Is 8.8-magnitude earthquake in Chile really serious warning for USA? Everyone has to think about. One of the really “Big Ones” to shake the United States was a magnitude-9.0 earthquake along the Pacific Northwest coast more than 300 years ago, before the arrival of huge numbers of people and development, that sent a catastrophic tsunami to Japan. According to Professor Bill McGuire of the Benfield Hazard Research Centre there are subduction zones all around the world, but mainly they occur around the rim of the Pacific. The so-called Ring of Fire. Most of the world’s big, really destructive earthquakes occur here. A subduction zone is where a continental plate is sliding below an adjoining plate causing a build-up of friction. When this friction builds to a critical point, the plate breaks free of the friction and slips. This slippage causes a mega-thrust earthquake.
As most mega-thrust earthquakes occur near, or below, the sea the huge energy release causes a tsunami. A tsunami is different from a normal wave, in that with a normal wave only the surface water is moving, with a tidal wave the whole water column is moving. Millions of tons of water! The combination of massive earthquakes and tsunamis makes subduction zones a deadly geological hazard. So, it should have been a cause for some concern that the Cascadia Subduction Zone, a 600 mile long fault, lies right off the Pacific north-west coast. The strange thing was, that Cascadia didn’t seem to be a danger at all.
For years, scientists have been studying seismic activity along the Cascadia Subduction Zone. They found that, unlike other subduction zones, it was virtually silent. If the adjoining plates were sliding smoothly then no friction would build and no earthquakes would occur. This appears to be backed up by 200 years of records, for as long as Europeans have lived here, there is no record of earthquakes from Cascadia.
• The state of Oregon last month announced 13 school buildings and 11 emergency management facilities would be retrofitted to withstand earthquakes. More than 1,000 such buildings are at high or very high risk of collapse during an earthquake, according to a 2007 report from the Oregon Department of Geology and Mineral Industries.
• Washington state’s Department of Transportation announced in 2008 a two-year project to retrofit 19 bridges on Interstate 5 to current seismic standards, part of a plan to improve more than 900 bridges to withstand earthquakes.
• Naval Hospital Bremerton completed a seismic retrofit project in 2007 to improve its ability to withstand a large earthquake and provide medical care during and after a crisis, according to the hospital public affairs division.
The spate of recent earthquakes, starting with the magnitude-9.3 Indian Ocean event in 2004, follows a 50-year cycle of earthquake activity, McNutt says. The last cycle, in the 1960s, produced the two other record holders for recorded earthquakes — the magnitude-9.5 quake near Valdivia and a magnitude-9.2 quake in Alaska’s Prince William Sound.
“We know earthquakes are not uniformly distributed in time; they cluster,” McNutt says. “Now suddenly the earthquakes are lighting up again.”
Even with the knowledge that a Big One is inevitable, retrofitting buildings and requiring better building practices is a tough sell, even in parts of the country where quakes are facts of life, says Mark Benthien of the Southern California Earthquake Center at the University of Southern California.
“Improvements to our building codes have often followed the earthquakes that we have had,” Benthien says. “They are very difficult to pass in other times.”
A report commissioned recently by the Earthquake Engineering Research Institute and the U.S. Geological Survey concluded that many of the deaths in Haiti’s earthquake could have been prevented by using earthquake-resistant designs and construction, as well as improved quality control in concrete and masonry work of affected buildings.
“The massive human losses can be attributed to a lack of attention to earthquake-resistant design and construction practices, and the poor quality of much of the construction,” according to the report. It added: “Indirect evidence suggests that the earthquake did not produce ground motions sufficient to severely damage well-engineered structures.”
Chile shows that earthquake-resistant building codes don’t mean that people will be able to return to buildings, “just that they won’t fall on them,” Christian adds. The unfolding scenario of millions of displaced Chileans would likely occur in the USA as well, after a major earthquake, he says.
“We could build things to completely survive earthquakes,” Christian says. “They would all look like nuclear power plants. And cost as much.”
No predictions are possible for when an earthquake will strike, but the pattern of recent events does worry U.S. planners.
McNutt points to Alaska as the closest copy to Chile on U.S. shores. With a very active fault in the Aleutians and a population hugging the coasts, Anchorage and Juneau are susceptible to similar “subduction” earthquakes, where the Pacific Ocean plate dives under the North American crust.
But it is Puget Sound, with its population and potential to funnel in a tsunami, which is being watched closely.
“Seattle is another area of concern,” McNutt says.
Off Washington state’s coast, large earthquakes have struck every 500 years or so, with the 1700 quake the last major one. The resulting tsunami tore cedar trees from the ground along Puget Sound and was written about in Japan. The Juan de Fuca plate moves about 40 feet in a century, which means about 120 feet worth of energy is coiled up in the fault now, says Brian Atwater, a U.S. Geological Survey expert on historical tsunamis.
“That gives you about a one in 10 chance of (another large quake) across the next 50 years,” he says. “That’s enough for society to make some serious decisions about how we build schools and hospitals.”
via http://www.usatoday.com/
