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This study delves into Horizontal-to-Vertical Spectral Ratio (HVSR) and directional HVSR (D-HVSR) using continuous 30-min ambient seismic noise (ASN) signals recorded at 68 sites in the Chalco lakebed zone of the Valley of Mexico (VM). Our aim is to explore the shallow stratigraphy and subsurface structures in that zone. The isoperiod map using the HVSR, and the ensuing velocity structure inversion reveals a basin of irregular shape and shallow depth in the surroundings. Additionally, we explore azimuthal effects by computing the D-HVSR by rotating the records to obtain the time series for a given direction. We found different polarizations at the measurement points, and focused on the frequencies in which the effect is more pronounced. The cases with significant polarizations at the dominant frequencies suggest an irregularity at the basement level. This is supported by preliminary synthetic results that uses the three-dimensional (3D) Indirect Boundary Element Method (IBEM) to assess the effects of interface irregularities like a channel or dike. In the present stage of development, the D-HVSR is a powerful tool to reveal the existence of lateral variations. The complexity of the lakebed zone response in the VM and its irregular characteristics show the potential of the D-HVSR approach as well as the need to deeply explore into the modeling for its better understanding and interpretation. • This work explores shallow stratigraphy of Lake Chalco in the Valley of Mexico. • Conventional HVSR allows for soil periods and isoperiod maps estimations. • Directional HVSR detects ground motion polarizations due to lateral irregularity. • Preliminary results of 3D synthetic HVSR confirm their directional behavior. [ABSTRACT FROM AUTHOR]

Completing the earthquake recurrence period of the Marmara segment, one of the two seismic gaps of the northern Anatolian fault zone, necessitates earthquake hazard studies for Istanbul, considering the population density. This study focuses on determining the soil properties of the study area in the Maltepe district of Istanbul. The Maltepe, with its large population density, stands very close to this fault zone and is vulnerable to earthquake damage. The soil properties such as Vs 30 , dominant period (T 0), and amplification ratio (A 0) were determined with the MASW from surface wave analysis methods at 48 points and the HVSR approach of microtremor analysis methods at 26 points. These point-based properties were mapped in the GIS environment, and spatial distribution maps were prepared. In addition, seismic vulnerability (Kg) and effective shear strain (γ e) calculations for different acceleration values were carried out to detect deformation-failure on the surface. It was understood from the maps and the 2D cross-sections produced to examine the lateral and vertical changes that the soil conditions in the study area developed under the control of the Maltepe Beykoz fault (MBF). Accordingly, the two sides of the MBF lineament were determined with different geophysical outputs by their different geological origins. Namely, the alluvial part in the coastal part was defined by low Vs 30 , high period, and Kg values; on the contrary, the more rigid parts located at high elevations were described by relatively high Vs 30 , low period, and Kg values. In addition, it was understood that the deltaic sediments spread loosely and undulated in the alluvium at the SE part of the MBF lineament. These results obtained in the study are significant for predicting disaster damages and taking precautions before disasters. In addition, the scientific data potential of the outcomes of this study will make future studies, which are carried out to determine other parameters to be used during the taking of the precautions, more accessible. • The study includes the assessment of the integrated HVSR (Nakamura technique) and MASW method. • The soil properties of the Maltepe district of Istanbul were determined. • Vulnerability and effective share strain were obtained from acceleration scenarios. • The outcomes of each method were compared and mapped GIS environment. • According to the results, the evolution of the area was shaped under the control of en-echelon NW-SE trending MBF. [ABSTRACT FROM AUTHOR]

