Arizona is on shaky ground.
The more geologists look for tremors in the Grand Canyon State, the more they find. In the past few months, the Arizona Geological Survey has recorded more than 60 small earthquakes in the northwestern corner of the state.
More recently, a single seismometer in Greenlee County has recorded 300 small tremors that appear to be centered near the Arizona town of Duncan at the New Mexico state line.
A similar phenomenon revealed itself in the White Mountains about 10 years ago when a traveling array of seismometers caught another swarm in an area that was not supposed to be geologically active, said Mike Conway, chief of geologic extension for the Arizona Geological Survey.
Geologists believe it is all a part of the settling of the southern part of the Basin and Range Province, which has been stretching like taffy for 25 million years.
Periodically, it seems, most of the state is shaky. Our Sky Island Mountains are still rising and our basins still sinking, adding about 3,000 square feet of surface to Arizona each year, a recent study by University of Arizona geoscientists found.
The recent swarms of earthquakes may be a new phenomenon or simply the result of increased monitoring.
“We may be seeing things that have been going on all the time,” said Arizona state geologist Lee Allison.
Cracks and crashes
Many of the quakes being recorded would not be picked up on national networks of detectors run by the U.S. Geological Survey.
They do register, however, on eight stations run by the Arizona Geological Survey for the past decade and they show up when portable stations are brought in to record aftershocks following a larger quake.
That’s what happened after a magnitude 5.3 earthquake just south of Duncan on June 28, 2014.
That earthquake, which caused some minor damage, was felt in Tucson. The Arizona Geological Survey originally moved in six portable seismometers and recorded a number of aftershocks.
One, which was left in place and monitored by geologist Jeri Young, has picked up the 300 recent earthquakes, all less than 3.0 in magnitude and many under 2.0.
That kind of shaking is negligible, said geologist Phil Pearthree, but “people in the Duncan area do feel these.”
Larger earthquakes have occurred in Arizona, he said. The Flagstaff area, which has several known faults, recorded three earthquakes greater than 6.0 in the early 20th century, he said.
An earthquake with an estimated 7.2 magnitude shook Arizona in 1887. It was centered in Mexico, 40 miles south of Douglas, and it was felt in Phoenix and Albuquerque.
In the Tucson area, boulders crashed down nearby mountains and started brush fires. The mountains were shrouded in dust and smoke for days. The quake caused cracking in the bell tower of Mission San Xavier. Some springs vanished, others appeared.
Plenty of faults
Geologists can’t predict earthquakes, said Pearthree, but they can tell you whether an area is capable of producing them. “I wouldn’t say the probability of having a large earthquake is high here, but they do occur,” he said.
The fault lines beneath Arizona soil are quite capable of producing earthquakes in the 6.0 to 7.0 range, he said.
Pearthree’s favorite fault is just south of Tucson, along the edge of the Santa Rita Mountains, extending as far as the development at Corona de Tucson, just south of Tucson.
Pearthree mapped the escarpment — a miles-long ledge created when the basin dropped relative to the Santa Rita Mountains — as part of his graduate studies.
It showed evidence of a large earthquake, possibly within the last 50,000 years — a short span of geologic time.
Allison has been documenting the swarm of small earthquakes along the borders of Arizona, Nevada and Utah for weeks on his Arizona Geology blog.
Seismometers in Arizona, Nevada and Utah have recorded more than 60 earthquakes in the area since March 28, with the largest being a 3.8 magnitude quake on May 6. There are known faults in the area at the southern end of the intermountain seismic belt, Allison said.
David Brumbaugh, a geophysicist at Northern Arizona University and director of the Arizona Earthquake Information Network, said the swarms in the northwest portion of the state are a new phenomenon, but not surprising. “That’s the first swarm, but not the first earthquake activity,” he said.
Likewise, the Duncan area has been active in the past, he said.
He doesn’t think they are harbingers of big quakes to come, but said, “I don’t think you can rule anything out.”