An artist’s rendering of underwater earthquake waves. Image credit: Caltech
Although climate change is most evident to humans as unusually warm winter days or melting glaciers, 95 percent of the extra heat that greenhouse gases store on Earth is stored in the world’s oceans. Because of this, monitoring the temperature of ocean waters has been a priority for climate researchers, and now Caltech researchers have found that seismic rumble sounds on the ocean floor can provide them with yet another tool to do this.
In a new publication in scienceThe researchers show how they can use existing seismic monitoring equipment as well as historical seismic data to determine how much the temperature of the Earth’s oceans has changed and continues to change even at depths that are usually out of reach of traditional tools.
To do this, they listen to the sounds of the many earthquakes that regularly occur under the ocean, says Jörn Callies, assistant professor of environmental science and technology at Caltech and co-author of the study. According to Callies, these earthquake sounds are powerful and travel long distances through the ocean without significantly weakening, making them easy to monitor.
Wenbo Wu, postdoctoral researcher in geophysics and lead author of the work, explains that during an earthquake under the ocean, most of its energy flows through the earth, but some of this energy is transmitted into the water as sound. These sound waves propagate outward from the epicenter of the quake, just like seismic waves that travel through the ground, but the sound waves travel at a much slower rate. As a result, bumps reach a seismic monitoring station first, followed by the sound waves, which appear as a secondary signal of the same event. The effect is somewhat similar to how you can often see lightning flash before you hear its thunder.
“These sound waves in the ocean can be clearly recorded with seismometers at a much greater distance than thunder – from thousands of kilometers away,” says Wu. “Interestingly, they are even louder than the vibrations that travel deep in the solid earth and are more commonly used by seismologists.”
The speed of sound in water increases with increasing water temperature. The team realized that the time it takes a sound to travel a certain distance in the ocean can be used to infer the water temperature.
“The key is that we use repeated earthquakes – earthquakes that keep coming back in the same place,” he says. “In this example, we look at earthquakes that hit Sumatra in Indonesia and we measure when they arrive in the central Indian Ocean. It takes about half an hour to cover this distance, with the water temperature making about a tenth of a second difference. It’s a very small fractional change, but we can measure it. ”
Wu adds that by using a seismometer that has been in the same location in the central Indian Ocean since 2004, they can look back on the data they have collected from every earthquake in Sumatra, for example, and determine the temperature of the ocean at the same time.
“We use small earthquakes that are too small to cause harm or even be felt by humans,” says Wu. “However, the seismometer can record them from great distances and thus monitor large-scale changes in sea temperature along a certain path in one measurement.”
Callies says the data they analyze confirms that the Indian Ocean has warmed, as other data collected by other methods has shown, but that it may be warming even faster than previously thought.
“The ocean plays a key role in the speed at which the climate changes,” he says. “The ocean is the most important energy reservoir in the climate system, and the deep sea in particular is important for monitoring. One advantage of our method is that the sound waves scan depths below 2,000 meters, where there are very few conventional measurements. ”
Depending on which earlier data they compare their results with, ocean warming appears to be up to 69 percent higher than assumed. However, Callies cautions against drawing immediate conclusions because more data needs to be collected and analyzed.
With underwater earthquakes happening around the world, Callies said it should be possible to expand the system he and his colleagues developed to monitor water temperatures in all oceans. Wu adds that because the technology uses existing infrastructure and equipment, it is relatively inexpensive.
“We believe we can do this in many other regions,” says Callies. “And in this way we hope to contribute to the data on how our oceans are warming.”
The paper, titled Seismic Ocean Thermometry, describing the research appears in the September 18 issue of science. Co-authors are Wenbo Wu, postdoctoral researcher in geophysics; Zhongwen Zhan (PhD ’13), Assistant Professor of Geophysics; and Shirui Peng, PhD student in environmental science and engineering, all from Caltech; and Sidao Ni (MS ’98, PhD ’01) from the Institute of Geodesy and Geophysics of the Chinese Academy of Sciences.
Reference: “Seismic Ocean Thermometry” by Wenbo Wu, Zhongwen Zhan, Shirui Peng, Sidao Ni and Jörn Callies, September 18, 2020, science.
DOI: 10.1126 / science.abb9519