Abstract The Lonar crater in India is the only known hyper-velocity impact crater formed in basalt target on the Earth formed by an extra-terrestrial meteorite impact, with a distinct shock-induced metamorphosed signature. To understand the effects of impact deformation, fracturing, emplacement of the shock-metamorphosed ejecta material and sedimentation structure in the target basalt around the crater, we investigate the sub-surface shear velocity structure around the Lonar crater region. We use three components, 24-hour records of ambient noise (or microtremor) H/V spectral ratio (HVSR) and Rayleigh wave dispersion data through cross-correlation of ambient noise, obtained from 20 broadband seismic stations around the Lonar crater. The HVSR pattern across the Lonar crater does not present common frequency peaks, suggesting a laterally varying lithological arrangement around the crater. We observe that the HVSR curves at the stations show one or more peaks, below 1 Hz and above 2 Hz. To obtain the average sub-surface shear velocity models, we use the joint inversion of the phase dispersion curves of fundamental and higher mode Rayleigh wave with HVSR data sets, using neighborhood algorithm, in the frequency range 0.2–20 Hz and explored up to 750 m depth. As a reliability-check for the shear velocity models, we compute the theoretical HVSR by forward modeling. The theoretical and observed HVSR peaks match fairly well at each broadband station spread across the Lonar crater. The results show the existence of the fractured/weathered and sedimentary layers and also of the fundamental and higher modes of Rayleigh waves altering the shape of the HVSR and the Rayleigh wave dispersion curves. A pattern of the multiple layers in the sediment–package is visible in the shallow and deeper parts up to 600 m depths across the Lonar crater, but with spatial variations, which may be due to variations in the extent of the fractured zone. This spatial asymmetry of shear velocity profiles might also be the effect of an obliquity impact at the Lonar crater. The significance of such a joint analysis of different datasets of surface waves and HVSR is intended to reduce non-uniqueness involved in the non-linear inversions and present more acceptable sub-surface models to enhance the near-surface geophysical understanding of the impacted structure around the Lonar Crater. Highlights • The study uses joint inversion of Ambient Noise H/V Spectral Ratio (HVSR) and Rayleigh wave dispersion curves for near surface shear velocity characterisation. • The study region is the Lonar crater in India, which is a unique geophysical entity, formed by the high-velocity impact of a meteorite on basalt target. • As a reliability-check, computation of the theoretical HVSR curves by forward modelling, matching fairly well with the observed HVSR peaks. • This approach reduces non-uniqueness involved in the non-linear inversions. [ABSTRACT FROM AUTHOR]

The engineering bedrock depth (h) and time-weighted average shear wave velocity till that depth (V S,h) are recognized to be of primary relevance for earthquake hazard assessment. In fact, in contemporary seismic construction codes, they are regarded as proxys for the site response evaluation. Therefore, time-, cost-efficient and reliable investigation strategies are required for their determination, particularly when numerous surveys have to be interpreted for the reconstruction of the geological setting in large study areas and within micro-zonation studies. With this aim a workflow based on the application of a recently published linear wavelength-depth transformation (W D transform) is evaluated in this paper. The workflow allows to directly derive both h and V S,h from surface waves dispersion and HVSR curves, avoiding the formal solution of the inverse problem,. Obtained results show for the first time the effectiveness and reliability of the proposed workflow for various bedrock depths and overlying velocity structures, and discuss advantages and drawbacks in its application in relation to the uncertainty of the inversion techniques. • W-D transform is used to obtain bedrock depth and time-weighted average Vs from surface wave dispersion and HVSR curves. • The accuracy of the procedure is demonstrated comparable to standard surface wave processing approaches. • Advantages associated to ease of use of the procedure and related assumptions are discussed. [ABSTRACT FROM AUTHOR]

The use of the ratio of microtremor spectra, as computed by the Nakamura's technique, was recently proved successful for the evaluating the thickness of sedimentary covers laying over both shallow and deep rocky bedrocks thus enabling bedrock mapping. The experimental success of such application and its experimental uncertainties are today reported in many publications. To map bedrock, two approaches exist. The first is to assume a constant shear wave velocity profile of the sediments. The second, and most preferable, is Ibs-von Seht and Wohlenberg's, based on correlating Nakamura's curves main peak and wells information. In the latter approach, the main sources of uncertainty addressed by authors, despite the lack of formal proof, comprise local deviations of the subsurface from the assumed model. I first discuss the reliability of the simplified constant velocity approach showing its limitations. As a second task, I evaluate the uncertainty of the Ibs-von Seht and Wohlenberg's approach with focus on local subsurface variations. Since the experimental basis is well established, I entirely focus my investigation on numerical simulations to evaluate to what extent local subsurface deviations from the assumed model may affect the outcome of a bedrock mapping survey. Further, the present investigation strategy suggests that modeling and inversion, through the investigation of the parameters space around the reference model, may reveal a very convenient tool when lateral variations are suspected to exist or when the number of available wells is not sufficient to obtain an accurate frequency-depth regression. [ABSTRACT FROM AUTHOR]

