Joint Seismic and Gravity Data Inversion to Image Intra-Crustal Structures: The Ivrea Geophysical Body Along the Val Sesia Profile (Piedmont, Italy)

Abstract

We present results from a joint inversion of new seismic and recently compiled gravity data to constrain the structure of a prominent geophysical anomaly in the European Alps: the Ivrea Geophysical Body (IGB). We investigate the IGB structure along the West-East oriented Val Sesia profile at higher resolution than previous studies. We deployed 10 broadband seismic stations at 5 km spacing for 27 months, producing a new database of ∼1000 high-quality seismic receiver functions (RFs). The compiled gravity data yields 1 gravity point every 1–2 km along the profile. We set up an inversion scheme, in which RFs and gravity anomalies jointly constrain the shape and the physical properties of the IGB. We model the IGB’s top surface as a single density and shear-wave velocity discontinuity, whose geometry is defined by four, spatially variable nodes between far-field constraints. An iterative algorithm was implemented to efficiently explore the model space, directing the search toward better fitting areas. For each new candidate model, we use the velocity-model structures for both ray-tracing and observed-RFs migration, and for computation and migration of synthetic RFs: the two migrated images are then compared via cross-correlation. Similarly, forward gravity modeling for a 2D density distribution is implemented. The joint inversion performance is the product of the seismic and gravity misfits. The inversion results show the IGB protruding at shallow depths with a horizontal width of ∼30 km in the western part of the profile. Its shallowest segment reaches either 3–7 or 1–3 km depth below sea-level. The latter location fits better the outcropping lower crustal rocks at the western edge of the Ivrea-Verbano Zone. A prominent, steep eastward-deepening feature near the middle of the profile, coincident with the Pogallo Fault Zone, is interpreted as inherited crustal thickness variation. The found density and velocity contrasts of the IGB agree with physical properties of the main rock units observed in the field. Finally, by frequency-dependent analysis of RFs, we constrain the sharpness of the shallowest portion of the IGB velocity discontinuity as a vertical gradient of thickness between 0.8 km and 0.4 km.

A new approach to construct 3-D crustal shear-wave velocity models: method description and application to the Central Alps

“…After that point, the Adriatic Moho depth gently increases to the South East. While this feature is well resolved both by our and the ANT results, the plate boundary and IGB zone (Scarponi et al 2020(Scarponi et al , 2021, at ca. 200-300 km distance, is better resolved using our sub-vertical rays, and it appears less clearly and shifted or smeared to the SE in the Lu et al (2018) model together with toodeep reaching low-velocities.…”

“…traveltime tomography, Ammon et al 2004;Kiselev et al 2008; ambient noise studies, Chen et al 2019;Guo et al 2015;Mroczek and Tilmann 2021;surface wave dispersion, Bodin et al 2012;Chen and Niu 2016;Julià et al 2000;Shen et al 2012) and gravity data (e.g. Basuyau and Tiberi 2011;Scarponi et al 2021;Tiberi et al 2003) are complementary in terms of resolution area and sensitivity to the only receiver functions investigations.…”

Moho depths beneath the European Alps: a homogeneously processed map and receiver functions database

“…We then apply the last quality control to the calculated RFs, similar to (Hetényi et al, 2015;Singer, 2017;Hetényi et al, 2018b;Scarponi et al, 2021), that is based on the signal to noise ratio and the amplitude of the direct wave signal. For the first part of this quality control, we calculate (1) the rms of the noise from 30 to 10 s before the P-wave arrival (rmsnoi), and (2)…”

Hamiltonian Monte Carlo Probabilistic Joint Inversion of 2D (2.75D) Gravity and Magnetic Data

“…Potential field data in the form of gravity and magnetic anomalies have been used for long to characterize the structure of the subsurface, ranging from applications to the small scale as in the context of exploration geophysics (e.g., Hinze et al., 2013; Y. Li & Oldenburg, 1998; Y. Li & Oldenburg, 2000; Nabighian et al., 2005; Zunino et al., 2009) to the continental scale, particularly to estimate heat flux or to infer crustal thickness (e.g., Baranov et al., 2018; Milano et al., 2020; C.‐F. Li et al., 2017; Llubes et al., 2018; Maule et al., 2005; Martos et al., 2017; Pappa et al., 2019; Scarponi et al., 2021; van der Meijde et al., 2013). Particularly popular for the easiness of use and interpretation are two‐dimensional (2D) models, which allow the study of vertical cross‐sections through the Earth.…”