Earthquake scientists stressed Thursday that the Wells, Nev., quake was not related to the Wasatch Fault that runs through highly populated areas of Utah and that it was extremely unlikely to trigger any movement of that fault.
"There's nothing that I understand to suggest anything will be happening on the Wasatch Fault," said Kristine Pankow, assistant director of the University of Utah Seismograph Stations.
The system of faults involved in the Nevada event was the Independence Valley Fault Zone, not Utah's Wasatch Fault. But Pankow said it should serve as a wake-up call because "we're in earthquake country, and these events though infrequent happen here." And experts agree that the Wasatch Fault is due for a stronger earthquake.
U. earth scientists drove toward the epicenter of the Wells quake on Thursday, taking portable seismic detectors to record detailed information about the depth, direction, intensity and ground movement of aftershocks.
By 10 a.m. MST, less than three hours after the first strong shaking, there were around 20 aftershocks of magnitude 3.0 and larger recorded by the stations' equipment in Utah, Pankow said.
Throughout Thursday the National Earthquake Information Center in Denver listed several aftershocks smaller than 3.0, although at 4:57 p.m. Utah time, a sizable aftershock was recorded at 4.6 magnitude. Aftershocks were occurring "primarily west of the main shock," according to Susanne Janecke, associate professor of geology at Utah State University.
The center listed the Nevada main earthquake as magnitude 6.0, located 12 miles east of Wells and 151 miles west of Salt Lake City. The break occurred about 5.6 miles beneath the surface at 6:16 a.m. local time (7:16 a.m. in Utah). Residents reported they felt it as far away as Salt Lake City, Ogden, Logan and Delta in Utah, and in more distant locations in Idaho.
Pankow said scientists "want to know where the aftershocks are occurring with a higher precision, and with the current distribution of stations we can't do that." Together with specialists from the University of Nevada at Reno and the U.S. Geological Survey, the Utahns were setting up specialized instruments near the epicenter.
The U. planned to deploy five to 10 monitors, she said. Small aftershocks could continue for weeks or months, according to Pankow.
The Wells event is considered a moderate earthquake. "But we have to keep in mind that damage relates more to the built environment," she said. "So if you have a lot of older buildings or weak construction, a magnitude 6 can do some significant damage."
Janecke concurred that the quake was "definitely not on the Wasatch Fault system. But it occurred on one of the many Basin and Range 'normal' faults, between the Wasatch Front and the Sierra Nevada."
The Basin and Range is a geographical region including the Great Basin and associated mountain ranges. A normal fault, she said, is a scientific term applying to the sort of faults that are present in Utah. It means earthquakes tend to generate more up-and-down ground movement than lateral movement. The Wells quake, however, did have a small lateral slip, she said.
By tracking areas where people were calling in to report they felt the shaking, and gauging how intense it seemed, scientists were able to chart an oblong shape that represents the ground movement. At least 2,400 people had called in.
If the ground was uniformly dense, the intensity lines would be round. But the ground isn't uniform. "The sedimentary basins like Salt Lake Valley tend to amplify the shaking ... and the duration of the shaking is longer," she said.
Utah has experienced a handful of earthquakes at least as large as the Wells event since the pioneers arrived in 1847. "There were magnitude 6's and low 7's," she said.
Between full numbers in the magnitude scale, ground shaking intensity increases tenfold, meaning a magnitude 7 has ground shaking that is 10 times as violent as a magnitude 6.
Wells' quake was not a big one, Janecke said. "In fact, it's at the size where it may not have ruptured all the way to the surface."
Asked if the quake could trigger a movement of the Wasatch Fault, Janecke said, "Oh, tiny, little chance, but it's pretty unlikely."
Huge earthquakes have set off quakes far from the original epicenter. But the Nevada shaking probably was too weak to affect the Wasatch Fault in any dire way, she said.
According to the U.S. Geological Survey's Earthquake Hazards Program, huge quakes are infrequent in the vicinity of Nevada's earthquake.The USGS Web site adds that "a recent geological study to investigate the history of fault movements (in the Independence Valley fault zone) estimated that the most recent large earthquake that caused surface rupture occurred at least 42,000 years ago. ... Smaller earthquakes that didn't rupture the ground surface likely have occurred more frequently on the Independence Valley fault zone."