Originally Published: October 25, 2011 9:59 p.m.
Soon after an earthquake struck northern Yavapai County at 11:20 a.m. Tuesday, the U.S. Geological Survey initially reported that it was a 3.6 magnitude earthquake and its epicenter sat just northwest of Chino Valley.
But by late afternoon, the Arizona Earthquake Information Center revised the strength and location of the tremor.
A preliminary USGS map showed the tremor centered close to the far east end of Dillon Wash Road, which originates at the Crossroads Ranch subdivision just east of Williamson Valley Road. That road dead ends just northwest of West Road 4 North in Chino Valley.
The USGS has only one earthquake-measuring device in Arizona, while the Arizona Earthquake Information Center has a dozen, enabling its estimate to be more accurate.
By late afternoon, the Arizona Earthquake Information Center had calculated that the quake measured 3.26 on the Richter scale and was centered about 10 miles to the northeast of the USGS estimate. It shows an epicenter along Prescott National Forest Road 9005N a few miles north of East Verde Ranch Road in the Paulden area.
What was likely a pre-tremor hit almost the same exact spot about 4 a.m. Sunday, Arizona Earthquake Information center Director David Brumbaugh said. It registered only a 2.1 magnitude.
Although at least two faults are located in the Chino Valley and Paulden area, Brumbaugh said it's highly unlikely a damaging earthquake will occur there during our lifetimes.
The Big Chino fault is located in the Paulden area, while the Little Chino fault is located along the east side of Chino Valley. It's possible that Tuesday's earthquake came from an undiscovered fault, Brumbaugh said. It sometimes takes an earthquake to discover them.
The Arizona Geological Survey only recently discovered the Little Chino fault during a routine survey because it was visible in a road cut.
The Arizona Geological Survey has two new videos out about the Big Chino and Little Chino faults. See them online at azgs.az.gov.
"We know there are active faults in Arizona, but we don't know how often they occur and what kind of risk they pose," Arizona Geological Survey scientist Brian Gootee says in the Little Chino fault video.
A quake with a magnitude of 6 to 6.5 likely occurred about 10,000 to 15,000 years ago on the Little Chino fault, judging by the way the ground shifted, he said.
About five to 10 large quakes over the past 200,000 years helped create the Big Chino Basin, Arizona Geological Survey scientist Phil Pearthree said in the Big Chino fault video.
The largest quake in that region over the last 50 years was a 5.0-magnitude quake in the Chino Valley area on Feb. 4, 1976, he said.
Standing on the courthouse plaza at the end of his video, Pearthree noted that a large quake could cause substantial damage to the old buildings in downtown Prescott.
Brumbaugh wasn't surprised that people reported hearing something like a sonic boom Tuesday when the earthquake occurred.
"Earthquake sounds are not unusual," he said. "Basically, it's just a shock wave popping out into the air and becoming an acoustic wave."
Brumbaugh asks people who felt or heard Tuesday's earthquake to email him with details at firstname.lastname@example.org with information. He hopes they will fill out a short form he sends back. People also can write to him at Arizona Earthquake Information Center, Northern Arizona University, P.O. Box 4099, Flagstaff, AZ 86011.