Geologists look to pinpoint a big one

This is an archived article that was published on sltrib.com in 2008, and information in the article may be outdated. It is provided only for personal research purposes and may not be reprinted.

WILLARD - Geologists studying two trenches gouged from the foothills here might as well be peering through windows on the past - and into the future.

Christopher DuRoss, of the Utah Geological Survey (UGS), says the data being collected on this section of the 240-mile Wasatch Fault can help pinpoint when a big earthquake might happen again and how severe it might be.

Ultimately, that information will help people prepare for the hazard, especially those living between Brigham City and North Ogden.

"The Brigham segment has the highest probability of a large-magnitude quake," DuRoss says.

The UGS and their counterparts at the U.S. Geological Survey have already looked at two other groups of trenches on the Brigham segment this spring to study past faulting.

Initial research on the Wasatch Fault more than 20 years ago was done under the National Earthquake Hazards Program, which Congress established in the mid-1970s. It suggested that the Brigham City segment saw severe earthquakes every 1,300 years and that the last one had been about 2,100 years ago.

"So, we're about 800 years overdue for a quake on this fault," DuRoss notes.

A new look, using new scientific tools and techniques, will help the geologists determine if that's right.

Storm clouds unfurled over the Great Salt Lake just beyond Interstate 15 to the west. A warm, sage-tinged breeze swept up from the valley below to trench walls where DuRoss, USGS geologist Anthony J. Crone and their team study barely perceptible patterns of soil and rock.

In one 100-foot-long wall they have marked two deep cracks that formed the last time there was an earthquake here. A pattern of colored tape nailed into the wall dots where, in one place, the hillside dropped 10 feet and, in another, it dropped 6 feet.

"We live in a dynamic environment," says Crone.

He explains that the 500 miles between Salt Lake City and Reno stretches apart by about 11 millimeters - or less than a half inch - each year. Over time, the tension builds like a rubber band that eventually snaps and results in an earthquake.

"This has been going on in the Basin and Range for millions of years," adds DuRoss. And that means that by studying the patterns of the past, geologists can make educated predictions about the future.

"We are trying to characterize the behavior in time - where time is the geologic record - and space - where space is the distance along the Wasatch Fault - and use that as a way to look into the short-term future," says Crone.

Estimates suggest that up to 6,200 people might be killed on the Wasatch Front if there is a magnitude-7 earthquake, and the damage to buildings would be about $42 billion. Roughly 80 percent of Utahns would be affected.

The Utah Seismic Safety Commission is developing a priority list of public buildings - including multilevel masonry schools like the ones that killed hundreds of children in China last week - that need to be replaced or upgraded to withstand the ground-shaking hazard.

DuRoss and Crone's teams have collected several bags full of soil that will go to a laboratory. Charcoal in it will be tested using carbon dating. About six months from now, when the data come back, the geologists hope to be able to say with greater certainty how severe the quake might be and when it is likely to occur.

The information could help save lives by suggesting which buildings need attention first and what precautions are needed, said DuRoss.

Similar studies a few years ago near Nephi showed that the most recent quake was about 300 years ago. And that meant the likelihood is low that there will be a quake in the next 1,000 years or so.

fahys@sltrib.com