What if great quake hit here in Utah?

Mar 12 2011 - 1:01am

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(Kyodo News/The Associated Press) A vessel sits after it was washed away by tsunami into urban area in Kesennuma, Miyagi, northern Japan, on Saturday after Japan's biggest recorded earthquake slammed into its eastern coast Friday.
(Kyodo News/The Associated Press) A father and child who lost their home stand in front of debris in Sendai, northern Japan, on Saturday after Japan's biggest recorded earthquake slammed into its eastern coast Friday.
(Kyodo News/The Associated Press) A vessel sits after it was washed away by tsunami into urban area in Kesennuma, Miyagi, northern Japan, on Saturday after Japan's biggest recorded earthquake slammed into its eastern coast Friday.
(Kyodo News/The Associated Press) A father and child who lost their home stand in front of debris in Sendai, northern Japan, on Saturday after Japan's biggest recorded earthquake slammed into its eastern coast Friday.

The Wasatch Front is unlikely to suffer a magnitude 8.9 earthquake like the one that devastated Japan, but the maximum magnitude quake we are likely to get -- 7 to 7.5 -- could easily kill thousands of people and cause billions of dollars in damage, experts say.

And that maximum quake we face by living on the Wasatch fault line could come tomorrow or 500 years from now, said Adolph Yonkee, Weber State University geosciences professor.

"We don't have as big of earthquakes here, but a major earthquake in Utah would still be serious and produce a lot of damage."

Tony Lowry, a Utah State University geophysicist, said Japan and Utah have different varieties of earthquakes.

Japan's earthquake was the result of shifting tectonic plates. Utah, being farther away from a plate boundary, would have an earthquake in which a smaller patch of rock would shift.

"The key for the amount of energy released is the amount of slip, or how far the rock moves, times the area it slips," Lowry said.

The entire Wasatch fault, which runs about 240 miles from the south of Idaho to central Utah, is 19 miles or less in depth. It's unlikely that more than a 30-mile section of the fault would fail in any single seismic event, Yonkee said.

Preliminary measurements from Japan's earthquake indicate the portion of the fault that failed was about 300 miles long and 75 miles in depth. Japan's earthquake Friday was more than 30 times more powerful than a Utah maximum predicted quake of magnitude 7.5, Lowry said.

The big problems in Utah would be ground shaking, landslides and soil liquefaction, a circumstance in which soil fails to support solid objects and can become like quicksand, causing surface structures and objects to tilt or sink.

Yonkee said what would determine damage levels in Utah would be the location of the epicenter, the magnitude of the quake and, in specific spots, the types and strengths of buildings and soil conditions.

Soil with a higher water, sand and silt content is more likely to liquefy than very rocky soil, he said.

All other things being equal, large cities would face the most damage because of their greater number of buildings, roads, people and infrastructures.

The greater Salt Lake City area has Utah's greatest potential for loss and damage, Yonkee said. In all urban areas, most damage would be caused by the ground shaking.

The Weber segment of the Wasatch fault, which includes both Weber and Davis counties, has suffered at least five major earthquakes in the past 6,500 years, according to the Utah Geological Survey.

The last was about 500 years ago, and the one before that was about 1,000 years ago.

On average, the Wasatch fault's major earthquakes occur about every 1,000 years, Yonkee said.

The Brigham City segment of the fault has suffered four major earthquakes in the same time period, with the last one about 2,500 years ago.

Lowry said a major earthquake would devastate Utah and take down many buildings, especially those built before 1989, when earthquake safety requirements were put into place for new buildings. Older brick and stone buildings would be especially vulnerable to shaking.

"We're not as careful as they are in Japan, because they do more to make sure everything is seismically sound than any other place on the planet," Lowry said.

Yonkee said geoscientists cannot predict precisely when the next major earthquake will hit Utah. The most informed forecast is that, over a 50-year time frame, there's a 20 percent chance of a major earthquake along the Wasatch Front, Yonkee said.

"You don't panic, but there's enough of a chance that you make the decisions on how you will be prepared, and how much money you will put into training and into making something seismically safe. If a forecast tells you there's a reasonable chance of an earthquake, you do what you need to to prepare."

For a Utah Seismic Safety Commission booklet on earthquake preparedness, visit www.ussc.utah.gov/publications/roots_earthquake.pdf.

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