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.
The aftershocks are still coming so we need to be careful.
How many aftershocks have you felt today?
The earthquake was followed by aftershocks.