UA-Based AZGS Is Making Noise About Earthquakes
Other than the Sonora earthquake of 1887, little is known about Arizona's seismic history. But researchers at the UA-based Arizona Geological Survey have some new tools to look into the future.

By Robin Tricoles, University Communications
Oct. 17, 2016

Bavispe church.jpg

The ruins of a church in Bavispe, Mexico, after the earthquake of 1887 in Sonora, Mexico
The ruins of a church in Bavispe, Mexico, after the earthquake of 1887 in Sonora, Mexico (Photo: Arizona Historical Society)

A low rumble rose from the Teras Mountain region in Sonora, Mexico, like that of "heavy ore teams in passing," wrote a witness. Seconds ticked by, and the rumble turned to noise, "like a continuous roll of heavy firing, with occasional short peals like a sharp clap of thunder." Water spurted from newly formed fissures in old streambeds. Boulders rolled down the mountainsides, leaving clouds of dust in their wake. Fires erupted. Railroad tracks were bent and broken.

Such was the Sonora earthquake of May 3, 1887, according to witness George E. Goodfellow, a physician and onetime Tucson resident. Reporting from Tombstone, Arizona, Goodfellow's initial account of the quake appeared in a letter to the editor in the May 20, 1887, issue of Science.

Goodfellow went on to say that the quake destroyed the town of Bavispe, Sonora, Mexico, killing and injuring dozens. In essence, the quake had ruptured the Earth's crust, creating telltale fault scarps, as powerful earthquakes are wont to do.

But precisely which seismic phenomena occurred before, during and after the quake is in question. After all, the area's population was sparse, and the era of seismological instrumentation was not yet underway.

In fact, relatively little is known about Arizona's seismic past — but researchers from the Arizona Geological Survey are about to shake that up.  

The researchers want to capture, record and analyze seismic data in and around Arizona using new and existing seismometers, which measure ground motion. Doing so will help researchers determine if the area's seismic activity is frequent enough to warrant concern about powerful earthquakes in the offing, which could affect people as well as infrastructure.

"The goal is to monitor the areas and try to figure out where the active faults are, the seismicity that's occurring and how much ground shaking occurs when we do have an event," says Jeri Y. Ben-Horin, research geologist with the AZGS, now based at the University of Arizona. "It takes many decades to figure out what the seismicity looks like for a particular state or region."

However, scientists do know that some parts of Arizona are at higher risk for earthquakes than others, with Yuma being one of them, followed by the Flagstaff-Prescott area, says Phil Pearthree, AZGS interim director. In fact, the Flagstaff-Prescott area has experienced many moderate earthquakes with magnitudes up to 6.2.

By comparison, the earthquake that shook the Teras Mountains southeast of Douglas, Arizona, had an estimated magnitude of 7.6. Evidence of the quake's power can be seen in a 60-mile-long rupture, which displaced the Earth's crust by as much 20 to 25 feet in some places, Pearthree says.

"You could go there today, and it would be very obvious," Pearthree says. "There's still a fresh scarp where the rupture occurred, and thousands, tens of thousands of years later, you'd be able to see where it occurred if you knew where to look."

Like a considerable portion of the Southwest, Tucson is sitting in an area known as the basin and range, says geoscientist Susan Beck, UA professor of global seismology and tectonics. It's "basically a chunk of crust," which is about 35 kilometers thick, she says.

"So, we see mountain ranges, and we see basins and valleys," Beck says. "We've got the Catalina Mountains next to us, and we've got the Tucson Mountains, and they used to be together, and over geologic time they've pulled apart. And if you look at the whole Western U.S. from Salt Lake City to the Sierra Nevada Mountains, it's pulling apart in an east-west sense. "

Although scientists are unable to predict earthquakes, paying attention to their patterns and behavior — especially that of smaller ones — can tell scientists about the likelihood that larger quakes are imminent.

Until recently, though, there weren't enough seismometers to detect smaller quakes throughout Arizona.

