Evaluation Of Site Effects Research Articles

Evaluation of local site effect in the western side of the Suez Canal area by applying H/V and MASW techniques

The soft sediments are one of the most important factors responsible for the amplification of the seismic ground motion in an area of study. Three components, single-station microtremor measurements were performed at 61 sites along the Suez Canal to estimate the fundamental frequencies of the soil and corresponding H/V amplitude ratios by using the horizontal-to-vertical spectral ratio (HVSR) method. We have applied the investigations of the shear wave velocity for supplementing the existing seismic microzonation of the Suez Canal. The multichannel analysis of surface wave (MASW) tests were done along the Suez Canal in the three cities, Suez, Ismailia, and Port Said using 24 channels digital engineering seismograph with 4.5 Hz geophones from September 2014 to January 2015 to get the shear wave velocity VS30. The SeisImager/SW software was used for analyzing the data, and 1D-shear wave velocity model have achieved for each site. The HVSR curves show that the fundamental frequency values are ranging from 0.57 to 1.08 Hz, and H/V amplitude ratios are ranging from 4.05 to 6.46. The average values of VS30 are (548, 301), (241, 319), (194, 110, 238) for Suez, Ismailia, and Port Said respectively. The average of shear wave velocity up to 30 m depth is estimated and used for site classification based on the National Earthquake Hazard Reduction Program (NEHRP) classification. The majority of the sites was classified as Class D (stiff soil) except one site at Port Said city is classified as Class E (soft soils), and another site in the Suez city is classified as Class C (hard rock).

Estimation of site effects using strong motion data of BYTNet array in Turkey

Simultaneous estimation of effects of source, propagation path, and local site amplification was carried out using observed strong motion records in a frequency range from 0.8 to 20 Hz for the purpose of empirical evaluation of the local site effects in different geological conditions in the northwestern part of Turkey. The analyzed data are S-wave portions of 162 accelerograms from 39 shallow events observed at 14 sites of BYTNet array. A spectral separation method was applied to the observed S-wave spectra. The solutions for source spectra, inelasticity factor of propagation path for S-waves (Q s-value), and factor of site amplification at each site were obtained in a least squares sense. In the analysis, we assumed that the factor of the site amplification at a reference site is the same as that of theoretical amplification of S-waves to the soil model whose bottom layer has an S-wave velocity around 2.15 km/s. The estimated Q s-value of the propagation path is modeled as Q s(f) = 87.4f0.78. The estimated site amplifications are characterized into three groups. The sites in the first group belong to rock site with no dominant peaks at a frequency range of 2 to 10 Hz. The second group of hard soil sites is characterized with moderately dominant peaks at a frequency of 5 Hz. The last group for soft soil sites has common peaks at a frequency of 4 Hz with larger amplitudes than those in the hard soil group. We, then, compare the amplifications with average S-wave velocity in top 30 m of the shallow S-wave profiles and proposed linear empirical formula between them at each frequency. We, furthermore, inverted the observed amplification factors into S-wave velocity and Q s-value profiles of the deep soil over the basement.

Site-specific ground response analysis and liquefaction assessment of Vijayawada city (India)

Safety against earthquake hazards can be achieved by considering site safety against geotechnical problems and structural safety against dynamic forces. Geotechnical problems such as amplification, landsliding and soil liquefaction are dangerous in regard to the scale of damage. Assessment of risk against such problems is a part of seismic microzonation process. Seismic risk is highly affected by risk factors due to geological environment and geotechnical properties of soils. In this study, it is attempted to identify and develop hazard maps against seismic hazard for Vijayawada city, India, where the subsoil consists of thick alluvial formations along the river. In particular, the study aims at the evaluation of site effects using the available geotechnical data and carrying out microtremor testing in the study area. The geotechnical zonation of the subsoil of the city suggests a high vulnerability of the physical environment in addition to site amplification of the ground motion phenomena. The ground response analysis at the surface was conducted by 1D equivalent linear method and by microtremor testing. From the ground response analysis, peak acceleration, frequency and amplification parameters were assessed and the city was divided into different zones corresponding to each value. Liquefaction hazard was quantified using SPT-based method in combination with the dynamic analysis. These findings supply an indispensible tool for the seismic retrofitting of the buildings and for the mitigation of the seismic risk for small to medium earthquakes in the city.

