Seismic sound waves crossing the ocean depths could serve as a new thermometer

Zoom in / The seismometer on the island of Diego Garcia (left) can calculate the ocean temperature of earthquakes near Sumatra (right).

Geophysics has shown that accurate measurements and a little bit of modeling can do wonders, such as showing the detailed structure of Earth’s interior despite the fact that it’s buried under hundreds of kilometers of rock. This is possible because seismic waves produced by earthquakes subtly change speed or direction as they pass through different materials. A new research paper shows that something similar can actually measure small changes in temperature in the deep ocean.

The idea of ​​using sound waves from human-made sources was actually floated several decades ago but faded after some experiment. A team led by Wenbo Wu at the University of Toronto realized that earthquakes could be leveraged in the same way, removing the expensive logistics of constantly launching booms for measurements, as well as concerns about impacts on marine life.

There are actually several types of seismic waves from earthquakes, and each behaves slightly differently. The P-wave (P for “primary” because it is the first to arrive) is similar to a sound wave, because it compresses rocks in the same direction they are moving. So when this wave hits the sea floor or the surface of the earth, the rocks can act like a huge loudspeaker, producing a low-frequency sound wave in the air or water.

The same process can work in reverse as well. Suppose there is an earthquake below the sea floor just off the coast. The movement of the sea floor can create a sound wave that travels across the ocean basin and re-enters the sea floor rock of an opposite coast. This can actually be detected by seismometers as a very late vibration – because the wave travels through the water much slower than it does through rocks. Since it appears after the primary and secondary seismic waves, this peculiar acoustic marker is called the third wave or T wave.

T-wave travel speed is sensitive to water temperature, as warm water slows it down. (To complicate matters further, they are also slightly sensitive to changes in salt concentration or current motion, but researchers say the effect of temperature is dominant.) Seismometers are sensitive enough to detect very small differences in timing, so they are able to measure changes that are more than Necessary. Below 1 ° C.

But to calculate the change, you obviously need at least two measurements. This means you need an earthquake roughly the same as a previous one – what researchers call “duplicate”. However, these earthquakes don’t have to be massive, so this isn’t as difficult as you might imagine.

It’s not a coral problem

To prove this works, the researchers used the seismograph station at Diego Garcia, a small atoll in the Indian Ocean about 3,000 kilometers from Sumatra. The boundaries of the tectonic plate there are incredibly active, so there is no shortage of earthquakes to work with. Between 2004 and 2016, there were more than 4,000 earthquakes of magnitude 3.0 or higher near the Nias Island region of Sumatra. The researchers treated all of these events carefully to find replicates that were similar enough to calculate the temperature. They found more than 2,000 such pairs based on 900 earthquakes.

Zoom / This is an example of two nearly identical earthquakes, but the T waves took a little longer to arrive in the second earthquake because the ocean was much warmer.

If this part of the Indian Ocean warms by 1 degree Celsius, then the T-waves generated by those earthquakes would take 5.4 seconds to reach this seismometer. The observed changes are smaller than that but are coherent – there is both an annual cycle and an incremental warming trend that appears similar to other, more traditional data sets.

However, the trend calculated by “seismic ocean temperature” is slightly larger. Researchers compare estimates from the Argo robotic buoy array and NASA’s ECCO dataset, which collects data from various sources. During the same time period, Argo shows a warming trend of 0.026 ° C per decade in this region, while ECCO shows 0.039 ° C per decade. The seismic estimate comes in at 0.044 ° C per decade.

Zoom in / Here’s how to align the seismic temperature record (blue) with two datasets based on measurements from things like buoys and satellites.

It is not about declaring an estimate to be the best; It’s about proving that the technology is reasonably useful. Measuring temperature change completely independently is valuable in itself. Rather than relying on a finite number of point measurements with, say, a float, it integrates the average temperature change through this entire volume of water. This is unique.

It also measures a deeper portion of the ocean that is difficult to reach, and this depth may be somewhat adjustable depending on the specific frequency of the T wave you are working with.

The researchers state that underwater (aquatic) microphones could be more sensitive, taking advantage of smaller earthquakes. (An array of hydrophons already exists to support nuclear test ban treaties.) But perhaps the most interesting thing here is that current seismometer data can be linked to this technology. Therefore, using a defined method, it is possible to search for historical measurements rather than just future measurements. It is usually much cheaper to obtain data without using a set of new tools than publishing a set of new tools.

It turns out that physics can tell you a lot if your tools are accurate enough and know how to use them.

Science, 2020. DOI: 10.1126 / science.abb9519 (About DOIs).

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