The S-wave velocity (V S) in the Bengal Basin, Bangladesh has not been resolved from the ground surface to an intermediate depth in a regional context despite its importance for seismic hazard and risk evaluation. For this reason, we estimated V S profiles beneath 19 seismic stations in Bangladesh to a depth approximately 2800 m by employing full horizontal to vertical spectral ratio (HVSR) curve inversion under the diffuse field theory for the noise wavefield. The seismic stations are concentrated in three tectonic zones within the basin: the Surma basin (SB, Zone 1), Bengal Foredeep (BF, Zone 2), and Chittagong Tripura Fold Belt (CTFB, Zone 3). Full HVSR analysis (from 0.2 to 10 Hz) allowed us to obtain deep profiles with combined insights from shallow geotechnical boreholes and deep P-wave velocity (V P) information from active seismic surveys. From the resultant V S profiles, engineering bedrock (V S > 760 m/s) depths were also identified throughout the study area for the first time. The V S profiles within the Holocene to Miocene sedimentary sequences showed rapid variations from location to location. This is due to the highly variable near-surface geology caused by the dense and complex river network and tectonic deformation in Bangladesh. Except for three stations, the engineering bedrock depth exceeded 30 m at all stations. These results indicate the existence of deep soft soil in the study area, where V S 30 based site characterization is inappropriate. Furthermore, seismic site response was estimated at a station (DHAK) by simulating a subduction zone earthquake. The resulting response spectrum (RS) exhibited ground motion amplification over a longer period, suggesting that multistory buildings at the site may be at risk if subjected to large earthquakes. The outcomes of this study can serve as useful guidelines for seismic risk reduction planning in Bangladesh. • Full HVSR was inverted for shear wave velocity in the Bengal Basin, Bangladesh. • The depth of engineering bedrock was estimated from the velocity profiles. • Bedrock was found at depths >30 m in most of the parts of the deep basin. • Response spectrum indicated ground motion amplification over a longer period. [ABSTRACT FROM AUTHOR]

The Wasatch fault (WF) has been the subject of intensive study. Adjacent to a large and growing urban corridor, it poses a distinct seismic hazard for ∼2.5 million residents who live on its hanging wall, and has numerous Quaternary scarps and produced a M5.7 quake in March of 2020. Thus, the shallow architecture of the fault is important for understanding its earthquake hazards. Prior work has revealed shallow bedrock shelves in the hanging wall near the fault. To detect and image these shelves, we have employed the HVSR method (horizontal-to-vertical spectral ratio) at three locations along and across the WF. This method is powerful in that data can be collected rapidly in an urban setting. However, this method requires knowledge of the V s of sediment overlying bedrock in order to estimate bedrock depth. We employed the MASW (multichannel-analysis of surface waves) method to obtain the characteristic V s of sediment overlying bedrock. At Corner Canyon in Draper, Utah, Lake Bonneville sands and gravels overlie intrusive rocks of the Little Cottonwood Stock. Fault offsets inferred from HVSR correlate to mapped scarps. At Traverse Ridge, an HVSR profile co-located to a prior seismic reflection profile also detected shallow bedrock beneath alluvial fan sediment, including a graben system. However, at this site the geology is more complex. Fan sediment is underlain by altered volcanic rocks with subjacent quartzite. The HVSR method detected and characterized these boundaries. A majority of data were collected across the WF in the vicinity of Rock Canyon, Utah in order to create a 3D surface of the bedrock shelf. This area is heavily developed, but with numerous mapped scarps. A 3D model of the bedrock shelf indicates that it is broad, >1 km from the main trace of the WF. This width was not previously known. The HVSR results conform to mapped fault offsets, but also reveal a large, buried (and previously unknown) graben. One main finding of this study is that HVSR is a useful tool for mapping the shallow architecture of bedrock shelves in the hanging wall of normal faults, as long as a reasonable estimate of the V s of sediment overlying bedrock can be made. Mapping can be accomplished rapidly in uneven topography or in urban settings as the instrumental footprint is very small. A second main finding is that large normal faults may commonly have concealed bedrock shelves. These shelves may affect engineering inputs like V s30. However, they may also have other unanticipated effects on the nature of ground shaking, perhaps due to the trapping of energy or reverberation between the ground surface, bedrock shelf, and bedrock in the foot wall. • The Wasatch fault poses hazards to about 2.5 million residents in Utah, USA. • The fault is capable of producing large earthquakes. • Shallow bedrock shelves that may affect ground shaking during earthquakes. • Shelves were mapped in 2D and 3D using MASW and HVSR. [ABSTRACT FROM AUTHOR]