Model Selection for CorrelatingVS30with Average Shear‐Wave Velocities at Lower Depths Based on the Iranian Data

Abstract The time‐averaged shear‐wave velocity to a depth of 30 m ( V S 30 ) is widely used in various geotechnical and seismic issues such as ground‐motion prediction equations, determination of site class in building codes, and evaluation of site effects. However, in many cases the velocity is not measured as deep as 30 m. Therefore, obtaining a correlation between V S 30 and velocities at depths lower than 30 m ( V Sz ) seems to be necessary. The present article proposes a rigorous model selection approach to find the best model for estimation of V S 30 by V Sz based on 257 Iranian profiles. The candidate models include the Boore et al. (2011) and Chandler et al. (2005) relations, as well as two types of power‐form correlation presented by the authors. The presented approach includes hypothesis testing based on goodness‐of‐fit measures of normalized residuals, the likelihood method of Scherbaum et al. (2004), and finally the log‐likelihood (LLH) criterion based on the information theory. The results show that the best model obtained by each of these methods is not necessarily the same. Furthermore, it is observed that while hypothesis tests are somehow inconsistent and noninformative, the LLH criterion presents a coherent method for ranking various candidate models and selecting the best one.

Site-specific response spectra for seismic motions in half-plane with shallow cavities

In this paper deals with the evaluation of 2D site effects due to the presence of subterranean structures specifically underground cavities using time-domain BEM. The medium is assumed to have a linear elastic constitutive behavior subjected to vertically propagating incident SV waves. The first objective of this paper is to provide a comprehensive parametric study to assess the seismic amplification functions of the surface ground motion due to shallow subterranean cavity. The second objective is providing quantitative estimates of the seismic ground response spectra above the various types of the underground structures based on the real earthquakes using a simple approach. It is observed that the amplification of the ground surface underlain by a shallow cavity is increased in long periods. The variation of the response spectra versus the buried depth and different shapes of the cavity at the superficial points above a cavity are depicted in comparison to the EC8 spectra as the major results of this study.

Evaluation of site effect within the tectonic basin in the northern side of Ankara

This study mainly concentrates on the determination of site effects for the Plio-Quaternary and especially alluvial soils of the Çubuk district and its close vicinity in Ankara, Turkey. In the study area, particularly in the vicinity of the asymmetric graben that has formed due to the tectonic activities along the NE–SW trending normal faults, a microtremor survey was implemented to identify the local site response under a possible seismic event. The results were compared and correlated by using passive and active surface wave measurements, engineering geological and geotechnical deep borehole logs along with information on local geology and geological tectonic setting. Regarding the microtremor results, larger fundamental periods were acquired than expected over the area and the effect of the tectonic deformation on the stiffness of the soils was also observed at either the H/V curves or the shear wave velocity profiles. The results have demonstrated that geotechnical information down to a depth of 30m may not be compared with the H/V parameters. The H/V amplitudes were not always accompanied with the higher periods in the Quaternary sediments. This could be due to the basin and basin edge effects observed particularly at the center and boundary of the basin, respectively. Additionally, the microtremor survey showed that the spectral ratio amplitudes derived by the H/V ratio are debatable and are not a direct indicator of local soil behavior under a seismic excitation.

Numerical and experimental evaluation of site effects at ridges characterized by complex geological setting

The paper presents the results of site effects investigation at two villages, Castelvecchio Subequo and Castel di Ieri (Central Italy), located about 2 km from each other, on top of carbonate ridges characterized by complex geological/structural characteristics. Both villages suffered of an anomalous high damage degree determined by the Mw = 6.1, 2009 L’Aquila earthquake, highlighting the occurrence of significant ground motion amplification phenomena. Studying seismic response at rock sites characterized by complex geological setting is a challenging issue, as different factors can alter the expected ground motion. The difficulties arise from both the reconstruction of a suitable and thorough geological/geotechnical subsoil model and limitations of geophysical and numerical methods in such contexts. In order to overcome these difficulties, we here proposed an approach which integrates numerical modelling, based on the definition of a subsoil setting built from an accurate geological field analysis, with active and passive geophysical techniques. In particular, the calibration of numerical models based on transfer functions computed from Standard Spectral Ratio applied to noise recordings provided encouraging results.

