Archive for February, 2010

Chilean Quake

Even now, a day after the huge 8.8 magnitude quake rocked Chile, the USGS is still monitoring aftershocks upwards of 5.5 on the Richter Scale. The earthquake was caused by a thrust of the Nazca Plate as it subducted under the South American Plate. As in 1960 when Chile was decimated by a 9.5 moment magnitude earthquake, strain and deformation buildup provoked by the subduction of the Nazca Plate finally ruptured, allowing the Nazca Plate to move under the South American Plate. This 8.8 magnitude quake lies 870 km South of the 8.5 magnitude earthquake that shook Chile in 1922, and about 230 km North of the 1960 earthquake(http://earthquake.usgs.gov/earthquakes/eqinthenews/2010/us2010tfan/#summary, 2010).

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Low-Angle Normal Faults

New evidence has shown that some faults that should not slip are weaker than scientists thought.  Studies have shown that faults that dip less than 45 degrees should not slip because it is easier to form a new fault than to slip.  New studies have shown that these faults do slip and they may act more like weak faults.  The fabric in the rocks that are in the faults had too much friction for the faults to slide easily.  Scientists tested the material found in the fault to find something close to what it occurs naturally.  They did this by taking the rocks in the fault and grinding it into a powder substance, which is then tested in a machine that applies shear forces to the powder, and it measures the amount of force it would take to move the sides of the fault.  The movements show that low angle faults contain too much friction to move, however they do move.  The scientists conducted another study  from a fault in Elba where they took the powder and produced frictional forces to prevent slipping.   They made small squares from the rock an dfound that material that was prepared in this way were weak and sheared in one direction, which resulted in low friction, and this was because it contained clays that allowed the material to slide.  This causes the faults to move a little, which could cause small earthquakes.

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New Zealand is a fascinating place. If you get the chance rent a movie called “Black Sheep ”  Its a cheesy New Zealand movie about sheep killing people, but the scenery in the background is amazing.

This article discusses the large active thrust fault . The shallow zone of this thrust lies near Hikurangi subduction zone near the North Island. This is said to be an excellent place to image active fault processes.  The subduction thrust has a shallow dip,  facilitating  the  delineation  of  spatial  variations  in  fault  zone  properties  using  surface  measurements.

    The tomography results suggest that geological structure in the overlying plate has a major affect on  how  fluid  escapes  from  the  subducted  plate.

You can find this article at http://www.see.leeds.ac.uk/peachandhorne/posters/Reyners.pdf

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The faint tug of the sun and moon on the San Andreas Fault stimulates tremors deep underground, suggesting that the rock 15 miles below is lubricated with highly pressurized water that allows the rock to slip with little effort, according to a new study by University of California, Berkeley, seismologists.

“Tremors seem to be extremely sensitive to minute stress changes,” said Roland Bürgmann, UC Berkeley professor of earth and planetary science. “Seismic waves from the other side of the planet triggered tremors on the Cascadia subduction zone off the coast of Washington state after the Sumatra earthquake last year, while the Denali earthquake in 2002 triggered tremors on a number of faults in California. Now we also see that tides – the daily lunar and solar tides – very strongly modulate tremors.”

Map of the San Andreas Fault (Red Line) - copyright by David Lynch (click to enlarge).

Republished from a December, 2009 press release by University of California, Berkeley


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Yes, it is 10 o’clock on a Saturday night, and I am blogging.  Mainly because it just occured to me that I am two hours away from the weekly deadline for structural.  While searching for a story to talk about, I came across this one which was written over a year ago, however I still find it pretty neat.  Scientists have created a way to moniter the strength of faults, which could lead in earthquake prediction, and showing if a particular fault will produce an earthquake or not.  For this study the scientists used seismometers that were high in sensitivity and they detected small changes in earthquake waves in the San Andreas Fault zone within the past 20 years.  The fault strength is helpful because earthquakes occur when a fault fails due to stress build-up or due to the fault weakening.  Within the San Andreas Fault zone they found fractures that were fluid filled and the areas shifted from time to time, and during this time the earthquakes became smaller and more frequent which indicated a weakened fault, and the fluid causes the fault to weaken.  The shift was caused by pressure from seismic waves from large, distant earthquakes, which caused the fluids to flow.  The scientists concluded that the strength of faults may be affected by siesmic events that occur on the other side of the planet.

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Although not really pertaining to geologic aspects, I came across this story that was published on Feburay 5, 2010 naturally i thought it was pretty cool. “Five crates of Scotch whisky and two of brandy have been recovered by a team restoring an Antarctic hut used more than 100 years ago by famed polar explorer Ernest Shackleton.”

Explorers’ century-old whisky found in Antartic

The Associated Press

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As the 142th Anniversary of the last big earthquake on the Hayward Fault approaches, new U.S. Geological Survey studies provide mounting evidence that the Bay Area should get ready for another big quake soon.

The Hayward Fault has ruptured about every 140 years for its previous five large earthquakes. October 21, 2008, marked the 140th Anniversary of the 1868 approximate Magnitude 7 earthquake. Two and half million people now live along the Hayward Fault and seven million people in the region would feel a repeat event of the same magnitude.

This large earthquake caused extensive damage to the San Francisco Bay Area and remains the nations 12th most lethal earthquake, resulting in about 30 fatalities.  The shaking from this earthquake was the strongest that the the new towns and growing cities of the Bay Area had ever experienced, producing devastating effects on brick buildings and walls and cracking buildings as far away as Napa, Santa Rosa, and Hollister.  Analysis of triangulation data suggest that the fault moved as far north as Berkeley with an average inferred slip rate of 1.9 m.  The average interval between the 10 earthquakes before 1868 is 170 years, with the last five earthquakes having an average interval of only 140 years.  The population at risk from a large Hayward fault earthquake is now 100 times greater than in 1868 and the infrastructure in the San Francisco Bay Area has been tested only by the relatively remote 1989 magnitude 6.9 Loma Prieta earthquake.

Tom Brocher


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