An earthquake happens somewhere in the world once every thirty seconds.
Sometimes there are many small earthquakes before the big one. These small ones are called foreshocks. After the big earthquake, the mainshock, again there may be many small quakes. These are called aftershocks.
You would feel the ground shake in a magnitude 3 quake.
The deadliest known quake in history shook China in 1556, taking about 830,000 lives.