Slow moving faults don't cause earthquakes

TUCSON, Sept. 3 (UPI) -- U.S. researchers say they've determined some slow-moving faults may help protect some regions of Italy and other parts of the world from earthquakes.

University of Arizona postdoctoral researcher Sigrun Hreinsdottir said until now, geologists thought when a 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," Hreinsdottir said.

Assistant Professor Richard Bennett, who led the study, said scientists can now "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 that translates into a big earthquake.

Hreinsdoottir and Bennett say they have shown the gently sloping fault beneath Perugia is moving steadily at the rate of approximately one-tenth of an inch a year.

They said Perugia has not experienced a damaging earthquake in about 2,000 years because the fault is actively slipping and might not be collecting strain.

"To have an earthquake, you have to have strain," Hreinsdoottir said.

The research appears in the August issue of the journal Geology.