Japan wires the ocean with an earthquake-sensing ‘nervous system’
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Within months of the 2011 earthquake, the Japanese government began to build S-net (Seafloor Observation Network for Earthquakes and Tsunamis). S-net wired the nation’s earthquake-detection network to the Japan Trench, the seismologically active offshore region where the 2011 earthquake began. Roughly 3,540 miles of cable now zigzag across 116,000 square miles of ocean to connect 150 observatories on the ocean floor. Each contains 14 distinct sensing channels, including seismometers and accelerometers, as well as pressure gauges to measure waves passing overhead. This network—the first part of the larger network that was completed in June 2025—was finished in 2017. When a magnitude 6.0 quake struck the following year, alerts reached the cities before the first jolt hit—a full 20 seconds before the nearest land seismometer rang its alarm—allowing precious time to slow bullet trains and broadcast warnings.
A much smaller seafloor network, the DONET (Dense Oceanfloor Network System for Earthquakes and Tsunamis) had been started in 2006 along a section of the Nankai Trough, another geologically active zone, where the Philippine Sea plate pushes beneath southwestern Japan. This zone had been considered Japan’s most urgent seismic threat. The last pair of magnitude 8.0-plus ruptures had occurred there in 1944 and 1946. And because historical intervals for major earthquakes in that area occur at an average of 100 to 200 years, stress between the plates was assumed to be nearing its breaking point. The Nankai megathrust zone lies only 40 to 60 miles off the densely populated hubs of Osaka and Nagoya and the Tōkai industrial belt—and the area’s trench geometry happens to aim tsunamis straight at the shore. Disaster plans project hundreds of thousands of casualties and economic losses of more than $1 trillion if warnings arrive only after land sensors are alerted. In 2013 DONET was expanded to include more than 460 miles of cables. And in 2019 the now recently completed N-net (Nankai Trough Seafloor Observation Network for Earthquakes and Tsunamis) was begun; it presently covers the rest of the Nankai megathrust zone. Connected by more than 1,000 miles of cable, N-net’s 36 observatories complete Japan’s larger earthquake-detection system.
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