Answer:
Earthquakes happen all the time. There are probably several happening right now at different places around the world. But they're all small ones, and are mostly not noticed. We only know about most of them because we have electronic instruments that can detect them. Some slightly larger ones happen less often, and are noticed, though they don't do any damage. Even less often is a large one that does some noticible shaking and some minor damage. The really big ones are least frequent. Fortunately.
The crust of the earth is made up of gigantic tectonic plates, and they are always in motion against, along or away from each other. The gradual movement of these plates releases energy, and we see these releases of energy as earthquakes. Along the plate boundaries we see what are called fault lines, and the quakes usually center on or around them.
As regards when the next "big" quake will come, we don't know. But it is coming. Big quakes have punctuated all of recorded history, and will continue to do so. We live on a living planet. Even the earth itself is alive. It can move - and it does.
Explanation:
