Horizontal To Vertical Spectral Ratio Research Articles

Characterizing Surface Geology, Liquefaction Potential, and Maximum Intensity in the Kachchh Seismic Zone, Western India, through Microtremor Analysis

Abstract This study presents results of microtremor analysis carried out over diverse geological formations and liquefaction sites in the Kachchh seismic zone of western India, which is host to the deadliest intraplate earthquake ( M w 7.7). Microtremors were recorded at 36 sites with single stations and 7 sites with an array. The microtremor array measurements were made using triangular arrays with sensors kept at 30–90 m radii. The Rayleigh‐wave velocities are estimated from the array data using high‐resolution frequency–wavenumber ( f ‐ k ) analysis, and the 1D shear‐wave velocity structure is determined by inversion. We found a good correlation between surface geology, resonant frequency and amplitude of the horizontal‐to‐vertical spectral ratio (HVSR) curve, and shear‐wave velocity. The ground vulnerability index ( K g ) values in the liquefied areas are found to be higher than those in the adjacent areas devoid of liquefaction. Broad or multiple peaks in the HVSR curves and large variations in the shear‐velocity models could be attributable to the structural and tectonic complexity in the maximum intensity area. Further, our results show that the average fundamental frequencies are around 0.32 Hz for the Quaternary, 0.80 Hz for Tertiary, 1.8 Hz for Cretaceous, 1.8 Hz for Jurassic, and 2.3 Hz for the Deccan trap formations. The inverted S ‐wave velocity varies from 190 to 1350 m/s down to a depth of 200 m for the Quaternary, 220 to 680 m/s down to 100 m for Tertiary, 236 to 1160 m/s for Cretaceous, 670 to 1780 m/s for Jurassic, and 260 to 2440 m/s for the Deccan trap sites. The results of microtremor analysis corroborate the observations from the available geophysical, geological, and borehole data.

Passive Methods for the Fast Seismic Characterization of Structures: The Case of Silea Bridge

This paper provides useful information about a passive method, usually applied on soils, for defining the frequencies of most infrastructures. Today, the eigen-frequency determination is one of the most significant and binding requirement aspects, especially in relation to the recent earthquakes in Italy (Accumoli, Norcia 2016; Finale Emilia 2012; L’Aquila 2009). The development of inexpensive, fast and reliable procedures to define the eigen-frequency of construction in general and of infrastructures in particular, is the aim of modern civil engineering. Currently, experimental tests are widely applied to evaluate the dynamic behavior of bridges. While the natural frequencies of a structure can be determined using different methods, passive methods are attractive because of their low costs and easy and fast procedures. In this paper, two passive methods, called standard spectral ratio (SSR) and horizontal to vertical spectral ratio (HVSR), are introduced and analyzed. The two methods are applied to estimate the frequencies in the transversal direction of a composite steel-concrete viaduct recently built in Italy. The comparison of the results obtained from SSR and HVSR and a finite element model confirms the possibility of applying the two methods for the dynamic characterization of bridges. In particular, the SSR method provides a correct estimation of the lower order natural frequencies and their degree of amplification. HVSR, which is typically used only on soil studies, provides a reliable early estimation of the frequency of a structure, if the latter is flexible, compared to the soil characteristics. The HVSR method is directly applied to the viaduct, so that an analogy is created between the soil layer and the structural elements. This paper is intended to show how the HVSR method typically applied to soil can reach good results in the dynamic characterization of Silea steel-concrete viaduct.

