First results from a borehole network for the seismic characterization of the Einstein Telescope Euregio Meuse-Rhine candidate site

Abstract

The low-frequency sensitivity of the Einstein Telescope depends on the seismic environment of its site, both through the mechanical coupling of ground motion to the test masses and through Newtonian noise. We present the first results of the seismic characterization of the Euregio Meuse-Rhine candidate site based on a network of recently instrumented boreholes covering the first seven months of 2026 at four underground stations and their co-located surface sensors. Two of these boreholes were deliberately sited next to railway infrastructure, so that the levels reported here describe how anthropogenic transients propagate to depth at different distances from the source. The analysis was carried out with \texttt{SiteChar}, a Python framework developed for this work to provide a common processing pipeline across different sensors, networks and observation periods. It processes the data in sequential chunks and separates the noise floor from the contribution of transients. We present a characterization of the catalogue of observed transients and a tentative classification. These measurements form part of an integrated site characterization approach that combines the observed seismic field with three-dimensional geological and wave-propagation models to predict Newtonian noise and ultimately guide its mitigation at the Euregio Meuse-Rhine candidate site.