April 4, 2013
The Earth Is Lazy When Developing Fault Lines, Say Researchers
April Flowers for redOrbit.com - Your Universe Online
University of Massachusetts Amherst geoscientist Michele Cooke and her colleagues have taken an uncommon approach to modeling the development of fault lines in the Earth´s crust, and their so-called "Earth is lazy" approach is providing new insights into how these faults grow. The team studies irregularities along strike-slip faults, which are the active zones where plates slip past each other such as at the San Andreas Fault of southern California.Among geologists there has been a great deal of uncertainty about the factors that govern how new faults grow in regions where one plate slides past or over another around a bend. Based on modeling in a clay box, Cooke and colleagues offer the first systemic exploration of fault evolution around fault bends. The results of this study were published in an early online edition of the“¯Journal of Structural Geology.
It is impossible to test ideas about how Earth's crust behaves in real time because geologic events typically unfold over many thousands or even millions of years. So far, reconstructing events after the fact has had only limited success. Finding a good analog for laboratory experiments has been a goal for geologists for decades.
“Geologists don´t agree on how the earth´s crust handles restraining bends along faults. There´s just a lack of evidence. When researchers go out in the field to measure faults, they can´t always tell which one came first, for example,” Cooke explained.
Cooke and her team take a ℠mechanical efficiency´ approach to studying dynamic fault systems' effectiveness at transforming input energy into force and movement. This is a novel approach for most geoscientists. The fact that a straight fault is more efficient at accommodating strain than a bumpy fault is one reason that Cooke is interested in understanding how the efficiency of fault bends evolves with increasing deformation.
According to her data, the crust behaves in accord with "work minimization" principles at restraining bends. Cooke calls this the "Lazy Earth" hypothesis. “Our approach offers some of the first system-type evidence of how faults evolve around restraining bends,” she says.
The lab at UMass Amherst is one of only a handful around the world to use a relatively new modeling technique. This new technique employs kaolin clay, also known as china clay, rather than sand to better understand the behavior of the Earth's crust.
Cooke's team used a clay box or tray loaded with kaolin, prepared so that its viscosity scales to that of the Earth's crust. When the clay is scaled correctly, data from experiments conducted over several hours in a table-top device are useful in modeling restraining bend evolution over thousands of years and at the scale of tens of kilometers.
Sand doesn't "remember" faults the way clay does, Cooke explained. Sand will just keep forming new faults when trying to model a bend in a fault, whereas clay remembers an old fault until it is so inefficient at accommodating the slip that a new fault will eventually form in a manner similar to what is observed on the ground.
Cooke and her team also use a laser scan to map the clay's deformation over time. This allows them to collect quantitative data about the system's efficiency.
“It´s a different approach than the conventional one,” Cooke acknowledges. “I think about fault evolution in terms of work and efficiency. With this experiment we now have compelling evidence from the clay box experiment that the development of new faults increases the efficiency of the system. There is good evidence to support the belief that faults grow to improve efficiency in the Earth´s crust as well. ”
“We´re moving toward much more precision within laboratory experiments,” she adds. “This whole field is revolutionized in past six years. It´s an exciting time to be doing this sort of modeling. Our paper demonstrates the mastery we now can have over this method.”
Cooke says that it is very revealing that a fault's active zone can shift locations significantly over 10,000 years. This observation has important implications for understanding seismic hazards. The better understanding that geologists have of fault development, the better they may be able to predict earthquake hazards and understand Earth's evolution.