Ground motion simulations for seismic stations in southern and eastern Romania and seismic hazard assessment

This research focuses on the evaluation of soil conditions for seismic stations in southern and eastern Romania, their influence on stochastic finite-fault simulations, and the impact of using them on the seismic hazard assessment. First, the horizontal-to-vertical spectral ratios (HVSR) are evaluated using ground motions recorded in 32 seismic stations during small magnitude (MW ≤ 6.0) Vrancea seismic events. Most of the seismic stations situated in the southern part of Romania exhibit multiple HVSR peaks over a broad period range. However, only the seismic stations in the eastern-most part of Romania have clear short-period predominant periods. Subsequently, stochastic finite-fault simulations are performed in order to evaluate the influence of the soil conditions on the ground motion amplitudes. The analyses show that the earthquake magnitude has a larger influence on the computed ground motion amplitudes for the short- and medium-period range, while the longer-period spectral ordinates tend to be influenced more by the soil conditions. Next, the impact of the previously evaluated soil conditions on the seismic hazard results for Romania is also investigated. The results reveal a significant impact of the soil conditions on the seismic hazard levels, especially for the sites characterized by long-period amplifications (sites situated mostly in southern Romania), and a less significant influence in the case of sites which have clear short predominant periods.

Seismic Hazard Analysis based on Earthquake Vulnerability and Peak Ground Acceleration using Microseismic Method at Universitas Negeri Semarang

Microseismic is a harmonic vibration of land that occurs continuously at a low frequency. The characteristics of microseismic represents the characteristics of the soil layer based on the value of its natural frequency. This paper presents the analysis of seismic hazard at Universitas Negeri Semarang using microseismic method. The data acquisition was done at 20 points with distance between points 300 m by using three component’s seismometer. The data was processed using Horizontal to Vertical Spectral Ratio (HVSR) method to obtain the natural frequency and amplification value. The value of the natural frequency and amplification used to determine the value of the earthquake vulnerability and peak ground acceleration (PGA). The result shows then the earthquake vulnerability value range from 0.2 to 7.5, while the value of the average peak ground acceleration (PGA) is in the range 10-24 gal. Therefore, the average peak ground acceleration equal to earthquake intensity IV MMI scale.

Long‐Period Amplification in Deep Alluvial Basins and Consequences for Site‐Specific Probabilistic Seismic‐Hazard Analysis: An Example from the Po Plain (Northern Italy)

This study deals with the impact of deep‐soil discontinuities on ground‐motion amplification in the Po alluvial basin (northern Italy), and consequently, on long‐period probabilistic seismic‐hazard analysis estimates. The focus is on Castelleone, where a seismic station of the Italian National Seismic Network has been deployed since 2009. To define a detailed shear‐wave velocity ( V S ) profile, extensive active and passive geophysical surveys were carried out. The active measurements included seismic profiles examined through standard refraction techniques and multichannel analysis of surface waves. The passive methods included ambient vibration measurements in both single and array configurations. In particular, three microtremor arrays with increasing apertures were performed to capture the experimental dispersion curve down to 0.2 Hz. The horizontal‐to‐vertical spectral ratio (HVSR) showed two main peaks, at 0.17 and 0.70 Hz. A joint inversion of the experimental phase‐velocity dispersion and HVSR curves was performed to obtain the V S profile. The data show two main discontinuities at ∼160 and 1350 m in depth. According to the Italian and European seismic codes, the shallow V S discontinuity can be ascribed to the seismic bedrock (i.e., V S >800 m/s), whereas the deeper one is observed at the transition between Pliocene–Quaternary deposits and Miocene marls. Preliminary site‐specific seismic‐hazard analysis in terms of displacement response spectra for periods up to 10 s shows that neglecting the effects of the deeper discontinuity implies underestimation in hazard estimates of up to about 49% for mean return period (MRP) of 475 years and about 57% for MRP of 2475 years, with possible consequences regarding the design of very tall buildings and large bridges.

