DIVEnet: a local seismographic network monitoring the lower continental crust drilling activities for the ICDP-DIVE project

Abstract

The ICDP DIVE project (Drilling the Ivrea‑Verbano zonE) addresses fundamental questions about the nature of the lower continental crust and its transition to the mantle. In its first phase, the project has drilled two, scientific, fully cored boreholes in the Ivrea Verbano Zone (IVZ) in Italy. The IVZ, considered the world’s best outcrop of lower crustal continental rocks, is structurally and historically connected to the underlying Ivrea Geophysical Body, a major, high density and high seismic‑velocity anomaly studied since the 1960s and a characteristic feature of the Western Alps. The two boreholes were conducted between 2022‑2024 in Val D’Ossola: the first in Ornavasso and the second in Megolo, 7 km apart. Within this framework, a dedicated seismographic network, named DIVEnet, has been monitoring natural earthquakes and possible operation‑related seismic activity for three years. Eleven seismographic stations (short period and broadband), provided by INGV and the University of Lausanne, were installed within a maximum distance of 15 km from the midpoint between the two drilling sites. All stations recorded data continuously and 10 provided data in real time. One broadband sensor was installed in a borehole, and its horizontal components’ orientation was determined by multiple methods. The stations occupied 14 different locations and operated from autumn 2021 to summer 2024, with varying recording durations. Being in an area characterized by low natural local seismicity and a relatively sparse distribution of seismographic stations, it is particularly important to record background activity and noise for as long as possible, especially before and after the start of drilling activities. Daily monitoring was conducted at INGV in quasi real time, and probabilistic power spectral density distributions (PPSD) have been computed. In total, 28 events with magnitudes ranging from 0 to 2.6 MLv were recorded within a distance of about 20 km from the boreholes, most of them aligned with the Insubric Line, that is thought to be tectonically inactive, and 612 events were recorded in the larger study region. No events were related to the drilling activities, which have only very slightly increased noise levels, mostly in the in the 0.1-0.3 Hz frequency range