Undersea earthquakes rock climate science

Photo: An artist showing earthquake waves under the sea. Show more

Credit: California Institute of Technology

Although climate change is most apparent to people as unusually warm winter days or melting glaciers, up to 95 percent of the additional heat trapped by greenhouse gases on Earth falls in the world’s oceans. For this reason, monitoring the temperature of ocean water has been a priority for climate scientists, and now California Institute of Technology researchers have discovered that seismic rumbling on the sea floor could provide another tool for them to do so.

In a new research paper published in the journal Science, researchers show how they are able to leverage existing earthquake monitoring equipment, along with historical seismic data, to determine how warming Earth’s oceans are and continue to change, even at the depths. That is usually out of the reach of traditional tools.

They do so by hearing the sounds of the numerous earthquakes that regularly occur under the ocean, says Jorn Callis, associate professor of environmental science and engineering at the California Institute of Technology and co-author of the study. These earthquake sounds are powerful, Calise says, and travel long distances across the ocean without weakening them too much, making them easier to monitor.

Winbo Wu, a postdoctoral researcher in geophysics and lead author of the research paper, explains that when an earthquake occurs under the ocean, most of its energy travels through the ground, but part of that energy travels into the water as sound. These sound waves propagate outward from the epicenter just like seismic waves travel across the Earth, but sound waves travel at a much slower speed. As a result, the ground waves will reach the seismic station first, followed by the sound waves, which will appear as a secondary signal for the same event. The effect is roughly the same as how the flash saw from seconds of lightning before thunder was heard.

“These sound waves in the ocean can be clearly recorded by seismometers at a distance much longer than thunder – from thousands of kilometers away,” says Wu. “Interestingly, they are” higher “than the vibrations that travel deep into the solid Earth, which seismologists use widely.

The speed of sound in water increases as the temperature of the water increases, so the team realized that the length of time it takes for a sound to travel a certain distance in the ocean can be used to infer the temperature of the water.

“The key is that we use frequent earthquakes – earthquakes that happen over and over again in the same place,” he says. “In this example, we look at the earthquakes that happened off Sumatra, Indonesia, and we measure the time they reached the central Indian Ocean. It takes about half an hour to travel that distance, with the water temperature causing about a tenth – a difference of a second. It’s a very small partial change.” But we can measure it. “

Since they have used the same-site seismometer in the central Indian Ocean since 2004, Wu adds, they can look at the data they have collected every time an earthquake happened in Sumatra, for example, and thus determine the ocean’s temperature at the same time.

“We are using small earthquakes that are too small to cause any harm or even be felt by humans at all,” Wu says. “But a seismometer can detect them from great distances, allowing us to observe changes in ocean temperature over a wide range over a specific path in a single measurement.”

Kales says the data they analyzed confirms that the Indian Ocean is warming, and other data gathered through other methods has indicated, but that it may be warming faster than previously expected.

“The ocean plays a major role in the rate of climate change,” he says. “The ocean is the main reservoir of energy in the climate system, and the depth of the ocean in particular is important for its monitoring. One of the advantages of our method is that the sampling of sound waves is at a depth of less than 2000 meters, as there are very few conventional measurements.”

Based on the previous data set that they compare their results to, it appears that the oceans are warming 69 percent more than previously thought. Callies cautions, however, that any immediate conclusions should be drawn, as more data needs to be collected and analyzed.

With undersea earthquakes occurring all over the world, Calise says it is possible to expand the system that he and his fellow researchers developed so that it can monitor water temperatures in all oceans. Because the technology utilizes existing infrastructure and equipment, it is relatively low-cost, Wu adds.

“We think we can do this in a lot of other areas,” says Kales. “By doing so, we hope to contribute data on how our oceans are warming.”

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The paper describing the research titled “Seismic Ocean Temperature Measurement” appears in the September 18 issue of the journal Science. The co-authors are Wenbu Wu, postdoctoral researcher in geophysics. Zhongwen Zhan (Ph.D.13), Assistant Professor of Geophysics; Cheroy Ping, a graduate student in Environmental Science and Engineering, all from California Institute of Technology; And Sidao Ni (MS ’98, PhD ’01) from the Institute of Geodesy and Geophysics of the Chinese Academy of Sciences.

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