Site - building resonance response in a complex geological setting: Ground motions recorded in the centre of Paleohora Basin and at a rock fractured outcrop site close to the basin edge (SW Crete, Greece)

Seismic site characterization is an important parameter in earthquake hazard assessment for risk mitigation studies, which are essential in engineering design practices and urban planning providing useful information to governmental or private civil protection authorities. Recent advances from site effect studies envisage for the future the importance of incorporating microzonation as a tool for determining resonance prone buildings especially for the most earthquake hazardous municipalities. This study aims to assess the frequencies of vibration of selected masonry and reinforced concrete buildings and to examine potential soil - building resonance in a complex geological setting of the small scale Paleohora Basin (southwest Crete). Ambient noise survey was performed in masonry and reinforced concrete buildings, on soil foundation and on soil at several distances from the base of the selected buildings. The selected public and private (masonry and RC) building constructions are located in the centre of the Basin which is characterized by heterogeneities induced by large scale E-W fault and at rock fractured (of complex orientation and opening of fractures and joints) outcrop site close to the margins of the Basin in the southeast bordered by an NNE-SSW fault. The spatial horizontal to vertical spectral ratio (HVSR) technique using ambient noise reveals that soil-building resonance phenomena could be inferred in the small scale alluvial Paleohora Basin.Two amplified peaks in the centre of the Basin and one amplified peak at the rock fractured outcrop site close to the Basin margins are observed from the data sets. In the centre of the Basin, the amplified peak at medium frequency (3.85–7.11 Hz) is related to the near subsurface irregularities locally induced by E-W faulting, while the low frequency (0.75 Hz) is related to the overlain Quaternary deposits. At the rock fractured outcrop site close to the margins of the Basin the one amplified peak at medium frequencies (2.5 Hz) is related to the fractured limestone outcrop. Weak motion earthquake data following the main earthquake event that occurred near to the southwest coast of Paleohora are recorded in the centre of the Basin (at the schoolyard) and on rock fractured outcrop close to the margins of the Basin. The HVSR of the weak motion earthquake data present in the center of the Basin two amplified peaks at low (0.75 Hz) and medium frequencies (6.56 Hz) and at the rock fractured outcrop site close to the Basin edge one amplified peak at medium frequencies (2.58 Hz). This study conducted in the complex small scale and dense populated Basin, highlights the necessity of incorporating the determination of prone resonance buildings into urban planning for risk mitigation studies, specifically in this earthquake hazardous municipality.

Assessment of a geological model by surface wave analyses

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.

Microtremor HVSR study of site effects in the urban area of the town of Mytilene, Lesvos (Greece) - Preliminary results.

Mytilene is the capital of Lesvos, the eighth largest island in the Mediterranean Sea and the largest in the North Aegean. The region of North Aegean is a geotectonically complex area, because its geodynamic status is directly affected by the North Anatolian Fault Zone. In the present paper, microtremor data have been analyzedfor the city of Mytilene using Nakamura technique of Horizontal to Vertical Spectral Ratio (HVSR) to ascertain the structure in terms of the predominant frequency. 100 microtremor measurements have been performed in the city of Mytilene. At each point of microtremor measurement, the natural frequency and amplification factor have been determined. The predominant frequency varies from 0.4 Hz to 6.6 Hz. The amplification factor in 0.4-8.07 range has been obtained from the HVSR analysis. The results are presented in terms of maps, including the spatial variability of the predominant frequency and developed GIS database. The results of this study make it clear that the characteristics of microtremors depend on the type of soil deposits.

Geoelectric prospecting in University Campus region for detection of possible geological discontinuities, Rio, Patra, Greece.

