Aftershock
noun
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.
まだ余震が続いているので、気をつけてください。
There are still aftershocks, so please be careful.
今日は何回ぐらい余震を感じましたか？
How many aftershocks have you felt today?
その地震の後に、余震が相次ぎました。
The earthquake was followed by aftershocks.