3.9 Quake Shakes Up Rim Area

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If the earth moved for you early Sunday morning, it wasn't your imagination. According to the Arizona Earthquake Information Center (AEIC) there was a magnitude 3.9 earthquake in Arizona at 4:37 a.m. Sunday.

The epicenter was about 40 miles north-northeast of Payson, 30 miles from Pine-Strawberry. Sunday's quake was centered 5 kilometers (3.1 miles) beneath the surface in the Chevelon Butte Area near Blue Ridge Reservoir. A 3.8 magnitude quake occurred in the same region at 3:37 p.m. Friday; and an aftershock with a magnitude of 2.4 was recorded at 6:38 p.m. Friday. That aftershock was of such a minute magnitude, the U.S. Geological Survey categorized it a "micro earthquake."

According to Dr. David Brumbaugh of the AEIC office in Flagstaff, a similar series of small quakes shook the area from March through July of 1989. The largest temblor in that cluster was 3.0.

Sunday's shallow quake, only about 3 miles in depth, is quite typical for a continental tremor.

Brumbaugh said landlocked earthquakes rarely exceed 15 miles in depth.

In comparison, the massive 9.0 quake near Indonesia on Dec. 26, 2004 originated more than 18 miles underground. That temblor spawned the deadly tsunami responsible for nearly a quarter of a million fatalities across 11 countries.

Brumbaugh added that while it's impossible to be certain, he suspects that Sunday morning's event near Payson was "the main event," and there probably won't be any further temblors in that area for a while. He also revised the quake's magnitude down from the U.S. Geological Survey's report of 4.0, saying that the seismographic evidence showed a reading of 3.9 on the Richter scale.

Each tenth of a point increase on the Richter scale represents a factor of 10 in the strength of a quake.

Earthquakes occur over fault lines -- points in the earth's surface where two tectonic plates meet and move against one another. Brumbaugh explained, in this instance, the region surrounding the quake is riddled with volcanic residue, a fact that makes it virtually impossible to distinguish a clear, single fault line.

The remote area doesn't reveal any visible surface breakage, nor would any be expected in such low-magnitude tremors.

He said there may, in fact, be several faults cutting through the terrain.

While Arizona is seldom thought of as "earthquake country," the National Earthquake Hazard Reduction Program has classified the state as "high risk," its second most serious category.

The U.S. Geological Survey says the largest recorded quake in Arizona history was a 5.6 magnitude temblor in 1959, centered at the far northern border near the community of Fredonia.

Brumbaugh said that his office maintains more extensive records than the federal agency, and AEIS has documented three temblors with a magnitude of 6.2, all north of Flagstaff -- in 1906, 1910 and 1912.

The state has a long history of geological upheaval. The Native American community of San Carlos, located some 25 miles east of Globe, is built on a field of basalt, a mineral composite created by volcanic flow. Deposits of the semi-precious gemstone peridot, mined on the San Carlos Apache Reservation, are the result of its volcanic past.

North of Flagstaff, Sunset Crater erupted about 1,000 years ago, around 1064 A.D.

Sunset is a cinder cone, formed of rock and dust spewed in that eruption.

To the west of Sunset Crater, the San Francisco Peaks are believed to have once soared to nearly 16,000 feet, substantially higher than their current elevation of 12,633 feet.

Humphreys Peak, the highest point of the San Francisco Peaks, is a stratovolcano, formed by alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs (superheated rocks thrown up during eruption.)

Humphreys Peak is in the same category as Mount St. Helens, its last eruption produced a violent pyroclastic blast that took an estimated 3,500 feet off its height.

The Superstition Mountains, east of Phoenix, also have volcanic origins, though they were last active somewhere between 17 and 25 million years ago.

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