Japan's magnitude 9.0 earthquake on March 11 was not only impressive because of its size — it was one of the world's largest — but also for its unsettling rumble.
"It was the loudest sound we've ever recorded," said Robert Dziak of the Pacific Marine Environmental Laboratory and Oregon State University in Newport, Ore., part of a team that picked up the rumble of the quake with a microphone stuck in the ocean. [ Listen to the sound of the quake.]
Dziak and his colleagues monitor the Earth's underwater action. They record undersea volcanoes and earthquakes just like any other sound — using microphones.
The sound from the fourth-largest recorded earthquake of all time was heard as the energy from the earthquake spread through the water. That energy set off a tsunami that devastated Japan's Tohoku coast, crippled nuclear reactors and killed an estimated 27,000 people. Hundreds of aftershocks have since ruptured, including more than 60 of magnitude 6.0 or greater, and three above magnitude 7.0.
The recording's brutal noise is unnerving, especially knowing the devastation that the disaster caused.
"Frightening is a good word," Dziak told OurAmazingPlanet.
The amazing sound was captured by a so-called hydrophone array, which is basically a bunch of underwater microphones. The U.S. National Oceanic and Atmospheric Administration (NOAA) has about 10 deployed around the world.
The hydrophone array that recorded the Japan quake was located near Alaska's Aleutian Islands, about 900 miles (1,500 kilometers) from the quake's epicenter. The research group is usually listening for volcanoes on the seafloor, but they often hear activity at remote volcanic islands.
The hydrophones are always recording, which allowed them to catch the unprecedented sound of the earthquake.
"We hear some of those volcanoes, but they're not nearly as loud as this," Dziak said.
The massive energy during the March 11 quake was released during what's called thrust faulting. Thrust faulting happens when one tectonic plate dives under another. In this case, the Pacific Plate is diving under the North American Plate.
The research group has recorded almost 50,000 earthquakes in the North Pacific over the past 20 years, Dziak said. An earthquake's noise comes from the different energy waves produced by the quake.
"All the seismic energy is transferred into the ocean as sound energy," Dziak said. "The ocean allows those waves to propagate with little energy loss."
Hydrophones are dropped into the water from ocean vessels, attached to a float and anchored to the seafloor. Scientists can detect earthquakes that strike in the middle of the ocean, far away from land-bound seismometers.
Any sound they record comes into the monitoring lab with about a 5- to 10-minute delay. Big subwoofers rattle the lab as the low-frequency sound is played. Because of the time lag, the monitoring team knew they were about to hear something loud.
"We saw it on the news reports so we knew it was a big one," Dziak said.
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