Horizontal To Vertical Spectral Ratio Research Articles

Relevance of rock slope deformations in local seismic response and microzonation: Insights from the Accumoli case-study (central Apennines, Italy)

Based on a National Government Decree, detailed seismic microzonation studies were carried out after the seismic sequence of Central Italy (2016–2017) in support of post-earthquake reconstruction. During these activities mass rock creep deformations of slopes were observed in the municipality of Accumoli (Latium Region), involving two urbanized areas on the left bank of the Tronto River. The geological and geomorphological surveys as well as the geomechanical characterization of rock masses revealed a deformation process of flexural toppling which involves the sub-vertical thick sandstone layers down to 50 m of depth. Geophysical investigations, consisting of downhole tests, active surface-wave testing and seismic ambient noise measurements, were carried out in the areas involved in the deformational processes. The results highlighted that the rock mass involved in the flexural toppling exhibits peaks of the horizontal-to-vertical spectral ratios (HVSR) in a frequency band ranging from 1.5 up to 2.5 Hz. In one of the two areas, H/V calculated on weak-motions recorded during the seismic sequence within the deforming rock mass, showed a clear peak in the H/V frequency range 1.5–2.0 Hz, the same range pointed out by the ambient vibration analysis. Due to local morphological, lithological and bedding conditions, such an evidence cannot be related to a simple 1D resonance due to the seismic impedance contrasts originated from the local stratigraphy. Nevertheless, it seems more reliable that the HVSR peaks can be related to the 3D volume of intensely jointed rock mass, in agreement with the field evidence of flexural toppling, as these peaks result confined within the deforming mass. In addition, the lack of a clear polarization of the particle motion in the seismic ambient noise dataset suggests that the observed effect should not be linked to the interaction of seismic surface waves with persistent discontinuities in an anisotropic rock mass. According to the National guidelines for seismic microzonation studies, these rock masses have not been identified as unstable, i.e. involved in conventional landslide processes, but have been identified as seismic amplification prone microzones with associated amplification factors of at least 1.5 in the period range 0.1–0.5 s. The identification of such microzones within areas involved in gravity-induced slope deformations is a significant prerequisite for planning strategies of post-earthquake reconstruction.

On the use of the coda of seismic noise autocorrelations to compute H/V spectral ratios

SUMMARYThe horizontal to vertical spectral ratio (HVSR) of seismic ambient noise has been proven to be a fast and efficient method for characterizing the 1-D resonance frequency of the local subsurface in a practical framework. Over the last decades, theories have been developed in order to extend the exploitation of HVSR beside the frequency of its first peak, notably the diffuse field assumption (DFA) which links the HVSR to the Green’s function of the local medium assuming the diffuseness of the seismic ambient noise wavefield. However, the underlying assumption of the seismic ambient noise being a diffuse, equipartitioned field may not be satisfied under certain circumstances. In order to exploit the contribution of scattering in forging diffuse wave fields, we leverage the advantages of coda waves and present a novel procedure for computing the HVSR, using the coda part of ambient noise correlations. We applied this technique to data gathered at the plio-quaternary sedimentary basin of Argostoli, Greece. Results on this data set show the potential of the method to improve the temporal stability of the HVSR measurements compared to the classical computation, and the fit with the theoretical HVSR curve derived from the DFA theory. These results suggest that this procedure could help in extracting physical information from the HVSR and thus could lead to an extended use of these measurements to characterize the mechanical properties of the medium.

Multiple Geophysical Techniques for Investigation and Monitoring of Sobradinho Landslide, Brazil

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.

