Earthquakes turn earth into 'loudspeakers'

Earthquakes can "pump" the ground like a giant subwoofer to create infrasound below the level of human hearing, U.S. seismologists say. That infrasound can reveal important details about an earthquake and could be used to measure the amount of ground shaking in the immediate region above the source, which would normally require an array of many seismometers, they said. "It's basically like a loudspeaker," said Stephen Arrowsmith, a researcher with the Geophysics Group at Los Alamos National Laboratory in Santa Fe, N.M. "In much the same way that a subwoofer vibrates air to create deep and thunderous bass notes, earthquakes pump and vibrate the atmosphere, producing sounds below the threshold of human hearing," he said in presenting the findings at the annual meeting of the Acoustical Society of America in Kansas City. The researchers tested their "loudspeaker" model by comparing its predictions to actual data collected from a magnitude 4.6-earthquake that occurred on Jan. 3, 2011, in Circleville, Utah. Their predictions were in close agreement with the actual data, they said. "This was very exciting because it is the first such clear agreement in infrasound predictions from an earthquake," Arrowsmith said. "Predicting infrasound is complex because winds can distort the signal and our results also suggest we are getting better at correcting for wind effects."