When the earth moves under our feet

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One of the most breathtaking geologic events is a major earthquake. In just a few moments, shaking of the earth can result in billions of dollars of damage and the loss of thousands of lives.

Many earthquakes are related to the movement of tectonic plates, the large chunks of the Earth’s outer surface that move in relation to each other. Plates are “born” in places like Iceland, where magma comes up from below and creates oceanic plate material. Plates “die” where one plate dives beneath another and ultimately is pulled and pushed down so deeply into the earth it melts away to nothing.

Plates vary a bit in how fast they move, but about an inch or two a year is not uncommon. That’s roughly the speed that your fingernails grow. That may not sound like much, but when you consider it’s the whole surface of the planet that’s moving, you get an appreciation for the great forces that are involved in tectonic movement.

One example of where the rubber meets the road regarding tectonic movement is in the Pacific Northwest. The Cascadia subduction zone (visit ubne.ws/ZIQm7O) is the area where what’s called the Juan de Fuca plate is diving under the North American plate. The movement generates major earthquakes from time to time. The most recent mega-quake occurred in 1700. Geologists think the region is about due for another similar event.

Does that mean the Pacific Northwest faces grave seismic danger like California? The situations in the two regions are a bit different, because California’s San Andreas fault creates movement at and near the surface — where we people live. But there’s no doubt Cascadia poses a major hazard. That’s true in part because the Pacific Northwest is not as well prepared for earthquakes as California is.

Recently I was reading in Science News about new information regarding earthquakes and plate movement. In the Cascadia region something called “slow slip” happens about every 15 months. Slow slip occurs when the rocks on either side of a major fault move about the same amount as in a major earthquake, but they do so over weeks to months rather than almost instantaneously as in an earthquake. Slow slip can generate as much movement as an earthquake that measures 7 or more on the Richter scale.

“It’s like an earthquake, only slower,” geophysicist Kelin Wang of the Geological Survey of Canada told Science News.

“There’s a lot we don’t know,” seismologist John Vidale of the University of Washington told Science News.

Now that geologists and geophysicists know what they are looking for, slow slip events have been identified around the world. In Japan, some slow slip events have been documented that occur about every three to five years and last a few months, while others occur much more frequently.

The evidence for slow slip was documented first for Cascadia in the bedrock of Vancouver Island, just over the international border in British Columbia.

Slow slip is often too slow to create seismic waves. But on occasion the rock on either side of the fault may move quickly enough to generate seismic waves that are just large enough to be above background noise. In those cases, the slow slip generates what scientists call tremor.

A confusing point is that sometimes tremor occurs before or after the movement of the slow slip. Sometimes slow slip occurs with no tremor at all. The reasons for these facts are not currently understood.

The longest period of slow slip yet detected anywhere occurred last year. It started in August under Vancouver Island in southwest Canada. It began as tremors there, then moved south. It crossed the international border, moving to and then beyond the Seattle region. All together, the event lasted 42 days.

At first glance it might seem that slow slip relieves stress on faults and could help us avoid major quakes. But some geologists think that slow slip events transfer stress to areas that then are more likely to rupture when a mega-quake occurs in a region. There’s some evidence to suggest that slow slip may have helped trigger Japan’s March 2011 quake, the terrible event that resulted in a destructive tsunami.

So even when it comes to slow motion earth movement, we’ve got to hang on to our hats.

E. Kirsten Peters, Ph.D., 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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