One more time Italy has to face an earthquake that caused casualties and heavy damage, particularly to the artistic heritage. It is common to verify the reliability of the scientific knowledge after an earthquake; sometimes a strong event modifies even the approaches to the seismic risk: let's think about the lessons of the Northridge earthquake in California and of the Kobe earthquake in Japan.

For these reasons we present the seismic hazard estimates obtained by the National Group for the Defence against Earthquakes, delivered to the Civil Protection Department in 1996, in the light of the recent events. No new analysis has been done. The results are shown at regional scale, and with non-standard representations (the standard product is the map of shaking expected not to be exceeded at 90% probability level in 50 years, accepted by Eurocode EC8). Further details can be obtained from: http://emidius.irrs.mi.cnr.it/GNDT/PS.html#PERICOLOSITA.
Seismic hazard is the expected shaking at a certain site in a given period due to earthquakes. In Italy we adopted two quantities to represent the shaking: the horizontal peak ground acceleration (PGA) and the macroseismic intensity. The former describes the physics of the phenomenon, and has engineering application in planning; the latter approaches the social-economic aspects as it represents the expected damage. The computation refer to average soil and vulnerability conditions; no site effects have been taken into account.

Seismic hazard map in PGA

In Fig. 1 the peak horizontal acceleration with 90% non-exceedence probability is reported. It is expressed in g (gravity acceleration). The values refer to an average ground. The epicentral area of earthquakes of the present seismic sequence is located in the Central Apennines near the area with maximum seismic hazard (values larger than 0.36 g). The areas in the background are the seismogenic zones of the seismotectonic model of Italy, developed ad hoc for the seismic hazard computation. The epicentres of the main earthquakes of the present seismic sequence are reported as well: they have been computed by the European Mediterranean Seismological Centre. White symbol for September 26 at 02:33 (Italian time), yellow for September 26 at 11:40, green for October 3 at 10:55, and blue for October 7 at 01:24.

Seismic hazard map in macroseismic intensity

In the standard map (Fig. 2), the earthquake sequence falls inside the area corresponding to IX degree MCS. An elongated maximum cover the whole axial part of the Central and Southern Apennines: such high values are expected in Friuli region and in Calabrian Arc, where the absolute maxima are located. In Fig. 3 we represent the probability to exceed intensity VII-VIII in 50 years. No other region inside national borders has similar high probability values: the spot tends to disappear at greater intensities. This is compatible with a seismicity model of relatively frequent but moderate events.