A GIS‑based approach for planning investigations in Seismic Microzonation studies of Level 3: methodology and application in the Etnean area, Italy

Active volcanic areas located within densely urbanized regions require reliable geophysical methods to characterize the subsurface and support seismic hazard assessment and monitoring strategies. The Campi Flegrei area (Southern Italy) represents a paradigmatic example, where hydrothermal activity, fluid circulation, and strong lateral heterogeneities, combined with urban constraints, make subsurface velocity modeling particularly challenging.In this framework, and in response to the seismic sequence that began in 2023, an extensive campaign of experimental geophysical investigations was promoted and funded by the Italian Department of Civil Protection in the Campi Flegrei area. We present a multi-method seismic investigation based on 66 Horizontal-to-Vertical Spectral Ratio (HVSR), 11 2D seismic array and 20 Multichannel Analysis of Surface Waves (MASW) measurements at 20 sites. Shear-wave velocity (Vs) profiles were derived at each site through joint inversion of the dispersion curve (retrieved from 2D seismic array data and MASW) with ellipticity of the Rayleigh waves.All HVSR measurements consistently exhibit a low-frequency peak (f0 ≈ 0.2–0.5 Hz), interpreted as the response of the deep caldera fill sediments. Further peaks at frequencies above 0.5 Hz may be associated with shallow impedance contrasts. The fundamental frequency (f0) seems reflecting lateral variations in the depth and stiffness of the caldera fill. Significant variability in HVSR amplitude, sharpness and polarization, reflects the interplay between geological heterogeneity and urban noise sources.The joint inversion approach reduces model non-uniqueness and provides well-constrained Vs profiles, improving the physical interpretation of HVSR features in terms of stratigraphy and velocity contrasts. This study highlights the potential of HVSR-based methods in active volcanic and urbanized settings and emphasizes the importance of combining passive and active methods to address geological complexity and anthropogenic interference, paving the way for further multi-scale studies and their application in urban volcanic contexts worldwide.The resulting Vs velocity profiles provide further information for interpreting the stratigraphic features and discontinuities of the caldera fill, useful for integration with other type of studies (hydrothermal alteration) and other type of geophysical dataMoreover, the results of this study offer valuable tools for geohazard assessment and constitute a preliminary step towards the seismic microzonation of the area.

A set of horizontal to vertical spectral ratio (HVSR) and multichannel analysis of surface waves (MASW) measurements, carried out in the Altavilla Milicia (Sicily) area, is analyzed to test a geological model of the area. Statistical techniques have been used in different stages of the data analysis, to optimize the reliability of the information extracted from geophysical measurements. In particular, cluster analysis algorithms have been implemented to select the time windows of the microseismic signal to be used for calculating the spectral ratio H/V and to identify sets of spectral ratio peaks likely caused by the same underground structures. Using results of reflection seismic lines, typical values of P-wave and S-wave velocity were estimated for each geological formation present in the area. These were used to narrow down the research space of parameters for the HVSR interpretation. MASW profiles have been carried out close to each HVSR measuring point, provided the parameters of the shallower layers for the HVSR models. MASW inversion has been constrained by extrapolating thicknesses from a known stratigraphic sequence. Preliminary 1D seismic models were obtained by adding deeper layers to models that resulted from MASW inversion. These justify the peaks of the HVSR curves due to layers deeper than MASW investigation depth. Furthermore, much deeper layers were included in the HVSR model, as suggested by geological setting and stratigraphic sequence. This choice was made considering that these latter layers do not generate other HVSR peaks and do not significantly affect the misfit. The starting models have been used to limit the starting research space for a more accurate interpretation, made considering the noise as a superposition of Rayleigh and Love waves. Results allowed to recognize four main seismic layers and to associate them to the main stratigraphic successions. The lateral correlation of seismic velocity models, joined with tectonic evidences, allowed to plot two geological sections, showing the main pattern of geological formations and tectonic structures.

This study employs the HVSR method to evaluate subsurface conditions and infer soil properties at Cheras area. The HVSR technique identifies the fundamental resonance frequency (F₀) and peak amplitude (A₀), which are critical for differentiating soil types. Results indicate that F₀ values between 2–5 Hz correspond to fine-grained soils, while values between 5–10 Hz suggest coarse-grained or compacted materials. Complemented by MASW, the study provides further insights into shear wave velocity (Vs) distribution and corresponding Standard Penetration Test (SPT-N) values. The MASW analysis identified four key zones: soft to firm soil (Vs = 0–200 m/s, SPT-N = 0–8), stiff to very stiff soil (Vs = 200–300 m/s, SPT-N = 8–25), hard/dense soil (Vs = 300–400 m/s, SPT-N = 25–35), and weathered rock (Vs > 400 m/s, SPT-N > 35). The undulating nature of the rock profile suggests differential weathering and potential instability zones, consistent with findings from other studies in granite terrains. The correlation between HVSR and MASW results confirms a complex subsurface with significant lithological and structural variations, including weak zones characterized by lower shear wave velocities. These weak zones highlight potential geotechnical challenges that may require further validation through borehole investigations. These findings indirectly contribute to evaluating soil stability and load-bearing capacity, demonstrating the utility of HVSR and MASW as cost-effective, non-invasive tools for preliminary soil investigations, particularly in urban areas.

The strong correlation between earthquake damage and the local geological conditions in an area has long been established, and site characterization studies are common practice for realistic estimation of seismic hazard and mitigation of the respective seismic risk. Estimation of the time‐averaged S‐waves velocity of the topmost 30 m (VS30) is recognized as a basic proxy for site characterization and has been incorporated in several building codes. In our work, we examine the inversion of single‐station ambient noise horizontal to vertical spectral ratio (HVSR) curves to estimate the local one‐dimensional (1D) VS(Z) structure. The inversion is performed based on the diffuse field assumption, which allows for the inversion of HVSR, including the contributions of both body and surface waves. Although ambient noise data acquisition is immensely cost‐effective, HVSR curve inversion is subject to the solution non‐uniqueness issue, offering ambiguous results. To provide a possible solution for this matter, additional near‐surface geophysical methods such as the multichannel analysis of surface waves and the electrical resistivity tomography (ERT) were applied to derive information on the shallow subsurface structure. The acquired 1D seismic velocity profile was implemented as initial model in the HVSR inversion to constrain the velocities of the upper meters. Additionally, the inferred stratigraphy from the ERT electrical profile was utilized to constrain the thicknesses of the layers. Tests were conducted with and without a priori information from complementary techniques to explore how the HVSR inversion procedure is facilitated. The proposed methodology was applied at the locations of six accelerometers in the city of Thessaloniki, northern Greece, under different geological conditions. The reliability of the inverted VS(Z) models was checked by performing 1D numerical simulations of ambient noise waveforms generated by distributed sources and by direct comparison between synthetic ambient noise HVSR and single‐station earthquake HVSR curves.

Geophysical methods have a varying degree of potential for detailed characterization of landslides and their dynamics. In this study, the application of four well-established seismic-based geophysical techniques, namely Ambient Noise Interferometry (ANI), Horizontal to Vertical Spectral Ratio (HVSR), Multi-Channel Analysis of Surface Waves (MASW) and Nanoseismic Monitoring (NM), were considered to examine their suitability for landslide characterization and monitoring the effect of seasonal variation on slope mass. Furthermore, other methods such as Ground Penetrating Radar (GPR) and DC Resistivity through Electrical Resistivity Tomography (ERT) were also used for comparison purpose. The advantages and limitations of these multiple techniques were exemplified by a case study conducted on Sobradinho landslide in Brazil. The study revealed that the geophysical characterization of the landslide using traditional techniques (i.e., GPR, ERT and MASW) were successful in (i) the differentiation between landslide debris and other Quaternary deposits, and (ii) the delineation of the landslide sliding surface. However, the innovative seismic based techniques, particularly ambient noise based (HVSR and ANI) and emitted seismic based (NM), were not very effective for the dynamic monitoring of landslide, which might be attributed to the short-time duration of the data acquisition campaigns. The HVSR was also unsuccessful in landslide site characterization i.e., identification of geometry and sliding surface. In particular, there was no clear evidence of the light seasonal variations, which could have been potentially detected from the physical parameters during the (short-time) ambient noise and microseismic acquisition campaigns. Nevertheless, the experienced integration of these geophysical techniques may provide a promising tool for future applications.

Geophysical techniques were employed to analyze one of the landslides that affected the main access road to Pujilí (Ecuador). A passive seismic technique was utilized to test a total of 70 horizontal to vertical spectral ratio (HVSR) points, complemented by an active seismic-refraction profile and a multichannel analysis of surface waves (MASW) survey. The results from the active geophysical surveys facilitated the determination of the shear-wave-velocity value for the surface materials that were in motion. However, the HVSR provided the fundamental frequency fo and amplification Ao values of the ground. The Nakamura (1989) relationship was applied to obtain the thickness of the sediments over a compact material from the fundamental frequency of the terrain in a two-layer model. Additionally, constrained models of the shear-wave velocity (Vs) distribution in the landslide area, obtained from the active seismic surveys, were used to invert the ellipticity curves. The results from this inversion were compared with those obtained by applying the Nakamura equation. The landslide-rupture surface was delineated for each type of analysis, which verified the correlation and minimal differences between the results of the three proposed studies, thus validating the procedure. The directivity of the microtremor HVSR signals was also analyzed, demonstrating a relationship with the internal structure of the sliding material. Furthermore, the ability to slide concerning the Kg parameter (vulnerability index, Nakamura, 1989) was studied. The usefulness of the directivity analysis in defining the internal structures in landslide materials and in determining the areas with the most significant instability was demonstrated. Overall, the HVSR is considered valuable when conducting early landslide studies and is helpful in determining the rupture plane while remaining a simple, fast, and economical technique.

In the shallow subsurface analysis, the potential hazard is identified and evaluated through the site investigation studies which can be determined either by conducting standard penetration test, cone penetration test, and multichannel analysis of surface wave (MASW). While the three methods stated are widely established and highly reliable, the result produced is sometimes unconvincing and lack of details. In this study, the horizontal to vertical spectral ratio (HVSR) analysis method is proposed instead of the more established methods. HVSR is a technique that based on the spectral analyses of recorded ambient noise in order to estimate the site effect parameters such as fundamental frequency and the amplification factor of local soil. The HVSR method was implemented in Bukit Tinggi, Pahang, Malaysia, which known to experience several small earthquakes between 2007 to 2009. Since the earthquake in central Peninsular Malaysia is uncommon, the tremor has caused panic and requires a thorough investigation of the soil effect in the region. The work conducted is motivated by this after effects with focus on the potential fault line investigation using the spectral ratio method (HVSR). The ambient noise signal measurements were performed at 20 sites along the probable fault line in order to calculate the HVSR. Receiver array arrangement and dimensions are chosen based on the prior information about the earthquake location as experienced by the local resident. At the end of the work, the amplification spectra for the soil column is evaluated at each site location, and the nominal frequency obtained, and maximum amplitudes are contoured showing the corresponding value trends in the region.

Site investigation studies provide the basic soil and engineering properties, which usually determined using many investigations as standard penetration test, cone penetration test, and multichannel analysis of surface wave (MASW), to identify and evaluate a potential hazard. The horizontal to vertical spectral ratio (HVSR) analysis method is a tool based on the spectral analyses of recorded ambient noise; it is widely used to estimate site effect parameters (fundamental frequency and the associated soil amplification), and many surveys using this technique to study the site effect have provided convincing results. Saqqara pyramid lies to the southwest of Cairo near the epicenter of Cairo earthquake that happened on 12 October 1992. The pyramid is severely damaged by the earthquake with epicenter at about 14 km away; as a result, numerous efforts are exerted to restore the pyramid and to prevent it from total collapse. The current work is motivated by such efforts with focus on the potentiality of using the spectral ratio method (HVSR) to define subsurface monuments. The microtremor measurements were performed at 15 sites distributed in front of the southern side of the Zoser pyramid region in order to calculate the HVSR. Array shape and dimensions is chosen based on the prior information about the location and extension of the tunnel underneath. The tunnel extended about 20 m underneath the area adjacent to the southern gate of the pyramid. The amplification spectra have been evaluated for the soil column at each site location, and the fundamental frequency obtained and peak amplitudes are contoured showing some low value trends at the area adjacent to the southern gate. This may lead to the possibilities of the HVSR method to explore the subsurface monuments. However, this statement is still in the early stage and further theoretical investigation is required.

This paper shows the results of bedrock depth analysis around the largest cover-collapse sinkhole that occurred during the 2020-2021 Petrinja earthquake sequence. Horizontal to Vertical Spectral Ratio (HVSR) data was collected by the Geotechnical Extreme Events Reconnaissance (GEER) team after the Mw 6.4 December 2020 Petrinja earthquake in Croatia. In addition, the GEER team collected other data to assess the damage and geologic conditions, including two geotechnical boreholes with field and laboratory data and Multichannel Analysis of Surface Waves (MASW) profiles. Out of 61 HVSR readings performed during reconnaissance, 15 are around the largest sinkhole, S001, about 25 m wide and 12 m deep, with vertical walls and groundwater. The soil in the area consists of a clayey cover that is 4.0 m to 10.0 m thick, with sporadic gravel lenses. Clays are mostly over-consolidated, with varying degrees of saturation with intensely karstified carbonate rocks underneath. The HVSR data was analyzed using the HVSRweb platform and associated Python-based modules incorporating various statistical assessment models include single azimuth, multiple-azimuth, and geometric mean. The geometric mean results based on resampling frequencies between 3 Hz and 10 Hz indicate karst depths between 12.0 m and 18.0 m, which is generally consistent with the bedrock assumed from the sinkhole depth. Furthermore, an evaluation of the spatial variability of the resonance frequencies and the corresponding depth estimates assesses the presence and orientation of karstic features around S001. Based on the assessed data, HVSR measurements appear to be a helpful tool for evaluating variations in subsurface impedance contrasts and can be used to augment geotechnical data and other geophysical measurement techniques due to the relative ease of deployment and rapid data acquisition.

Application of HVSR technique in the site effects estimation at the south of Marsa Alam city, Egypt

In this study, shallow seismic surveys, including seismic refraction, Multichannel Analysis of Surface Waves (MASW), Refraction Microtremor (ReMi), and Microtremor measurements were conducted to estimate site characterization at 26 strong-motion stations of AFAD (Disaster and Emergency Management Presidency) in the province of Hatay, situated in one of the most seismically active regions in southern Turkey. The Horizontal to vertical spectral ratio (HVSR) technique was applied, using smoothed Fourier spectra derived from a long duration series to determine dominant frequency values at different amplification levels. Shear wave velocity up to 30 m of the ground was detected with MASW analysis. In the ReMi analysis, up to 80 m was reached with a corresponding average of 650 m/s shear wave velocity. The shear wave velocities estimated by the MASW method up to 30 m were compared with those found by the ReMi method, and they were observed to be very compatible. The province of Hatay was classified according to Vs30 based NEHRP Provisions, Eurocode-8, the Turkish Building Earthquake Regulation (TBDY-2018), and Rodriguez-Marek et al. (2001). The shear-wave velocity (Vs30), Horizontal to Vertical ratio’s (H/V) peak amplitude, dominant period, and site class of each site were determined. The H/V peak amplitudes range between 1.9 and 7.6, while the predominant periods vary from 0.23 sec to 2.94sec in the study area. These results are investigated to explain the consistency of site classification schemes.