Geophysical prospecting is a non catastrophic technique, applicable on a wide range of problems, including archaeological, environmental and geological problems. At Campus University of Patras, a detailed geophysical investigation applied for detection of possible existing geological discontinuities, which produced serious problems at buildings and main roads of Campus. As main technique used the electric mapping and electric imaging. These were applied on already prepared geophysical grids by measuring parallel profiles along and perpendicular to the geomagnetic north. The two geophysical grids were separated 100 meters away each other. Firstly, an electric mapping procedure took place by using twin-probe array with four electrodes in distance between 0.5-3 meters. As result was the recording of soil resistance on horizontal layer with constant depth. By processing the data through Geosoft Oasis Montaj software, the distribution of this physical property was illustrated on color scale maps. Secondly, electric imaging technique applied with twenty-five equal space electrodes along straight lines, with one meter space byusing the hybrid arrangement Wenner-Schlumberger (Stephanopoulos, 2002). As result was the recording of distribution of soil apparent resistivity on a vertical layer in eight separated depths. Resistivity calculated by processing imaging data through 2D mathematical algorithm based on least squares inversion (Res2Dinv). Furtherprocessing by Oasis Montaj, had as a result the production of horizontal slices (Stephanopoulos 2002) and 3D maps, where the resistivity distribution was illustrated on separated depths, in color and grey schedule format. The combined geophysical investigation with the contribution of the HVSR (Horizontal to Vertical Spectral Ratio) technique confirmed the existence of geological discontinuity.

Earthquake ground motion simulation at Zoser pyramid using the stochastic method: A step toward the preservation of an ancient Egyptian heritage

Strong ground shaking during earthquakes can greatly affect the ancient monuments and subsequently demolish the human heritage. On October 12th 1992, a moderate earthquake (Ms=5.8) shocked the greater Cairo area causing widespread damages. Unfortunately, the focus of that earthquake is located about 14km to the south of Zoser pyramid. After the earthquake, the Egyptian Supreme council of antiquities issued an alarm that Zoser pyramid is partially collapsed and international and national efforts are exerted to restore this important human heritage that was built about 4000years ago. Engineering and geophysical work is thus needed for the restoration process. The definition of the strong motion parameters is one of the required studies since seismically active zone is recorded in its near vicinity. The present study adopted the stochastic method to determine the peak ground motion (acceleration, velocity and displacement) for the three largest earthquakes recorded in the Egypt’s seismological history. These earthquakes are Shedwan earthquake with magnitude Ms=6.9, Aqaba earthquake with magnitude Mw=7.2 and Cairo (Dahshour earthquake) with magnitude Ms=5.8. The former two major earthquakes took place few hundred kilometers away. It is logic to have the predominant effects from the epicentral location of the Cairo earthquake; however, the authors wanted to test also the long period effects of the large distance earthquakes expected from the other two earthquakes under consideration. In addition, the dynamic site response was studied using the Horizontal to vertical spectral ratio (HVSR) technique. HVSR can provide information about the fundamental frequency successfully; however, the amplification estimation is not accepted. The result represented as either peak ground motion parameters or response spectra indicates that the effects from Cairo earthquake epicenter are the largest for all periods considered in the present study. The level of strong motion as indicated by peak ground acceleration reaches the value of 250gals which is considerably high. At the end, it is worth to mention that the information resulted from the present work may be useful for the planned restoration decision of the Zoser pyramid site.

HVSR analysis of seismic site effects and soil-structure resonance in Varaždin city (North Croatia)

Varaždin is an old baroque city located in the North of Croatia along the Drava River basin (alluvial sediments). Local site effects due to alluvial soft sediments (clays, sands, gravels) can play a crucial role in the amplification of seismic wave ground motions. Ambient noise (microtremors) measurements using the Horizontal-to-Vertical Spectral Ratio (HVSR) method were applied to determine the seismic ground response of the area, expressed by the fundamental/natural frequency (f0) and the site amplification (A0) of the sediments. The results of the free-field measurements show variations of the soil fundamental frequencies (0.47–5.19Hz), site HVSR amplifications (1.95–4.65), Vs,30 distributions (212–340m/s) and the sediments cover thicknesses (16–116m) above bedrock in Varaždin city area. The obtained results are found in good agreement with the geological map, the thicknesses of the sediments from lithological cross sections, existing boreholes data and MASW measurements. It was observed that the directional HVSR analysis and the polarization angles of the microtremors are affected by the horizontal polarization of the ground motion near, along and across the faults and correspond to the earthquake mechanism of the fault-plane solution from the past earthquakes in the area.The potential danger of the soil-structure resonance was estimated by comparison of the microtremor measurements in buildings of different heights (3–11 floors) and nearby free-field locations, using the Standard Spectral Ratio and Floor Spectral Ratio. The fundamental frequency of buildings ranged from 1.78Hz for taller buildings up to 5.64Hz for smaller buildings. If the building fundamental frequency is close to the soil fundamental frequency, the seismic ground motion will create possible resonance. Smaller buildings up to 6 floors showed lower potential of soil-structure resonance due to very low soil fundamental frequencies for the entire city area, while taller buildings up to 11 floors showed higher potential of soil-structure resonance. The directional HVSR analysis demonstrated how buildings in the studied area receive majority of the ambient noise energy from the polarization direction which is comparable with the observed polarization angles from free-field measurements.The microtremor HVSR method proved to be useful for estimation of seismic site effects and the potential danger of soil-structure resonance in an area with low seismicity like Varaždin city. Presented results are of great importance concerning monumental heritage and urban planning in Varaždin city.

