Seismographs have been installed around the world to monitor earthquakes, events that show up as big spikes between small wiggles on a seismogram. The small wiggles, called microseisms, represent ambient noise but could be the next tool to track changing climate patterns.
Seismic noise is caused by wind, oceanic movements and even foot stomping during big football games. Seismologists studying seismic noise have noticed that many parts of the world are getting noisier because of more intense ocean activity, characterized by hurricanes and tropical storms.
Keith Koper, director of seismograph stations at the University of Utah, pulled data from about 500 seismographs in the United States from Oct. 18 to Nov. 3 last year, the days that Superstorm Sandy tore through the Caribbean and the U.S. East Coast.
While a number of the devices are scattered across the United States, the majority are concentrated east of the Minnesota-Texas line and west of Lake Erie and Florida. Koper and his team found that as the storm turned north toward New Jersey, the seismometers lit up with microseismic activity. Sandy was literally shaking the country.
There are two ways by which ocean waves can create the seismic waves, Koper explained. One is by a wave hitting the coastline, the weaker of the two forces. The one that generates more seismic energy is when two ocean waves interact. "And so what happens in case of Sandy is the waves come in and they actually reflect off of the coastline and then they interfere with the other incoming waves, and it is that interference pattern that generates the stronger, larger seismic waves," he said.
There is no scale to measure microseisms, but the waves generated by Sandy carried enormous amounts of energy. "The waves themselves probably are equivalent to waves you'd see from a magnitude-3 earthquake, but they just happen for so long that the energy never built up," Koper said.
Detecting 'noisier' oceans
Sandy is just one example of how ocean waves trigger seismic waves, said Richard Aster, professor of geophysics at the New Mexico Institute of Mining and Technology. "We can also look at the integrated effect of all the storms in the world, something I've been working on, to see if the world is getting statistically noisier as a result of global warming or other changes in the intensity of storms and where storms are tracking. So it's just a unique proxy for the state of the world's ocean waves," he said.
Aster has studied microseisms going back at least two decades. In a study conducted in 2010, he found that some parts of the Northern Hemisphere were getting noisier due to increased ocean wave activities.
Seismic signals of ocean activity depend on the time of year. Aster noticed a big upsurge in background seismic noise in the world's oceans from the Northern Hemisphere and then from the Southern Hemisphere as big, wave-generating events occurred in the local winter. El Niño also showed up as increased microseismic activity. El Niño both creates more intense storms and directs those storms into areas where waves can bounce off coastlines or off each other.
"So what we have to do is look at many years of those cycles, and we look at the trends. And what we've seen recently is that in the last few years the signals from the Pacific in particular, although the number of really extreme events hasn't necessarily increased, the wave years we saw has become longer so the high energy excitation has started a bit earlier and lasted a bit longer," Aster said.
Piecing together changes in wave climate
Aster is now updating his 2010 analysis with more information from seismographic stations around the world and with records that go back to the 1970s.
"It does look like some parts of the world are getting noisier. A few are getting quieter, but the number that seem to show an increase in the background seismic microseism far outweighs the number that show a decrease," he said. He is also developing techniques to comb through reams of paper records that go to the early part of the century.
The science of tracking seismic waves from storms goes back a long way. "Believe it or not, it goes all the way to even in World War II before there was good satellite data and coverage that seismologists tried to forecast severe storms in the ocean. The Navy was interested in that," Koper said.
While the climate-wave-wind system is not understood well enough to make predictions, Aster and other seismologists are working with climate scientists to piece together changes in the world's wave climate. These changes could have big implications for storms, sea-level rise and coastal erosion.
The advantage of using seismographs to track storms, Aster said, is that they cover more ground -- such as in Antarctica, where the ice prevents putting buoys in the ocean, and when clouds prevent satellite observation.
"It turns out it's the most ubiquitous signal. You can put a seismograph in the middle of Asia and easily see the world's oceans," Aster said.