SUBSOIL

SEISMIC MICROZONING OF NAPOLI

The seismic microzoning of the urban area of Napoli has been defined with a hybrid method based on the modal summation technique for the one-dimensional part of the model (from the seismic source to the borders of the two-dimensional section) and the finite difference technique within the 2D-section (Nunziata, 2004). The computed seismograms include the focal mechanisms and the magnitude of the possible source and the complete propagation wavefield, volume and surface waves, in a visco-elastic medium whose physical parameters, mainly the shear wave velocities V_S, are known.

The main phases of the research have regarded the historical and recent seismicity study for the definition of the scenario earthquake (Nunziata, 2007), the collection of stratigraphies and physical parameters of the soils, the shear wave velocity measurements with non invasive techniques suitable for the urban area (FTAN method and hedgehog non linear inversion) for the evaluation of the site effects (Nunziata et al., 2004).

In particular, the 23 November 1980 Irpinia earthquake (M_S=6.9) is of primary importance because it was the first strong earthquake recorded near Napoli (Torre del Greco accelerometer station). It was used to successfully test the computation method, based on the modal summation technique and the propagation medium from the Appenninic belt to Napoli, on outcropping rock (ref. in Nunziata, 2004). This earthquake has allowed to correlate the seismic intensity, used for the historical earthquakes, with the magnitude of the earthquake.

Taking in to account the stratigraphies, the urban area of Napoli has been divided in 6 geologically homogeneous zones (Nunziata, 2004).

Seismograms have been computed for the 1980 earthquake (M_S=6.9) along representative sections of the recognized six geological zones, assuming different V_S models measured in each zone to evaluate the ground motion effect. Local seismic amplification effects have been estimated in terms of spectral amplification, defined as the ratio between the response spectra computed along the two-dimensional section and the response spectra defined for the reference one-dimensional model. For an engineering practical use, the mean and maximum spectral amplification have been computed among all the sections. For the response spectra computation a 2% damping factor, for the masonry structures, and a 5% value, for the armed concrete structures, have been used.

The zone 1, located in the western sector of the city, is characterized by a cover, sometimes tmore than one hundred metres thick, of recent pyroclastic material overlying marine sands or occasionally Neapolitan Yellow Tuff.

The zone 2 is characterized by recent pyroclastic products and man made grounds overlying Neapolitan yellow Tuff. The top of the tuff is on average at 15 m of depth.

The  zone 3 is in the eastern sector of the city: the northern part, zone 3N, is characterized by ancient tuffs below a cover of Neapolitan yellow tuff in soil facies and Campanian Ignimbrite. The southern part, zone 3S, is characterized by vesuvian tuffs in soil and lithoid facies.

The zone 4 was until the last century under the sea level and subsequently the subsoil is constituted by marine sands on Neapolitan Yellow Tuff, at about 30 m of depth.

The zone 5 includes the historical centre of Napoli and is characterized by a man made grounds cover, up to 20 m thick, and a pozzolana layer overlying Neapolitan Yellow Tuff, with several cavities (Nunziata et al., 2002).

The zone 6 has been studied in more detail for the building of the Centro Direzionale, after the 1980 earthquake. This zone was a lacustrine area drained for the urban expansion; it is characterized by important lateral variations. The subsoil is constituted by succession of man made grounds, alluvial soils (ashes, sands, peats) loose and poorly lithified pozzolana, Neapolitan Yellow Tuff and marine sands (Nunziata et al., 2000).