by Timothy Oleson Monday, July 23, 2012
The sounds we associate with earthquakes tend to be those induced aboveground. Low-pitched rumbles, rattling windows and car alarms might be heard during small temblors, while more terrifying sounds like the crumbling of concrete and the cacophony of people trying to reach safety sometimes accompany large earthquakes. But what does an earthquake itself sound like, as rock grinds against rock in a rupturing fault and large amounts of energy are released? Thanks to some recent efforts, we may be starting to get an idea.
In truth, it is difficult to know what earthquakes underground sound like because typical seismic waves have frequencies below the audible range for humans, which is between 20 and 20,000 Hertz. Now, however, a group of researchers has given voice to one seismic monster: the March 11, 2011, magnitude-9.0 Tohoku earthquake that rocked Japan and was detected on seismometers worldwide.
By converting seismic data from instruments around the planet to sound files and then playing the recordings back at faster-than-real-time speeds, the researchers boosted the seismic frequencies into the audible range. The technique also condenses the signals so that processes that occurred over several minutes or hours can be heard on much shorter timescales. Depending on where the seismometers that recorded a particular dataset were located, the resulting sounds of the main shock and the aftershocks resemble everything from booming explosions to distant thunderclaps and gentle rainfall to the sound of furniture being rolled across a hardwood floor.
The researchers, led in part by seismologist Zhigang Peng of Georgia Tech, detailed their technique recently in Seismological Research Letters. Sample sound files are also posted online (http://geophysics.eas.gatech.edu/people/zpeng/Japan_20110311/index.html) for all to enjoy.
Beyond the novelty of “hearing” earthquakes, making seismic activity audible serves other important purposes, according to Peng. For example, it might be useful to scientists studying the physics and triggering behavior of earthquakes, or to science educators teaching general audiences about the science behind large temblors. “People are able to hear pitch and amplitude changes while watching seismic frequency changes. Audiences can relate the earthquake signals to familiar sounds such as thunder, popcorn popping and fireworks,” Peng said in a press release.
In a separate effort, an enterprising YouTube user has come up with another way to audibilize earthquakes: assembling the worldwide catalog of earthquakes greater than magnitude-4.5 in 2011 to create a sort of time-lapse audio-visual seismic collage. The technique is similar to one used for educational purposes in the 1990s by the Smithsonian Institution and others. But now, it’s been updated for the Internet Age. The video, entitled “World Earthquakes 2011 Visualization Map,” was posted to the site by user StoryMonoroch in January and by mid-May had already amassed more than 620,000 views.
Each of the more than 9,000 events, pulled from the U.S. Geological Survey’s earthquake archive, was assigned an audible tone — sounding like a computerized thump — the volume and pitch of which were based on the earthquake’s magnitude and depth, respectively. As 2011 zips by over the course of six minutes, the constant drone of small events is occasionally punctuated with louder magnitude-7.0 and greater earthquakes.
In addition to the thump, each quake is projected on a world map as a yellow dot surrounded by a red circle of varying size, representing the hypocenter and magnitude, respectively. The visual display provides a useful accompaniment to the audio, clearly demarcating the plate boundaries and other tectonically active areas where the vast majority of seismic activity is concentrated.
If you are not interested in sitting through the entire year’s recap, make sure to at least listen past the 1:50-mark in the video. This is where the collage gets really interesting. A couple of large foreshocks immediately precede a jolting boom that occurs seemingly out of nowhere on, you guessed it, March 11. The magnitude-9.0 blast sets off a cascade of rapid-fire aftershocks that — if your speakers are turned up — might make you feel like you stepped onto the set of “Tron.” Also interesting is the notable increase in the rate of worldwide seismicity, albeit dominated by the aftershocks concentrated near Japan, in the weeks following the Tohoku quake.
Not surprisingly, written descriptions do not do these projects justice. And while neither is likely to rock your world, literally or figuratively, the videos do make for a few fascinating and awe-inspiring listens. Just be careful: You might want to turn down your computer’s volume first.
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