October 1, 2009
New Method To Measure Faults May Help Predict Quakes
Researchers at the Carnegie Institution say that they have discovered a method of measuring and monitoring geological fault lines beneath the Earth's crust"”a development that could significantly enhance scientists' ability to accurately predict earthquakes.
Geologists previously had no means with which to detect and measure the fluctuating strength of faults that are hidden beneath the Earth's surface. Now, however, researchers from the Carnegie Institution's Department of Terrestrial Magnetism have teamed up with experts from Rice University and the University of California, Berkeley and have successfully used a hyper-sensitive seismometer to measure barely perceptible changes in earthquake waves moving through the San Andreas Fault zone in California.
The group's research"”conducted over the last 22 years"”has pointed to a progressive weakening of the fault, corresponding to an increased instance of smaller, more frequent earthquakes along the fault line.
"Fault strength is a fundamental property of seismic zones," explained Taka'aki Taira of the University of California, Berkeley.
"Earthquakes are caused when a fault fails, either because of the build-up of stress or because of a weakening of the fault. Changes in fault strength are much harder to measure than changes in stress, especially for faults deep in the crust. Our result opens up exciting possibilities for monitoring seismic risk and understanding the causes of earthquakes."
The portion of the San Andreas Fault near Parkfield, California"”dubbed the "Earthquake Capitol of the World""”is one of the most-studied fault lines on the planet and has served as a sort of natural laboratory for seismologists since the advent of the field of study.
The researchers observed that their seismographic readings from small earthquakes indicated areas of fluid-filled fractures within the fault zone. Puzzling, however, was the fact that these small areas appeared to move periodically, corresponding to a series of more frequent earthquakes of smaller magnitude"”something the experts recognized as a tell-tale sign of a weakening fault.
"Movement of the fluid in these fractures lubricates the fault zone and thereby weakens the fault," explained Fenglin Niu of Rice University.
"The total displacement of the fluids is only about 10 meters at a depth of about three kilometers, so it takes very sensitive seismometers to detect the changes, such as we have at Parkfield."
Turning to the question of what caused the movement of these fluids, the team stumbled upon their most remarkable observation.
They observed that in at least two separate incidences, the fluid shifts came after there was a disturbance in the fault zone caused by seismic waves from severe but distance earthquakes"”the most significant of which was the 9.1 giant quake that rocked western Sumatra in December 2004, some 5,000 miles away. The researchers believe that the force of these waves rippling below the Earth's crust may have been strong enough to cause the subterranean fluids to flow.
According to Niu, "it is possible that the strength of faults and earthquake risk is affected by seismic events on the other side of the world""”a possibility that could forever change the way seismologists understand and predict earthquakes.
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