In this study, we face the problem of local seismic response in explosive volcanic setting by using an
integrated geological-geophysical-geotechnical approach in the test area of Stracciacappa maar (Sabatini
Volcanic District, central Italy). Our aim is to understand if the horizontal and vertical chaotic heterogeneity
typical of the volcanic deposits influence site response.
The Stracciacappa maar is an active eruptive centre characterised by a crater of about 1 km in diameter and a
crater floor of about 30-40 m below the rim (De Rita et al. 1983; Sottili et al. 2012). The ring is mainly
composed by the pyroclastic succession belonging to the last phreatomagmatic activity. This pyroclastic
succession generally bends outward the rim with low angle dip (10-20°). It consists of at least 25 metres thick
alternation of decimetre- to centimetre-thick layers of fine-medium ash and small lapilli. The crater depression
is filled by epiclastic debris deposits and by recent and present-day lacustrine muds. The epiclastic debris
deposits, of alluvial and delta origin, consist of alternation of cm-thick reworked fine-grained and
coarse-grained volcaniclastic material, dipping with low-angle (1s) motions, compared to tectonic events of
equivalent magnitude (Jousset and Douglas 2007); the unscaled recording at Bronte Station (BNT in ITACA
database, http://itaca.mi.ingv.it) of the ML=4.4 October 27, 2002 event was employed; 2) a high magnitude
far-field “tectonic” event (tectonic scenario), whose reference spectrum was built with Ground Motion
Prediction Equations (Ambraseys et al., 2005) assuming M=6.5 and distance of 70 km. These conditions are
compatible with seismogenic sources located in central Apennines of Italy. Three unscaled recordings of events
characterised by magnitude and distance in the range of 6-7 and 60-90 km, respectively, were extracted from
ITACA database (http://itaca.mi.ingv.it), matching on average the reference spectrum
Two subsoil models have been considered: a detailed model (based on distribution of the lithotypes unravelled
by the geological survey) and a simplified one (obtained by grouping interfingering lithotypes resting below the
lacustrine silty clays). The result show that the two models have similar response in all range of the interesting
period (0.1-1.0s); the damping properties of soft clays and sands deposits in the upper meters reduce the
difference in the seismic response at the surface of both models. The results suggest the possibility to simplify
the heterogeneous distribution of deposits in this volcanic context for assessment of seismic response
purposes.
Finally, we carried out both linear and equivalent linear analyses in one-dimensional and bi-dimensional
conditions, in order to investigate the bidimensional effects and the role of nonlinearity on the seismic
response,. In linear case the behaviour of soils was assumed linear visco-elastic with small strain damping ratio
values D0; amplification factors higher than 10 were reached at 2 Hz at soft clays surface in correspondence of
the centre and western edge of the maar in bi-dimensional analysis, whereas the 2D/1D ratios were in the
order of 2-3 around 2 Hz with maximum values at the maar edges. In the nonlinear analyses, the maximum
amplifications dropped below 10 and the 2D effects (i.e., 2D/1D ratios) were generally lower than 2 in the
whole range of frequency
