It is not that earthquakes themselves were new, but rather that an earthquake would have been directly caused by God to punish Korah in front of everyone that would have been new.
Numbers 16:28 - 29 sets this up:
Then Moses said, “This is how you will know that the LORD has sent me to do all these things and that it was not my idea: If these men die a natural death and suffer the fate of all mankind, then the LORD has not sent me.
The sons of Korah (this is often called "Korah's rebellion") wanted new leadership, rather than simply relying on Moses' word. Moses was willing to put their claims to the test. Moses was even willing to give them the benefit of the doubt - if nothing bad happens when Korah tries to do its thing, then I'm not talking to the Lord.
But if the LORD brings about something totally new
i.e. if something bad happens, then yes, in fact the Lord is supporting me (Moses).
and the earth opens its mouth and swallows them.
In other words, if something new happens in the lives of the sons of Korah, then this is God talking. In a way, its no different than Gideon's fleece, only for the whole of Israel. Moses is stacking the situation so that God must show himself strong on account of Moses, or else God isn't talking to Moses.
...you will know that these men have treated the Lord with contempt.
In any event, earthquakes themselves had most likely happened before this.
- Genesis 7 talks about the fountains of the deep opening up - which could be an earthquake, and
- in Exodus 19:18, Mount Sinai itself was said to have "trembled greatly" when the Lord descended.
Again, what's new is the Lord making an event occur that is out of the ordinary, just as Korah speaks. This is also the first time in Scripture that the earth "swallows" someone up.
Incidently, according to this scientific study the Sinai has a long history of earthquakes, and they have been going on a long time.