"The idea is to be able to record earthquakes systematically at least down to magnitude 3," Pearthree says. "That gives us an idea of how frequently earthquakes are occurring in different parts of the state, and you can learn something about how frequently larger ones may occur by how many smaller ones occur."

Now, thanks to a grant from the Federal Emergency Management Agency, or FEMA, and a nationwide project known as the Transportable Array, Arizona acquired eight new seismometers. The Transportable Array, run by various universities and research institutions, is a network of 400 broadband seismometers and atmospheric sensors that operate from a grid of temporary sites that moves from west to east across the United States. Along the way, states had the option of acquiring them.

"They gridded the western half of the United States with the seismometers and then marched them across the country every two to three years," Ben-Horin says. "They just finished the eastern half of the country."

With FEMA's assistance, Arizona was able to acquire eight of the seismometers, which are now known as the Arizona Broadband Seismic Network. "It was the first time we had modern instruments, and they cover a good bit of the state that never had coverage before," says Ben-Horin, who is leading the effort to expand the network and put more seismometers in play.

"The new stations are filling some really big spatial gaps," Ben-Horin says. "If you don't have enough stations, the errors associated with location calculation can sometimes be too big."

By using several seismometers throughout an area, researchers can look at the arrival times of shock waves at specific sensors and triangulate the location of the earthquake. In fact, the more instruments in play, the more likely researchers can detect smaller quakes, some as low as a magnitude of 1, says Ben-Horin, whose job it is to monitor the sensors' health and well-being, remotely and in person.

"The seismometers are very sensitive, and they are very low voltage," Ben-Horin says. "Their equipment is very particular, and it has a lot of special needs."

For example, Ben-Horin determines whether their GPS clocks are running correctly, their bilge pumps are working, whether they are successfully reporting signals to a central database, and whether they have suffered weather- or heat-related damage.

In addition to listening to their seismometers and recording what they have to say, Ben-Horin and Pearthree also look to the land to track the Earth's movement.

"We look at how landforms are altered by earthquakes," Ben-Horin says. "When they break the surface, they make stairsteps. Those quakes must be 6.5 or larger to rupture the surface. You need a quake that's big enough to scarp the land surface. A really good tool for finding the scarp is Google Earth. We used to use air photos, which would take a long time to get. But now we can work with different aged photos that Google has."

In essence, Ben-Horin, Pearthree and their colleagues are marrying what they know about the faults and seismicity to characterize the region using multiple tools.

But even data gleaned from just the seismometers is a treasure trove of information that can be analyzed to study deep-Earth and crustal structures such as rocks and faults, or how the Earth's magma interacts with other geological structures.

In fact, Ben-Horin says, there are many data points, such as the depth of a quake or its direction, which influence how the ground shakes beneath our feet and beneath critical infrastructure such as hospitals, schools, power plants and dams.

"There's a lot of neat research that can be accomplished by having these seismometers and by just listening and recording earthquakes," Ben-Horin says.

Ben-Horin credits the late AZGS director Lee Allison for making the growing network of seismometers possible.

"He was a real visionary and had a real sense of the importance of capturing data," she says.

"Sometimes you're collecting data and have a sense that you're getting something done, but you don't have a full sense of what it can be used for, but you know it's important. And Lee had a lot of faith in that. He had a vision for sharing data, collecting it, getting science done in a big way. This research will go on for decades."

Extra info


Great Arizona ShakeOut


10:20 a.m. Thursday, Oct. 20

The 2016 Great Arizona ShakeOut will take place on Oct. 20. The shakeout is when millions of people worldwide will practice how to reduce their chances of injury during an earthquake by using a safety drill known as "Drop, Cover and Hold On."

Emergency preparedness experts recommend that, in the event of an earthquake, you should drop to your hands and knees, cover your head and neck with one arm and hand, and hold on until the shaking stops. To participate in the shakeout, register here.