Assessment of local seismic response of the Stracciacappa maar (Central Italy)

In this work, we assess the local seismic response in the volcanic setting of the Stracciacappa maar (Sabatini Volcanic District, central Italy), whose pyroclastic succession records phreatomagmatic eruptive phases ended about 0.09 Ma ago. Its present-day crater has a diameter of about 1 km and a crater floor about 30–40 m below the crater rim. New geological map and cross sections illustrate the complex geometric relationships between the thick pyroclastic succession and even meter-sized lava ballistic. A composite interfingering between lacustrine and epiclastic debris sediments fills the crater floor. A continuous coring borehole was drilled inside the crater, 45 meters deep from the wellhead, with sampling of undisturbed samples. Then, an Electrical Resistivity Tomography test and three 2D seismic passive arrays were carried out for constraining the geological model and the velocity profile of the Swaves, respectively. Moreover, fourteen recordings of about 30 minutes of ambient vibrations were collected in order to define the resonance frequency of sedimentary covers. Regarding the non linear properties of soils, the cyclic soil behaviour was investigated through simple shear tests. The collected dataset was used to define the subsoil model for 2D site response analyses. Two different input motions were selected: a moderate magnitude, volcanic scenario and a high magnitude, tectonic earthquake. Both earthquake scenarios revealed significant ground motion amplification (up to 15 in terms of spectral acceleration at about 1 s) essentially related to 2D phenomena in correspondence of sharp lateral variation of thickness and deposition facies within the Stracciacappa maar. Our results provide insights on the assessment of local seismic response in similar volcanic settings in highly urbanised environments elsewhere.