Shallow Structure of the Longmen Shan Fault Zone from a High-Density, Short-Period Seismic Array

ABSTRACTThe Longmen Shan fault zone that was shocked by the 12 May 2008 M 8.0 Wenchuan earthquake acts as the boundary between the western edge of the Sichuan basin and the steep eastern margin of the Songpan-Ganze block. In this study, continuous seismic data recorded by 176 temporary short-period seismic stations between 22 October and 20 November 2017 are used to study the shallow crustal structure of the Longmen Shan fault zone by applying ambient-noise tomography and horizontal-to-vertical spectral ratio (HVSR) analysis. From ambient-noise analysis, fundamental-mode Rayleigh-wave dispersion curves between 0.25 and 1 Hz are extracted. Then, the direct surface-wave tomographic method is used to invert surface-wave dispersion data for the 3D shallow shear-wave velocity structure. Our results show that low shear-wave velocities are mainly distributed around the surface rupture trace of the Wenchuan earthquake at least down to 2 km. From the HVSR method, the sites are sorted into two types according to the pattern of HVSR curves with single peak or double peak. By converting frequency to depth, the results show that the sediments are thicker near the surface rupture. The low-velocity zone based on ambient-noise tomography agrees well with the distribution of sedimentary cover estimated from HVSR, which are generally consistent with geological information. Our results provide high-resolution shallow crustal velocity structure for future detailed studies of the Longmen Shan fault.

Passive seismic horizontal to vertical spectral ratio (HVSR) surveying to help define bedrock depth, structure and layering in shallow coal basins

SummaryTrial passive seismic HVSR survey lines were carried out in the north-western part of the Wilga sedimentary basin in Western Australia, which is a small and shallow isolated sub-basin to the Perth Basin, sitting above rifted Archaean granite-gneiss basement rocks. The HVSR survey method was trialled to determine if it could resolve subsurface basin features quickly and cheaply. This trial HVSR survey was carried out as a series of transects along open walking tracks. It was very successful for mapping the general basin architecture by resolving bedrock depth to a maximum depth of close to 200 m along the survey transects, and defining a half-graben structure that forms the faulted eastern boundary of the basin, with the basin depth gently deepening toward the west. The HVSR data are also interpreted to have detected subtle layering within the basin sedimentary sequence to possibly map Permian black shale beds, coal measures and splits sitting below an unconformity of Cretaceous sedimentary cover. The passive seismic HVSR method has potential to be a robust survey method for defining basin architecture in shallow coal basins with poorly indurated basin fill deposits, especially in remote parts of the world with difficult access and limited exploration funds to carry out active seismic surveys.

Case studies on the application of passive seismic horizontal to vertical spectral ratio (HVSR) surveying for heavy mineral sand exploration

This study presents the results from several case studies on the application of passive seismic Horizontal to Vertical Spectral Ratio (HVSR) surveying methods for Heavy Mineral Sand (HMS) deposit s...

Application of passive seismic and AEM to 3D paleochannel imaging: Capricorn Orogen

This study uses shallow passive seismic HVSR (horizontal-to-vertical spectral ratio) technique to determine the depth and extent of a Cenozoic paleochannel composed dominantly of sand and clay inci...

Seismic response of cracking features in Jubao Mountain during the aftershocks of Jiuzhaigou Ms7.0 earthquake

Jiuzhaigou is a world-heritage site located in the plateau area of Northwest Sichuan Province, China. Serious slope failures in the epicentral area were triggered by the Ms7.0 Jiuzhaigou earthquake occurred on August 8, 2017. The source areas of the hazards are usually concentrated near ridge crests, revealing the possible occurrence of ground motion amplification phenomena. To explore the role of the amplification of ground motions in the formation of earthquake-triggered slope failures, two seismometers were installed, on the next day after the main shock, at the bottom of the slope of Jubao Mountain near the seismogenic fault. The two monitoring sites are located at elevations of 1414 m (J1) and 1551 m (J2, the top of the mountain). Five aftershocks were recorded by the monitoring instruments. We compared the mean levels of the peak ground acceleration (PGA) observed at different locations, and investigated the directional variations in the shaking energy by analyzing the polar diagrams of the Arias intensity (Ia). Then, in order to identify the directional resonance phenomenon and their frequencies and amplification coefficients, we examined the horizontal-to-vertical spectral ratio (HVSR) and the standard spectral ratio (SSR). Polar diagrams of the Arias intensity (Ia) indicated that the site response of Jubao Mountain showed a pronounced directivity (in the EW direction) with shaking maxima near the hill top oriented orthogonally to the elongation of the relief. We observed an obvious resonance phenomenon at site J2 at relatively low frequencies (2.5–9 Hz) and very weak spectral amplifications at site J1 at high frequencies (5–15 Hz), which suggested that the predominant frequency of monitoring site J2 was obviously attenuated and that the difference in the spectra was related to the influences of the local-scale site conditions of the whole mountain. The results of spectral ratio analyses (HVSR and SSR) showed that the direction of resonance was concentrated around an EW orientation, and the amplification factors near the hill top were larger than 2. It suggests that geologic factors also play a significant role in the anisotropic amplifications affecting the tops of slopes besides the topographic effects.