Insights into bedrock surface morphology using low-cost passive seismic surveys and integrated geostatistical analysis

The HVSR (Horizontal to Vertical Spectral Ratio) technique is very popular in the context of seismic microzonation and for the mapping of shallow seismic reflectors, such as the sediment/bedrock transition surface. This easy-to-deploy single station passive seismic technique permits the collection of a considerable amount of HVSR data in a cost-effective way. It is not surprising that some recent studies have adopted single station micro-tremor analyses in order to retrieve information on geological structures in 1D, 2D or even 3D reconstructions. However, the interpolation approaches followed in these studies for extending the punctual HVSR data spatially are not supported by a detailed spatial statistical analysis. Conversely, in order to exploit the informative content and quantify the related uncertainty of HVSR data it is necessary to utilize a deep spatial statistical analysis and objective interpolation approaches. Moreover, the interpolation approach should make it possible to use expert knowledge and auxiliary information. Accordingly, we present an integrated geostatistical approach applied to HVSR data, collected for retrieving information on the morphology of a buried bedrock surface. The geostatistical study is conducted on an experimental dataset of 116 HVSR data collected in a small thermal basin located in the Venetian Plain (Caldiero Basin, N-E Italy). The explorative geostatistical analysis of the data coupled with the use of interpolation kriging techniques permit the extraction of relevant information on the resonance properties of the subsoil. The utilized approach, based on kriging with external drift (or its extension, i.e. regression kriging), permits the researcher to take into account auxiliary information, evaluate the related prediction uncertainty, and highlight abrupt variations in subsoil resonance frequencies. The results of the analysis are discussed, also with reflections pertaining to the geo-engineering and geo-environmental context.

Local amplification and subsoil structure at a difficult site: Understanding site effects from different measurements

We present a detailed site effects study at a site (TYF) close to the Thermaikos gulf coast in Thessaloniki, northern Greece. Different types of data recorded by different instruments are analyzed. Empirical amplification is estimated using spectral ratios relative to a reference station (SSR) and horizontal to vertical spectral ratios (HVSR) using earthquake data. In addition, seismic noise records from different arrays were analyzed using HVSR. Our results show that earthquake data SSR fails for our data. The reason is the poor signal to noise ratio of our records. Better results were obtained using HVSR for earthquake data. Seismic noise HVSR were not useful due to the particular soil profile at TYF, with the exception of HVSR of seismic noise recorded in one of our arrays that were able to reflect a significant change in the coast line at our site. Although amplification at site TYF is relatively small, it is large enough to originate a difference of one intensity unit relative to firm ground motion. Amplification at TYF is caused by a deep soil structure (over 350m in depth) and therefore cannot be captured using measures like Vs30.

