In the tunnel under Tokyo Bay, lights flicker in the darkness. They aren’t signaling Godzilla’s arrival—they are detecting muons. When high-energy cosmic rays collide with atoms in Earth’s atmosphere, muons are produced, and as a live video shows, the detectors constantly flash when they’re picked up. Last year, however, the detectors helped find something different. In a first, they observed meteotsunami waves generated by Typhoon Mindulle in September 2021, according to a new study in Scientific Reports.
Submarine Particle Hunters
Identifying meteotsunamis, let alone monitoring them, can be difficult because these large waves caused by storms can be confused with seismic tsunamis, storm surges, or seiches. Plus, in bays and lakes where tools are limited or unavailable, measurements of meteotsunamis are challenging and, in some instances, impossible. But better characterization and monitoring of meteotsunamis are necessary because they can be damaging to communities and infrastructure.
The researchers saw an opportunity to glean insights into the meteotsunami that occurred on Tokyo Bay in 2021. Muons can pass through large objects easily; therefore, muography has been used to “see” the invisible, such as the interiors of volcanoes. It’s also found applications in areas ranging from archaeology to nuclear engineering, such as peering into the crippled Fukushima Daiichi nuclear power plant. By looking at muon data, the density or other properties of the target can be inferred.
The researchers decided to try muography on meteotsunami waves, which are created by air pressure changes associated with storms. Detectors were installed in the underwater section of the Tokyo Bay Aqua-Line, a 14-kilometer bridge and tunnel route connecting the city of Kawasaki with the city of Kisarazu. Dubbed the Tokyo-Bay Seafloor Hyper-Kilometric Submarine Deep Detector (TS-HKMSDD), the array consists of detectors that were set up along a 200-meter stretch of the tunnel in March and June 2021. When Typhoon Mindulle passed about 400 kilometers (almost 250 miles) south of the bay, the array picked up slight differences in the scattering of muons through the seawater. The oscillations of the meteotsunami reached a height of 15 centimeters and decayed within a few hours because of the bay’s shallow depth. This instance was the first such use of muography, say the researchers, who believe the technique is well suited for ocean studies.
“HKMSDD is very cheap, accurate, and stabler than any other technique,” said Hiroyuki Tanaka, a researcher at the Earthquake Research Institute and the International Muography Research Organization at the University of Tokyo and an author of the new study. “As well, it can detect offshore tides. Unlike tide gauges or buoys, HKMSDD doesn’t have to be exposed to harsh environments, and there aren’t any mechanically moving parts in HKMSDD. As a matter of fact, HKMSDD has operated without any intermittency (even 1 second) for more than a year. Realization of this stability is probably impossible for any other legacy technology.”
Tanaka has used muography to attempt to predict volcanic eruptions at Sakurajima, a very active stratovolcano in southern Japan, as well as to image ancient tombs, dams, and industrial furnaces. Muography has advantages over tide gauges because it can provide a 2D view of a tsunami as opposed to point data, he said, adding that satellite-based radar altimetry for observing tidal changes is limited by orbits and repeat periods and Global Navigation Satellite System (GNSS) buoys are expensive and subject to GPS signal failure.
Worth the Cost?
So is muography ready for wider use in meteorology and ocean studies? At least one outside expert says it’s too early to tell.
“This is an interesting and exotic study, which demonstrates the multipurpose usage of large science infrastructures,” said Ivica Vilibić, a senior scientist in the Division for Marine and Environmental Research at the Ruđer Bošković Institute in Zagreb, Croatia, who was not involved in the study. “However, there is still a lot of work that should follow this initial study, e.g., to properly quantify meteotsunami height with muography.”
Muography is currently too expensive to be used exclusively for detecting meteotsunamis—according to Tanaka, muon detectors cost about $5,000 each. Sea level sensors, on the other hand, are cheaper to deploy to locations that are prone to weather waves, added Vilibić. But that doesn’t mean muography arrays don’t show promise for future use in meteotsunami detection in Tokyo Bay and beyond. TS-HKMSDD has been upgraded to 50 sensors since the study was first submitted for publication. HKMSDDs are planned for the North Sea and the Gulf of Finland, where meteotsunami waves have been observed, said Tanaka.
“We anticipate that more global data will be obtained, leading to more global understanding of meteotsunamis,” Tanaka said.
—Tim Hornyak (@robotopia), Science Writer