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5:17 AM Mon, Jan. 21st

Unearthed: Newly discovered fault sheds more light on Upper Verde Basin

Arizona Geological Survey/Courtesy<br>Phil Pearthree, Joe Cook, Brian Gootee and Jeri Young of the Arizona Geological Survey study the newly discovered Little Chino Fault.

Arizona Geological Survey/Courtesy<br>Phil Pearthree, Joe Cook, Brian Gootee and Jeri Young of the Arizona Geological Survey study the newly discovered Little Chino Fault.

The newly discovered Little Chino Fault along the northeast side of the Town of Chino Valley will help scientists gain a better understanding of the complex geology and hydrology of the Upper Verde River Basin.

An Arizona Geological Survey crew was recently mapping near Chino Valley when research geologist Brian Gootee got lucky and discovered the Little Chino Fault, said Phil Pearthree, chief of the survey's Environmental Geology section.

The discovery "provides new insight into basin formation, faulting and drainage evolution of the Verde River and its tributaries," the Geological Survey's Little Chino Fault report states.

Gootee was looking for a vehicle crossing over Granite Creek when he got permission to use a private road at a sand-and-gravel operation. He saw signs of the LC Fault along a road cut and brought Pearthree in to check it out.

"I was very excited to see it," Pearthree said.

"The fault otherwise would have stayed buried and we wouldn't have recognized it," said Michael Conway, Arizona Geological Survey chief. "Road cuts can turn out to be invaluable things."

Active fault region

The Upper Verde region is one of the more active fault areas in Arizona, although the word "active" in geologic time is a lot different than human time.

The Arizona Geological Survey has long known of the Big Chino Fault, one of the larger faults in the state at 35 miles long, just north of the Little Chino Fault.

"The great length and apparent youthfulness of the (Big Chino) fault make it a very important feature for evaluating the size and frequency of earthquakes, not only in the Chino Valley area but also for the entire transition zone area between the Colorado Plateau and the Sonoran Desert," a survey publication on the fault states.

Quakes on the Big Chino and Little Chino faults might occur every 20,000-30,000 years, Conway said.

"The chances of that actually happening over 100 years is really low," Pearthree added.

"It's probably good to be aware," of the newly discovered fault, but Pearthree wouldn't suggest any special construction measures unless someone was talking about structures that could cause serious problems if they cracked, such as reservoir dams.

Strong quake potential

The Big Chino fault has the potential to produce an earthquake at a magnitude as large as 7, Pearthree said.

The Little Chino fault is smaller than the BC Fault. The report estimates it's about 7 miles long, but it's possible it could produce an earthquake as large as the BC Fault, he said.

A magnitude 7 quake could split the ground.

"It could produce a pretty good shake," Conway said.

For comparison, Conway produced a report about a Feb. 4, 1976, earthquake in the Chino Valley area that measured only 4.9. For every single digit increase, the magnitude increases 10-fold and the energy release increases 30-fold, he noted.

Chino Valley citizens reported a noise like a sonic boom on that winter day 35 years ago.

"Cattle, cats and dogs went howling and bawling away," the report stated. "Patrons of stores ran for cover. The proprietor of a gas station reportedly saw waves on the ground and nearly fell over. Bottles were thrown from shelves ... mirrors broke." At least two residents reported structural damage to a home and garage.

Much smaller quakes have occurred in the Chino Valley area as recently as Dec. 25, 2009, and May 5, 1999.

Region ranks third

According to the Arizona Earthquake Information Center, no subsequent quakes originating in Arizona have been larger than the Chino Valley quake of 1976, although Yuma is a much more dangerous area because it's near California's San Andreas Fault system.

The faults around Flagstaff rank second to Yuma, with this Upper Verde region coming in third, Pearthree estimated.

The Upper Verde faults are highly complex because they are located in a geologic transition zone, Conway said.

The BC and LC faults are probably at least six miles deep and located near important groundwater flow areas such as the Verde's headwater springs and Del Rio Springs, Pearthree said.

Such faults can be conduits for groundwater flow. However, the survey hasn't concluded whether other older faults closest to the springs are active or not, he said.

Earthquakes along faults have been known to cut off spring flows, although that's impossible to predict, he said.

Why map here?

Arizona contains 1,935 quadrangles and the Arizona Geological Survey can map only 6-7 per year with its limited staff, Conway said. Money from the U.S. Geological Survey and Arizona Department of Energy has helped immensely after the survey's budget was cut 45 percent over the last three years.

Most of the survey's mapping takes place in the more populated areas of Phoenix and Tucson.

But the survey's advisory committee recommended work in the Upper Verde Basin partly because it recognized that work could help with ongoing efforts to understand the area's hydrology, Pearthree said.

The mapping can help make a planned Upper Verde River Basin groundwater computer model more accurate, he said.

The faults in this region helped form the Big Chino and Little Chino groundwater basins over a period of 10 million to 15 million years.

Another reason for the work up here is the rapidly growing population, Pearthree said.

This is the first mapping the survey has conducted in this quadrangle since 1965, although the survey mapped the entire length of the Verde River riparian area last year for the ongoing Gila River adjudication process.

The quadrangle maps also help numerous other entities, from mining companies to subdivision developers to the Arizona Department of Transportation, Conway noted.

What's next?

The survey now is trying to gather more information about the size and past earthquake activity of the LC Fault, and see if it is connected with the BC Fault.

"Undoubtedly there are other faults in the area we haven't been able to map," Conway added.

Download the Big Chino Fault Report (11.1 MB)

Download the Little Chino Fault Report (2.4 MB)