GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 within the Upper Triassic dolomite (Fig. 1). The borehole S14, the same is 170 m long and drains within the same rocks as the borehole S13. The boreholes S13 and S14 were equipped with a 3-channel, 24-bit analog-digital converter ADC (model SL06 by SARA Electronic Instrument company, htp://www.sara.pg.it/ ) , with a very high hydraulic pressure sampling frequency (20 Hz). The hydraulic scheme of the experimental apparatus (showed in Fig. 5 in De Luca et al. 2018) is composed by (a) the horizontal boreholes; (b) an old analogic manometer; (c) a hydraulic valve always open during the data acquisiton periods; (d) hydraulic pressure sensor; (e) hydraulic valve not completely close to enable the measurement of temperature and electrical conductvity in a container (h); (f) temperature sensor; (g) electrical conductvity sensor; (h) transparent plastc container housing the temperature and electrical conductvity sensors. Water is expelled when reaching about three quarters of the volume of the container. The hydraulic pressure measured at the head of the 150-200 m long horizontal boreholes was about 0.5-0.7 MPa (piezometric height of 50-70 m), except for borehole S13, where a much higher pressure was recorded, about 2.0-2.5 MPa (piezometric head of 200–250 m). Inside UL, the INGV seismic staton GIGS, which was also adopted as part of the GINGER experiment used also to study the Rotatonal seismology (Belf et al., 2017; Di Virgilio et al., 2017), is equipped with two broadband seismometers (Nanometrics Trillium 240 s and Guralp CMG 3T 360 s). This instrumentaton is used both for contnuous microseismic monitoring of GSA and for recording global seismicity (Italian seismicity and teleseismic events). The main objectve of the work was to identfy and correlate the coincidences between earthquakes on a global, regional and local scale to the variatons in hydraulic pressure within GSA detected by the HPD installed in the boreholes S13 and S14. To this end, we proceeded by comparing the hydroseismograms obtained from the hydraulic pressure values recorded in the HPD of the boreholes S13 and S14 and the seismograms recorded by the INGV seismic staton GIGS. The tme interval for data acquisiton, observaton and analysis was between May 1 st , 2015 and December 31 st , 2022 and the monitoring is stll going on. 3. Hydraulic pressure versus seismic data discussion In the period May 2015 and December 2022, the HPD recognized 130 earthquakes compared to the 974 analysed. The event recogniton by HPD means that (i) the hydraulic pressure signal recorded by the sensors is atributable to a hydroseismogram and, (ii) the tme coincidence of the hydroseismogram with the seismogram of the same event recorded by the seismic staton GIGS (Fig. 2). Based on the elaboraton of the above-mentoned hydraulic pressure versus seismic data it was possible to plot the diagrams of Fig. 3 which summarize the results. The top plot of Fig. 3 shows two doted lines: the grey line represents the sensitvity limit for the detecton of seismic events reported by Manga and Wang (2015), as a result of a review of all similar studies untl 2015; the blue line, obtained from the observaton in the tme interval of 15 months (May 2015-September 2016) by De Luca et al. (2016), is more sensitve, regardless of distances, with respect to that of Manga and Wang (2015). The slope of the blue line by De Luca et