In order to investigate the sedimentary cover of a vast perilagoon area from the geotechnical point of view and test different possible and unconventional approaches to surface-wave analysis, a series of different acquisitions and analysis procedures are tested, always considering the need for a light equipment and simple field procedures. In one case a joint inversion of the Horizontal-to-Vertical Spectral Ratio (HVSR — passive seismics) and Rayleigh-wave group-velocity spectrum (active seismics) is performed and results eventually compared to simpler single-component inversion of the group-velocity spectrum only (in both cases the group-velocity spectrum is considered according to the Full Velocity Spectrum – FVS – approach). For a second site, active data collected by using a single 3-component geophone are holistically inverted by considering the group-velocity spectra of the radial- and vertical-components of Rayleigh waves, jointly with the Radial-to-Vertical Spectral Ratio (RVSR). A second procedure based on purely-passive data (HVSR curve and phase velocity dispersion curve determined via Miniature Array Analysis of Microtremors ) was also considered and results compared. In all cases, results appear extremely consistent providing mutual evidence of the efficiency of the considered methods and showing very homogenous stratigraphic conditions in the investigated area. [ABSTRACT FROM AUTHOR]

Abstract: The local geology and shallow S-wave velocity structure of a site are recognized to be key factors for the increase in the damaging potential of seismic waves. Indeed, seismic amplitudes may be amplified in frequency ranges unfavorable for building stock by the presence of soft sedimentary covers over lying hard bedrock. Hence, microzonation activities, which aim at assessing the site response as accurately as possible, have become a fundamental task for the seismic risk reduction of urbanized areas. Methods based on the measurement of seismic noise, which typically are fast, non-invasive, and low cost, have become a very attractive option in microzonation studies. Using observations derived from seismic noise recordings collected by two-dimensional arrays of seismic stations, we present a novel joint inversion scheme for surface wave curves. In particular, the Love wave, the Rayleigh wave dispersion and the HVSR curves are innovatively combined in a joint inversion procedure carried out following a global search approach (i.e., the Genetic Algorithm). The procedure is tested using a data set of seismic noise recordings collected at the Bevagna (Italy) test-site. The results of the novel inversion scheme are compared with the inversion scheme proposed by Parolai et al. (2005), where only Rayleigh wave dispersion and HVSR curves are used, and with a cross-hole survey. [Copyright &y& Elsevier]

Abstract: Surface Wave (SW) dispersion and Horizontal-to-Vertical Spectral Ratio (HVSR) are known as tools able to provide possibly complementary information useful to depict the vertical shear-wave velocity profile. Their joint analysis might then be able to overcome the limits which inevitably affect such methodologies when they are singularly considered. When a problem involves the optimization (i.e. the inversion) of two or more objectives, the standard practice is represented by a normalized summation able to account for the typically different nature and magnitude of the considered phenomena (thus objective functions). This way, a single cost function is obtained and the optimization problem is performed through standard solvers. This approach is often problematic not only because of the mathematically and physically inelegant summation of quantities with different magnitudes and units of measurements. The critical point is indeed represented by the inaccurate performances necessarily obtained while dealing with problems characterized by several local minima and the impossibility of a rigorous assessment of the goodness and meaning of the final result. In the present paper joint analysis of both synthetic and field SW dispersion curves and HVSR datasets is performed via the Pareto front analysis. Results show the relevance of Pareto''s criterion not only as ranking system to proceed in heuristic optimization (Evolutionary Algorithms) but also as a tool able to provide some insights about the characteristics of the analyzed signals and the overall congruency of data interpretation and inversion. Possible asymmetry of the final Pareto front models is discussed in the light of relative non-uniqueness of the two considered objective functions. [Copyright &y& Elsevier]

