What is a surprise earthquake? The example of the 2002, San Giuliano (Italy) event

Both in scientific literature and in the mass media, some earthquakes are defined as surprise earthquakes . Based on his own judgment, probably any geologist, seismologist or engineer may have his own list of past surprise earthquakes . This paper tries to quantify the underlying individual perception that may lead a scientist to apply such a definition to a seismic event. The meaning is different, depending on the disciplinary approach. For geologists, the Italian database of seismogenic sources is still too incomplete to allow for a quantitative estimate of the subjective degree of belief. For seismologists, quantification is possible defining the distance between an earthquake and its closest previous neighbor. Finally, for engineers, the San Giuliano quake could not be considered a surprise, since probabilistic site hazard estimates reveal that the change before and after the earthquake is just 4%.