Kinkyū jishin sokuhō (緊急地震速報), or Earthquake Early Warning (EEW), is a service provided by the Japan Meteorological Agency to provide advance warning when an earthquake is detected.

How the EEW system works

The Earthquake Early Warning system provides advance announcement of the estimated seismic intensity and expected arrival time of the principal motion of an earthquake, based on prompt analysis of the focus and magnitude of the earthquake using wave form data observed by seismographs near the epicenter.
An EEW is issued when an earthquake of seismic intensity (shindo / 震度) 5 or higher is expected, to areas where the seismic
intensity is expected to be 4 or higher.

How can I get Earthquake Early Warnings?

When issued, residents in areas that may be affected are notified via TV and radio, smartphones and mobile phones, and municipal emergency speaker systems, with an alarm sound and a recorded voice warning of an impending earthquake. Note that the alarm sound used for EEW notifications on TV, radio and emergency speaker systems (①an unsettling chime sound) differs from that used for smartphones (②a 'woop woop'-type alarm sound).

You can hear samples of the sounds online here:

① NHK: https://www.nhk.or.jp/sonae/bousai/ - A sample TV broadcast of an EEW

② docomo - The same sound is also used by other mobile phone service providers.

What should I do when an Earthquake Early Warning is issued?

You may only have a few seconds to act, so take immediate steps to protect yourself wherever you are before strong shaking begins.

At home

• Protect your head and get under a sturdy desk or table, or move to another safe place
• Don't rush outside
• Don't take risks by rushing to turn off gas and extinguish naked flames

On a train or bus

• Hold on firmly to hand straps and handrails

In an elevator

• Stop the elevator at the nearest floor and get out immediately

Outdoors (in urban areas)

• Beware of falling concrete block walls
• Beware of falling signs and falling broken glass

Driving

• Reduce speed but avoid braking suddenly
• Turn on the car's hazard lights to alert other vehicles

At a shopping mall or other facility

• Take steps to protect yourself, and follow the directions of staff