"Your Guide to Earthquake Safety"

Aftershock

"Aftershock" is a little earthquake we had continually after big earthquakes.
On the other hand we call big earthquake we have before having aftershock "main shock".

Aftershock is considered that, it occurs because of bedrocks’ being unstable condition dew to having main shocks.

Aftershock usually occurs immediately after having main shock and its number has becoming fewer and fewer properties, in other word, it decays softly.
So a lot of people in Japan have had impression that aftershock goes on indefinitely.

Moreover, "Aftershock area", where aftershock occurs, became wider and wider over time.
So, in the area which had big tremor has possibility of houses destroyed or mudslide, we need be carefully strictly.

Such earthquakes have main shock and aftershock is a pattern of earthquake, what is called "Main shock aftershock" type.
And big earthquake which has shallow hypocenter is mostly main shock- aftershock type.

By the way, there is a difference pattern of earthquake; it’s called "Swarm-type".
It doesn’t have big tremor remarkably.
But in this pattern, seismicity becomes intense or calm.