Comparison of Field Data Processing Methods for the Evaluation of Topographic Effects

This paper presents a comparison of field data processing methods used to evaluate topographic effects. A data set of 52 weak motion events recorded using a dense array of seismometers located on steep, complex topography is analyzed. Both time domain and frequency domain analysis methods are evaluated for effectiveness at resolving the amplitude and frequency range associated with topographic effects. Results from this study indicate the median reference method (MRM) provides a more stable estimate of the frequency range and expected amplification value than the standard spectral ratio (SSR) or horizontal-to-vertical spectral ratio (HVSR) methods.

Study of Site Effect at Seismic Station Located in Undermined Area of Karviná Region (Czech Republic)

The Karvina region is well known as an area with an intensive mining induced seismicity. The local geological pattern, especially subsurface sedimentary layers, belongs to one of the most important factors that influence the amplification of seismic effect on the surface. In order to investigate the amplification effect, there are used methods of spectral ratio that enable to analyse records of vibrations. In the present study, two methods, signed as SSR (standard spectral ratio) and HVSR (horizontal to vertical spectral ratio), were used for the site effect evaluation. The analysis was performed in the populated Doubrava locality where high seismic loading on the surface due to mining induced seismicity is documented.

Seismic microzonation of the broader Chania basin area (Southern Greece) from the joint evaluation of ambient noise and earthquake recordings

In the present work, a combination of single station and multi-sensors array measurements of ambient noise have been performed in the urban area of Chania and its southern basin, in order to determine the main subsurface geophysical model properties and assess the local site-effect on seismic motions. Furthermore, Horizontal to Vertical Spectral Ratio on Earthquake recordings (HVSRE) has been employed to validate the ambient noise results. Simulation of ambient seismic noise has been also used in an effort to cross-validate the experimental methods applied. The results indicate a highly-varying subsurface model due to the complex geological setting. Horizontal to Vertical Spectral Ratio (HVSR) reveals a low frequency resonance peak associated with the seismic contrast of the overall Quaternary/Neogene sediment cover with the underlying bedrock. A second higher HVSR frequency peak, usually disregarded in standard HVSR analysis, is observed for most HVSR data. This peak is found to be associated with the strong velocity contrast of the low velocity Quaternary sediments and the underlying stiffer Neogene deposits, using both empirical correlations and appropriate modelling. The available array measurements verify the clear separation of these Quaternary deposits and the underlying Neogene sediments in the southern Chania basin, as well as the more loose surficial Neogene formations and the underlying stiffer layers within the main city complex. Simulated ambient noise signals processed with the same HVSR settings verify the findings of the experimental approaches. Earthquake recordings processed with the HVSRE method show a good agreement with the expected amplification pattern of the final geological-geophysical structural model of the broader Chania area.

Estimation of site amplification functions for the National Capital (Delhi) Region, India

The site amplification functions at 48 sites of NCR have been estimated in this study using the waveforms of locally recorded 23 earthquakes. Due to the absence of a suitable reference site in the region, the widely used horizontal-to-vertical spectral ratio (HVSR) technique has been used for this purpose. The maps showing the spatial distribution of predominant frequencies and the site amplifications at different frequencies corresponding to the natural frequencies of the different-storey buildings have been presented. The predominant frequencies in general are found to be in the range 2.5–7.5 Hz with an average of 4.4 Hz for the region having older alluvium sediments and in the range 1.1–6.4 Hz with an average of 3.3 Hz for the region with the younger alluvium deposits. The average value of the site amplifications for the frequency band 3.0–10.0 Hz is in the range 2.0–5.3 for the sites with significant soil cover, while the spectral amplification corresponding to the predominant frequency varies from 2.5 to 7.5 at most of the sites. The spectral amplification level lies in the range 2.0–3.0 for the sites with less or no sediment cover. The spectral amplification levels presented for the different-storey buildings may be used for the mitigation of seismic hazard in the region. The estimated site amplification functions may be used in the simulation of the site-specific strong ground motions and therefore useful for the evaluation of seismic hazard of a region.

