous amounts of energy can be released over the span of just seconds, with resulting damage to buildings, roads, power lines and pipelines.
The Cascadia region of the Pacific Northwest is in danger of large earthquakes because it’s a subduction zone — a place where ocean crust dives underneath the overriding North American plate. Worldwide, subduction zones harbor the greatest threats for truly enormous earthquakes, with magnitudes from 8 to 9 and even higher. In 1960 a quake off the coast of Chile had a magnitude of 9.5 — the highest ever on record. Quakes that enormous have major ground shaking that lasts for a terrifyingly long time, and they can create large tsunami at sea. In addition, such quakes have numerous aftershocks, quakes that in their own right are significant.
The scale used by geologists to measure earthquakes has its complexities. In California, the Loma Prieta quake of 1989 had a magnitude of 6.9. In 2002 a quake with magnitude 7.9 struck Denali Park, Alaska. The Alaskan quake, measuring a single unit higher on the magnitude scale, released more than 30 times more energy than the smaller Loma Prieta quake.
The most recent mega-quake in Cascadia is estimated to have had a magnitude between 8.7 and 9.2. It occurred on Jan. 26, 1700. We know about it both from physical evidence here in our country and from written records of a tsunami that arrived in Japan some hours after the quake. The sobering fact is that we could have a similar event again, and at any time.
We can’t predict the date of the next major earthquake in the U.S. but we can anticipate some likely impacts it will have. In Cascadia the dangers spring both from ground shaking and flooding along the coasts and estuaries due to tsunami. The Oregon Legislature commissioned a report that estimated more than $30 billion in property could be lost when the next Big One hits. The death toll might stand at around 10,000 from such an event.
It behooves those of us who live in earthquake country — whether in the lower Midwest, California or Cascadia — to educate ourselves about risks. Having several days worth of food and water on hand, and a way to cook up some vittles, are simple goals most of us can achieve.
Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human and Natural Resource Sciences at Washington State University.


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E. Kirsten Peters

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