Free-floating observatories record seismic waves to help study Earth's interior.
Two small, mobile torpedo-shaped buoys plying the waters of the Mediterranean Sea have captured the seismic signature of a magnitude-7 earthquake occurring some 10,000 kilometres away. The earthquake, in the Aleutian Islands near Alaska on 24 June, was documented by two floating seismic observatories nicknamed MERMAIDs (for Mobile Earthquake Recorder in Marine Areas by Independent Divers), and reported in the journal Eos this week1 by a team led by Yann Hello of the Géoazur laboratory at the University of Nice in Villefranche-sur-Mer, France.
Hydrophones aboard the free-floating MERMAIDs record seismic P waves, one component of earthquake waves that travel through the centre of Earth. The experimental buoys that the team released for a few weeks this summer drifted with the sea's currents at depths of up to 2 kilometres, and the two recorded the same Aleutian event from different locations in the Mediterranean. Once they have a fleet of buoys at sea, that offset will allow Hello and his colleagues to compare the P wave paths to see how the waves travel from the source of an earthquake to hydrophones on the other side of the world.
Differences in the paths should highlight structural variations deep within Earth, as the speed of the waves changes as they travel through different structures such as the mantle and core. A hot plume like the one that is thought to feed the formation of the Hawaiian Islands, for example, would slow down the waves because of differences in temperature and composition with the surrounding mantle. The test run proves that the MERMAIDs can 'see' distant earthquakes, and Hello and his team hope to launch a fleet of the buoys, which would provide enough data to detect differences in Earth's interior.
Most seismic monitoring is currently done by land-based stations, which leaves waves that end at the ocean-covered parts (about two-thirds) of the planet unmonitored, says geophysicist Jonathan Berger of the Scripps Institution of Oceanography in La Jolla, California, who is not involved in the MERMAID project. "Imagine getting a CAT scan of your head, and instead of getting good coverage of your brain, you only get half — because the receivers aren't there," Berger says.
Having a large mobile array of monitoring buoys travelling the oceans would hugely increase the amount of data available for tomographic models of Earth's interior structures. Berger says that "the big advantage is that [MERMAIDs] will get many, many more [P wave] paths that end up in the ocean. The more paths you get, the more detailed the picture."
Richard Allen, a seismologist at the University of California at Berkeley, says: "The whole concept of floating observatories could change the game. Over time we could start to collect data from every point on the surface of Earth." That data could move seismology research on local earthquakes to a global scale, using three-dimensional structural models of the entire planet. A MERMAID flotilla would be much less expensive than setting up large arrays of ocean-bottom seismometers, and would complement those already on the ocean floor, which continuously record seismic data, says Berger.
But, cautions Guust Nolet, one of the lead investigators for MERMAID, "the price you pay is that the data are very noisy", as the ocean vibrates with everything from rainfall to ship noises. The team is working on modelling techniques to filter out the noise, and to help the autonomous buoys decide when to surface to report only the largest earthquakes and thus save battery life.
The team envisions a network like that of the Argo project, which has more than 3,000 buoys in oceans worldwide that measure water temperature and other environmental conditions. The MERMAID researchers plan to deploy half a dozen buoys next year, each at a cost of €20,000 (US$27,000).
Meanwhile, geophysicist Frederik Simons at Princeton University in New Jersey is working on the next generation — Son-O-MERMAID — which will harvest energy from ocean waves to eliminate the need for heavy batteries and lengthen the life of the buoys. And earlier this week, Berger and his colleagues at Scripps received funding from the US National Science Foundation to build their own wave-powered travelling buoys, which will visit and retrieve data from stationary ocean-bottom seismometers.
Hello, Y., Ogé, A., Sukhovich, A. & Nolet, G. Eos 92, 337 (2011).
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Lubick, N. MERMAIDs detect distant earthquakes. Nature (2011). https://doi.org/10.1038/news.2011.583