Japan has taken a major step forward in disaster preparedness by becoming the first country in the world to fully cover all its earthquake faultlines with underwater sensors and cables. These are connected to hundreds of observatories, creating a warning system that detects earthquakes and tsunamis before they strike.

This network acts like a giant nervous system under the sea, picking up even the smallest signs of seismic activity. Experts say the system can now send out an alert about an earthquake 20 seconds before it hits, and warn of a tsunami up to 20 minutes in advance.

This early warning could give emergency teams more time to issue alerts, help people evacuate, and prepare buildings and infrastructure for the coming impact.

The idea for this massive expansion came after the devastating 2011 Great East Japan Earthquake, one of the worst natural disasters in the country’s history. The magnitude 9.0 quake killed nearly 20,000 people, lasted six minutes, and unleashed a massive 130-foot tsunami that moved at speeds of 700 km/hour.

The quake began 72 km off the coast, but because the sensors were only on land at the time, the warning systems couldn't detect the full size of the earthquake or the tsunami that followed. Many people had just 10 minutes or less to escape.

This tragedy made Japan realise its biggest blind spot was beneath the ocean.

Seismologist Harold Tobin, director of the Pacific Northwest Seismic Network, says this new system is far more than just an advanced alert setup.

“By wiring up the offshore fault zone, we’re constantly able to listen to it. 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.”

The first part of this network, called S-net (Seafloor Observation Network for Earthquakes and Tsunamis), was finished in 2017. It focused on wiring Japan’s earthquake detection systems to the Japan Trench, the same area where the 2011 quake began.

This system now includes 5,700 kilometres of cables, reaching across 116,000 square miles of ocean and connecting to 150 seabed observatories. These observatories use seismometers, accelerometers, and pressure gauges to track underwater movement.

In 2018, a magnitude 6 quake hit Japan. Thanks to S-net, the warning alert reached cities 20 seconds before land-based systems picked it up.

Japan didn’t stop there. In 2019, it launched another system called N-net, focused on the Nankai Trough—a highly active earthquake zone that hasn’t had a major quake since 1946.

This trench is about 800 km long and lies where the Philippine Sea Plate is sliding under Japan. A report has warned that if a large earthquake hits this region again, it could kill up to 300,000 people and cause $2 trillion in damages.

The Nankai Trough even made headlines recently because of an old 1999 manga comic that predicted a giant earthquake and tsunami on July 5, one that would be even worse than the 2011 disaster.

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