From the Cabinet Office and the Japan Meteorological Agency (JMA).
The Cabinet Office says that about 20% of the world’s earthquakes of magnitude 6.0 or more are said to happen in Japan, and that about 2,000 active faults are known from Hokkaido to Kyushu.
Large earthquakes are expected in the near future along the Kuril Trench and the Japan Trench, directly under the Tokyo area, along the Nankai Trough, and directly under the Chubu and Kinki areas.
But large earthquakes can also come from other faults. For the fault zone that caused the 2016 Kumamoto earthquake, the chance of a magnitude 7.0-class earthquake within 30 years had been under 1%.
The JMA’s seismic intensity scale, shindo, has 10 levels: 0, 1, 2, 3, 4, 5-lower, 5-upper, 6-lower, 6-upper and 7.
In the past, shindo was judged from how the shaking felt and what happened around people. Since April 1996, it has been measured automatically by seismic intensity meters.
The shindo that the JMA announces comes from observation points set up across Japan by the JMA, local governments and the National Research Institute for Earth Science and Disaster Resilience.
Right after an earthquake starts, an Earthquake Early Warning forecasts when strong shaking will reach each place and how strong it will be, and announces this as fast as possible.
People use it to protect themselves, to slow down trains, and to control machines in factories before the strong shaking arrives.
The time between the warning and the strong shaking is very short, from a few seconds to a few tens of seconds at most. Close to the center of the earthquake, the warning cannot arrive in time.
There is no single right answer. Talking it over with someone at home is a good way to learn.