Site Effect Evaluation In Sellano, Italy, By 1-D And 2-D Numerical Analyses

Site Effect Evaluation In Sellano, Italy, By 1-D And 2-D Numerical Analyses

The Effect of Shallow Quaternary Deposits on the Shape of the H/V Spectral Ratio

In the last two decades, the horizontal-to-vertical (H/V) spectral ratio of seismic noise technique has been widely used for site-effect estimation and geophysical exploration through the soil fundamental frequency. Usually, only one peak is observed in the H/V spectral ratio, but in some cases, a second peak can also be obtained. Nevertheless, to date, the peaks at higher frequencies are rarely studied in detail. Geological and geophysical data are especially needed to better explain the presence of this second peak, which normally is neglected. An extensive survey of H/V measurements was conducted in the Llobregat river delta, located to the south of Barcelona. At most sites, two clear peaks were identified: one at low frequencies ( 1 Hz). To understand this behaviour, a seismic noise array and active surface wave measurements have been conducted to obtain a shear-wave velocity profile (V s) up to the bedrock. Two impedance contrasts have been detected: the first one at a shallow depth and the second one between the soft sedimentary cover and the bedrock. During the modelling process, the theoretical H/V computed from the obtained V s models fits well with the experimental H/V peaks. The results from this study show that the structure of shallow quaternary layers can clearly change the shape of the H/V ratio, producing two clear peaks in some situations. In this case, the contact between the low-velocity clay layer and the gravels with a high seismic wave velocity produces a shallow impedance contrast related to the second peak observed in the H/V ratio. Comprehension of these secondary peaks could avoid a misreading of the soil fundamental frequency that could produce errors in a site-effect evaluation or in the calculation of the bedrock depth. Finally, we show that passive seismic techniques provide the quaternary overburden and bedrock geometry in urban areas and allow for the limitations of other geophysical techniques in these environments to be overcome.

Evaluation of Site Effects on Strong-Motion Records in Concepcion during the 2010 Maule, Chile, Earthquake

In the 2010 Maule, Chile, earthquake ( M w 8.8), many high‐rise buildings in downtown Concepcion (CONC) sustained severe damage. The accelerogram observed at CONC had large amplitudes and long periods. The spectra contained strong peaks with periods around 2 s, whereas the spectra from a neighboring site (CCSP) did not exhibit strong peaks. Because CONC and CCSP are located in the Concepcion basin and at the edge of the basin, respectively, these observations suggest that the spectral differences are due to site geological effects. To quantitatively evaluate site effects, microtremor array observations were conducted as a subsurface structure survey in Concepcion. The S ‐wave velocity profiles were estimated from the inversion of the Rayleigh‐wave phase velocity and the horizontal‐to‐vertical ratio. The different spectral characteristics of the records at CONC and CCSP could be explained by the theoretical amplification factors computed from the velocity profiles. Further studies on seismic microzoning of the city with an emphasis on the subsurface structure should improve disaster planning in Concepcion.

On the behavior of site effects in central Mexico (the Mexican volcanic belt – MVB), based on records of shallow earthquakes that occurred in the zone between 1998 and 2011