Preliminary site characteristics for urban seismic hazard in pasuruan fault from microtremor measurements

The Pasuruan fault is one of active faults in East-Java with the minimum estimated earthquake magnitude to be 6.6 Mw. Although the Pasuruan fault is relatively small in term of estimated earthquake magnitude, because the area is highly populated, earthquake along the fault could pose a significant hazard to the densely populated region and infrastructure. This study mainly focuses on site characteristics of along the Pasuruan fault based on microtremor Horizontal-to-Vertical spectral ratios (HVSR) as a part of the seismic hazard assessment. Through this measurement, the output the site natural frequency, the amplification and the soil vulnerability maps which will be used to make an urban seismic hazard map for along Pasuruan fault. The microtremor measurements were carried out at 48 locations to investigate the natural frequencies and the amplifications at sites. The results showed that the natural frequency (f0) values range from 1 Hz to 10 Hz and the amplification factor (A0) values range from 2 to 8. Correlations between the natural frequency with the geological formation and topographic were established. Variations of both parameters are also indicated as a result of variations in surface soil parameters. The vulnerability index (Kg) is indicated the soil damage level due to ground motions. The weak zone failed during an earthquake on the Southwest site was identified by the highest Kg value.

Estimation of horizontal-to-vertical spectral ratios (ellipticity) of Rayleigh waves from multistation active-seismic records

The horizontal-to-vertical spectral-ratio (HVSR) analysis of ambient noise recordings is a popular reconnaissance tool used worldwide for seismic microzonation and earthquake site characterization. We have expanded this single-station passive HVSR technique to active multicomponent data. We focus on the calculation of the HVSR of Rayleigh waves from active-seismic records. We separate different modes of Rayleigh waves in seismic dispersion spectra and then estimate the HVSR for the fundamental mode. The mode separation is implemented in the frequency-phase velocity ([Formula: see text]-[Formula: see text]) domain through the high-resolution linear Radon transformation. The estimated Rayleigh-wave HVSR curve after mode separation is consistent with the theoretical HVSR curve, which is computed by solving the Rayleigh-wave eigenproblem in the laterally homogeneous layered medium. We find that the HVSR peak and trough frequencies are very sensitive to velocity contrast and interface depth and that HVSR curves contain information on lateral velocity variations. Using synthetic and field data, we determine the validity of estimating active Rayleigh-wave HVSR after mode separation. Our approach can be a viable and more accurate alternative to the empirical HVSR analysis method and brings a novel approach for the analysis of active multicomponent seismic data.

Bedrock depth evaluation using microtremor measurement: empirical guidelines at weathered granite formation in Singapore

Detection of bedrock depth is one of the critical site investigation procedures for seismic hazard analysis and underground developments that may encounter varying rock formation. The most common practice to detect bedrock is to directly drill boreholes. However, the intrusive borehole-based site investigation process is often expensive and time consuming and provide limited information from discrete boreholes. In the present study, a surface wave based technique, microtremor array measurement (MAM) and microtremor measurement (MM) is used to find the depth of bedrock at Bukit Timah granite formation in Singapore. Based on a series of MAM and/or MM surveys, four interpretation approaches, i.e., (1) Bilinear intersection method; (2) Preselected shear wave velocity (VS) based approach; (3) Normalized phase velocity approach; and (4) Horizontal to vertical spectral ratio (HVSR) analysis, are proposed for bedrock depth detection. The first three approaches are based on surface wave inverted VS profile using a vertical component of MAM. The fourth approach is to utilize the natural frequency of the ground through HVSR using three components microtremor measurements (MM). By compiling experimental data at nine different testing sites, it was demonstrated that non-invasive surface wave-based approaches can be effectively used for bedrock detection. Especially, a promising empirical correlation between the natural frequency of ground and the depth of bedrock is subsequently proposed.

