Hearing the sounds of the Earth

ANCHORAGE - Alaska's mountains speak to those who listen.

Posted: Friday, November 19, 2004

The northern lights sometimes whisper at the approach of dawn, while North Pacific cyclones whine with a strange and subtle tone.

This is not some poetic babble. A research team at the University of Alaska Fairbanks has been using sensitive arrays of microphones near campus and in Antarctica to eavesdrop on the cries and murmurs of earthquakes, marine weather, alpine winds, shimmering auroras and explosions of all kinds.

By recording and analyzing sounds that ripple below the threshold of human hearing, scientists have found innovative ways to measure complex natural phenomena. At the same time, the research has opened an ear on a world most people scarcely imagine.

A representative from the Geophysical Institute's Infrasound group presented reports in November on earthquake and auroral sounds during the annual American Geophysical Union meeting in San Francisco. They were among nearly 50 papers describing infrasounds produced by meteors, the space shuttle Columbia disaster and other jolts around the world.

"We're discovering things that people never saw before because of the tools we have now," said Fairbanks physicist John Olson, the group's principal scientific investigator. "These (sounds) are around us all the time. For instance, in Alaska, one of the biggest things we see day in, day out are the signals from marine storms. They're almost like a tone with a period of about five seconds."

It's all part of an infrasound renaissance made possible by modern computing power and an international effort to intercept the faint rumble of illegal nuclear blasts.

Based in Vienna, Austria, the Comprehensive Nuclear Test Ban Treaty Organization has sponsored a global network of sensors to monitor air, land and sea for vibrations or fallout. Among those will be about 60 arrays of infrasound devices, some of which are still being installed.

The array of eight microphones in Fairbanks went online in 2002 with a tightly spaced inner triangle surrounded by a pentagram of microphones spread over about half a mile. Another array was installed in 2001 in Windless Bight near the U.S. McMurdo Base in Antarctica.

Designed by UAF professor emeritus Charles "Buck" Wilson, interpreted by Olson, and operated by engineer Dan Osborne and a crew of technicians, these ultrasensitive microphones feed data around the clock to computers in Austria. But they also record subtle Earth harmonics rocking the hills overlooking Fairbanks.

"We do it for the natural signals. That's what our interest really is," Olson said.

Like all acoustic energy, infrasound moves in waves of rising and falling pressure. But while a faint tone at the threshold of hearing might tremble the eardrum at 20 cycles per second, each infrasound wave might arrive only once every few seconds, or even once a minute or more. Those frequencies are too slow for people to hear, with wavelengths that sometimes span miles.

But these long-period waves can travel across continents without losing strength, sometimes circling the globe before fading away. The signal from a nuclear bomb detonated in China in 1980 took at least six hours to reach an earlier array in Fairbanks, only to reappear almost 40 hours later, Wilson said.

Gently pulsating auroras in the morning appear to produce their own unique sound as they heat the upper atmosphere, something that Wilson and Olson have just begun to investigate with auroral researchers.

Winds tearing over the crests of the St. Elias and Alaska ranges set up pulses of sound that spread across the state and get picked up distinctly by the Fairbanks array. And that's not all.

"The Fairbanks data is flooded with man-made signals," Olson said. "We see every aircraft that takes off at the airport and lands. ... One signal that we use almost for calibration comes from the Fort Knox gold mine. (Every day) they set off a surface explosion. They've been a good barometer for us."

One of the strongest signals the array ever received was a long-lasting grumble from the Denali Fault earthquake on Nov. 3, 2002, Olson said.

Within about 27 seconds, the microphones in Fairbanks recorded the seismic jolt produced by the quake, which initially struck on a previously unknown fault about 75 miles south of Fairbanks. The rupture then continued southeast along the Denali and Totschunda faults, moving through the Alaska Range at almost 2 miles per second, unzipping glaciers and mountainsides along a track about 210 miles long.

As the mountains themselves rocked back and forth almost 30 feet, they produced a colossal acoustic signal that surged through the air at about 1,100 feet per second, the speed of sound. It arrived at the microphone array about 12 minutes after the quake first struck and continued for the next 10 minutes.

"As the rupture propagated, it was shaking the mountains nearby, and those mountains were acting like speaker pistons and sending the signals back our way," Olson said.

These waves had frequencies of a minute - 1,200 times too slow for human hearing - and wavelengths of up to 12 miles, Olson said. Their direction of travel corresponded with the movement of the rupture through the Alaska Range, giving seismologists a unique measure of how the quake behaved.

"In general, these infrasound sensors help us understand the process," said state seismologist Roger Hansen, a co-author on the talk at the conference. "In this case, we're looking at the rupture velocity, and the infrasound is another piece of information for something that's difficult to measure."

For the presentation in San Francisco, Olson had his team speed up the Denali quake's sound so that it could be heard. It's a common practice used with infrasounds because it translates an inaudible phenomenon into human terms.

"My feeling is that the human ear and mind is a tremendous instrument for hearing signals," he said. "That's because we listen to people's voices, and we hear intonations and interpret them."

Hansen said the scratchy hiss of the Denali quake startled and amazed him and other scientists at the conference. In a phone interview, he even tried to reproduce it.

"It's not very exciting in terms of having any kinds of whistles or chirps, or rising or falling tones," Olson said. "But it is a big, loud rumble."



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