Towards a high-resolution vS crustal velocity model for the Ivrea Geophysical Body: constraints from seismic ambient noise tomography

The arc of the Western Alps is characterized by a complex crustal structure. Lower-to-middle crustal composition outcrops are exposed in the Ivrea-Verbano Zone (IVZ) and a major crustal anomaly, known as Ivrea Geophysical Body (IGB), presents dense and seismically fast rocks right below the surface. Understanding better their relation provides a key to refine our understanding of orogeny formation mechanisms.

We performed seismic ambient noise tomography using data from the IvreaArray and the AlpArray Seismic Network, selected within a radius of ca. 100 km around the study area. Previous seismic investigations provided knowledge on the crustal structure in the Western Alps, by means of active refraction seismics and of more recent local earthquake and ambient noise tomography at regional scales (e.g. Solarino et al. 2018 Lithos, Lu et al. 2018 GJI). Recently, gravity data and receiver function analysis imaged the IGB as a dense and fast seismic anomaly, related to upper mantle material, reaching up to few km depth below sea level (Scarponi et al. 2021 Frontiers). However, local high-resolution constraints on the absolute v_S distribution remain unknown.

We used raw summer seismic data (June to September) across 3 years of recording, and computed daily ambient noise cross-correlation traces, for all the available station pairs (61 stations in total) in the 2-20s period range. Daily cross-correlations were stacked and processed to extract Green’s functions. Subsequently, we performed frequency-time analysis to get group velocity dispersions for the fundamental mode of surface Rayleigh waves. We computed 2D surface group velocity maps at each period, which clearly show the slow sediment area of the Po Plain, and the fast IGB structure within the crust.

We are going to use the 2D group velocity maps to derive local dispersions curves and invert for 1D v_S-depth profiles with the use of the Neighborhood Algorithm, to produce a 3D v_S velocity model for the IVZ at high-resolution. This will also provide new geophysical constraints in the target area of the scientific drilling project DIVE (www.dive2ivrea.org) and reliable information for crustal corrections, which are necessary for upper mantle studies in such a complex area.