Extreme Ocean Storms On the Rise, Tremors Show

Kimberly Johnson, National Geographic News 25 Apr 08;

Extreme ocean storms have ramped up in frequency over the past 30 years, according to new research based on small tremors.

The faint tremors, called microseisms, are periodic movements of Earth's surface that can last anywhere from 5 to 30 seconds.

Unlike earthquakes, which are caused by movements of Earth's tectonic plates, microseisms are created by the incessant beating of waves along the coasts.

The phenomena are usually dismissed as background noise by scientists studying earthquake readings.

"The gist is that we monitor pervasive seismic tremors observed around the world that arise from wind-generated waves to [assess] Earth's wave climate," said study co-author Richard Aster, a geophysics professor at the New Mexico Institute of Mining and Technology.

The new findings, while still in their early stages, could be used to test long-held theories about whether global warming leads to more violent ocean storms, Aster added.

Stormier Seas

Aster and colleagues studied microseisms at 22 seismographic stations scattered across the world, from Antarctica to Pittsburgh, Pennsylvania.

They also looked at worldwide ocean-borne microseism data taken from 1972 to 2008.

The researchers found that the microseisms' power increased with time, perhaps as storm winds intensified or changed direction.

The data also showed that each of the 22 seismographic stations registered an uptick in extreme ocean gales.

"The waves we are studying are ordinary ocean waves, not seismically generated ones," Aster said.

"We are basically using the long-term seismic record to see if Earth is getting systematically ['noisier'] due to changing ocean waves."

Though the signals are small, "we do see these trends of a few percent a decade that appear to be robust," Aster added.

The study appeared in the March/April edition of the journal Seismological Research Letters.

Seismic Noise

Paul Earle, a research geophysicist for the U.S. Geological Survey, was not associated with the study.

He said the research shows how microseisms can be used to study patterns in ocean-storm intensity.

"It is interesting that microseisms are more commonly called 'seismic noise' and early instruments were specifically designed to suppress and remove this signal," Earle said.

"This study highlights the importance of archiving and preserving continuous geophysical measurements—even if their use is currently unknown."