Investigation of the relation between Vs30 and site characteristics of Iran based on horizontal-to-vertical spectral ratios

It is well known that the ground-motion amplitude of an earthquake could be extensively influenced by site effect. This study aims at providing a simple and new site amplification model for Iran based on the site fundamental frequency (fpeak) obtained from horizontal-to-vertical spectral ratio (HVSR). For this purpose, as a more reliable predictor of site amplification, HVSR have been calculated from pseudo-spectral acceleration of Iranian ground motions database consisted of 560 records from 30 events with moment magnitude ranging from 3.7 to 7.7 at epicentral distance of 1.7–297 km. Proposed HVSR model for Iran has been compared with the other regional models developed for California, Japan, Taiwan, Italy and China. In addition, the correlation of measured time-averaged shear wave velocity in the upper 30 m (VS30) in different seismograph stations with site fundamental frequency is evaluated and a new predictive relation for VS30 has been proposed for the study area.

Assessing soil amplifications in Groningen, the Netherlands

Abstract Since the shallow part of the Dutch subsurface is practically always unconsolidated, the elastic waves generated by deeper (~3000 m) seated earthquakes will be subjected to transformation when arriving in these layers. Since, the number of induced seismic events has increased over recent decades, a better understanding of site response of the Dutch subsurface is required. Local site amplification can directly be measured due to the presence of sensors at multiple depth levels in the Groningen borehole network. Amplification factors from 73 local events have been calculated for each borehole location to quantify earthquake site response. Furthermore, horizontal-to-vertical spectral ratios (HVSR) from the ambient seismic field are calculated. A relationship has been established between the composition of the upper Holocene sediments and the size of the amplitudes of HVSR and earthquake site response. Highest amplitudes are measured where the Holocene sediments are composed of clay, fine sands, silts and peat. We can conclude that HVSR from the ambient seismic field can be used as a first-order proxy to get an indication for wave amplification during a seismic event. This allows a first assessment on wave amplification at sites without sensors at multiple depth levels and without abundant local seismicity.

Analysis of Influencing Factors and Numerical Simulation of Horizontal-to-Vertical Spectral Ratio Method

To improve the calculation accuracy of the horizontal-to-vertical spectral ratio (HVSR) method, this study theoretically analyzed the influencing factors of Rayleigh wave polarizability. The phase difference of the horizontal component and the phase difference of the vertical component are found to play a key role in calculating the polarizability. The influence mechanism of the superposition of body waves and different Rayleigh waves on the polarizability of the Rayleigh wave is derived. The effects of the body wave, amplitude, frequency and Rayleigh wave superposition of different sources on the polarizability are verified by numerical simulation. The results show that the body wave significantly interferes with the polarizability of the Rayleigh wave. When a signal contains more than one set of Rayleigh waves, the superposition of the same-source Rayleigh waves does not affect the ratio. However, the superposition of Rayleigh waves from different sources significantly interferes with the calculation of the polarizability. This provides a technical method and a theoretical basis for accurately extracting the Rayleigh wave polarizability dispersion curve from a seismic record signal. This would help improve the detection accuracy of the HVSR method for ground pulse signals.

Approximate analytical HVSR curve using multiple band-pass filters and potential applications

The Nakamura method, which utilizes the Horizontal to Vertical Spectral Ratio (HVSR) analysis, is widely used for seismic microzonation studies. The HVSR is an easy tool for estimation of site response resonances based on recorded ambient noise; however, it gives amplifications at resonant frequencies that are poorly correlated to the actual amplifications during strong ground motion.Generally, the site response, including any resonant effects, depends on the amplitude, frequency and duration of ground motion. An approach was proposed previously by McGuire [1], in which the transfer function of the soil response was approximated as a Single Degree of Freedom (SDOF) oscillator with one resonant frequency, obtained from the maximum in HVSR. A new approach is developed here, in which the entire HVSR curve is approximated by a manageable set of parallel band-pass resonators. Each individual oscillator is defined by three parameters: center frequency, gain, and steepness (Q factor). This approximation allows for the development and use of an analytical model of the HVSR curve.The application of the new approach is demonstrated on data recorded by the stations of the Southern Ontario Seismic Network (SOSN/Polaris), which have well studied characteristics and site response [2,3]. Data collected at each site consists of noise recordings to obtain the HVSR, as well as earthquake records. The analytical HVSR curves for each station are used to remove the site effect component from the recorded seismograms.

