Some slow-moving faults may help protect some regions of Italy and other parts of the world against
destructive earthquakes, suggests new research from The University of Arizona
in Tucson.
Until now, geologists thought when the crack between two
pieces of the Earth's crust was at a very gentle slope, there was no movement
along that particular fault line.
"This study is the first to show that low-angle normal
faults are definitely active," said Sigrún Hreinsdóttir, UA geosciences
research associate.
Richard A. Bennett, a UA assistant professor of geosciences,
wrote in an e-mail. "We can show that the Alto Tiberina fault beneath Perugia is steadily slipping as we speak--fortunately, for
Perugia,
without producing large earthquakes."
Perugia is the capital city
of Italy's Umbria region.
Creeping slowly is unusual, Bennett said. Most faults stick,
causing strain to build up, and then become unstuck with a big jerk. Big jerks
are big earthquakes.
For decades, researchers have known about the Alto Tiberina
and similar faults and debated whether such features in the Earth's crust were
faults at all, because they didn't seem to produce earthquakes.
Hreinsdóttir and Bennett have now shown that the gently
sloping fault beneath Perugia
is moving steadily at the rate of approximately one-tenth of an inch (2.4 mm) a
year.
Perugia
has not experienced a damaging earthquake in about 2,000 years, Hreinsdóttir
said. Because the fault is actively slipping, it might not be collecting
strain, she said. "To have an earthquake, you have to have strain."
Other towns in the region that lie near steeply sloping
faults, including L'Aquila and Assisi, have experienced large earthquakes
within the last 20 years.
The team published their paper, "Active aseismic creep
on the Alto Tiberina low-angle normal fault, Italy," in the August issue of
Geology. The National Science Foundation funded the research.
In the same issue of Geology, Geoffrey A. Abers terms the UA
team's work "a fascinating new discovery." Abers, of Lamont-Doherty
Earth Observatory of Columbia University in Palisades, N.Y.,
was not involved in the research.
The UA team became interested in the Alto Tiberina fault
because previous research suggested the fault might be moving.
To check on the fault, the UA team measured rock movements
in and around Perugia
using a technique called geodesy.
Geodesy works much like the GPS system in a car.
Geoscientists put GPS units on rocks, Bennett said. Just as the car's GPS uses
global positioning satellites to tell where the car is relative to a desired
destination, the geodesy network can tell where one antenna and its rock are
relative to another antenna.
Taking repeated measurements over time shows whether the
rocks moved relative to one another.
In some cases, the GPS sites are too far apart to attribute
very small movements of the Earth to an individual fault such as the Alto
Tiberina, Hreinsdóttir said. However, the University of Perugia
established a dense network of GPS stations in the region in 2005.
The UA team analyzed data from 19 GPS stations within
approximately a 30-mile (50 km) radius around Perugia. Having such closely spaced stations
and several years of data were key for detecting the fault's tiny motions, she
said.
"This study is one more piece in the puzzle to
understand seismic hazards in the region and can apply to other regions of the
world that have low-angle normal faults," Hreinsdóttir said.
Bennett said there are numerous examples of such faults that
are thought to be inactive, including the western U.S.,
Italy, Greece and Tibet.
He and UA geosciences doctoral candidate Austin Holland are
now investigating similar faults in Arizona.
One such fault, the Catalina Detachment, was involved in the formation of the
Santa Catalina and Rincon Mountains that surround Tucson to the north and the east.
"No large earthquakes are known to have occurred on the
Catalina detachment in historic times, so we don't really know if that fault is
active or not," Bennett said. "Based on the results from the Alto
Tiberina, it's possible the Catalina Detachment fault just slides very slowly
and doesn't produce earthquakes."
The motion would be so slow as to be undetectable until the
most recent technological advances in geodesy, he said. "The technology
has evolved so far that we are now confident we can see little motions."
To better assess the earthquake risk in the Tucson
region, his team is using geodesy throughout southern Arizona to recheck the markers that the
National Geodetic Survey measured in the late 1990s.
"Now we can go out and repeat measurements to see how
the positions have changed in ten years," he said.
Bennett will soon be able to say how fast the Tucson area's mountains
are moving—his team took measurements earlier this year and is analyzing the
data now.
Rick Bennett
Sigrún Hreinsdóttir
UA Dept. of Geosciences
SOURCE: University of Arizona