Implications of earthquake return periods on the building quality

In the pre-modern age, before scientific computational procedures were formulated, effective earthquakeproof structural solutions were developed following rules simply based on "common sense" and experience. This kind of knowledge, handed down from generation to generation, led to the definition of structural solutions suitable to resist a variety of critical situations. In this way, both the original morphology and the evolution in time of most effective construction techniques are directly related to the level of criticality of local actions: in areas affected by severe and frequent natural disasters, special systems had to be developed. In order for this to happen, it is necessary that the memory of the natural event and the knowledge of the damage are not lost. The risk consciousness induces local society not to forget nor abandon traditional construction techniques, but rather to maintain and improve them. In this work, a couple of examples are selected, representing this ambivalence: Castel del Monte (on the southern slope of the Gran Sasso Mountain, Italy) and Lefkas (one of the Ionian Islands, Greece) are both characterized by high seismic hazard but with different return periods: 200 and 30 years, respectively. This has led, in the first case, to alternately forget and rediscover the tradition and, in the Greek one, to the development of a structural system able to resist simultaneously two kinds of criticality, the geotechnical and the seismic one.