Japan Wires the Ocean with an Earthquake-Sensing ‘Nervous System'

Scientific American

09-07-2025

If the ocean floor had a nervous system, it might look something like this: thousands of miles of fiber-optic cables connected to sensors set atop the fault lines where Japan's earthquakes begin. Completed in June, this system aims to stave off devastation like that of 2011—when a relentless six-minute-long temblor was followed by a 130-foot tsunami that reached speeds of 435 miles per hour and pounded cities into rubble. Delayed alerts gave some communities less than 10 minutes to evacuate and only warned of much smaller waves, based on inaccurate earthquake readings. Nearly 20,000 people died, with thousands more injured or missing. Reactor meltdowns at the flooded Fukushima Daiichi nuclear power plant irradiated the surrounding land and spilled radioactive water into the ocean.
The undersea, magnitude 9.0 'megathrust' earthquake—the worst in Japan's recorded history—began in the Pacific seafloor 45 miles off the country's eastern coast. Land-based sensors detected its first shock waves but couldn't immediately provide clear readings of its magnitude or that of the tsunami it created. Mere months later, Japan began expanding its earthquake-detection system to cover the ocean floor. With the system's completion last month, Japan has become the first country to achieve direct, real-time monitoring of entire subduction zones—adding minutes and seconds to evacuate people and brace crucial infrastructure for impact.
But the advanced warning system is not the entire story, says seismologist Harold Tobin, director of the Pacific Northwest Seismic Network. 'By wiring up the offshore fault zone, we're constantly able to listen to it,' he says. 'That means we can detect all sorts of subtle signals that tell us how faults work, such as the storage of stress and how it starts to be released at the beginning of an earthquake.'
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Japan Builds Its Ocean-Floor 'Nervous System'
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.
With the final N-net link set up this June, the complete system increases warning times by 20 seconds for earthquakes and a full 20 minutes for tsunamis—enough time to divert incoming flights and close sea gates in busy ports. And the project could provide seismologists with a treasure trove of useful new data. Of particular interest are slow-slip events, in which faults gradually release without earthquakes. 'If you wind the clock back 20 years, we basically thought faults were either locked and not moving at all or were having an earthquake and moving very, very fast,' Tobin explains. But slow-slip events reveal a third mode in which faults move faster than the steady plate tectonic rate but much slower than an earthquake. Whereas slow-slip events generally aren't present before small earthquakes, they often occur in the days before major ones—perhaps detaching 'enough of the fault zone that it prepares the system for a big earthquake,' Tobin says. 'That might end up being something we can use as an earthquake-precursor-detection system.' He's quick to point out, however, that not all slow-slip events are followed by earthquakes.
N-net technicians will spend the coming months calibrating instruments and folding their feeds into a single monitoring backbone that includes Japan's approximately 6,000 land-based sensors. But the hardest part is done: installing armored fiber-optic cables and observatories along the abyssal plain from ships and 'plowing' shallow seabed areas to bury cables and protect them from anchors and fishing gear. Underwater robots helped out in deeper waters and will now service the observatories and replace parts.
From Nuclear Bomb Detectors to Tsunami Alarms
The completion of Japan's network coincides with that of another tsunami-detection program at Cardiff University in Wales. GREAT (Global Real-Time Early Assessment of Tsunamis) came online in June and streams data from four of the 11 hydroacoustic ocean stations created for the Comprehensive Nuclear-Test-Ban Treaty Organization. Built to listen for clandestine nuclear bomb blasts, the globe-spanning system detects low-frequency acoustic-gravity waves. These pressure pulses sprint through seawater at roughly 3,355 miles per hour—more than 10 times faster than a tsunami's leading edge. Researchers at Cardiff University use machine-learning algorithms to interpret the hydrophone signals. Within seconds, the system estimates earthquake magnitude, fault slip type, and tsunami potential and sends out alerts, though researchers estimate that a total of two dozen hydrophone sites would be required to make coverage global.
Cascadia's Silent Megathrust: A Massive Quake Waiting Unseen
Even as such detection systems expand, however, one of the planet's most vulnerable faults remains among the least monitored: the Cascadia megathrust fault, which runs along the coast of the Pacific Northwest from Vancouver Island to northern California. Unlike Japan's faults, this one does not produce many small earthquakes—which initially led seismologists to believe it posed little risk. But recent research has shown it is prone to rare but massive quakes. In stark contrast to Japan, the Cascadia megathrust fault has only a single cable with three seismometers, though funding was recently secured to replace one of the seismometers and to add three more. (Canada also has a small cable system in place.) 'We have just the paltriest beginnings of what they have in Japan,' Tobin says. Early detection of a massive earthquake could give tens of millions of people along the Pacific Northwest coast more time to prepare, as might detection of slow-slip events in the fault. 'We actually understand really well now that it's just storing up the stress toward a very big—potentially magnitude-9-scale—earthquake, every bit as big as 2011, with the same tsunami hazard,' he says. 'It's pretty inevitable.'