The Horizontal-to-Vertical Spectral Ratio (HVSR) method and Multi-Channel Analysis of Surface Waves (MASW) method are commonly used as a joint fit technique to retrieve the 1-D shear wave velocity. The Kumaon Himalaya consists of major thrusts like MFT, MBT, SAT, NAT and MCT, from South to North) and other tectonic features. These geological structures are observed in the form of lineaments on the surface. In the present study, 2-D section of shallow shear wave velocity structure has been estimated along the transect crossing South Almora Thrust (SAT) in the Kumaon Himalaya to study the variation of shear wave velocity across the thrust. In the present work, the ambient noise survey and Multi-Channel Analysis of Surface Wave (MASW) survey has been conducted along the road profile crossing the South Almora Thrust (SAT) at equally spaced stations of 3 Km. The 1-D shear wave velocity has been used to prepare the 2-D section of shear wave velocity. The lineaments in this division have been identified by the variation in the two dimensional shear wave velocity section prepared from the so obtained 1-D shear wave velocity in this profile. The study shows that there is a good correlation between variation of shear wave velocity in the region and major tectonic features of the area. The geological sections in this area has been compared with the obtained 2D structure which give a fair amount of idea about dip of SAT in this area.

Integrated surface geophysical surveys were conducted along a 3 km line in the sabkha area at the Port of Duqm site in the Sultanate of Oman in order to model the thickness of the uppermost sabkha layer and to determine the existence of a paleo-channel in the area. The spatial location of the survey line was laid out by a geodetic land survey for accurate geophysical measurements. Three geophysical surveys, the shallow seismic refraction method, the 2D multichannel analysis of surface waves (MASW) and the horizontal-to-vertical spectral ratio (HVSR) survey, were conducted. Despite uncertainties that are known to be associated with each of the used methods, a good agreement was revealed between the geophysical results and data from three nearby boreholes in terms of thickness and seismic wave velocity variation. The results of the shallow seismic refraction and MASW methods show a section with three main layers; a near-surface layer with P-wave velocity varying from 300 m s−1 to 500 m s−1, S-wave velocity varying from 150 m s−1 to 350 m s−1 and thickness ranging from 2 m at the northwestern end to 10 m at the southeastern end. This low-velocity layer is interpreted as the sabkha formation. The second layer is characterized by P-wave velocity ranging from 1000 m s−1 to 1600 m s−1, S-wave velocities of 450 m s−1 to 600 m s−1 and thickness varying from 4 m to 8 m. The third layer shows P-wave velocity from 2200 m s−1 to 2700 m s−1 and a shear wave from 650 m s−1 to 850 m s−1. This could be considered as a soft rock layer. The fundamental frequencies indicated by the HVSR results at the southeastern part of the line close to the start point, where relatively thick recent alluvium deposits are present, are lower than those at the northwestern part towards the end point, where rocky outcrops of Tertiary limestone appeared. The results reflect the existence of a paleo-channel at the southeastern part of the profile in agreement with the geomorphological data.

The West Bohemia/Vogtland region is unique for its intraplate earthquake swarm activity. The aim of this study was to estimate the effect of local geology to seismic amplification at the West Bohemia Seismic Network (WEBNET) sites. Using multichannel analysis of surface waves (MASW) we obtained 1D seismic shear-wave velocity (VS) models and average seismic shear-wave velocity from the surface to a depth of 30 meters (VS30). To estimate the site response (amplification ratio), we correlated VS30 with observed horizontal to vertical spectral ratios (HVSR) of M6.4 Petrinja earthquake (December 29, 2020) recorded by the WEBNET network. The comparison between HVSR and VS30 showed that for seismic stations with high shear-wave velocities the amplification is low. For stations where VS30 is under 600 m/s the amplification ratio is much higher and with the velocity decrease it increases significantly. Despite many doubts the parameter VS30 seems in this case as an effective proxy for the site amplification. Note: This paper was accepted into the Technical Program but was not presented at IMAGE 2021 in Denver, Colorado.

Estimating the near-surface site response to mitigate earthquake disasters at the October 6th city, Egypt, using HVSR and seismic techniques