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Today I Learned

Japan, apparently, gets a 10-second warning before an earthquake hits — on every phone

Japan's Earthquake Early Warning system uses P-waves to alert phones, TVs, and trains seconds before the shaking arrives. Sometimes it's 30 seconds. Sometimes it's two.

· 2 min read · By Daichi
A Japanese smartphone displaying the JMA Earthquake Early Warning alert in red Japanese characters
◇  A Japanese smartphone displaying the JMA Earthquake Early Warning alert in red Japanese characters

The phones scream first, then the building moves

The first time it happens, you don’t know what’s going on. Every phone in the room rips out the same harsh two-tone alarm — pi-roron, pi-roron — followed by a robot voice saying “Jishin desu, jishin desu” (earthquake, earthquake). Then, three to ten seconds later, the lamp on the table starts swaying.

That gap is engineered. It’s the Kinkyū Jishin Sokuhō — Japan’s Earthquake Early Warning system.

The fact

Operated by the Japan Meteorological Agency (JMA), EEW has been issuing public alerts since 2007. It uses Japan’s network of over 1,000 seismometers to detect the P-wave — the fast, mostly-harmless first wave of an earthquake — and broadcasts an alert before the slower, destructive S-wave arrives at populated areas. The alert reaches TVs, radios, public address systems, Shinkansen control rooms, factory floors, elevators, and every smartphone in a target region — without an app, via cellular Cell Broadcast.

Lead time depends on how far you are from the epicenter: directly above, you may get only a second or two; 100 km away, more like 20–30 seconds. Enough time for trains to brake, for surgeons to step back from the table, and for you to get under the desk.

Why it works this way

Three things make it possible:

  1. Wave physics. P-waves travel at roughly 6 km/s; S-waves at roughly 3.5 km/s. The further you are from the epicenter, the larger the gap between them.
  2. Density of sensors. Japan sits on four tectonic plates and runs the most-instrumented earthquake-monitoring network in the world. JMA has hundreds of stations; NIED’s Hi-net adds nearly 800 more borehole sensors.
  3. Cell Broadcast as infrastructure. Carriers are legally required to relay the JMA alert. Your phone doesn’t need an app or a SIM-billing plan — even a tourist’s SIM-free phone often gets the alert, as long as it supports Japanese Cell Broadcast.

The Shinkansen system goes further: the network has its own dedicated coastal sensors that automatically cut power to trains when they trigger. During the 2011 Tōhoku earthquake, this is widely credited with stopping every running Shinkansen safely.

Where you’ll encounter it

  • Anywhere in Japan, with any phone bought locally. The alert sound and TV chime are unmistakable; both are deliberately designed to be unpleasant.
  • NHK channels — every TV broadcaster overlays the alert immediately when issued.
  • JMA’s official site and app — for non-emergency strong-shake forecasts and post-event maps.

Closing

I’ve gotten the alert maybe forty times in my life. Twice I had time to move. Most of the time the shaking is small and over before you’ve finished standing up. Apparently that’s the deal — false alarms are part of the price of the warning that one day actually saves you.

  • Flagship guide: Earthquake Safety in Japan for Travelers — the alert sound, what to do indoors, and which apps to install.