On April 27th at 19:03 GMT a magnitude Ms=5.1 happened in the Central part of the Gulf of Corinth (38.37-22.10) and not Patras as you mention. Following this more than 70 events were recorded above Ms=2.5. The b-values of the sequence=1.19 are very close to the normal "backround" b-values=1.14 and the mechanisms of these events are inhomogeneous indicating a swarm and not a foreshock sequence. The reason for this activity is the extensive stress regime N-S opening up the Gulf of Corinth by 1.5 cm/year and making this region the highest in seismicirty in Europe.
Best regards