Abstract. The Mexican volcanic belt (MVB) is a seismogenic zone that transects the central part of Mexico with an east–west orientation. The seismic risk and hazard of this seismogenic zone has not been studied in detail due to the scarcity of instrumental data as well as because seismicity in the continental regime of central Mexico is not too frequent. However, it is known that there are precedents of large earthquakes (Mw > 6.0) that have taken place in this zone. The valley of Mexico City (VM) is the sole zone, within the MVB, that has been studied in detail. Studies have mainly focused on the ground amplification during large events such as the 1985 subduction earthquake that occurred off coast of Michoacán. The purpose of this article is to analyze the behavior of site effects in the MVB zone based on records of shallow earthquakes (data not reported before) that occurred in the zone between 1998 and 2011. We present a general overview of site effects in the MVB, a classification of the stations in order to reduce the uncertainty in the data when obtaining attenuation parameters in future works, as well as some comparisons between the information presented here and that presented in previous studies. A regional evaluation of site effects and Fourier acceleration spectrum (FAS) shape was estimated based on 80 records of 22 shallow earthquakes within the MVB zone. Data of 25 stations were analyzed. Site effects were estimated by using the horizontal-to-vertical spectral ratio (HVSR) methodology. The results show that seismic waves are less amplified in the northeast sites of the MVB with respect to the rest of the zone and that it is possible to classify two groups of stations: (1) stations with negligible site amplification (NSA) and (2) stations with significant site amplification (SSA). Most of the sites in the first group showed small (<3) amplifications while the second group showed amplifications ranging from 4 to 6.5 at frequencies of about 0.35, 0.75, 15 and 23 Hz. With these groups of stations, average levels of amplification were contrasted for the first time with those caused by the subduction zone earthquakes. With respect to the FAS shapes, most of them showed similarities at similar epicentral distances. Finally, some variations of site effects were found when compared to those obtained in previous studies on different seismicity regions. These variations were attributed to the location of the source. These aspects help to advance the understanding about the amplification behavior and of the expected seismic risk on central Mexico due to large earthquakes within the MVB seismogenic zone.

Site effect assessment using microtremor measurement, equivalent linear method, and artificial neural network (case study: Babol, Iran)

Site effect assessment is an important procedure for a reliable site-specific hazard assessment. Fundamental frequency is a very important factor that must be considered in a construction site for examining the potential damage resulting from earthquake. In the two last decades, the microtremor H/V spectral ratio method has been widely used for site effect studies. Microtremor measurement is fast, applicable, and cost-effective. Microtremor measurement was undertaken at 60 stations in the Babol, north of Iran, during 2011 and 2012. Regarding Nakamura’s method, H/V spectral ratios, fundamental frequency, and amplification factor were calculated for all microtremor stations. The obtained results were controlled with SESAME guidelines. In order to assess the accuracy of the microtremor measurements and its application in site effect evaluation, a preliminary site response modeling was carried out using the equivalent linear methods at some stations. More than 18 boreholes in the study area were modeled as multilayer column, overlaid on bedrock. Also, the artificial neural networks (ANN) with different inputs including thickness, type of soil, unit weight, shear wave velocity, and max shear modulus of soil layer were trained. Then, the trained ANN predicts the fundamental frequency, and the output results were compared with microtremor measurement. The results showed that microtremor measurement approach provides an acceptable means of site effect evaluation in the study area.

Local site effect microzonation of Lorca town (SE Spain)

Local site effect assessment based on subsurface ground conditions is often the key to evaluate urban seismic hazard. The site effect evaluation in Lorca town (south-eastern Spain) started with a classification of urban geology through the geological mapping at scale 1:10,000 and the use of geotechnical data and geophysical surveys. The 17 geological formations identified were classified into 5 geological/seismic formations according to their seismic amplification capacity obtained from ambient vibration measurements as well as from simultaneous strong motion records. The shear-wave velocity structure of each geological/seismic formation was evaluated by means of inversion of Rayleigh wave dispersion data obtained from vertical-component array records of ambient noise. Nakamura’s method was applied to determine a predominant period distribution map. The spectral amplification factors were fourfold the values recorded in a reference hard-rock site. Finally, the capability of this study for explaining the damage distribution caused by the May 11th, 2011 Lorca destructive earthquake (Mw \(=\) 5.2) was examined. The methods used in this work are of assistance to evaluate ground amplification phenomena in urban areas of complex geology as Lorca town due to future earthquakes with applicability on urban seismic risk management.

