Sun. Oct 2nd, 2022
A seismometer on Diego Garcia's atoll (left) can calculate ocean temperature with earthquakes near Sumatra (right).
enlarge A seismometer on Diego Garcia’s atoll (left) can calculate ocean temperature with earthquakes near Sumatra (right).

Geophysics has shown that precise measurements and a little bit of modeling can work wonders, showing us the detailed structure of the Earth’s interior, despite being inaccessible buried under hundreds of miles of rock. This is possible because seismic waves produced by earthquakes subtly change speed or direction as they pass through different materials. A new article shows that something similar can even be small temperature changes in the deep ocean

An idea to use acoustic waves from man-made sources arose several decades ago, but died out after some trials. A team led by Wenbo Wu of the University of Toronto realized that earthquakes could be exploited in a similar way, eliminating the expensive logistics of constantly dropping booms to get measurements, as well as concerns about the effects on marine life. taken away.

There are actually different types of seismic waves released by earthquakes, and each behaves a little differently. The P wave (P for “primary” because it is the first to arrive) is analogous to a sound wave in that it compresses the rock in the same direction it travels. So where this wave reaches the seabed or ground surface, the rock can act like a huge loudspeaker and produce a very low-frequency sound wave in the air or water.

The same process can also work in reverse. Suppose there is an earthquake under the seabed, near a shoreline. The movement of the seafloor can create a sound wave that travels across an ocean basin and re-enters the seafloor rock of an opposing coast. This is actually detectable by seismometers as vibrations that arrive extremely late – because the wave travels much more slowly through water than through rock. As it appears after the primary and secondary seismic waves, this strange acoustic straggler is called a tertiary or T wave.

The travel speed of the T wave is sensitive to water temperature, while warmer water slows it down. (A tad more complicated is that it is also somewhat sensitive to changes in salt concentration or current movement, but the researchers say the temperature effect dominates.) Seismometers are sensitive enough to detect very small differences in timing, so this is able to detect changes of many less than 1°C.

But to calculate a change, you obviously need at least two measurements. That means you need an earthquake nearly identical to a previous one — what the researchers call a “repeater.” However, it is not necessary for these earthquakes to be huge, so this is not as difficult as you might think.

It’s no problem atoll

To show that this works, the researchers used a seismometer station on Diego Garcia, a small atoll in the Indian Ocean about 3,000 kilometers from Sumatra. The tectonic plate boundary there is incredibly active, so there’s no shortage of earthquakes to work with. Between 2004 and 2016, there were more than 4,000 earthquakes of magnitude 3.0 or greater near the Nias Island area of ​​Sumatra. The researchers carefully processed all of these events to find repeaters similar enough to perform the temperature calculation. They found more than 2,000 such pairs based on 900 earthquakes.

Here's an example of two nearly identical earthquakes, but the T waves took a little longer to get to the second because the ocean was warmer.
enlarge Here’s an example of two nearly identical earthquakes, but the T waves took a little longer to get to the second because the ocean was warmer.

If this part of the Indian Ocean were to warm up 1°C, T waves from those earthquakes would take 5.4 seconds longer to reach this seismometer. The observed changes are smaller than that, but they are coherent: there is both an annual cycle and a gradual warming trend similar to other more traditional data sets.

The trend calculated by this “ocean seismic thermometry” is a little bigger. The researchers compare it with estimates from the automated Argo array of floats and with NASA’s ECCO dataset, which combines data from several sources. Over the same period, Argo shows a warming trend of 0.026°C per decade in this area, while ECCO shows 0.039°C per decade. The seismic estimate comes out at 0.044 °C per decade.

Here's how the seismic temperature record (blue) matches two data sets based on measurements from things like floats and satellites.
enlarge Here’s how the seismic temperature record (blue) matches two data sets based on measurements from things like floats and satellites.

It’s not about labeling one estimate as the best; it is about demonstrating that the technology is plausibly usable. Measuring temperature changes completely independently is valuable in itself. And instead of relying on a limited number of point measurements from, say, a float, it integrates the average temperature change throughout that entire volume of water. That’s unique.

It also measures a deeper part of the ocean that is more difficult to access, and that depth can be tweaked slightly based on the specific frequency of the T-wave you’re working with.

The researchers state that underwater microphones (hydrophones) could be even more sensitive, taking advantage of smaller earthquakes. (A range of hydrophones already exist in support of nuclear test bans treaties.) But perhaps the most interesting thing here is that existing seismometer data can be probed with this technique. So with a nailed down method it is possible to look for historical measurements rather than just future ones. And getting data without deploying a set of new tools is usually a lot cheaper than deploying a set of new tools.

Physics, it turns out, can tell you a lot about whether your instruments are precise enough and you know how to use them.

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

By akfire1

Leave a Reply

Your email address will not be published.