Even bigger earthquake possible when Salt Lake's 'Big One' hits

New research suggests a seismic connection ties the Salt Lake portion of the Wasatch Fault with the parallel West Valley fault zone about six miles to the west. The two frame the Salt Lake Valley and a potential, simultaneous rupture could make Salt Lake's "Big One" a little bigger.

Chris DuRoss and Michael Hylland of the Utah Geological Survey reported these findings at the Seismological Society of America meeting Wednesday in Salt Lake City.

"We feel pretty strongly that these faults rupture co-seismically in big earthquakes with the Salt Lake fault being the driver," DuRoss told fellow earthquake researchers gathered at the Calvin L. Rampton Salt Palace Convention Center.

Utah seismologists have long suspected the two faults are connected. Now, DuRoss and Hylland have gleaned the data from trenches to back the idea up, according to Kristine Pankow of the University of Utah Seismograph Stations.

"It could affect the seismic hazard. It's tricky because earthquakes on these faults may be 100 years a part, 10 days apart, or the same day," she said.

The paleoseismic record Hylland and DuRoss built indicate the faults produced large quakes that ruptured the surface about 1,400, 2,200 and 5,500 years ago.

Much attention at this week's conference is directed at the Wasatch Fault, the 200-mile fault running along Wasatch foothills, marking the boundary between the Great Basin, falling away to the west, and the more geologically stable Rocky Mountain region to the east.

"We care about this fault because 80 percent of Utah's residents are concentrated along the Wasatch Front," DuRoss said. It is also the state's most active fault, generating many small and moderate quakes since the arrival of Mormon settlers. It is due for a major event, particularly on the Salt Lake section. Utah's only recorded quake greater than magnitude 6.5, the kind that opens the Earth's surface, roared through the Hansel Valley in 1934.

The Salt Lake section is the most active of the Wasatch Fault's 10 segments. Scientists believe it dives into the Earth in a western direction and the West Valley fault truncates into it about five miles beneath the Salt Lake Valley.

"Now that we know they are linked the next question is does [a big earthquake on the West Valley faulty] occur within a minute or does it occur as an aftershock?" DuRoss said.

In 2010, the Utah Geological Survey dug a new research trench across the Wasatch Fault at Penrose Drive north of the U. campus and three more across the West Valley fault at Baileys Lake seven miles to the west. The West Valley trenches clearly show deformation from ancient earthquakes.

The geologists used radiocarbon and optically stimulated luminescences to roughly determined when these faults broke.

DuRoss and Hylland identified eight big quakes in the past 12,000 years on the Salt Lake segment and six at Baileys Lake. Dating showed that all but five of the West Valley quakes occurred around the same time as a big event across the valley. The sixth occurred around 14,000 years ago, at a point in the record that is not complete, so it is possible that it also ruptured with a Salt Lake quake, according to the research.