. Earth Science News .

Earthquakes generate big heat in super-small areas
by Staff Writers
Providence RI (SPX) Oct 18, 2011

File image courtesy AFP.

Most earthquakes that are seen, heard, and felt around the world are caused by fast slip on faults. While the earthquake rupture itself can travel on a fault as fast as the speed of sound or better, the fault surfaces behind the rupture are sliding against each other at about a meter per second.

But the mechanics that underlie fast slip during earthquakes have eluded scientists, because it's difficult to replicate those conditions in the laboratory.

"We still largely don't understand what is going at earthquake slip speeds," said David Goldsby, a geophysicist at Brown, "because it's difficult to do experiments at these speeds."

Now, in experiments mimicking earthquake slip rates, Goldsby and Brown geophysicist Terry Tullis show that fault surfaces in earthquake zones come into contact only at microscopic points between scattered bumps, called asperities, on the fault. These tiny contacts support all the force across the fault.

The experiments show that when two fault surfaces slide against other at fast slip rates, the asperities may reach temperatures in excess of 2,700 degrees Fahrenheit, lowering their friction, the scientists write in a paper published in Science. The localized, intense heating can occur even while the temperature of the rest of the fault remains largely unaffected, a phenomenon known as flash heating.

"This study could explain a lot of the questions about the mechanics of the San Andreas Fault and other earthquakes," said Tullis, professor emeritus of geological sciences, who has studied earthquakes for more than three decades.

The experiments simulated earthquake speeds of close to half a meter per second. The rock surfaces touched only at the asperities, each with a surface area of less than 10 microns - a tiny fraction of the total surface area.

When the surfaces move against each other at high slip rates, the experiments revealed, heat is generated so quickly at the contacts that temperatures can spike enough to melt most rock types associated with earthquakes.

Yet the intense heat is confined to the contact flashpoints; the temperature of the surrounding rock remained largely unaffected by these microscopic hot spots, maintaining a "room temperature" of around 77 degrees Fahrenheit, the researchers write.

"You're dumping in heat extremely quickly into the contacts at high slip rates, and there's simply no time for the heat to get away, which causes the dramatic spike in temperature and decrease in friction," Goldsby said.

"The friction stays low so long as the slip rate remains fast," said Goldsby, associate professor of geological sciences (research). "As slip slows, the friction immediately increases. It doesn't take a long time for the fault to restrengthen after you weaken it.

"The reason is the population of asperities is short-lived and continually being renewed, and therefore at any given slip rate, the asperities have a temperature and therefore friction appropriate for that slip rate.

"As the slip rate decreases, there is more time for heat to diffuse away from the asperities, and they therefore have lower temperature and higher friction."

Flash heating and other weakening processes that lead to low friction during earthquakes may explain the lack of significant measured heat flows along some active faults like the San Andreas Fault, which might be expected if friction was high on faults during earthquakes.

Flash heating in particular may also explain how faults rupture as "slip pulses," wrinkle-like zones of slip on faults, which would also decrease the amount of heat generated.

If that is the case, then many earthquakes have been misunderstood as high-friction events. "It's a new view with low dynamic friction. How can it be compatible with what we know?" asked Tullis, who chairs the National Earthquake Prediction Evaluation Council, an advisory body for the U.S. Geological Survey.

"Flash heating may explain it," Goldsby replied.

Related Links
Brown University
Bringing Order To A World Of Disasters
When the Earth Quakes
A world of storm and tempest

Get Our Free Newsletters Via Email
Buy Advertising Editorial Enquiries


. Comment on this article via your Facebook, Yahoo, AOL, Hotmail login.

Share this article via these popular social media networks
del.icio.usdel.icio.us DiggDigg RedditReddit GoogleGoogle

Papua New Guinea jolted by 6.7 quake
Sydney (AFP) Oct 14, 2011
The Pacific island nation of Papua New Guinea was jolted by a 6.7-magnitude earthquake Friday, but no tsunami warning was issued. The US Geological Survey said the quake occurred at a depth of 45 kilometres (28 miles), 103 kilometres east of the mountain city of Lae and 326 kilometres north of the capital Port Moresby. Geoscience Australia, which measured the quake at 6.5 magnitude, said ... read more

A team for an emergency

Fukushima city begins decontamination of homes

Gas blast kills 11 miners in north China: Xinhua

Radioactive emissions from Fukushima plant fall: TEPCO

IBM stock sags on revenue target miss

Samsung seeks iPhone sales ban in Japan, Australia

A hidden order unraveled

RIM out to rev up BlackBerry with new apps

Researchers explore plankton's shifting role in deep sea carbon storage

Sea levels will continue to rise for 500 years

US rivers and streams saturated with carbon

War-damaged power cable cuts Tripoli water supply

CryoSat rocking and rolling

US probes mystery disease killing Arctic seals

NASA Continues Critical Survey of Antarctica's Changing Ice

Research shows how life might have survived 'snowball Earth'

Southern Africian farmers using fertilizer trees to improve food security

Chinese man charged in theft of US trade secrets

S Africa to release report on Iraq's oil-for-food

Method of studying roots rarely used in wetlands improves ecosystem research

Earthquakes generate big heat in super-small areas

Russian Ship Finds Tsunami Debris where Scientists Predicted

Central America toll from rains climbs above 90

Wary Bangkok bolsters flood barriers

Kenyan forces advance on strategic Somali rebel bases

Car bomb rocks Mogadishu during Kenyan ministers visit

Kenyan forces hunt militants deep inside Somalia

Planned Tanzanian soda ash plant threatens flamingoes

Children prefer cooperation

Differences in jet lag severity could be rooted in how circadian clock sets itself

100,000-year-old ochre toolkit and workshop discovered in South Africa

Children, not chimps, choose collaboration


The content herein, unless otherwise known to be public domain, are Copyright 1995-2011 - Space Media Network. AFP and UPI Wire Stories are copyright Agence France-Presse and United Press International. ESA Portal Reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. Advertising does not imply endorsement,agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement