Priolo, Enrico

The detection level of a seismic network is a measure of its effective ability to record small earthquakes in a given area. It can vary in both space and time and depends on several factors such as meteorological conditions, anthropic noise, local soil conditions—all factors that affect the seismic noise level—as well as the quality and operating condition of the instruments. The ability to estimate the level of detection is of tremendous importance both in the design of a new network and in determining whether a given network can recognize seismicity consistently or needs to be improved in some of its parts. In this article, we determine the detection level of the Cuban seismic network using the empirically estimated seismic noise spectral level at each station site and some theoretical relationships to predict the signal amplitude of a seismic event at individual stations. The minimum local detectable magnitude thus depends on some network parameters such as the signal‐to‐noise ratio and the number of stations used in the calculation. We also demonstrate the effectiveness of our predictions by comparing the estimated detection level with those empirically determined from one year of data (i.e., the year 2020) of the Cuban seismic catalog. Our analysis shows, on the one hand, in which areas the current Cuban network should be improved, also depending on the regional pattern of faults, and, on the other hand, indicates the magnitude threshold that can be assumed homogeneously for the catalog of Cuban earthquakes in 2020. Because the adopted method can use current measurements of the seismic noise level (e.g., daily), the proposed analysis can also be configured for continuous monitoring of network state quality.

Seismic monitoring in areas where induced earthquakes could occur is a challenging topic for seismologists due to the generally very low signal to noise ratio. Therefore, the seismological community is devoting several efforts to the development of high-quality networks around the areas where fluid injection and storage and geothermal activities take place, also following the national induced seismicity monitoring guidelines. The use of advanced data mining strategies, such as template matching filters, auto-similarity search, and deep-learning approaches, has recently further fostered such monitoring, enhancing the seismic catalogs and lowering the magnitude of completeness of these areas. In this framework, we carried out an experiment where a small-aperture seismic array was installed within the dense seismic network used for monitoring the gas reservoir of Collalto, in North Italy. The continuous velocimetric data, acquired for 25 days, were analysed through the application of the optimized auto-similarity search technique FAST. The array was conceived as a cost-effective network, aimed at integrating, right above the gas storage site, the permanent high-resolution Collalto Seismic Network. The analysis allowed to detect micro-events down to magnitude Ml = −0.4 within a distance of ~15 km from the array. Our results confirmed that the system based on the array installation and the FAST data analysis might contribute to lowering the magnitude of completeness around the site of about 0.7 units.

This paper describes the seismological analyses performed within the framework of the seismic microzonation study for the reconstruction of 138 municipalities damaged by the 2016–2017 sequence in Central Italy. Many waveforms were recorded over approximately 15 years at approximately 180 instrumented sites equipped with permanent or temporary stations in an area that includes all the damaged localities. Site response was assessed using earthquake and noise recordings at the selected stations through different parameters, such as spectral amplification curves, fundamental resonance frequencies, site-specific response spectra, and average amplification factors. The present study was a collaboration of many different institutions under the coordination of the Italian Center for Seismic Microzonation and its applications. The results were homogenized and gathered into site-specific forms, which represent the main deliverable for the benefit of Italian Civil Protection. It is remarkable that the bulk of this study was performed in a very short period (approximately 2 months) to provide quantitative information for detailed microzonation and future reconstruction of the damaged municipalities.

We present the activities carried out within the S2 2012-2013 Project, funded in the frame of DPCINGV Agreement, that concerns the mid-long term Seismic Hazard Assessment in Italy on two priority areas, the Po Plain and the Southern Italy. The Po Valley, an area hitherto considered of low seismological interest, has attracted the attention of the seismological community following the events of May 20, 2012. The day after the main shock that stuck eastern Emilia in 2012, the OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale deployed a temporary seismographic network in the Ferrara area. All the investigated locations were set on soft soils. The large amount of collected data allowed comparison between observed PGA and theoretical predictions, ShakeMaps and attenuation laws, but the lack of a reference site during the 2012 recordings did not allow for estimating the amplification factor. In order to accomplish this task, in February 2013, a new 5 stations recording array acquired data at four of the 2012 network sites and at the Casaglia reference site, north of Ferrara, where a borehole Very Broad-Band station is coupled with a midperiod sensor at the surface. This borehole reference station made possible the estimation of site amplification of the 2013 array sites. The reference methods allowed an easier identification of the resonance frequency, which peaks appear sharper than what displayed by single-station methods (both H/V on ambient noise and earthquakes), and relative soil amplification. A large amount of original seismological data has been recorded in a poorly instrumented area, including significant events of the 2012 Emilia and 2013 Lunigiana sequences. The entire dataset of continuous waveforms has been made available on the OGS web based OASIS Database, from the earliest stages of the project. For the largest events strong-motion parameters were calculated and published on the OASIS database. Event time series and metadata (site monographs) are available similarly to the ITACA Database.

This paper concerns the analysis of the site amplification that significantly influenced the non-uniform damage distribution observed at San Giuliano di Puglia(Italy)afterthe2002Moliseearthquake(MW=5.7).Infact,thehistorical coreofthetown,settledonoutcroppingrock,receivedlessdamagethanthemore recent buildings, founded on a clayey subsoil. Comprehensive geotechnical and geophysicalinvestigationsallowedadetaileddefinitionofthesubsoilmodel.The seismicresponseofthesubsoilwasanalyzedthrough2-Dfinite-elementand3-D spectral-elementmethods.Theaccuracyofsuchmodelswasverifiedbycomparing the numerical predictions to the aftershocks recorded by a temporary seismic network.Aftercalibration,theseismicresponsetoasyntheticinputmotionreproducing the main shock was simulated. The influence of site amplification on the damage distribution observed was finally interpreted by combining the predicted variation of ground motion parameters with the structural vulnerability of the buildings.

During the 1982–1984 bradyseismic crises in the Campi Flegrei area (Italy), the University of Wisconsin deployed a network of seismological stations to record local earthquakes. In order to analyse the potential of the recorded data in terms of tomographic imaging, a blind test was recently set up and carried out in the framework of a research project. A model representing a hypothetical 3D structure of the area containing the Campi Flegrei caldera was also set up, and a synthetic dataset of time arrivals was in turn computed. The synthetic dataset consists of several thousand P- and S-time arrivals, computed at about fourteen stations. The tomographic inversion was performed by four independent teams using different methods. The teams had no knowledge of either the input velocity model or the earthquake hypocenters used to create the synthetic dataset. The results obtained by the different groups were compared and analysed in light of the true model. This work provides a thorough analysis of the earthquake tomography potential of the dataset recording the seismic activity at Campi Flegrei in the 1982–1984 period. It shows that all the tested earthquake tomography methods provide reliable low-resolution images of the background velocity field of the Campi Flegrei area, but with some differences. However, none of them succeeds in detecting the hypothetical structure details (i.e. with a size smaller than about 1.5–2 km), such as a magmatic chamber 4 km deep and especially the smaller, isolated bodies, which represent possible magmatic chimneys and intrusions.

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The paper reports the comparison between 1D, 2D and 3D numerical simulations of seismic site response at San Giuliano di Puglia (Italy) and the amplification recorded in the aftershocks following the 31.X.2002 Molise earthquake (MW=5.7). The records were taken by mobile stations installed in the historical center on a soft rock outcrop and in the newer part of the town on a marly clay formation. The site response analyses by the 3D model involved a subsoil volume of about a 2000 x 2000 x 1500 m, including the extension of the whole town center at surface. 2D site response analyses were carried out on a section developing along the longitudinal axis of the town, and 1D simulations were referred to the subsoil profiles at the seismic stations. 1D and 2D models accounted for the heterogeneity of clayey formation, along both vertical and horizontal directions, as evidenced by a comprehensive geotechnical investigation. The results of numerical predictions are compared to the aftershocks records in terms of amplification functions and ground motion parameters (peak acceleration and Housner intensity).

Moderate-magnitude shallow earthquakes in the Atlantic Ocean, hundreds of kilometres southwest of Lisbon, can generate efficient suboceanic Rayleigh waves (SRW) that are well recorded in Portugal. Here we compare moderate-size earthquakes recorded by seismic stations in Portugal with the Tyrrhenian Sea earthquakes recorded in peninsular Italy where SRW were recently observed. In spite of a different behaviour of high frequencies due to the different tectonic setting of the two areas, similar results are found in the intermediate-period range, suggesting that this effect, if extrapolated to a magnitude larger than 8, could be devastating at regional distance in terms of ground motion amplitude and duration. Through 1D models, we explore the hypothesis that the high level of destruction and the long duration of shaking felt during the Great 1755 Lisbon earthquake were caused by SRW. In this preliminary study, we check the role of critical model parameters. We find that duration and amplitude are largest when the average thickness of the water layer is 2 km and shear-wave velocity of the ocean floor is close to the speed of sound in the water. Both conditions are realistic for a source in the Atlantic Ocean, few hundreds of kilometres southwest of Lisbon. Moreover, the propagation of SRW at regional distances accounts for durations of more than ten minutes as the effect of a single large earthquake.