Remote sensing’s contribution to earthquake risk assessment and mitigation

The dramatic increase of human population in areas exposed to seismic hazard (especially in developing countries) is leading to a large increase in earthquake risk. Remote sensing products can certainly contribute to following the rapid expansion of urban environments and the dynamics of risk evolution in areas of interest. In particular, remote sensing can contribute to the different portions of the risk assessment chain, including hazard and site effects evaluation. The contribution of remote sensing to mapping physical vulnerability and exposure is therefore very promising when coupled with advanced statistical approaches, as well as the possibility of rapid damage estimation in the immediate postdisaster stage, and the monitoring of the longer term recovery phase. However, while the positive results are very often shared with the rest of the interdisciplinary community involved in this field with a great deal of enthusiasm, I personally very often miss a deeper discussion on the drawbacks of the adopted procedure, including costs, the feasibility of analysing large areas over short time periods, the lack of data (or risk of missing data in the case of emergency) due to bad weather conditions, etc. This leads to the possibility of using these methods at a reasonable cost in nearly real time. InSar data have been exploited for some time in providing information about earthquake source processes, as well as being combined with seismological observations in joint inversions to better constrain source parameters. However, most of the used models are still too simple and, in my experience, I often miss a deeper analysis of the robustness and reliability of the solutions. As an engineering seismologist, I take a great interest in those results that show fairly good correlation between areas affected by subsidence and the surface geology. However, such results should not be used, as has been attempted, for a correlation with damages, forgetting the simple fact that it is not the site effects that determine damages, but the actual ground motion, which is the result of the seismic source process, wave propagation and, finally, site effects and the physical vulnerability of the built environment. Nevertheless, the reasonable correlation observed between subsidence and surface geology might be used for optimizing site effect studies by identifying areas that require a more thorough investigation since such areas are likely to be underlain by

Preliminary Analysis for Evaluation of Local Site Effects in Lima City, Peru from Ground Motion Data by Using the Spectral Inversion Method

Effects of local site, propagation path and source in ground motion records observed in Lima, Peru, were separated by the spectral inversion method proposed by Iwata and Irikura (1986 [1], 1988 [2]) to examine the relation between local subsurface conditions and local site amplifications in a frequency range from 0.5 to 20 Hz. S-wave portions of accelerograms in horizontal components observed at 5 stations for 11 events along the Pacific coast of Lima city, Peru, were analyzed. The Q factor was obtained from our inversion results as frequency-dependent functionQS( f )= 80.4f0.63. In terms of local site effects, stations located on alluvial gravel deposits were likely to suffer amplification at frequencies larger than 4 Hz, while one station (CAL site) located on soft soil sediment has different behavior of amplification. We also compared our results with 1-D theoretical computation, observed standard spectral ratio and observed H/V spectra in previous studies, finding that site responses determined by different methods are similar. In addition, we analyzed the relationship between average S-wave velocity in the top 10 meters and the average site amplification factor in a frequency range between 0.5 Hz and 10.0 Hz, showing a good correlation between the two parameters. We also calculated the average transfer function (AvTF) to compare it with the existing amplification map for Lima city, and found that our calculations differed from this map.

Shallow shear-wave velocity profiles and site response characteristics from microtremor array measurements in Metro Manila, the Philippines

This paper presents the outcome of reconnaissance surveys in metropolitan Manila (Metro Manilla), the Philippines, with the aim of mapping shallow shear-wave velocity structures. Metro Manila is a seismically active and densely populated region that is in need of detailed investigation of the subsurface structures, to assess local site effects in seismic hazard estimation. We conducted microtremor array observations and used the spatial autocorrelation method to estimate the shear-wave profiles at 32 sites in major geological settings in Metro Manila. We applied a hybrid genetic simulated annealing algorithm to invert phase velocity data from the spatial autocorrelation method to generate shear-wave velocity models near the global best-fit solution. The comparison between the inferred shear-wave velocity profiles and PS logging showed good agreement in terms of the fundamental mode of Rayleigh waves and site responses. Then, we utilised the inferred shear-wave velocity profiles to compute the site amplifications with reference to the motion in engineering bedrock. Subsequently, the site amplifications have been grouped, based on NEHRP site classes. The amplification factor has also been compared with the average shear-wave velocity of the upper 30 m at each site, to produce a power-law regression equation that can be used as a starting basis for further site-effects evaluation in the metropolis.

