The aftershocks are still coming so we need to be careful.
How many aftershocks have you felt today?
The earthquake was followed by aftershocks.
A surplus (that's an extra!) earthquake is a quake that comes after the main (non-surplus) quake. That's an aftershock.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.