Single and multi-channel passive source methods for calculating the shallow S-wave velocity structure and site effect parameters at 15th May City, Egypt

In this work, two passive seismic techniques were utilized for recording noise data at 57 sites to estimate near-surface soil response in 15th May city. Single seismic station of microtremor data was used to investigate the study area utilizing Nakamura method of Horizontal to Vertical Spectral Ratio (HVSR) to determine the structure in terms of the predominant frequency. At each point of microtremor measurement, the natural frequency and amplification factor were estimated. Velocities of transverse waves (VS) of the upper part section of the sedimentary succession are estimated using microtremor observations technique. The spatial autocorrelation (SPAC) method was employed for estimation dispersion curves, VS structure was resulting by genomic algorithm. The results demonstrate that, the fundamental resonance frequency is varying from 1.36 Hz to 1.607 Hz at the majority of the study area, amplification values ranges between 1.09 and 3.61 and the thickness of unconsolidated sediments were determined. The retrieved shear-wave velocity models were characterized by significant impedance contrasts for array site № 1 at 7 m, 13 m, and 36 m depths corresponding to velocities 142, 636 and 1047 m/s, array site № 2 at 2 m, 6 m, and 16 m depths corresponding to velocities 253, 614 and 942 m/s and array site № 3 at 2 m, 6 m, and 20 m depths corresponding to velocities 242 m/s, 608 m/s and 1027 m/s respectively. Value results of Fo were used in the calculation of bedrock depth at the measured sites. The parameters obtained in this study are matching well with the site geology.Based on the average shear wave velocity values for the top 30 m (VS30) for each array location, the site classes were estimated. The sites Ar_1 and Ar_2 belong to class C (very dense soil and soft rock), while Ar_3 fit in class B (rock).

Horizontal-to-Vertical Spectral Ratio (HVSR) Method for Earthquake Risk Determination of Jakarta City with Microtremor Data

Jakarta is the capital of the Republic of Indonesia which lies above a thick sedimentary basin. Geographically, Jakarta is 200 km away from the Indo-Australian subduction zone that sinks under the island of Java. There are many vital buildings and with the thick sediments underlying the city of Jakarta. Therefore, this region has a considerable seismic vulnerability. This will be dangerous if there is an earthquake that has the same frequency as the natural frequency of the building. It will cause a resonance resulting in amplification of seismic waves in the area. Each building has a different natural frequency, one that affects is the height of the building. To characterize the subsurface structure of the Jakarta Basin, microtremor data processing was obtained from the recording of 95 stations which was operated in October 2013 - February 2014 using Horizontal-to-Vertical Spectral Ratio (HVSR) method. HVSR is a method for obtaining subsurface information from single station measurements by comparing the Fourier spectrum of horizontal components to its vertical components. This ratio is a function of the frequency that will produce the H/V curve. The dominant frequency value on the HVSR curve represents the natural frequency of the area. The Seismic Vulnerability Index (Kg), which serves to determine the soil weak zone, can be calculated from the H/V curve. The dominant frequency value maps generated for the Jakarta area range from 0.2-0.22 Hz for low frequencies and 1-8.6 Hz for high frequencies. The large dominant frequency correlates with the thin sediment layer. Based on the frequency range, the south and northwest regions of Jakarta have relatively shallow basement depths compared to other regions. The resulting amplification value map can be divided into 4 maps with different period ranges. From the four maps, the North Jakarta area or the area around Jakarta’s coastline is most at risk of amplification with an H/V value up to 11 because the area is associated with alluvial deposits and coastal sediments. The seismic vulnerability distribution map in Jakarta City ranges from 15-850 relatively high in northeast and north of Jakarta.

