Scientists use Indian Ocean earthquake data to tell how fast it is heating

Chennai: In an advance that may lead to a relatively low-cost technique to monitor water temperatures in all of the oceans, scientists have developed a novel method to determine how fast the Indian Ocean is warming by analysing the sound from seabed earthquakes.

Researchers, including those from the California Institute of Technology (Caltech) in the US, have said as much as 95 per cent of the extra heat trapped on the Earth by greenhouse gases like carbondioxide is held in the world’s oceans, making it important to monitor the temperature of ocean waters.

As per the study, published in the journal Science, the scientists used existing seismic monitoring equipment, as well as historic data on earthquakes, to determine how much the temperature of the ocean has altered, and continues changing, even at depths that are normally out of the reach of conventional tools.

They assessed a 3000-kilometer-long section in the equatorial East Indian Ocean, and found temperature fluctuations between 2005 and 2016, with a decadal warming trend that “substantially exceeds previous estimates.”

By one estimate, the scientists said the ocean could be warming by nearly 70 per cent greater than had been believed. However, they cautioned against drawing any immediate conclusions, as more data need to be collected and analysed.

Jorn Callies, a co-author of the study from Caltech, noted that the method works by monitoring underwater quake sounds, which are powerful and travel long distances through the ocean without significantly weakening.

Wenbo Wu, postdoctoral scholar in geophysics and lead author of the paper, explains that when an earthquake happens under the ocean, most of its energy travels through the earth but a portion of it is transmitted into the water as sound.

These sound waves propagate outward from the quake’s epicenter just like seismic waves that travel through the ground, but the sound waves move at a much slower speed.

As a result, ground waves will arrive at a seismic monitoring station first, followed by the sound waves, which will appear as a secondary signal of the same event. The effect is roughly similar to how you can often see the flash from lightning seconds before you hear its thunder.

“These sound waves in the ocean can be clearly recorded by seismometers at a much longer distance than thunder — from thousands of kilometers away,” Wu says.

“Interestingly, they are even ‘louder’ than the vibrations traveling deep in the solid Earth, which are more widely used by seismologists.”

The speed of sound in water increases as the water temperature rises, so the length of time it takes a sound to travel a given distance in the ocean can be used to deduce the water’s temperature.

 

NT Bureau