The OTRIONS seismic network: instrumentation upgrade and borehole installation

The Gargano Promontory is one of the most seismically active regions of southern Italy. Here we installed the OTRIONS seismic network consisting, at its very beginning, of 12 short period stations. The network has operated in this region since 2013, and it has been upgraded during time. Five out of these stations have been equipped with broadband sensors. More recently, we installed a posthole seismometer at the site of Lucera at a depth of 30 meters to further reduce seismic noise. In this paper, we describe the OTRIONS network upgrade and focus basic guidelines for the installation of a borehole seismometer. We have performed a post‑installation orientation considering both the polarization analysis and the correlation of seismic recordings because of the impossibility of a mechanical orientation of the sensor that rotated during the descent phase. Both methods give coherent results. We have also analysed the noise level after the station’s upgrade. The maintenance and upgrade of the seismic network is fundamental for the continuous monitoring of this area.