Active and passive seismic techniques permit to draw images of relevant buried morphologies in mountainous areas. Here we apply the Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic technique to draw the shape of a buried valley in the core of the Italian Central Alps, at the confluence among the fault-controlled Valtellina, Valchiavenna and Lake Como depressions ("Colico knot"). Twenty-three single-station, ambient-noise measurements were acquired and the site-specific fundamental frequency f 0 was obtained by computing the HVSR in the 0.5 Hz to 5 Hz frequency band. Shear wave velocity (Vs)-versus-depth profiles were computed from the inversion of the HVSR curves, under the assumption of a 1D structure. To limit uncertainty, the results were calibrated by comparison with data from two sources: 1) HVSR profiles acquired in a comparable setting in Valtellina, at a site where estimates of Vs for the seismic basement and cover sediments were available; 2) borehole logs which crossed the sediments above the metamorphic bedrock at the Colico knot site. Independent data were used to constrain the best fit of the experimental HVSR peaks at the Colico knot site, in terms of frequency f 0 and HVSR amplitude, thus retrieving information about Vs variations related to the bedrock versus sediments unconformity. A SW-NE elongated buried trough, as deep as 150 m below sea-level and up to 2 km wide was drawn from the present-day Adda valley to the Lake Como depression to the South. The morphology of this abandoned and buried course of the Adda river is controlled by active deep-seated gravitational slope deformation along the adjacent mountain slope. The valley was abandoned before the Last Glacial Maximum (latest Pleistocene), plausibly owing to narrowing and uplift induced by deep-seated slope deformation. In fact, the LGM Adda glacier reshaped the valley slopes and the bedrock watershed that separates at present the post-Glacial Adda river course from the paleo-Adda trough. • HVSR method was applied to map Neogene buried paleo-valleys in the Central Alps • Narrow peaks to plateau-like HVSR peaks testify complex geometry in the subsurface • 1D inversion was supported by independent data to limit uncertainties • Paleo-valleys evolved under fault- and DSGSDs control of slope dynamics [ABSTRACT FROM AUTHOR]

An experimental dynamic characterization was carried out through the execution of non-invasive geophysical surveys, in order to obtain useful information for planning interventions aimed at protecting the structure. In particular, HVSR and MASW methods allowed to reconstruct the subsoil seismostratigrafic structure, and HSSR and RDM techniques was employed to obtain the structure dynamic properties. It was highlighted a complex dynamic of the building, probably linked to the numerous modifications it has undergone over the centuries. A clusters analysis, carried out on the fundamental frequency values detect in the measurement sites, allowed to identify structure parts with homogeneous dynamic behaviour. Comparing the vibration frequencies of the building with the ground ones, there seems to be no possibility that soil-structure resonance phenomena may occur. • Geophysical surveys allow a quick and non-invasive dynamic characterization. • Seismo-stratigraphic image highlight Impedance contrasts. • Cluster analysis allow identifying structure parts with homogeneous dynamic response. • Soil-structure resonance phenomena can be exclude from the comparison of the results. [ABSTRACT FROM AUTHOR]

