Although climate change appears to cause people to melt hot winters or glaciers from time to time, the world’s oceans retain about 95 percent of the extra heat trapped on Earth by greenhouse gases. For this reason, monitoring seawater temperature is a priority for meteorologists, and Caltech researchers have now found that seismic sounds on the coast could provide them with another tool for that.
In a new paper publication Science, Researchers show how current seismic monitoring equipment and historical seismic data can be used to determine how much the Earth’s ocean temperature has changed and is changing even at depths that are normally inaccessible. Of traditional equipment.
They do this by listening to the sounds of several earthquakes that regularly occur under the sea, says John Kallis, assistant professor of environmental science and engineering at Caltech and co – author of the study. Kallis says the seismic sounds are strong and can travel long distances across the ocean without being significantly weakened, making it easier to observe.
Wenbo Wu, a postdoctoral scholar and author of a paper on geophysics, explains that when an earthquake occurs under the ocean, most of its energy travels through the earth, but a portion of that energy is transmitted to water by sound. These sound waves propagate outwards like seismic waves traveling from the source of the earthquake to the earth, but the sound waves move very fast. As a result, ground waves first reach an earthquake observation station, and then sound waves, which appear as a secondary signal of the same event. The effect is similar to how you often see flash from lightning moments before you hear thunder.
“These sound waves in the ocean can be clearly recorded by earthquakes that are farther away than thunder – thousands of kilometers away,” Wu says. It is noteworthy that “they are much louder than the vibrations traveling deep in the solid earth, which is more widely used by seismologists.”
The speed of sound in the water increases as the temperature of the water rises, so the team realized that the time it takes for a sound to travel a certain distance in the ocean can be used to lower the temperature of the water.
“The important thing is that we use repetitive earthquakes – repetitive earthquakes in the same place,” he says. “In this example we are looking at earthquakes from Sumatra, Indonesia, which we estimate when they reach the Central Indian Ocean. It takes them about half an hour to travel that distance, and the water temperature is about one tenth of a second.
Wu added that since they have been using a seismometer in the same location in the Central Indian Ocean since 2004, they can look back at the data collected whenever there is an earthquake in Sumatra. At the same time the ocean.
“We use small earthquakes that are small enough to cause damage or even be felt by humans,” Wu says. “But the seismometer can detect them over long distances, so it allows us to monitor large-scale ocean temperature changes on a particular path.”
The data they analyzed confirm that the Indian Ocean is warming, as indicated by other data collected by other methods, but this may heat up faster than previously estimated.
“The ocean plays an important role in the rate at which climate is changing,” he says. “The ocean is the main reservoir of energy in the climate system, especially the deep ocean. One advantage of our method is that the sample of sound waves is less than 2,000 meters, where conventional measurements are very small.”
Depending on which set of previous data, and comparing their results, ocean warming is 69 percent higher than believed. However, as more information needs to be gathered and analyzed, Kallis warns against reaching immediate conclusions.
As earthquakes occur around the world, Kallis says he and his co-researchers could expand the system to monitor water temperatures in all oceans. It is relatively inexpensive as existing infrastructure and equipment utilize this technology.
“We think it can be done in many other areas,” Kallis said. “By doing this, we hope to contribute to data on how our oceans are warming.”
The dissertation entitled “Seismic Ocean Thermometry” appears in the September 18 issue of Science. Wenbo Wu, a postdoctoral scholar in geophysics, co-authored; Songwen Shan (PhD ’13), Assistant Professor of Geophysics; Shirui Peng, a graduate student in environmental science and engineering, all from Caltech; Sidao Ni (MS ’98, PhD ’01) from the Institute of Geodesy and Geophysics at the Chinese Academy of Sciences.
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