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."

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E-mail: bau@desnews.com