Dario Albarello

SWAPS is a MATLAB-based software for retrieving Rayleigh phase and group velocity dispersion curves by considering ambient noise recorded at pairs of sensors for near-surface applications (on the scale of tens of meters). By combining both phase and group velocity estimation, SWAPS resolves inherent phase ambiguity of two-station methods, without requiring reference models, prior constraints or long recordings. SWAPS also provides multi-station analysis (batch), through which dispersion curves can be automatically retrieved for all station pairs within a dense seismic array. Reliable two-station dispersion estimates make seismic site characterization feasible in contexts where dense arrays are prohibitive, while the batch analysis over multiple pairs of sensors supports the reconstruction of more complex subsurface structures. SWAPS is distributed under the MIT license and is designed for researchers and professionals involved in seismic site characterization.

In this work, Ambient Noise Tomography (ANT) is applied for a 3D subsurface reconstruction in terms of shear-wave velocity (Vs) beneath one of the main emissive vents of the Nirano Mud Volcanoes site (northern Italy). The adopted workflow is based on short ambient noise records (~1 h) acquired with a small seismic array composed of vertical geophones and implements an automated procedure to capture seismic heterogeneities in the subsoil volume. This procedure can be of general interest when the use of more invasive procedures is hampered by the logistic or economic limitations. In the case here considered a low shear-wave velocity anomaly is identified right below the emissive vent extending vertically from the surface up to approximately 30 m depth. A quantitative model is proposed to explain the correspondence of the low velocity anomaly and emitting vents in terms of increased porosity and saturation of hosting sediments, where deep seated fluids and gas may accumulate close to the surface. This may increase the possible hazard relative to potentially damaging eruptions in the case of a sudden release of the gas stored in this shallow reservoir.

A simple procedure is proposed to reduce horizontal to vertical spectral ratio curves relative to ambient vibrations measured at the sea bottom to equivalent curves measured inland. The procedure does not rely on specific assumptions about the nature of ambient vibrations and allows extending inversion techniques developed for the interpretation of spectral ratio measurements inland to sea floor observations.

Methodological aspects relative to the conversion of probabilistic hazard evaluations in terms of ground motion intensity into hazard in terms of macroseismic Intensity. As a first step, the probabilistic relationships between ground motion intensity and macroseismic intensity are critically re-examined to account for the different formal properties of these two dimensions of the earthquake intensity. Then these relationships are coherently considered in the conversion of hazard curves by accounting for the inherent binning of macroseismic intensity values and of their inherent probabilistic nature. It is shown that approximate procedures currently used to provide this conversion may provide biased outcomes when the dispersion of values relative to ground motion intensity corresponding to macroseismic Intensity is dismissed or not properly accounted for. The impact of this possible bias is evaluated by considering the seismic hazard at reference site condition at the city of L’Aquila in Central Italy.

Seismic risk assessment at regional scale requires hazard estimates accounting for seismostratigraphical amplification effects. When detailed data related to the local subsoil configuration are lacking, these effects can be inferred from numerical simulations fed with information available on at regional scale. A key aspect concerns the implementation of these outcomes including relevant uncertainty into probabilistic seismic hazard estimates relative to standard subsoil conditions. A coherent approach is here proposed, which coherently accounts for the inherent probabilistic character of reference hazard estimates and of uncertain 1D seismostratographical amplification effects inferred from geological maps. The proposed approach has been applied in Central Italy relative PGA values corresponding to an exceedance probability of 10% in 50y. It is shown that accounting for uncertainty affecting amplification estimates is of main importance for correct implementation into PSHA. The outcome of this analysis is not expected to be considered for anti-seismic design of single structures, which requires detailed, and sound estimates of site effects at the proper scale. Anyway, these estimates may play a role for the preliminary identification of most critical situations along lifelines or outside inhabited areas where seismic microzonation studies are not available.

The presence of a minimum within the low-frequency range in the average Horizontal (H) to Vertical (V) Spectral Ratios of ambient vibrations has been suggested to be representative of hydrocarbon reservoirs in active oil and gas fields in the Middle East and Europe. Similar evidence has been also found in correspondence of active mud volcanoes in Italy (Antunes et al., 2022; Panzera et al., 2016). In this view, the aim of the study is to explore the possibility of using ambient vibrations to identify and characterize reservoir corresponding to mud volcanoes vents in the Nirano mud volcanoes area (Norther Italy). Seismic surveys carried out in the area so far allowed the seismic characterization of the shallowest part of the subsoil involved in the emission process (Antunes et al., 2022; Brindisi et al., 2023; Carfagna et al., 2024). To extend downward this characterization and understand if the presence of the minimum detected in the Nirano Reserve (Antunes et al., 2022) is confined to the area of emitting vents, an extensive survey of ambient vibrations was performed. In particular, a dense network of velocimetric measurements has been deployed by considering single station and array configurations. Results obtained confirm that measurements with a clear HVSR minimum at around 0.5 Hz only characterize the area of emitting vents. In the assumption that this minimum depends on the characteristics of the reservoir responsible for fluid emissions, the Biot–Gassmann theory (Lee, 2004; Tinivella, 2002) for seismic waves velocities of gas hydrate-bearing sediments has been considered to infer reservoir characteristics from Vs and Vp profiles obtained by the inversion of the HVSR curves. The satisfactory fit of model outcomes with observations testifies one more the effectiveness of ambient vibration measurements to characterize mud volcanoes and relevant subsoil configuration. This opens new possibilities for the study of mud volcanoes and gas reservoirs both onshore and offshore by passive seismic observations.References Antunes V., Planès T., Obermann A., Panzera F., D’Amico S., Mazzini A., Lupi M.; 2022: Insights into the dynamics of the Nirano Mud Volcano through seismic characterization of drumbeat signals and V/H analysis. J. Volcanol. Geoth. Res., 431, 107619. https://doi.org/10.1016/j.jvolgeores.2022.107619.Brindisi A., Carfagna N., Paolucci E., Albarello D.; 2023: Fine structure of seismic emissions from Nirano mud volcanoes (northern Apennines, Italy): a phenomenological study. Bull. Geophys. Oceanogr, 20, 1-12. DOI 10.4430/bgo00437.Carfagna N., Brindisi A., Paolucci E., Albarello D.; 2024: Seismic monitoring of gas emissions at mud volcanoes: The case of Nirano (northern Italy). J. Volcanol. Geotherm., 446, 107993. https://doi.org/10.1016/j.jvolgeores.2023.107993.Lee M.W.; 2004: Elastic velocities of partially gas-saturated unconsolidated sediments. Mar. Pet. Geol., 21(6), 641-650. https://doi.org/10.1016/j.marpetgeo.2003.12.004.Panzera F., Sicali S., Lombardo G., Imposa S., Gresta S., D’Amico S.; 2016: A microtremor survey to define the subsoil structure in a mud volcanoes area: the case study of Salinelle (Mt. Etna, Italy). Environ. Earth Sci., 75, 1-13. https://doi.org/10.1007/s12665-016-5974-x.Tinivella U.; 2002: The seismic response to over-pressure versus gas hydrate and free gas concentration. J. Seism. Explor., 11(3), 283-305.

The Italian seismic code (NTC18) provides indications about the expected effects of some morphological configurations on the expected ground motion during earthquakes. In particular, two main 2D morphologies are identified as reference: cliffs and crests. Based on numerical simulations, the value of St is assumed to depend on the steepness of the cliffs and aspect ratio of the crest. A critical aspect of these estimates is that the considered configurations are defined in terms of steepness angles and aspect ratios, without any scale indication. Moreover, the considered morphologies are very schematic, and this prevents their simple application in the natural context: in most case an expert judgement is necessary, and this makes the final estimates potentially controversial and difficult to validate on the basis of empirical observations. To face this problem, in the frame of the PRIN project “SERENA”, a procedure has been developed for the automatic identification of areas prone to morphological amplification effects by following NTC18 prescriptions, based on the Digital Terrain Model. The proposed approach allows the full exploitation of topographical data at the maximum resolution available. After a first application to restricted areas, the proposed procedure has been applied at National scale at the seismometric and accelerometric sites managed by INGV. The aim is twofold: first comparing outcomes of the new approach with the ones proposed by other Authors at the same sites, second to provide a sound basis of a coherent and reproducible estimate of St values to be compared with possible empirical evidence. In the presentation, the results obtained about the first aim will be presented and discussed.

The complexity of the geological setting of the Italian peninsula implies variable site conditions affecting seismic hazard. Their extensive mapping is needed for a reliable assessment of the induced risk on residential areas, industrial agglomerations, infrastructures, strategic sites, cultural heritage spread on the entire national territory. The research meets the need for a national reference map providing co-seismic ground deformation at the scale of interest of land use planning and bridging the information gap now existing among the urbanized areas covered by seismic microzonation studies. The project aims at defining a multidisciplinary procedure for a multiscale mapping of the local seismic hazard of Italy, exploiting the potentialities of having spatially extensive information for combining site-specific to regional estimates of site effects. The key elements are: i) full exploitation of geological/geomorphological data, ii) extensive numerical modelling, and iii) empirical testing of local hazard estimates. The study deals with the regional scale analysis and classification of the landscape and the geological-technical properties of the near-surface stratigraphic configurations (GeoMorpho-Stratigraphic Units, GMSUs), which affect seismic hazard. The GMSUs are obtained by combining geomorphological analysis and automated landscape classification procedures, field-based lithostratigraphic constraints, instrumental signatures from geophysical investigations, and geotechnical parameters. Distinct parameterizations will be assessed for each GMSUs to feed simplified numerical models to quantify synthetic hazard indicators suitable for risk evaluations and land use planning. Outcomes will be managed through a coherent probabilistic approach to bound relevant uncertainty as a function of locally available data. Outcomes will be tested by considering site-specific analyses.

It is widely recognized that the amplification of ground motion during earthquakes is attributed to the interference of seismic waves trapped between the free surface and impedance contrasts in the shallow subsoil. Seismic Microzonation (SM) studies are devoted to evaluating these site effects, but their application in wider contexts is a hard and expensive task. To estimate seismic site effects at regional scale, the most viable approach is to utilize detailed geological and geomorphological data (1:10.000-1:50.000), which are available for large part of Italy.In the frame of the national research project “SERENA”, in this study a procedure is proposed and tested to constrain entity of 1D seismostratigraphical ground motion amplification based on geological information at the most detailed scale available. In particular, amplification factors are estimated for Seismically Homogeneous Microzones (SHM) defined on the basis geological information. Each SHM is represented as a stack flat homogeneous layers each characterized in terms of engineering-geological units by following the seismic microzonation standards. Seismic properties of each layer (shear waves velocity, density, damping and G/G0 curves) and respective range of variability are determined on the basis of the most recent literature.This information feeds a linear equivalent numerical approach and the Inverse Random Vibration Theory to compute the expected seismic response at each SHM. To account for the relevant uncertainty, 100 random profiles were generated for each SHM, which were compatible with available data. Outcomes of the relevant numerical simulations were considered to assess uncertainty affecting amplification estimates at each SHM.Through this procedure, approximately 4000 SHM were identified, distributed across over 80,000 formation outcrops mapped on Geological map of Tuscany Region, selected by dedicated ArcgisPro TM/Arcpy TM scripts elaborated for this aim. The 50th percentile of the amplification factor distribution for each SHM was taken into consideration. This process aimed to create a new map of amplification factors for the entire territory of Tuscany, achieving an optimal spatial resolution of 1:10,000.To assess the reliability of the results obtained from numerical simulations, and evaluate the possible presence of biases, outcomes of the numerical procedure here considered were compared with those from second and third levels of Seismic Microzonation studies available in Tuscany. Approximately 1500 benchmark samples were identified, revealing distinct trends among various SHM, particularly between those with outcropping sedimentary covers and those with exposed geological bedrock.In general, amplification estimates provided by the approach here proposed provide a slight overestimate of the ones provided by the detailed seismic microzonation studies (less than 10% on average). However, this overestimate is largely within the range of uncertainty affecting regional estimates and mostly concern SHMs where bedrock outcrops.It is worth to note that by no way the proposed approach should be considered as substitute of detailed local studies. Anyway it could be considered to provide ex-ante evaluations to be used as a preliminary reference for large scale risk analysis and for a preliminary assessment of expected ground motion effects where more detailed studies are not available so far.