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US researchers have found a way to monitor geological faults deep in the Earth that could help them predict an imminent earthquake more precisely than with other methods. This is the first time that scientists have been able to detect temporal changes in fault strength at seismogenic depth from the Earth’s surface.
The late Paul Silver and Taka’aki Taira of the Carnegie Institution’s Department of Terrestrial Magnetism, working with Fenglin Niu of Rice University and Robert Nadeau of the University of California, Berkeley, used highly sensitive seismometers to detect subtle changes in earthquake waves that travel through the San Andreas Fault zone near Parkfield, California, over a two-decade time span.
“Fault strength is a fundamental property of seismic zones,” explains Taira, who has moved to the Berkeley since the research was done. “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.”
Seismologists have focused the San Andreas Fault near Parkfield, the “Earthquake Capital of the World,” for years. The site has a sophisticated array of borehole seismometers, the High-Resolution Seismic Network, as well as other geophysical instruments in situ. Researchers consider it a natural laboratory for seismology because of the frequent quakes that occur there.
Earlier studies have suggested that there are fluid-filled fractures within the fault zone and that these shift. When this happens, “repeating” earthquakes apparently become smaller and more frequent, which researchers say is indicative of a weakened fault. “Movement of the fluid in these fractures lubricates the fault zone and thereby weakens the fault,” says Niu.
“The total displacement of the fluids is only about 10 metres at a depth of about three kilometres, so it takes very sensitive seismometers to detect the changes, such as we have at Parkfield.” Niu further explains that it seems to be distant earthquakes that cause the fluids to shift, such as the 2004 Sumatra-Andaman Earthquake, which led to tsunamis throughout the Indian Ocean that year.
The authors speculate that such large events should produce a temporal clustering of global seismicity, a hypothesis that appears to be supported by the unusually high number of large earthquakes occurring in the three years following the 2004 earthquake. The team presents additional evidence that a similar phenomenon occurred following the 1992 Landers earthquake.
An international team of researchers has developed a new magnetic carbon material that not only acts as a semiconductor but is also magnetic and could help scientists develop the next generation of microelectronic devices.
The new carbon material is based on graphene, which resembles graphite, the form of carbon found in pencil “lead”, but which exists as single sheet-like layers resembling nanoscopic chicken wire fencing. Graphene was first created by scientists in Manchester five years ago and is not only 200 times stronger than steel but because its electrons are highly mobile it has unique electro-optical properties. As such, some researchers think that graphene is the natural successor to silicon and could lead to the advent of spintronic devices that exploit electron spin and charge in computer memory and data processing.
Now, researchers from the Virginia Commonwealth University, USA, Peking University in Beijing, China, the Chinese Academy of Science in Shanghai, and Tohoku University in Sedai, Japan have used computer modelling to design a chemical cousin of graphene, which they call graphone. Experiments with the new material confirm the electromagnetic properties predicted by the computer models.
The team points out that while the properties of graphene can be modified relatively easily by introducing “defects” into its structure or by saturating it with hydrogen atoms, it has not proven easy to make it magnetic.
“The new material we are predicting – graphone – makes graphene magnetic simply by controlling how much hydrogen is put on graphene,” explains VCU’s Puru Jena. “One of the important impacts of this research is that semi-hydrogenation provides us a very unique way to tailor magnetism,” adds team member Qiang Sun, “The resulting ferromagnetic graphone sheet will have unprecedented possibilities for the applications of graphene-based materials.”
The team explains that graphene undergoes a transition from its original “metallic” state to semiconductor when all the carbon valencies are fully hydrogenated, to make graphane. However, density functional theory predicted that half hydrogenation (to make graphone) would result in a ferromagnetic semiconductor with a small indirect gap. This they confirmed experimentally.
“From graphene to graphane and to graphone, the system evolves from metallic to semiconducting and from nonmagnetic to magnetic. Hydrogenation provides a novel way to tune the properties with unprecedented potentials for applications,” the team says.
Nano Lett, 2009, in press