Now geologists will analyze the rock, dirt and patterns to try to predict when northern Weber County might again feel a major temblor.

"The past is key to the future," said Chris DuRoss, a Utah Geological Survey geologist who gave tours of the trench Tuesday.

The UGS and the U.S. Geological Survey have spent years studying various sections of the 220-mile Wasatch Fault, one of the nation's more active.

Last year, they trenched near Nephi and, this year, they are learning about the Weber section, which is in the middle of the fault that stretches from southern Idaho to central Utah along the west base of the Wasatch Range.

As hillsides along the Wasatch Front increasingly sprout more homes, geologists are fast losing opportunities for such digs.

They selected the North Ogden site because it clearly shows the result of past earthquakes: a once-gentle slope formed by alluvial mud flows or debris deposits that is now marked by several successive drops, or hills.

Evidence from the hillside trench, roughly 250 feet long by 30 feet wide by 14 feet deep, is consistent with what geologists have found in the Salt Lake City and Provo sections of the fault, DuRoss said.

By examining data from Brigham City to Nephi, geologists hope to piece together the fault's history.

Once geologists have dated the samples from the North Ogden trench and others along the fault, they expect to see patterns that will help them predict when future earthquakes will strike. They also will be better able to assess how much the ground might shake or whether homes, oil, gas and water lines built on or near the fault would be damaged.

"The impact of an earthquake would be large," DuRoss said.

Previous geologic studies, from the 1980s, indicate that the latest major quake in the Weber area tosok place about 500 years ago. The new research is expected to help determine if that is true.

Anthony Crone, a USGS geologist, cautioned against assuming earthquakes can ever be predicted with perfect accuracy. They're like teenagers, he said, "unpredictable."

Given that the Wasatch Fault slips by one or two millimeters a year on average, future earthquakes are inevitable, Crone said.

"It's an active fault."

kmoulton@sltrib.com