The recently exposed fault face at Seven Peaks Resort is an open-air classroom for geology students.
The most spectacular sight is the bare-faced rock that towers above a person standing at the base. Over millions of years the rock wall has been pushed to the surface by the geological forces that built the Wasatch Mountains.The surface, called slickenside, was polished as it rubbed against the block that was forced down. Scratch marks indicate that the movement was vertical, as is the case with dip-slip "normal" faults.
Provo geologist Lehi Hintze said the Wasatch Fault was thought to be a normal fault, but it is still gratifying to find another piece of evidence to confirm what scientists have believed about the fault activity.
Deputy state geologist Bill Lund said the scratch marks are vertical but also show the fault is slipping very slightly to the south. A straight slip would leave a scratch mark 90 degrees to the horizontal. "Most of the lines (on the fault face exposed at Seven Peaks) are at about 80 degrees. Pure dip slip is a straight down slip. An oblique slip is in-between," Lund said.
The San Andreas Fault in California is a strike slip fault, meaning the movement is lateral. The earthquake hazard along the Pacific Coast is greater than here because the fault is the boundary between tectonic plates. Tectonic plates are the gigantic building blocks of the Earth's crust.
The Wasatch Fault is part of the Intermountain Seismic Belt, which is an intraplate fault. Earthquakes are less frequent along an intraplate fault and also tend to be less severe.
Another geological feature visible along the trench dug at Seven Peaks is fault gouge, a soil layer formed by rock pulverized during an earthquake.
The black layer runs down the slope on the south side of the trench. Lund said the soil is an old sea bed deposit and is called Manning Canyon Slate.
Kent Compton, director of Mountain Operations for Seven Peaks, said the geologists that come to look at the fault spend most of their time just north of the huge slickenside. Geologists carefully study a slope of looser deposits, the alluvial soils left from Lake Bonneville.
While the slickenside records ancient history of the Wasatch Fault, the alluvial soils show the recent history.
Some of the soil is lake deposits from Lake Bonneville. Other layers have come from soil falling from the mountain. Lund said if the soil layers could be dated, the interval between earthquakes could be measured.
Trenches dug in three locations in Utah Valley show an earthquake with ground rupture occurring every 2,000 to 2,500 years, said Lund.
He said the Seven Peaks site is less than ideal for dating soils because most of the soil deposits were under Lake Bonneville. The lake washed away organic materials that could otherwise help with carbon dating to establish a time table.
Hintze said that the advantage to the Seven Peaks site is the depth. He said the other trenches went only as deep as 15 feet. Seven Peaks has unburied at least 30 feet, he said, so it goes back twice as many years.