As Maik says, there are small ones fairly regularly. The biggest in recent years was January/February 2014, when there were two big ones of more than 6.0. Over a three week period, there were many aftershocks, on average every 20 minutes. In some parts of the World this would cause major damage and loss of life, but with strong building regulations in Greece, the effect is mitigated and there was no loss of life.
At the time, we took the precaution to have a bag of clothes and other essentials outside the house, just in case!