Published online 8 January 2008 | Nature | doi:10.1038/news.2008.422

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Colliding waves in the North Atlantic can shake the Earth.

Colliding waves, like these ones off the coast of Greenland, create seismic vibrations. Igor Shkvorets

Researchers have located an area between the Labrador Sea and Iceland where waves collide and send vibrations to the ocean’s floor, creating a hum that can be detected for thousands of kilometres.

The results, published this week in the journal Proceedings of the Royal Society A1, confirm an old prediction that oceans can be powerful generators of seismic activity.

Often called a seismic hum, the signature buzz of wave-generated 'microseisms' has been observed since the early twentieth century, when seismologists began installing seismometers capable of detecting their slow vibrations: a typical microseismic wave can last more than 10 seconds. First dismissed as noise in the data, ocean microseisms have recently been heralded as a powerful way of probing the properties of the Earth's crust.

Most microseismic vibrations are generated near coastlines, where waves pulling back from the coast hit the next set of waves coming in. During the winter months, however, waves from sea storms produce seismic vibrations that can be many times more powerful.

Swell beginnings

Seismic sensors in Europe, North America, Iceland, and Greenland have long pointed to the North Atlantic as a possible hotspot for microseisms. To investigate, seismologist Sharon Kedar of the Jet Propulsion Laboratory in Pasadena, California, and his colleagues used information from satellites and buoys to connect what was happening on the ocean’s surface to the vibrations they detected on dry land.

The researchers pinpointed a microseismic hotspot stretching from the Labrador Sea to south of Iceland. In this zone, autumn and winter winds kick up rough ocean waves that travel in opposite directions.

The wavelength of the colliding surface waves and the 2-kilometre depth of the water here create a strong resonance, with the surface and bottom of the ocean forming the ends of a giant, water-filled organ pipe. The pressure from the periodic collisions creates sizeable seismic vibrations that can be detected on the coasts of North America, Greenland, and Europe.

The researchers looked for a similar hotspot in the North Pacific, but found that the ocean column under stormy waters was too deep to show a similar effect.

Making waves

The model predicting deep sea generation of seismic vibrations is more than 50 years old2, says Kedar, "but this is the first real confirmation" that the model is correct. "Until now, we didn’t have a good understanding of where that noise is coming from. People had the expectation that the sources were random in place and time, but they are not," he says. The work could help advance growing efforts track changes in the Earth’s structure with microseisms, says Kumar.

"This is really a first in terms of connecting real ocean-model data, which is done by oceanographers, to the seismic data, which is the realm of solid-earth scientists," says seismologist Toshiro Tanimoto of the University of California, Santa Barbara, who uses networked seismometers to image the Earth’s interior. "They have shown for the first time that the two different data sets point to the same place."

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Determining the effect of waves generated closer to shore on the ocean’s hum remains a tricky problem, because many complex factors — including beach slope and tide — affect the size and frequency of these waves and the pattern of wave collisions. The researchers say a more complete model is still on the horizon.