From regional seismic hazard to “scenario earthquakes” for seismic microzoning: a new methodological tool for the Celano Project

We present the results of a probabilistic seismic hazard assessment and disaggregation analysis aimed to understand the dominant magnitudes and source-to-site distances of earthquakes that control the hazard at the Celano site in the Abruzzo region of central Italy. Firstly, we calculated a peak ground acceleration map for the central Apennines area, by using a model of seismogenic sources defined on geological-structural basis. The source model definition and the probabilistic seismic hazard evaluation at the regional scale (central Apennines) were obtained using three different seismicity models (Gutenberg–Richter model; characteristic earthquake model; hybrid model), consistent with the available seismological information. Moreover, a simplified time-dependent hypothesis has been introduced, computing the conditional probability of earthquakes occurrence by Brownian passage time distributions. Subsequently, we carried out the disaggregation analysis, with a modified version of the SEISRISK III code, in order to separate the contribution of each source to the total hazard. The results show the percentage contribution to the Celano hazard of the various seismogenic sources, for different expected peak ground acceleration classes. The analysis was differentiated for close (distance from Celano o20 km) and distant (distance from Celano 420 km) seismogenic sources. We propose three different ‘‘scenario earthquakes’’, useful for the site condition studies and for the seismic microzoning study: (1) large (M > 6.6) local (Celano-epicentre distance 16 km) earthquake, with mean recurrence time of 590 years; (2) moderate (M > 5.5) local (Celano-epicentre distance 7.5 km) earthquake, with mean recurrence time of 500 years; and (3) large (M > 6.6) distant (Celano-epicentre distance 24 km) earthquake, with mean recurrence time of 980 years. The probabilistic and time-dependent approach to the definition of the ‘‘scenario earthquakes’’ changes clearly the results in comparison to traditional deterministic analysis, with effects in terms of engineering design and seismic risk reduction.