Study links earthquake faults to slow-moving depths

Nov 08, 2013
Study links earthquake faults to slow-moving depths
Fault scarps like this one in Italy’s central Apennine Mountains have allowed researchers to understand how the lower crust, nine miles below, influences earthquakes. Credit: Joanna Faure Walker, University College London

(Phys.org) —Most earthquakes erupt suddenly from faults near Earth's surface, and the big ones can topple cities. But miles below, rocks heated to the consistency of wax moving over thousands to millions of years may be the driving force behind some of these events. In a new study in the journal Nature Geoscience, scientists link rapid-fire destruction on the surface to the hyper-slow flow of rocks in Earth's lower crust. The study also shows that the weight of overlying mountains causes these deep rocks to strain at a rate that determines the frequency of large earthquakes on overlying faults; the scientists say that this relationship follows simple mathematical laws that were known from lab experiments.

"This tells us that the earthquake-prone faults in the shallow part of the crust are directly rooted into the flowing material at depth," said the study's lead author, Patience Cowie, a geologist at the University of Bergen in Norway.

The study is based on measurements taken from Italy's earthquake-prone Apennine Mountains, where a magnitude 6.3 earthquake in L'Aquila in 2009 killed more than 300 people. The Apennines bear scars from past seismic activity in the form of scarps, marking where land has slipped down on the faults. Working in the central and southern Apennines, Gerald Roberts and Joanna Faure Walker, geologists at University College London, measured how much the scarps had moved, and in which direction, to reconstruct deformation rates over the past 10,000 years.

Their results, presented at a conference in Greece in 2011, caught the eye of study coauthor Christopher Scholz, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory. At each fault, Scholz noticed that elevation appeared closely linked to the deformation rate, suggesting that the gradual flow of deep crust, more than nine miles down, was placing strain on the fault. The researchers hypothesize that stress from the weight of the mountains cause shear zones in the deep crust to deform, determining how often the fault will slip to produce earthquakes. The deformation rate data from the Apennines closely matched that from lab experiments showing that rocks in deep shear zones strain at a rate proportional to the cube of the stress.

In 2009, a magnitude 6.3 earthquake struck the village of L’Aquila, in the Apennines, killing more than 300 people. Credit: Enpasedecentrale/Wikicommons

"The higher the elevation, the faster the faults are sliding," said Scholz. "As long as you're pushing up on the mountains, they grow, but they always want to collapse because of gravitational forces. After the tectonics that's pushing them stops, as in the Apennines, they collapse under their own weight."

A related conclusion is that a doubling of the topographic elevation corresponds to an eight-fold increase in the deformation rate. That means faults located in lower elevation areas that have not ruptured in historical times may still be active but with longer periods of inactivity between earthquakes due to their lower deformation rate.

The study offers an elegant way to understand the nature of deformation in the lower crust and how it influences earthquakes, said Roland Bürgmann, a geophysicist at the University of California at Berkeley who was not involved in the research. "To understand the real world, we need to simplify it, which they do by making a lot of assumptions, but it does look like an intriguing relationship," he said.

It is unclear if the researchers' model can be fully applied to other -prone areas. Other mountain belts like the Himalayas are being deformed by horizontal stresses and are still in the mountain-building phase, while the Apennines are currently collapsing under the action of vertical stresses. Still, the Apennines have allowed researchers to discover how the deep crust behaves which can be applied anywhere, said Scholz.

Explore further: Seismologists puzzle over largest deep earthquake ever recorded

More information: www.nature.com/ngeo/journal/va… t/full/ngeo1991.html

Related Stories

Slow earthquakes: It's all in the rock mechanics

May 20, 2013

(Phys.org) —Earthquakes that last minutes rather than seconds are a relatively recent discovery, according to an international team of seismologists. Researchers have been aware of these slow earthquakes, ...

Researchers replicate supershear earthquakes in the lab

Jun 07, 2013

(Phys.org) —A team of geology researchers working in France has for the first time recreated the conditions in a lab that lead to a phenomenon known as a supershear earthquake. In their paper published ...

Double quake highlights Italy's seismic perils

May 29, 2012

Two killer earthquakes that struck northeastern Italy in nine days have shed light on the brutal but complex seismic forces that grip the Italian peninsula, scientists say.

Recommended for you

Canada to push Arctic claim in Europe

8 hours ago

Canada's top diplomat will discuss the Arctic with his Scandinavian counterparts in Denmark and Norway next week, it was announced Thursday, a trip that will raise suspicions in Russia.

Severe drought is causing the western US to rise

13 hours ago

The severe drought gripping the western United States in recent years is changing the landscape well beyond localized effects of water restrictions and browning lawns. Scientists at Scripps Institution of ...

A NASA satellite double-take at Hurricane Lowell

14 hours ago

Lowell is now a large hurricane in the Eastern Pacific and NASA's Aqua and Terra satellites double-teamed it to provide infrared and radar data to scientists. Lowell strengthened into a hurricane during the ...

User comments : 0