Assessment of the Seismic Site Amplification in the City of Ivanec (NW Part of Croatia) Using the Microtremor HVSR Method and Equivalent-Linear Site Response Analysis

The city of Ivanec is located between valley of the Bednja River and Mt. Ivanščica and this area can be prone to significant seismic site amplification due to local site characteristics. This study presents the first assessment of seismic site amplification for the city of Ivanec by the microtremor horizontal-to-vertical-spectral-ratio (HVSR) method and the equivalent-linear (EQL) site response analysis. Based on microtremor measurements and HVSR analysis, fundamental soil frequency and HVSR peak amplitude maps indicate potentially seismic danger zones. The 1-D EQL site response analysis was performed using multiple suites of earthquake ground motions scaled to the 95- and 475-year return periods of peak ground accelerations. Site amplification maps at the predominant peak frequency and ground surface indicate two microzones, one with high amplification in the central part of the city due to soft soil characteristics, and the other with small amplification in the transitional zone from alluvial basin towards the foothills of Mt. Ivanščica. HVSR peak amplitudes and site response peak amplifications showed similar spatial distributions with similar predominant peak frequencies but with different amplitude levels. Site amplification maps provided significant information about potential resonance effects for structures of certain heights that can be correlated with the local ground shaking characteristics.

On the efficient acquisition and holistic analysis of Rayleigh waves: Technical aspects and two comparative case studies

The present study illustrates a series of technical aspects regarding the holistic acquisition and analysis of Rayleigh waves acquired by a single multi-component geophone.Compared with the common multi-channel and multi-offset active and passive methodologies, this approach requires simple and straightforward acquisition procedures and takes advantage of the joint analysis of the group-velocity spectra of both the vertical and radial components together with the Radial-to-Vertical Spectral Ratio (RVSR), i.e. the ratio between the amplitude spectra of the radial and vertical components.In this way we can fully describe the Rayleigh-wave propagation both in terms of velocities (group-velocity spectra) and relative amplitudes (RVSR) and jointly invert the data by means of a three-objective optimization scheme based on the Pareto optimality.Since the group-velocity spectra of the vertical and radial components are analyzed according to the Full Velocity Spectrum (FVS) approach, data inversion is performed without interpreting the velocity spectra in terms of dispersion curves.The results of the described methodology are assessed by considering two case studies and by comparing the retrieved shear-wave velocity profiles with the ones obtained through the analysis of the dispersion retrieved from standard active and passive multi-channel and multi-offset data also considering the Horizontal-to-Vertical Spectral Ratio (HVSR).Eventually, through a synthetic dataset, the effect of significant lateral variations is discussed with respect to the group-velocity spectra and the RVSR and RPM (Rayleigh-wave Particle Motion) curves.

Seismic Site Classification from the Horizontal-to-Vertical Response Spectral Ratios: Use of the Spanish Strong-Motion Database

Normally, the average of the horizontal-to-vertical (H/V) ratios of the 5% damped response spectra of ground motions is used to classify the site of strong-motion stations. In these cases, only the three-orthogonal as-recorded acceleration components are used in the analysis, and all the vector compositions that can generate a different response for each period oscillator are excluded. In this study, the Spanish strong-motion database was used to classify the sites of accelerometric stations based on the predominant periods through the average horizontal-to-vertical spectral ratios (HVSR) of recorded ground motions. Moreover, the directionality effects using the vector composition of the horizontal components of ground motions were also considered in the estimations of H/V ratios. This consideration is a relevant novelty compared to the traditional H/V ratios methods. Only earthquakes with magnitudes above 3.5 and hypocentral distances below 200 km were selected, which resulted in 692 ground-motion records, corresponding to 86 stations, from events in the period between 1993 and 2017. After the analysis, a predominant-period site classification was assigned to each station. On the whole, the obtained mean and standard deviation values of the spectral ratios are comparable to those shown by other researchers. Therefore, the advantages of the proposed procedure, which takes the directionality effects into account, can be summarized as follows: (a) The obtained information is richer and gives enables more sophisticated and realistic analyses on the basis of percentiles and (b) it is easier to detect anomalous stations, sites, and/or accelerograms. Moreover, the method eliminates the effect of directionality as a contributor to epistemic uncertainty.