Despite what Utah politicians would have you believe, the real mover and shaker in the state's future likely will be the Wasatch Fault.
And though no one knows for sure when the next earthquake will strike along the fault, members of the U.S. Geological Survey and Utah Geological and Mineral Survey believe the state is overdue.Geologic officials partially base their predictions on what they've discovered from digging trenches along segments of the Wasatch Fault zone. Last week they showed off their latest cuts, at the mouth of Rock Canyon east of Provo and in a nearby stream bed, to a handful of media representatives and city and county leaders.
"The geological record says we're overdue," said Bill Lund, USGS research geologist. "We're talking a major one from 7 to 7.5 on the Richter scale."
The trenches give officials a look into the state's recent geologic past, helping them determine how many quakes have occurred in the past few thousand years and how strong they were. Data also help officials determine when the quakes occurred and when the next one can be expected - give or take a couple hundred years.
Lund said information obtained from the latest trench will be coupled with data gathered from previously dug trenches near Mapleton and American Fork. Soilsamples from the Rock Canyon trench, dug in June, are still being examined. But preliminary data indicate the trenches all are part of the same fault segment.
Along the south face of the Rock Canyon trench, Lund said, officials found evidence of the area's last major earthquake. "My guess is that it occurred less than 1,000 years ago. But that's just a guess."
Data from the Mapleton and American Fork trenches indicate that the last big earthquake _ measuring about 7.5 on the Richter scale _ happened between 550 and 750 years ago, with previous quakes occurring at approximately 2,000-year intervals. If the latest fault zone is determined to be part of that same segment, then officials would expect future major quakes along the segment to occur at the same interval _ that is if the fault behaves in a regular pattern.
"If this (trench area) is part of the same segment, then we can say with some confidence that it will be another 1,000 years before the next surface faulting," Lund said.
The Wasatch Fault has seven or eight active segments of varying lengths, meaning "rupture length" of earthquakes along the fault are likely to be relatively short. However, that segmentation means Utahns, most of whom live near the fault, can expect a higher frequency of quakes.
Lund said a Wasatch Fault segment at Brigham City is the most likely to undergo a quake because it is the most overdue. If a major quake were to hit Brigham City, he said, ground ruptures could be expected in and around the city, but Utahns living along other sections of the fault could expect major shaking.
Typically, more earthquake deaths result from severe ground shaking than ground rupturing, he said.
Utah County Geologist Bob Robison said information gathered from geological studies helps cities and counties discourage or prevent construction in dangerous areas. He said USGS surface fault-rupture maps will be incorporated into the county's hazard ordinance.
Commission Chairman Malcolm Beck has an even more practical use of the information. "It will help us make sure we know when the next earthquake will happen, so we'll know when to leave."
Lund said geologists' knowledge of the past is only as deep as the trenches they have dug. The farther down they go, the more they will discover.
"Twenty feet only buys us about 2,000 years," Lund said. He said officials are planning to dig a deep "megatrench," which they hope will provide additional information about the distant past.