Subscribe free to our newsletters via your
. Earth Science News .


Subscribe free to our newsletters via your




















SHAKE AND BLOW
Shallow earthquakes and deeper tremors along southern San Andreas fault
by Staff Writers
San Francisco CA (SPX) Jan 28, 2016


Researchers led by McGill University's Rebecca Harrington install temporary seismometer stations near Cholame, California, along the San Andreas Fault. Image courtesy Rebecca Harrington. For a larger version of this image please go here.

Seismologists working along California's San Andreas Fault near Cholame and Parkfield now have a better idea of how and where friction changes along the fault to produce both shallow earthquakes and the deeper earth tremors called low-frequency earthquakes (LFEs).

In their report published online 26 January in the Bulletin of the Seismological Society of America (BSSA), the team led by Rebecca Harrington of McGill University shows that the depth of this boundary between earthquakes and LFEs varies along the San Andreas strike, but roughly follows along a zone deep in the crust where temperatures hover around 350 degrees Celsius.

There is also a somewhat puzzling, five-kilometer wide gap of seismic quiet between the deepest earthquakes and the shallowest LFEs analyzed in the study, the researchers noted. If this gap holds up under further scrutiny, seismologists will study its possible role in transferring seismic stress during a large earthquake, said Harrington.

Slip along the San Andreas occurs through either aseismic creep or earthquakes in the shallow parts of the fault (less than 15 kilometers deep) while slip takes the form of stable sliding in the deeper part of the fault lying 35 kilometers below the surface. The zone between these two regions--where LFEs occur--is called the brittle-ductile transition zone, where increasing heat and pressure cause rocks to become less prone to fracture and more prone to bending and flowing deformation.

"Our study illuminates a possible gap in activity at the top of the transition zone, between the deeper LFEs and shallower earthquakes, which may be important to the transfer of stress into the seismogenic part of the fault," Harrington noted. "We need to better understand how slip evolves across this boundary throughout the seismic cycle."

Cholame may be best known as the site where iconic actor James Dean died in a 1955 car crash, but the Cholame section of the San Andreas Fault is also where the last major Southern California earthquake on the fault may have originated. The 1857 Fort Tejon earthquake measured magnitude 7.8, causing the fault to rupture the earth continuously for over 350 kilometers (225 miles).

Harrington and her colleagues deployed a temporary network of 13 seismic stations near Cholame to look more closely at the relationship between LFEs and earthquakes, since the area has been the site of vigorous LFE and tremor activity. Analyzing data collected by the temporary network and other permanent seismic stations, the scientists were able to precisely locate 34 earthquakes and 34 LFEs that occurred in the area between May 2010 and July 2011.

The depth at which fault slip changes from earthquake to LFE varies along the strike of the fault, they concluded. These variations could be caused by differences in the type of rock, the presence of fluids or other factors that affect the frictional properties of the fault along its strike.

In their analysis, the seismologists also identified two clusters of small earthquakes, one near the 2004 magnitude 6.0 Parkfield earthquake and one near the northern boundary of the Fort Tejon rupture, just south of Cholame. These clusters may represent areas of mixed frictional properties along the fault, adding support to the idea that earthquakes may originate in these types of frictional environments.

As for the mysterious gap separating earthquakes and LFEs, Harrington said it's puzzling why there wouldn't be more of a gradual transition between the two types of events as the fault deepens. One possibility, she says, is that the slip is aseismic in this zone. It might also be that the gap is accumulating strain, and won't show signs of seismic slip until the strain rates are higher.

Harrington and her colleagues are working on further studies that could help detect more different types of LFEs, and to determine whether the gap is a real one, she said.

.


Related Links
Seismological Society of America
Bringing Order To A World Of Disasters
When the Earth Quakes
A world of storm and tempest






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

Previous Report
SHAKE AND BLOW
Alaska hit by 6.8-magnitude earthquake: USGS
Washington (AFP) Jan 24, 2016
A 6.8 magnitude earthquake struck southern Alaska early Sunday, US seismologists said. The US Geological Survey, updating its earlier location for the quake, said it struck at 1030 GMT and was centered about 83 kilometers (51 miles) east of Old Iliamna. The USGS, which initially had assessed the magnitude of the quake as 7.1, estimated that it had a depth of 127 kilometers (79 miles). ... read more


SHAKE AND BLOW
Ten El Faro families settle with owners of sunken US ship

China pushes inferno documentary into purgatory

Charities warn of 'desperate' plight of refugees in snow

Nepal quake rebuilding to take years, new chief says

SHAKE AND BLOW
Research reveals mechanism for direct synthesis of hydrogen peroxide

Chanel swaps bling for eco-inspired haute couture

Beetle-inspired discovery could reduce frost's costly sting

Material may offer cheaper alternative to smart windows

SHAKE AND BLOW
Climate change: Ocean warming underestimated

Pressure building on global water supply

MH370 hunt loses hi-tech sonar probe to undersea volcano

Natural oil seeps encourage microbial life in Gulf of Mexico

SHAKE AND BLOW
New gravity dataset will help unveil the Antarctic continent

Melting Greenland ice sheet may affect global ocean circulation, future climate

Mounting evidence suggests early agriculture staved off global cooling

Ancient underwater volcanoes may have ended 'Snowball Earth'

SHAKE AND BLOW
Developing countries bear brunt of nitrogen pollution: study

Soybean has greater energy value when fed to pigs than previously known

Fatty acids from GM oilseed crops could replace fish oil

Weed blasting offers new control method for organic farmers

SHAKE AND BLOW
Alaska hit by 6.8-magnitude earthquake: USGS

Warmer Oceans Could Produce More Powerful Superstorms

More than 1,200 flee as Indonesia volcano spews ash, gas

Kobe marks 21 years since killer quake

SHAKE AND BLOW
Burkina arrests 11 failed coup soldiers after arms depot raid

Horn of Africa port Djibouti signs China trade deals

UN reduces size of peacekeeping force in Ivory Coast

Several dead as Shebab storm African Union base in Somalia

SHAKE AND BLOW
Chinese scientists create 'autistic' monkeys

The indications of a new geological epoch marked by human impact are clear

Why are habits so hard to break

Evidence of a prehistoric massacre extends the history of warfare




Memory Foam Mattress Review
Newsletters :: SpaceDaily :: SpaceWar :: TerraDaily :: Energy Daily
XML Feeds :: Space News :: Earth News :: War News :: Solar Energy News








The content herein, unless otherwise known to be public domain, are Copyright 1995-2016 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news 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 All images and articles appearing on Space Media Network have been edited or digitally altered in some way. Any requests to remove copyright material will be acted upon in a timely and appropriate manner. Any attempt to extort money from Space Media Network will be ignored and reported to Australian Law Enforcement Agencies as a potential case of financial fraud involving the use of a telephonic carriage device or postal service.