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SALT LAKE CITY -- Scientists have discovered evidence of four big earthquakes that rocked the Salt Lake Valley long before pioneers arrived -- and they're not on the well-known Wasatch Fault on the East Bench.
Geologists are getting their first good look at the West Valley Fault zone, just west of the Salt Lake International Airport. They've wanted to dig trenches in the area for years.
"We suspected that there had been numerous earthquakes in this region," said Chris DuRoss of the Utah Geological Survey.
But since at least the 1980s the area on the margins of the Great Salt Lake has been too soggy to dig.
Over the last decade the shrinking lake receded several miles from the fault zone, and the water table has dropped. Now that the Utah Geological Survey has been able to dig three trenches, the water table is visible 10 feet below the surface.
Recently geologists had their first look underground, and it confirms their suspicions. The horizontal layers of sediment show clear signs of being broken by fault movements. It's clear evidence of four big earthquakes in the last 15,000 years.
"We can say that these were probably a magnitude 6.5 or so earthquake," said Mike Hylland of the Utah Geological Survey. "Pretty strong."
The West Valley Fault zone is actually comprised of a dozen different fractures. They lie roughly between Redwood Road and 5600 West from about 1700 North to 4800 South.
The evidence in the trenches suggests that each of the four earthquakes on the West Valley Fault system was strong enough to break the ground. They displaced the earth vertically by an average of about 18 inches. That is significantly less displacement than the five or six quakes known to have shaken the Wasatch Fault during the same 15,000-year time period.
The next step for the scientists is to answer a key question: Are the four West Valley quakes the same ones that rattled the Wasatch Fault, or are they entirely different events?
As Hylland, put it, "When the Wasatch moves, does the West Valley Fault move as well, at the same time? We have a fairly informed hunch that that is what happens, but we don't have good solid data to say that's what happens. "
The answer could turn out to be significant. If the two fault systems move during the same earthquake, it suggests they are both triggered by the same source deep underground. That would imply bigger quakes than scientists previously suggested.
But if quakes on the two faults are unrelated, it implies that quakes are probably more frequent in the valley than thought; there must have been four on the west side and five or six different ones on the east side.
"It's really not reason to either relax or to be more concerned," DuRoss said. "I think the best answer is that we should all be prepared for an earthquake at any time, whether it's on the West Valley Fault zone or the Wasatch Fault zone."
If they can figure out whether the two fault zones are structurally related, it could sharpen scientists' understanding of what to expect in the future.
"The type and strength and duration of ground-shaking may change with this work," DuRoss said. He thinks that could ultimately affect building codes.
It's expected to take months of lab work before the answers start coming in.
E-mail: hollenhorst@ksl.com
