What is a seismic gap?

It is a scientific theory under investigation (experiments, statistical analysis) this moment which is based on the thought that earthquake in a region is a time repeated phenomenon. Therefore, by drawing maps including many neighboring seismic regions with the date that an earthquake hit each one of them, one could create the sequence of these events. Finally, by following that sequence in a long term basis, one would realise that there will be regions in the map which await an earthquake as they compose part of the mentioned sequence.

In simple Words, it is like you want to build a 500 pieces puzzle, only now someone else is telling you where to put each piece (the earthquake). The puzzle is the map thus the more pieces you put, the more you can see on your puzzle (map) what is missing. Therefore the more accurate you become to your prediction of the location of the next piece (earthquake).