Environment / Earth Science

No matter how closely you listen, you will not hear the Earth hum—but humming it is.

Far, far below the range of human hearing, waves of energy are coursing through the crust, causing the ground beneath your feet to rise and fall about three-millionths of an inch every few minutes. First detected by networks monitoring seismic activity in 1998, the tiny ripples were initially chalked up to the many small earthquakes that occur each day around the world. But studies over the past decade have proved that the hum is far too constant for that explanation.

In February, oceanographer-turned-seismologist Spahr Webb, of Columbia University’s Lamont-Doherty Earth Observatory, reported finding the hum’s likely origin: ocean waves colliding over continental shelves. When two trains of waves, traveling in different directions, smash into one another, they send a continuous cacophony down to the seafloor. That energy triggers vibrations that ripple through the planet, producing an inaudible ringing. “It’s the equivalent of a magnitude 6 earthquake occurring every day,” Webb says. “That’s the kind of energy we’re talking about.”

A spectrogram of ocean sounds
records a tremor from an underwater
volcano near Japan. Each column represents a
three-minute pulse of an ultralow sound.

Image courtesy of Kate Stafford/University of Washington
and Robert Dziak/NOAA Vents Program/
Oregon State University

Earth’s hum turns out to be just one of the many enigmatic signals resonating in the range known as infrasound. Broadly defined as sound waves longer than 56 feet, infrasound lies below the rumbling 20-hertz bass notes at the threshold of human hearing. Hurricanes, tsunamis, and tornadoes also generate their own characteristic low-frequency noise. Scientists are now studying infrasound to learn more about those powerful natural events and how to predict them—a skill that certain animals may already have. In fact, some animals generate their own ultrabass tones for a unique type of long-distance communication.

A big reason for the surge in interest is the growing number of monitors capable of picking up reverberations from nuclear tests, a mandate of the 1996 Comprehensive Nuclear Test Ban Treaty. In 2004, the stations helped scientists prove that what was feared to be a North Korean nuclear test was in fact the detonation of a train carrying explosives. The same basic technology can document a previously unheard world of natural sound as well. For example, physicist Alfred Bedard of the NOAA Earth System Research Laboratory in Boulder, Colorado, is working on a network of sensors that can identify a tornado before it reaches the ground.

The NOAA team has set up several arrays of sensors, spaced hundreds of miles apart, that register slight variations in air pressure—a sign of a lengthy sound wave passing by. (Scientists are not sure why tornadoes pump out infrasound, but Bedard thinks it may result from a rapid expanding and contracting of the vortex.) The readings are relayed to a laboratory in Boulder, where they are analyzed for advance warning of an approaching tornado. “The goal of the weather service is to get an average warning of tornadoes of about 10 minutes,” Bedard says. “We’re detecting signals that are arriving at our sites 30 minutes prior to a tornado report.

At the University of Mississippi, physicist Henry Bass leads a group using infrasound to help predict developing storms. Hurricanes stir up ocean waves and may contribute to the planet’s overall hum. Picking out the infrasound pulses from individual hurricanes could be “a way of supplementing the information available from satellites and airplanes,” Bass says. “We don’t know if it will work, but at this point the state will try almost anything.”