Shear-wave velocity profiling according to three alternative approaches: A comparative case study

The paper intends to compare three different methodologies which can be used to analyze surface-wave propagation, thus eventually obtaining the vertical shear-wave velocity (VS) profile. The three presented methods (currently still quite unconventional) are characterized by different field procedures and data processing.The first methodology is a sort of evolution of the classical Multi-channel Analysis of Surface Waves (MASW) here accomplished by jointly considering Rayleigh and Love waves (analyzed according to the Full Velocity Spectrum approach) and the Horizontal-to-Vertical Spectral Ratio (HVSR).The second method is based on the joint analysis of the HVSR curve together with the Rayleigh-wave dispersion determined via Miniature Array Analysis of Microtremors (MAAM), a passive methodology that relies on a small number (4 to 6) of vertical geophones deployed along a small circle (for the common near-surface application the radius usually ranges from 0.6 to 5m).Finally, the third considered approach is based on the active data acquired by a single 3-component geophone and relies on the joint inversion of the group-velocity spectra of the radial and vertical components of the Rayleigh waves, together with the Radial-to-Vertical Spectral Ratio (RVSR).The results of the analyses performed while considering these approaches (completely different both in terms of field procedures and data analysis) appear extremely consistent thus mutually validating their performances.Pros and cons of each approach are summarized both in terms of computational aspects as well as with respect to practical considerations regarding the specific character of the pertinent field procedures.

Detailed site effect estimation in the presence of strong velocity reversals within a small-aperture strong-motion array in Iceland

The rock site characterization for earthquake engineering applications in Iceland is common due to the easily exposed older bedrock and more recent volcanic lava rock. The corresponding site amplification is generally assumed to be low but has not been comprehensively quantified, especially for volcanic rock. The earthquake strong-motion of the Mw6.3 Ölfus earthquake on 29 May 2008 and 1705 of its aftershocks recorded on the first small-aperture strong-motion array (ICEARRAY I) in Iceland showed consistent and significant variations in ground motion amplitudes over short distances (<2km) in an urban area located mostly on lava rock. This study analyses the aftershock recordings to quantify the local site effects using the Horizontal to Vertical Spectral Ratio (HVSR) and Standard Spectral Ratio (SSR) methods. Additionally, microseismic data has been collected at array stations and analyzed using the HVSR method. The results between the methods are consistent and show that while the amplification levels remain relatively low, the predominant frequency varies systematically between stations and is found to correlate with the geological units. In particular, for stations on lava rock the underlying geologic structure is characterized by repeated lava-soil stratigraphy characterized by reversals in the shear wave velocity with depth. As a result, standard modeling of HVSR using vertically incident body waves does not apply. Instead, modeling the soil structure as a two-degree-of-freedom dynamic system is found to capture the observed predominant frequencies of site amplification. The results have important implications for earthquake resistant design of structures on rock sites characterized by velocity reversals.

Separation of High‐ and Low‐Level Ambient Noise for HVSR: Application in City Conditions for Greater Toronto Area

Abstract The Nakamura method, which involves horizontal‐to‐vertical spectral ratio (HVSR) analysis, is widely used for seismic microzonation studies. The noise from local traffic in city conditions presents a challenge for the application of HVSR analysis. This article presents a technique developed for separation of the transient noise due to local traffic (high‐level noise) and background ambient noise (low‐level noise) and the application of the HVSR analysis to both partitions of the noise. This approach is applied to identify the predominant frequencies for almost 200 noise samples from the Greater Toronto area. The results demonstrated that the developed technique is effective and allows estimation of the fundamental resonant frequency in the HVSR in urban environment, even in the presence of intensive nearby traffic. The interpretation of the obtained results showed that, most probably, the lower (fundamental) frequency appears due to multiple reflections from the overburden/bedrock boundary. In some cases, a resonance with higher amplitude is dominant, and it is due to a contrast boundary between soil layers in the overburden.