Warm water source of Therma

There are two major causes for hot water sources. One is volcanic activity. The other is rocks which are under pressure (e.g. due to tectonic activity). This area has both, although the last active volcanic activity dates from a while ago (Miocene), I guess it is linked to rocks under pressure due to the tectonic activity. Remember that the last huge earthquake on the island dates from less than 100 year ago.