Slow Earthquakes Take Weeks To Occur
May 21, 2013

Understanding Why Slow Earthquakes Occur

Lee Rannals for - Your Universe Online

Researchers have been aware of slow earthquakes for only the past decade, but little has been understood about them. However, new tools may help explain what triggers these quakes.

Scientists thought slow earthquakes were related to the type of rock in the fault, believing that clay minerals are important in slip behavior. A team writing in the journal Nature Geoscience performed laboratory experiments using natural samples taken where slow earthquakes occur to gain a little more understanding about the natural event.

"New technology has shown us that faults do not just fail in a sudden earthquake or by stable creep," said Demian M. Saffer, professor of geoscience, Penn State. "We now know that earthquakes with anomalous low frequencies -- slow earthquakes -- and slow slip events that take weeks to occur exist."

He said usually when you shear clay-rich fault rocks in the laboratory in the way rocks are sheared in a fault, as the speed increases, the rocks become stronger and self-arrests the movement.

"Initially the rocks reacted as expected, but these clays got weaker as they slid further. They initially became slightly stronger as the slip rate increased, but then, over the long run, they became weaker," Saffer said.

One of the team's experiments closely matched the velocity at which slow earthquakes occur in nature. They also found that water content in the clays helped influence how the shear occurred.

"From the physics of earthquake nucleation based on the laboratory experiments we would predict the size of the patch of fault that breaks at tens of meters," said Saffer. "The consistent result for the rates of slip and the velocity of slip in the lab are interesting. Lots of things point in the direction for this to be the solution."

Experts worry about slow earthquakes because there is evidence that shows low frequency events can actually trigger larger earthquake events.

In 2011, scientists found that slow earthquake areas and the ultra-slow velocity layers cluster together, and that regions of non-volcanic tremors are adjacent to those clusters. They echoed Saffer and his team's findings by saying that the events could be tied to high levels of water saturation.