Abstract
Nowadays, policies addressed to prevention and mitigation of seismic risk need a consolidated methodology finalised to the assessment of local seismic response in explosive volcanic settings. The quantitative reconstruction of the subsoil model provides a key instrument to understand how the geometry and the internal architecture of outcropping and buried geological units have influence on the propagation of seismic waves. On this regard, we present a multidisciplinary approach in the test area of the Stracciacappa maar (Sabatini Volcanic District, central Italy), with the aim to reconstruct its physical stratigraphy and to discuss how subsoil heterogeneities control the 1D and 2D local seismic response in such a volcanic setting. We first introduce a new multidisciplinary dataset, including geological (fieldwork and log from a 45-m-thick continuous coring borehole), geophysical (electrical resistivity tomographies, single station noise measurements, and 2D passive seismic arrays), and geotechnical (simple shear tests performed on undisturbed samples) approaches. Then, we reconstruct the subsoil model for the Stracciacappa maar in terms of vertical setting and distribution of its mechanical lithotypes, which we investigate for 1D and 2D finite element site response analyses through the application of two different seismic scenarios: a volcanic event and a tectonic event. The numerical modelling documents a significant ground motion amplification (in the 1–1.5 Hz range) revealed for both seismic scenarios, with a maximum within the centre of the maar. The ground motion amplification is related to both 1D and 2D phenomena including lithological heterogeneity within the upper part of the maar section and interaction of direct S-waves with Rayleigh waves generated at edges of the most superficial lithotypes. Finally, we use these insights to associate the expected distribution of ground motion amplification with the physical stratigraphy of an explosive volcanic setting, with insights for seismic microzonation studies and local seismic response assessment in populated environments.
Highlights
The assessment of local seismic response is a key issue for policies finalised to prevention and mitigation of seismic risk (e.g. Sitharam and Anbazhagan 2008; SM Working Group 2008; Mihalić et al 2011; Santos et al 2011; Working Group Fivizzano 2019)
By comparing the results of linear 1D and 2D site response analyses and by examining the experimental horizontal to vertical noise spectral ratio (HVNSR) spectral ratios, we suggest that the amplification effects are essentially related to 1D resonance of the very soft lacustrine/epiclastic deposits on the stiffer underlying deposits, significantly enhanced by 2D aggravation effects related to the interaction of direct S-waves and Rayleigh waves generated at the maar edges
The study shows that electric resistivity tomography is a prospection suitable to highlight the main morphologic features and helps to identify lithotypes within the maar, while 2D seismic arrays provide suitable average values of S-wave velocity
Summary
The assessment of local seismic response is a key issue for policies finalised to prevention and mitigation of seismic risk (e.g. Sitharam and Anbazhagan 2008; SM Working Group 2008; Mihalić et al 2011; Santos et al 2011; Working Group Fivizzano 2019). Bibby et al 1995; Rowland and Sibson 2004; Lorenz and Kurszlaukis 2007; De Benedetti et al 2008; Martín-Serrano et al 2009; Árnason et al 2010; White and Ross 2011; Isaia et al 2015) Through this extreme variability of thickness and depositional facies, seismic spectral amplification and peak ground acceleration can significantly change over a volcanic apparatus (Mora et al 2001; Almendros et al 2004; Chávez-Garciá et al 2007; Nunziata 2007; Tramelli et al 2010; Panzera et al 2011; Santos et al 2011; Chávez-García and Kang 2014; D’Amico et al 2016; Galluzzo et al 2016). Understanding factors defining subsoil setting in volcanic environments is extremely relevant for highly urbanised areas
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