Estimation of an upper limit on ground tilt noise in an underground research infrastructure as a benchmark for the Einstein Telescope

In this work, we present the results of an investigation into low-frequency seismic noise in an underground laboratory infrastructure, serving as a case study for the future Einstein Telescope. Specifically, we focus on the low-frequency noise measured at the Gran Sasso National Laboratory (Laboratori Nazionali del Gran Sasso, LNGS), expanding upon previous findings at the same location. Finally, we estimate upper limits on the expected tilt noise, considering only the ground contribution (i.e., without considering any other contributions from specific experimental setup), for frequencies below 70 mHz, using seismic data from LNGS and KAGRA. Finally, these underground upper bounds are superimposed and compared directly with the official active seismic isolation requirements of the Einstein Telescope. Our findings confirm that the ambient underground tilt levels comply with ET suspension control targets, providing vital engineering guidance for active tilt-attenuation strategies in next-generation gravitational-wave observatories.