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The Daily Need

U.S. is due for a sizable quake, but not because of Japan’s

Image adapted from U.S. Geological Survey seismic hazard map released in 2008.

Yes, a quake of at least 6.7 magnitude will likely hit the United States soon, experts say, but the disaster in Japan is not an indication that said quake will hit any sooner.

The West Coast is home to two geologic features that make it ripe for major temblors: the San Andreas fault in California and the Cascadia subduction zone in the Pacific Northwest. Both have lain silent for more than a century. Is the U.S. overdue?

Many experts think the answer is “possibly, yes.” The U.S. Geological Survey says there is a 99 percent probability that California, in particular, will experience a  6.7 or higher magnitude quake in the next 25 to 30 years. The probability of a quake of 7.5 or higher in that time frame is 46 percent.

Despite speculation to the contrary, however, the recent disasters in Japan, Chile, New Zealand and Haiti haven’t done anything to increase the odds.

During the week and a half since Japan was shaken by the 9.0-magnitude quake, many academics and science journalists have appeared on television discussing the Ring of Fire, a collection of tectonic plates under the Pacific that includes, among others, the Pacific plate, the Nazca plate (off the western coast of South America) and the Filipino plate (off the eastern coast of the Philippines). The question is frequently posed: Does increased activity in one part of the “Ring” have an affect on the other parts?

Simon Winchester, an author and earthquake expert, suggested on MSNBC last week that since earthquakes have recently wreaked havoc in three corners of the Pacific – an 8.8 in Chile last year, a 6.3 in New Zealand in February and now a 9.0 in Japan — the fourth corner — that is, America’s West Coast — will likely soon be seeing the compensating tectonic movement.

According to Steven Wojtal, an earthquake expert and professor of geology at Oberlin College who has studied fault lines – in particular, the San Andreas fault — for years, things aren’t quite that simple.

First, the Chilean earthquake was not caused by the same plate as the earthquakes in Japan and New Zealand. In Chile, the quake was caused by a shift in the Nazca plate, while in Japan and New Zealand quakes were caused by the Pacific plate.

Courtesy U.S. Geological Survey

And, though North America’s West Coast is also on the Pacific plate, the U.S. and Canada are so far from Japan and New Zealand (and on such a different part of the plate) that, on this side of the Pacific, we are unlikely to feel any tectonic backlash from the tremors abroad, Wojtal explained.

But “predicting earthquakes will never be like predicting the weather,” he said.

And, if Japan’s disaster isn’t a geologic warning to California, the century-long silence on the San Andreas fault is. The last earthquake along the fault, in April 1906, is remembered as one of the worst natural disasters in American history. The quake and resulting fires took the lives of some 3,000 people, the largest loss of life from a natural disaster in California’s history. In 2008, the USGS predicted that California would see a large earthquake within the next three decades. And many fear the state is not sufficiently prepared.

The Cascadia subduction zone, off the coast of Oregon and Washington, is less worrisome. The subduction zone causes extremely intense earthquakes, but much more irregularly — on average, every 500 to 600 years. The last was, according to a USGS professional paper, on January 26, 1700 — about 311 years ago. Europeans had not yet settled the Pacific Northwest, and there are no Native America records of the quake, but, by studying tree rings and Japanese accounts of the resultant tsunami, researchers have estimated the quake was between an 8.7 and a 9.2 – one of the worst in our continent’s (relatively) recent history.

For now, most experts, including Wojtal, recommend that the West Coast should remain wary – specifically, those on the West Coast must ensure that their building codes are up to snuff. When it comes to the toll on human life, “natural disasters are only part of the story,” said Wojtal. It’s not the earthquakes that kill people, he pointed out, it’s collapsing buildings. The difference between Haiti — where a 6.3-magnitude quake reduced much of the country to rubble — and Japan — where most buildings withstood a 9.0-magnitude earthquake (if only to be leveled by the tsunami) — was preparedness.

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