ocean waves
Crashing waves in the deep ocean can generate enough energy to create a seismic "hum." Image courtesy: Bruce Molnia/U.S. Geological Survey
The latest buzz in Earth science literally comes from out of the blue-the deep blue seas. Scientists have long known about microseisms--small Earth tremors created when ocean waves traveling in opposite directions merge together. But no one could figure out where they came from. Now, for the first time, scientists have pinpointed a specific area in the North Atlantic where this mysterious hum is emitted from the depths of the ocean.

Microseisms were first recorded as a strange, continuous buzz on the earliest seismometers, devices that measure earthquakes. Every year, the cumulative energy released by these small vibrations equals the amount of energy released globally from earthquakes. Records of microseismic activity give scientists 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. Scientists are interested in learning where these microseisms originate because the information can help them monitor stress in Earth's crust.

The theory of the origin of microseisms was first introduced in 1950 by Michael Longuet-Higgins from the University of Cambridge in England. Longuet-Higgins suggested that the vibrations originated in places where ocean waves were traveling in opposite directions toward each other at the same frequency and at a certain ocean depth. According to his theory, when these waves collide, they combine to form stationary waves that remain in a constant position 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. The vibrations generate seismic surface waves, which spread out thousands of miles and are detected by seismometers as noise.

A science team led by Sharon Kedar of NASA's Jet Propulsion Laboratory, Pasadena, Calif., and including Longuet-Higgins, used Longuet-Higgins' theory to predict regions of the ocean where microseisms could originate. The challenge was in finding an area of the ocean with just the right conditions.

"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," said Frank Webb, a JPL geophysicist who is a co-author of the new study, appearing in the March 8 issue of the Proceedings of the Royal Society, Series A. "Conversely, you could have a section of ocean floor at a depth favorable to microseisms, but then you 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 in the same area.

"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."

Webb said that while this region is not the only one in the world to produce microseisms, it is the first region in which the source of microseisms has been definitively located.

Webb said the project brought people from very different fields together to address a complex problem, including oceanographers and seismologists. "That's something that has rarely been done since we first started to look for areas where microseisms originate. 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.

Other institutions participating in the study include the University of California, San Diego; the California Institute of Technology, Pasadena, Calif.; and the Hydrologic Research Center in San Diego.

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Written by Diya Chacko
Media contact: Alan Buis/JPL 818-354-0474