Analysis: Jonathan Amos, BBC science correspondent
Japan is one of the most seismically active areas on Earth, accounting for about 20% of global quakes of magnitude 6.0 or greater. Seismometers are recording some kind of event every five minutes, on average.
It is through bitter experience that Japan has learnt the strategies to mitigate damage, injury and death. Not only does it implement some the best building construction practices but it has also established an early warning network.
This system relies on the lightning analysis of the developing quake, establishing its location and strength. Alerts are then broadcast that can give people more distant from the epicentre vital seconds' notice.
Just 10 seconds is more than sufficient to drop and get under a sturdy table or open the doors of a fire station.
The prospect of buildings already damaged in Thursday's quake toppling over in this latest tremor will be a concern.