In recent decades, numerous observations of the connection between sedimentary basins structure and severity and variability of damages during earthquake events have sparked a series of studies for characterization of such subsurface structures. However, these investigations were always accompanied with cost concerns and executive restrictions. Among non-destructive and cost effective approaches which have been developed in seismic response evaluations, usage of microtremor has attracted considerable attentions. In the current study, we have focused on inversion of microtremor acquired in the Qom basin, Iran. As the area is known to be seismically active, our main contribution is the construction of a three-dimensional model of the sedimentary basin in terms of shear wave velocity, which extends and integrates the previous knowledge (limited to the Vs30) down to the bedrock. Additionally, we provided further evidence of several possible faults in the area, one of which has been introduced earlier but poorly investigated so far: the "Qomrud" Fault. The results of a series of previous studies on the area, comprising down-hole surveys, electrical resistivity surveys, surface seismic refractions and surficial bore-hole excavations have been used, either to form a more precise initial model for inversion process or to evaluate the outputs. Additionally, we investigated possibility of obtaining the distribution of the Vs30 in the area directly from inversion of HVSR curves. It revealed that, although for relatively shallow bedrocks (80–100 m) the differences of achieved velocities is lower than 10–15%, in deep sediments, the Vs30 from microtremor is largely overestimated. Furthermore, we discussed the applicability of empirical relationships for estimating bed-rock depth in the investigated basin. • Horizontal to vertical spectral ratio is inverted to extract velocity structure. • 2-dimensional and 3-dimensional subsurface structure of Qom basin is presented. • Credibility of microtremor inversions for shallow velocity structures is discussed. • The results show some trends which could be evidences of possible faults. [ABSTRACT FROM AUTHOR]

To assist in the identification of fundamental-mode dispersion curves of Rayleigh waves in dispersion diagrams, we explore the relation between the shape of the horizontal-to-vertical spectral-ratio (HVSR) of ambient seismic noise and the shape of the dispersion curves for phase and group velocities in a stratified medium. We propose to use the information coming from the HVSR to identify the osculation zones and multi-mode effects and to locate inflection points and critical points in the observed phase and group dispersion diagrams of Rayleigh waves. The relationship between these curves has been numerically investigated for some models consisting of one and two homogeneous layers overlying a half-space, with velocities increasing downwards. It is primarily found that the first minimum in the HVSR appears close to the frequency of the inflection point of the fundamental mode of phase velocity. In addition, the osculation and multimode effects occur between frequencies of the fundamental peak and the first minimum of the HVSR. On the other hand, the frequencies of the minima in HVSR closely approximate the critical points of the fundamental-mode group-velocity dispersion curve, even better than the inflection points of the fundamental-mode phase-velocity curve. Finally, we show an example of experimental identification of fundamental-mode phase and group dispersion curves supported by the shape of the HVSR, obtaining a reliable velocity profile through the simultaneous inversion of these three curves. • HVSR helps to pick phase and group dispersion curves of fundamental Rayleigh mode. • Osculation and multimode effects expected between the main peak and trough of HVSR. • Inflection and critical points in dispersion curves linked to HVSR characteristics. [ABSTRACT FROM AUTHOR]

Ambient vibrations spectral ratios at forty-five sites and shear wave velocity profiles obtained by inverting surface waves dispersion curves were used to assess seismic site response properties in the Floridia village located in southeastern Sicily. The results of the horizontal-to-vertical spectral ratios analysis often indicate fundamental in the frequency range 1.5–4.0 Hz. Conversely, where the bedrock outcrop, spectral ratios are flat. The calcarenites outcropping in the north-west sector of the village and the soft sediments in the south-east are characterized by shear wave velocity of about 700 m/s and ranging between 200 and 600 m/s, respectively. The geological substratum has instead shear wave velocity of about 1500 m/s. The achieved geophysical and geological data help to build a subsoil model of the studied area, classifying the main lithotypes above the bedrock and determining the hidden morphology. Moreover, amplification functions were computed for zones with homogenous geological and geophysical properties. Therefore, these site amplification functions were used to simulate ground motion scenarios corroborating the efficiency of the obtained results. The used strategy, involving geophysical and geological methods, represents an important step to derive ground motion scenarios and seismic risk maps. Moreover, the characterization of amplification due to local site effects could help to understand the pattern and the damage distribution of historical earthquakes. • Using HVSR and Vs profiles we assessed seismic site response properties. • Geophysical/geological models were defined for the investigated area. • Amplification functions were computed for zones with homogenous properties. • Earthquake scenarios were simulated to corroborating the efficiency of the obtained results. [ABSTRACT FROM AUTHOR]