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http://www.wired.com/news/technology/0,1282,66495,00.html?tw=wn_tophead_2
The Earth hums. Although inaudible to human ears, powerful ocean waves produce a quasi-harmonic humming sound in the ground that can be detected just about anywhere with seismometers.
Long dismissed as "background noise" by seismologists, a new look at this constant hum is opening a window on ocean activity, providing insight into the Earth's structure that may one day be used to give advance warning of earthquakes.
The restless seas generate enormous amounts of energy that, as it turns out, unsettle the mighty continents, especially during storms.
"Ocean waves hitting the coast of Labrador, Canada, generate seismic waves in California," says Sharon Kedar, a geophysicist at the Jet Propulsion Laboratory and co-author of the study, published Thursday in Science.
These aren't ordinary ocean waves but unusual pressure waves that form when water waves with similar frequencies, but from opposite directions interact, said Kedar. Unlike normal waves, the pressure waves do not lose their strength as they travel to the ocean bottom. When one of these waves interacts with the sea bottom, it produces a small tremor, or microseism, that travels thousands of miles through the solid Earth at about 2.5 miles per second.
Winter storms and steep coastlines produce the right conditions to generate pressure waves, which create these ocean microseisms, he said. Microseismic data readings from a few locations allow scientists to triangulate the location of individual storms in the middle of the Pacific or Atlantic. Some researchers hope they can use the phenomena to assess the strengths of El Niño conditions in the Pacific when little or no direct ocean data is available.
Collectively, these ocean microseisms have an annual cumulative seismic energy comparable to that generated from earthquakes, said Kedar. All that constant low-level energy has another use: detecting changes in the Earth itself.
To understand what's going on deep in the Earth requires seismic data. Until now, only the occasional tremors from earthquakes and big explosions generated such data. Nor were seismometers designed to detect low-frequency tremors or vibrations.
"Newer digital broadband seismometers can detect the ocean microseisms," said Rob Clayton, a geophysicist at the California Institute of Technology Seismological Laboratory.
A constant flow of seismic data from a region over a long period of time allows the detection of very subtle changes in the Earth, said Clayton. "I think we will be able to monitor a lot of things this way."
Using data from 155 digital broadband seismometers, Clayton said they should be able to detect the seasonal cycle of the filling and pumping out of aquifers in the Los Angeles Basin. That may prove useful in monitoring groundwater levels one day.
But by far the biggest application is in learning more about earthquakes and as a possible early-warning tool. The constant stream of data from ocean microseisms may allow seismologists to detect small vibrations deep within the Earth itself.
"Direct monitoring of the stress changes in a fracture zone is the holy grail of seismology," said Clayton.
In other words, a variation in the Earth's hum in a particular location could be the harbinger of an earthquake. Or it may be just another big storm in the North Pacific. Or it might be something else entirely. It will take some time to figure out how to use this new information, Clayton said.
"If I had to guess, (I'd say) in a year from now we'll have some interesting stuff on how useful this will be."
http://www.wired.com/news/technology/0,1282,66495,00.html?tw=wn_tophead_2
The Earth hums. Although inaudible to human ears, powerful ocean waves produce a quasi-harmonic humming sound in the ground that can be detected just about anywhere with seismometers.
Long dismissed as "background noise" by seismologists, a new look at this constant hum is opening a window on ocean activity, providing insight into the Earth's structure that may one day be used to give advance warning of earthquakes.
The restless seas generate enormous amounts of energy that, as it turns out, unsettle the mighty continents, especially during storms.
"Ocean waves hitting the coast of Labrador, Canada, generate seismic waves in California," says Sharon Kedar, a geophysicist at the Jet Propulsion Laboratory and co-author of the study, published Thursday in Science.
These aren't ordinary ocean waves but unusual pressure waves that form when water waves with similar frequencies, but from opposite directions interact, said Kedar. Unlike normal waves, the pressure waves do not lose their strength as they travel to the ocean bottom. When one of these waves interacts with the sea bottom, it produces a small tremor, or microseism, that travels thousands of miles through the solid Earth at about 2.5 miles per second.
Winter storms and steep coastlines produce the right conditions to generate pressure waves, which create these ocean microseisms, he said. Microseismic data readings from a few locations allow scientists to triangulate the location of individual storms in the middle of the Pacific or Atlantic. Some researchers hope they can use the phenomena to assess the strengths of El Niño conditions in the Pacific when little or no direct ocean data is available.
Collectively, these ocean microseisms have an annual cumulative seismic energy comparable to that generated from earthquakes, said Kedar. All that constant low-level energy has another use: detecting changes in the Earth itself.
To understand what's going on deep in the Earth requires seismic data. Until now, only the occasional tremors from earthquakes and big explosions generated such data. Nor were seismometers designed to detect low-frequency tremors or vibrations.
"Newer digital broadband seismometers can detect the ocean microseisms," said Rob Clayton, a geophysicist at the California Institute of Technology Seismological Laboratory.
A constant flow of seismic data from a region over a long period of time allows the detection of very subtle changes in the Earth, said Clayton. "I think we will be able to monitor a lot of things this way."
Using data from 155 digital broadband seismometers, Clayton said they should be able to detect the seasonal cycle of the filling and pumping out of aquifers in the Los Angeles Basin. That may prove useful in monitoring groundwater levels one day.
But by far the biggest application is in learning more about earthquakes and as a possible early-warning tool. The constant stream of data from ocean microseisms may allow seismologists to detect small vibrations deep within the Earth itself.
"Direct monitoring of the stress changes in a fracture zone is the holy grail of seismology," said Clayton.
In other words, a variation in the Earth's hum in a particular location could be the harbinger of an earthquake. Or it may be just another big storm in the North Pacific. Or it might be something else entirely. It will take some time to figure out how to use this new information, Clayton said.
"If I had to guess, (I'd say) in a year from now we'll have some interesting stuff on how useful this will be."