High resolution 2-D image of the Ivrea-Geophysical Body: a joint inversion of new seismic and gravity data.

Abstract:

We present a high-resolution image of one of the most prominent geophysical anomalies of the European Alps: the Ivrea Geophysical Body (IGB). We here investigate the IGB and the associated crustal structure in 2-D along Val Sesia by means of receiver functions (RFs) migration, and inversion together with the latest gravity data collected in the region.

The IGB constitutes an open terrain for geophysical investigations: previous gravity surveys and refraction seismic experiments –which lead to the iconic Bird’s Head model-, together with reflection seismic experiments and recent local earthquake tomographies (LET) still present open questions concerning the IGB’s geometry. The IGB is considered as a sliver of the Adriatic lower lithosphere, emplaced at shallow depth along the inner arc of the Western Alps and partially outcropping in the Ivrea-Verbano Zone (IVZ). The regional structure has been imaged over a 25 x 25 x 15 km grid by LET (Diehl et al. 2009) and in 3-D by by gravity data modelling (Scarponi et al. 2020).

To achieve higher-resolution along Val Sesia, we collected new seismic data along a West-East profile of 10 broadband seismic stations – at 5 km spacing – for 2 years, together with new gravity data at 1 point per 1 to 0.5 km along the same profile. We used teleseismic arrivals to build a database of ca. 780 RFs, which have been migrated and inverted for seismic discontinuity locations and velocity contrasts associated with the body. As a result, we observe shallow seismic interfaces with eastward dip, together with possible fault structures along the profile itself. Furthermore, the RFs’ frequency-content dependence sheds new light on the thickness of the seismic velocity transitions that are the discontinuities.

The latest IGB gravity model, locating the anomaly as shallow as 1-2 km, is compared with the results obtained via joint inversion of seismic and gravity data. For this, we calibrated ad-hoc the existing equations for the relationship between density and seismic rock properties.

In conclusion, we propose a new 2-D image for the IGB, whose geometry has been better resolved by the joint use of newly collected seismic and gravity data. Moreover, the geological information collected across the IVZ gives further constraints on the rock properties used in the joint inversion of our geophysical observations.