Franceschina, Gianlorenzo

Estimation of local seismic response plays a key role in assessing local seismic hazard and particularly in the design of shaking scenarios. Modelling local seismic response involves knowing of the shear wave velocity (Vs) and quality factor (Qs) profiles for the site in question. The many techniques that have been developed to assess Vs in surface deposits produce reliable measurements of Vs , but these rarely correspond to direct measurements of Qs . The latter is often considered through damping measures from laboratory tests on small-scale soil samples, which can provide information primarily on intrinsic attenuation, neglecting the contribution of scattering effects. In this paper, using seismic recordings obtained at the surface and in boreholes at 100 m depth, we estimate an average value of Qs of some characteristic alluvial deposits of the Po Plain (northern Italy). Data come from a microseismic network which sampled an almost uniform lithology in the central Po Plain and consisted of three surface and four borehole stations with an interstation distance of about 2 km. The average value of Qs of the shallowest 100 m of the sedimentary strata, Qs100, is estimated by considering: (1) the high-frequency attenuation of seismic waves due to propagation through the corresponding stratigraphy and (2) the interference between incident and surface-reflected waves observed at borehole stations. We parametrize the first through k0_100, the difference between the values of the spectral decay parameter kappa (k) estimated at the surface and at the boreholes depth, respectively. We use the second in order to compute Vs100, the time-averaged Vs referred to the uppermost 100 m stratigraphy. We obtain: k0_100 = (11 ± 3) ms, Vs100 = (309 ± 11) m s −1 and Qs100 = 31 ± 10. At the surface, the estimated values of the site-specific kappa, k0, are found to range from 75 to 79 ms. As expected, these results are in good agreement with studies performed in other sites characterized by sandy or clayey lithologies, and can be usefully used in site response analysis at sites where the rigidity is mainly controlled by lithostatic pressure.

Detection analyses are necessary to plan microseismic networks for use in the monitoring of anthropic activities. In 2014, the Italian Ministry of Economic Development (MiSE) issued guidelines for the monitoring of microseismic activity, ground deformations, and reservoir pore pressure. In 2016, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) was commissioned to carry out specific investigations, aimed at assessing the guidelines applicability at the pilot site of the gas storage concession "Minerbio Stoccaggio" (Bologna). In this work, we present an overview of detection analysis, performed by the INGV team, during the experimental phase of the above mentioned guidelines. Measurements of ambient seismic noise, performed from 1 January 2018 to 31 March 2019, were used to assess detection thresholds of different configurations of the Minerbio Integrated Seismic Network (MISN). Detection analysis is particularly relevant for the Inner Domain of Detection (IDD), the crustal volume centred on the reservoir, within which it is vital to ensure the highest network performance. The results obtained in this work, validated through data recorded by the MISN during the analysed period, show that in the worst noise conditions observable in the area, the final configuration of the MISN enables localising ML ≥ 1.0 events occurring in the whole IDD, in line with the monitoring requirements prescribed by the guidelines.

The storage concession "Minerbio Stoccaggio" (Bologna, Northern Italy) covers a 69 km 2 area, 65% of hich is located in the Minerbio municipality. Since 1979, a microseismic network for the monitoring of seismicity, eventually induced by gas storage activities, has been installed in this area. The network was operated by Stogit S.p.A, a subsidiary company of Snam, which is the largest storage operator in Italy. In 2016, the microseismic network, consisting of three surface stations and one 100-m-deep borehole sensor with minimum interstation distances of about 3.0 km, was integrated with 12 regional stations installed in an 80 × 80 km 2 area centered on the surface projection of the reservoir. In 2018, the microseismic network was enhanced by adding one surface and three 150-m-deep borehole stations. In this work, we evaluate the detection improvement of the microseismic network, integrated with the regional stations. We define two crustal volumes for earthquake detection: the inner domain of detection, IDD (10 × 10 × 5) km 3 , within which we should ensure the highest network performance, and the extended domain of detection, EDD (22 × 22 × 11) km 3 . By comparing the simulated power spectral density of hypothetical seismic sources located in EDD with the average power spectra of ambient seismic noise observed at each station site, we calculate detection and localization thresholds for the two above-mentioned networks. Under unfavourable noise conditions, we find that the present operative seismic network allows locating earthquakes with M L ≥ 0.8 occurring at the depth of the reservoir and with M L ≥ 1.0 if located within IDD.

In August 2016, a magnitude 6.0 earthquake struck Central Italy, starting a devastating seismic sequence, aggravated by other two events of magnitude 5.9 and 6.5, respectively. After the first mainshock, four Italian institutions installed a dense temporary network of 50 seismic stations in an area of 260 km2. The network was registered in the International Federation of Digital Seismograph Networks with the code 3A and quoted with a Digital Object Identifier ( https://doi.org/10.13127/SD/ku7Xm12Yy9 ). Raw data were converted into the standard binary miniSEED format, and organized in a structured archive. Then, data quality and completeness were checked, and all the relevant information was used for creating the metadata volumes. Finally, the 99 Gb of continuous seismic data and metadata were uploaded into the INGV node of the European Integrated Data Archive repository. Their use was regulated by a Memorandum of Understanding between the institutions. After an embargo period, the data are now available for many different seismological studies.

In this paper we present a feasibility study for the surface seismic monitoring of the “Sant’Alberto” gas reservoir (Po Plain, Northern Italy). The project was commissioned to INGV in the framework of the activities planned by the oil & gas E&P company PoValley Energy (PVE), aiming to exploit the underground deposit. After estimating the ambient seismic noise of the area, we hypothesized a microseismic network composed of five stations installed in a 8.0 x 8.0 km 2 area centered on the surface projection of the reservoir. Moreover, due to the high noise levels observed, we also considered the possible installation of a borehole station at 200 m depth. Detection and localization thresholds were estimated by comparing the simulated power spectral density of hypothetical seismic sources located in the crustal volume surrounding the reservoir, with the mean power spectra of the ambient seismic noise observed (or hypothesized) at each station-site. Two crustal volumes for the earthquakes detection were considered: the inner domain of detection, DI (8.6 x 8.6 x 4.0 km 3 ), within which we should ensure the highest network performance, and the extended domain of detection, DE (18.6 x 18.6 x 9.0 km 3 ). After estimating the background level of ambient seismic noise, simulations show that: 1) five surface stations installed within the surface projection of DI ensure localization thresholds ranging between M L 0.7 and 0.8 within DI; 2) the installation of the borehole station allows to improve the detection threshold everywhere within DI, and down to M L = -0.4 nearby the reservoir; and 3) localization thresholds obtained in the crustal volume included in DE, but not in DI, show values around M L = 1.0.

During the August-September 2016 Amatrice, Mw 6.0, seismic sequence, the real time INGV strong mo-tion data sharing was assured by the INGV Strong Motion database (ISMD). Starting on August 24th, the main task of the web portal was to archive, process and distribute the strong-motion waveforms rec-orded by the permanent and temporary INGV accelerometric stations for the earthquakes with magnitude ≥ 3.0, occurring in the Amatrice area and surroundings. At present (i.e. September 30th, 2016), ISMD freely provides more than 21.000 strong motion waveforms to all users. In particular, about 2.200 strong motion waveforms were recorded by the temporary network installed for the earthquake sequence monitor-ing in the epicentral area by SISMIKO and EMERSITO working groups. In addition, for each permanent and temporary recording site, the web portal provides a comprehensive description of the necessary infor-mation to properly use the strong motion data.