A detailed study of the site effects in the volcanic area of Campi Flegrei using empirical approaches

Campi Flegrei is a highly populated active caldera in the south of Italy. Several hundred thousand people live within this area, which is characterized by seismicity and ground deformation episodes, known as 'bradyseism'. For this reason, this area falls into a high-risk category and thus the Italian Civil Defence requires a detailed site-effect estimation. To determine the local amplification of the seismic waves for a high number of sites, we have analysed the seismic recordings of three seismic networks that have been deployed in the Campi Flegrei area over different time periods. The first network was deployed during the bradyseismic crisis of 1982-1984. We selected 22 of the highest magnitude earthquakes that were recorded during this crisis. An additional 22 seismic events were selected from those recorded by the mobile seismic network that has been in operation in the Campi Flegrei area since 2006. The third data set comprises noise recorded by 34 seismic stations that were deployed during the active SERAPES experiment in 2001 September. The generalized inversion technique and the H/V spectral ratio method were applied to the S waves and coda waves of the earthquakes recorded by the first two seismic networks, to determine the site-transfer functions of the recording stations. The seismic noise recorded by the third network was analysed using the Nakamura's technique. The results show that the high topographical and geological heterogeneity of the sites located inside the caldera has an important influence on the seismic-wave amplification. Consequently, the site-transfer functions can be different even at sites close to each other. The transfer functions of the sites located outside the caldera are much more regular, apparently due to the more regular topography and geology.