October 19, 2012
Scientists Show Earth’s Surface Acts As A Giant Loudspeaker
Brett Smith for redOrbit.com — Your Universe Online
In the midst of an earthquake, buildings sway back and forth, terrain pops and locks, and the ground seems to shuffle underfoot.
Acoustic scientists have shown in a new study that the Earth´s surface and atmosphere act as a giant loudspeaker. This “loudspeaker” soundtracks these geologic raves in both the audible range of hearing and in infrasound, or frequencies below the range human hearing can detect.
According to the computer modeling, sound recordings and seismic data used in the study, an earthquake “pumps” the surface and the atmosphere above it, sending sound waves radiating from the epicenter.
"It's basically like a loudspeaker," said Stephen Arrowsmith, a researcher at Los Alamos National Laboratory in Santa Fe, N.M., who will present his team's findings at the 164th meeting of the Acoustical Society of America (ASA) next week. "In much the same way that a subwoofer vibrates air to create deep and thunderous base notes, earthquakes pump and vibrate the atmosphere producing sounds below the threshold of human hearing."
The infrasound made by an earthquake can provide detailed information about the event, according to the researchers. In particular, it can reveal the amount of shaking that is occurring directly above the source of the quake. Accurate analysis of these sound waves could provide information that is typically gathered using an array of seismometers. This could make infrasound detection a key tool for assessing the damage and studying the mechanism behind a seismic event.
In creating their computer models, the acoustics team assumed the surface and atmosphere would pump like a piston and, during a seismic event, act in the same way as a loudspeaker or subwoofer. Subwoofer technology was first introduced in the 1960s as a way to convey audio signals in the 20 to 200 Hz range, and it uses a small-volume box to generate the desired sound and pressure levels.
To test this model, the research team collected acoustic and seismic data during a 4.6-magnitude earthquake that occurred on January 3, 2011 near Circleville, Utah. The data was recorded at the University of Utah that maintains seismograph stations equipped with infrasound recording devices.
After analyzing the data, they found it closely matched the results produced by their loudspeaker-based computer models.
"This was very exciting because it is the first such clear agreement in infrasound predictions from an earthquake," said Arrowsmith in a statement. "Predicting infrasound is complex because winds can distort the signal and our results also suggest we are getting better at correcting for wind effects."
The researchers were able to demonstrate the significance of “pumping” the ground and how it generates infrasound during a seismic event.
Infrasound is typically described as sound with a frequency less than 20 Hz, the lowest frequency for human hearing. Other natural events can produce infrasound, including avalanches, lightning, and waterfalls. According to the National Oceanic and Atmospheric Administration (NOAA), infrasonic arrays can be used to detect deadly avalanches in the Rocky Mountains as they happen and tornadoes as they form in the upper atmosphere before they touch down.