In a science first, a cosmic ray sensor detected tsunami waves

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We are all aware of the devastating impact that tsunamis can have, and the potential for tragedy continues to drive research into improved detection methods. A new study reports on a more common approach to tsunami detection: muon monitoring.

These high-energy elementary particles are created when cosmic rays arrive from space, are everywhere in the atmosphere, and can pass harmlessly through just about anything, including you: 100,000 of them will pass through your body during you will read this sentence.

Importantly, they can be very slightly moved in their path by large natural forces, including tsunamis.

An incredibly sensitive instrument is needed to detect muon motion, which brings us to the Tokyo-Bay Seafloor Hyperkilometer Deep Underwater Detector, or TS-HKMSDD for short. It is installed inside the Tokyo Bay Aqua-Line expressway tunnel.

TS-HKMSSD had the honor of detecting tsunami waves through muon ripples for the first time. Significant from the perspective of earlier warnings, the detection occurred in real time and was found to be very accurate.

“The Tokyo Bay Seafloor Hyper-Kilometer Deep Underwater Detector is the world’s first underwater muon observatory, and it detected variable muon activity during the tsunami,” says geophysicist Hiroyuki Tanaka, of the University of Tokyo in Japan.

“This variation is consistent with ocean swells that have been measured by other methods. Combining these readings means we can use muographic data to accurately model changes in sea level, bypassing other methods that exhibit disadvantages.”

These other methods include tide gauges, buoys in the water, satellite imagery taken from above, and various sensors in the sea itself. Muon detection, however, promises to be faster, cheaper and easier to maintain than these approaches.

The new research describes how the TS-HKMSDD system detected a mild tsunami crossing Tokyo Bay in September 2021, caused by a typhoon approaching Japan from the south. As the ocean swelled, the number of muons changed slightly, dispersed by the volume of water.

Now that TS-HKMSSD has shown it can detect these muon shifts, the researchers suggest that instruments like this could be installed in other tunnels in tsunami-prone areas and used with equipment such as tide gauges as part of early warning systems.

“Thanks to the success of early tests like this, similar systems are already being tested in the UK and Finland,” says Tanaka.

“Obviously, an undertaking like this presents challenges, and installing delicate instruments in a busy tunnel can be difficult. But we are grateful for the cooperation of the agencies responsible for the Tokyo Bay Tunnel. “

The muon detectors that make up TS-HKMSSD are actually quite small, about 2 meters (6.5 feet) long. Currently, 20 of them are placed along the road tunnel under Tokyo Bay, working together to create the overall system.

In addition to detecting approaching tsunamis, a system like this could be used to search for natural gas reserves and reveal old earthquake patterns.

For now, the researchers are excited to have the TS-HKMSSD operational as an accurate tsunami detector, which, with some additional work, will be available to alert experts in the event of a natural disaster.

“To my knowledge, the tunnel is now the first active national road in the world defined as a laboratory,” says Tanaka.

The research has been published in Scientific reports.


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