Scientists have long known about microseisms, but no one could figure out where they came from - until now. They were first recorded as a strange, continuous buzz on the earliest seismometers, devices that measure Earth vibrations over periods from one to several seconds long. Scientists use seismometers to "hear" everything from earthquake tremors to these tiny microseismic vibrations of the ocean floor.

Every year, the cumulative energy of these small vibrations equals the combined annual energy release from earthquakes. Finding out where ocean microseisms originate could help scientists monitor stress in Earth's crust with a technique called "noise tomography." The technique uses seismic waves to image sections of the crust.

Records of microseismic activity give us a history of wave interaction in Earth's oceans since the early 20th century. They are also used to examine the history of storms over the ocean, according to Frank Webb, a geophysicist at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Webb has studied this phenomenon extensively and is co-author of a new study on microseisms appearing in the March 8 issue of the Proceedings of the Royal Society, Series A. JPL's Sharon Kedar led the interdisciplinary science team, which included researchers from JPL; University of California, San Diego; the California Institute of Technology, Pasadena; and the Hydrologic Research Center in San Diego.

"It's been an interesting project, because people from very different fields were working together to address this problem," Webb said, adding that the team included both oceanographers and seismologists. "That's something that has rarely been done since we first started to look for areas where microseisms originate."

The theory of the origin of microseisms was first introduced in 1950 by Michael Longuet-Higgins from the University of Cambridge in England, who also worked on this recent project. Longuet-Higgins suggested that the vibrations originated in places where ocean waves were traveling at the same frequency opposite to each other at a certain ocean depth. According to his theory, the interacting waves combine to form stationary waves over large areas of the ocean.

These waves create tall, pulsing columns of pressure that repeatedly beat down on the ocean floor, causing it to vibrate at double the frequency of the wave. The vibrations generate seismic surface waves, which propagate thousands of miles and are detected by seismometers as noise.

Longuet-Higgin's theory was used to predict regions of the ocean where microseisms could originate. Webb said that actually finding an area of the ocean with the right conditions to generate microseisms was difficult.

"You could have two opposing waves generating these pressure fluctuations, but they have to be interacting at exactly the right depth for the ocean floor to resonate," Webb said. "Or you could have a section of ocean floor at a depth favorable to microseisms, but you then need storms to generate opposing waves that meet right over that area."

Using ocean wave models that determine the states of the ocean in different areas, the team located a region of the ocean that matches the criteria from Longuet-Higgin's theory in a region of the North Atlantic that extends from the Labrador Sea (between Greenland and the northeast coast of Canada) to the south of Iceland. The team found the region by comparing opposing wave interactions to seismic data recorded at the same area.

"With Longuet-Higgin's theory, we located areas of the ocean with high potential for microseisms, but then we needed to see if storms in those areas generated the right waves," Webb said. "The area we found in the North Atlantic Ocean had the right depth and the right storm system to generate microseisms."

While this region is not the only one to produce microseisms, it is the first region in which the source of microseisms has been located.

Webb said it was a privilege to be on a team with the originator of the theory of how microseisms form.

"It's been an honor to work with Michael Longuet-Higgins. He was really happy to revive this project, even a few decades later," Webb said.

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