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The Verde Independent | Cottonwood, Arizona

home : latest news May 26, 2016


1/30/2011 12:30:00 PM
Big Chino Fault has a long history of big quakes
Arizona’s Transition Zone is a relatively narrow geologic province located between the basin and range in the south half of the state and the Colorado plateau to the north. Rugged mountains high elevation basins characterize the zone.
Arizona’s Transition Zone is a relatively narrow geologic province located between the basin and range in the south half of the state and the Colorado plateau to the north. Rugged mountains high elevation basins characterize the zone.
The Big Chino Fault located on the margin between Big Black Mesa and the Big Chino Valley, is believed to have been the source of earthquakes in the 7.0 or larger magnitude, the most recent taking place in the last 10,000 years or so
The Big Chino Fault located on the margin between Big Black Mesa and the Big Chino Valley, is believed to have been the source of earthquakes in the 7.0 or larger magnitude, the most recent taking place in the last 10,000 years or so

Steve Ayers
Staff Reporter


BIG CHINO - At five minutes past five p.m. on the afternoon of Feb. 4, 1976, a noise was heard across the Big Chino Valley that some compared to an exploding boiler. To others it sounded like a sonic boom.

Cattle bawled, dogs howled and cats took off running. Patrons of a local market ducked for cover as bottles and cans fell from the shelves.

The owner of a gas station in Chino Valley reported seeing the ground buckle into waves that nearly knocked him over.

Wells across a wide portion of the landscape turned muddy in an instant.

Mirrors broke, glasses fell from shelves and flowerpots came crashing to the ground in many residences of the sparsely populated area. Cracks appeared in cinder block walls, and plaster fell from ceilings.

The shaking was reported as far away as Ash Fork, 35 miles north.

The 1976 earthquake in Chino Valley is the largest recorded in Arizona in the last 50 years. The ground continued to shake for three more days, as aftershocks of 2.5 magnitude and higher continued to rock. Microseismic events continued to occur about every three days for at least the next three years.

The epicenter is believed to have been along the Big Chino Fault, a relatively young and potentially dangerous, 35-mile crack in the earth's crust that runs along the base of Big Black Mesa, on the northern edge of the Big Chino Valley.

The fault is the northern edge of an immense graben, a keystone shaped block of crust. As the graben has dropped over the last few million years, it has filled on top with alluvial material and water, creating the rich deep aquifer that is the source of water for the upper Verde River.

According to Phil Pearthree, research geologist for the Arizona Geological Survey, old riverbeds visible on the landscape demonstrate that the fault has been active in recent geologic times. Some of that activity, he says, has resulted in fairly large earthquakes.

"Several terraces, stair stepped on the sides of hills are indicators that the ground beneath has slipped downward on several occasions in the not too distant past.

"Some of what is seen can be caused by down cutting of the stream, but it looks as though its been helped by some major seismic events," Pearthree says.

Interpreting the terraces is complex, he points out, but by all indications the Big Chino Fault has fallen numerous times, slipping between six and nine feet along the center of the fault during each of the major events.

"It's probably been 10,000 years since one of these major events occurred. The force created by a 35-mile long fault dropping like that calculates to earthquakes in the magnitude of 7.0 to 7.3," Pearthree says. "The 1976 quake measured a 5.1 magnitude. A 6.1 magnitude quake releases 30 times as much energy as a 5.1, and a 7.1 earthquake would release 30 times more than the 6.1. When you get in the seven-magnitude range you are talking a major event that can cause considerable destruction.

Taylor Waste
Related Stories:
• Verde River basin remains geologically active
• Earthquake close to home


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Reader Comments

Posted: Friday, July 13, 2012
Article comment by: jonny rocket

i remember the 1976 quake. i was 19 years old at the time. i was looking out the window and saw a shock wave moving away from me. it was a trip.

Posted: Tuesday, October 25, 2011
Article comment by: Justin Gilchrist

names justin i go to chino high and a 3.6 earthquake hit the chino-prescott area today...

Posted: Tuesday, February 1, 2011
Article comment by: Win Hjalmarson

Enjoyed. The fault with recent (geologically speaking) movement of the valley floor can be seen at the heads of a few alluvial fans. Even an amateur geologist can see the displacement of the earth's surface from a distance.

Posted: Monday, January 31, 2011
Article comment by: The goatherder

That's the end of the story? This thing reads like it's missing a huge piece, like the bottom chunk fell with a falling fault block. Come on. You can do better.



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