Experimental Relationship Between Ambient Vibration H/V Peak Amplitude and Shear-wave Velocity Contrast

Originally proposed by Nogoshi and Igarashi (1970, 1971), the ambient vibration H/V spectral ratio (HVSR) method consists of estimating the ratio between the Fourier amplitude spectra of the horizontal (H) to vertical (V) components of ambient noise vibrations recorded at a single station. This method, widespread by Nakamura (1989, 2000), is now widely used for evaluation of site effects and in seismic micro‐zoning studies (e.g., Gueguen et al. , 2000; Alfaro et al. , 2001; Duval et al. , 2001; Guillier et al. , 2004; Panou et al. , 2005; Chatelain et al. , 2008a; Bensalem et al. , 2010; Hellel et al. , 2010). These studies are based on the HVSR peak‐frequency value and distribution, which reflect both the value and spatial variation of the soft sediment topmost‐layer resonance frequency. The frequency of the HVSR peak is related to the topmost‐layer thickness, while its amplitude is proportional to the shear‐wave velocity contrast between the surficial layer and the underlying seismic bedrock. The shape of the HVSR curve is controlled by the S ‐wave transfer function between the underlying rock and the surface sediments. Theoretical investigations of 1D structure from synthetic noise have shown that the HVSR peak is pronounced around the S ‐wave fundamental frequency when the surficial layer has a sharp shear‐wave velocity contrast with the seismic bedrock (Field and Jacob, 1993; Lachet and Bard, 1994; Lermo and Chavez‐Garcia, 1994; Wakamatsu and Yasui, 1996; Tokeshi and Sugimura, 1998). To our knowledge, no experimental study has yet been completed on this subject. A second peak may appear on HVSR curves, indicating the …

Topographic effects on the Monte Po Hill in Catania (Italy)

The study regards an evaluation of site effects on and near the Monte Po hill, located in the north-eastern part of the city of Catania (Italy), an area at high seismic risk. At the beginning of 2007 a seismic station was located in a school building, situated at the slope toe, but no seismic events have been recorded as yet. Therefore, synthetic seismograms have been used to evaluate the ground response analysis at the surface. Because the average slope is moderate (less than 15°), 1-D computer codes have been used to model the equivalent-linear earthquake site response analyses of layered hill deposits, as generally performed by professionals. However, the slope response has also been analysed in greater detail, using a 2-D computer code and the soil characterisation has been evaluated accurately by means of borings, Down-Hole tests, SDMT tests and laboratory tests. Comparing 1-D with 2-D results the stratigraphic site amplification and the Topographic Aggravation Factor (TAF) have also been computed. The aim of the study is that it will form a basis for the design of works to remediate the damage caused by a landslide reactivated by the earthquake in Eastern Sicily on December 13, 1990 (ML=5.4).

Site effects evaluation in Málaga city’s historical centre (Southern Spain)

In this paper we present a site effects analysis carried out in Malaga city’s historical centre (Southern Spain). Two different methodologies have been used: an experimental technique using ambient noise measurements and a 1D numerical method. Soil fundamental frequencies have been obtained from the first technique, and soil transfer functions have been calculated from the numerical methodology. In order to use these results in vulnerability studies, intensity increments for each type of soil have also been estimated. From this information, a seismic microzonation has been proposed for the city centre, classified in six types of soils. Soil fundamental frequencies vary between above 5.0Hz at the hills of the city (where rock arises on the surface), and 1.0Hz near Guadalmedina river. The results show regions with high intensity increments (ΔI = +1.5) corresponding to areas which suffered heavy damage in the 1680 earthquake (Imax = VIII–IX). Moreover, most of the monuments and historical buildings in the city are located in these high risk areas. Results underline the importance of this kind of studies for seismic risk mitigation, historical preservation and emergency planning in the main cities’ historical centres.

Soil and Sediments Microzonation for Evaluation of Site Effects on Earthquake Damages in Mobarakeh, Esfahan, Iran

As an important step in effectively reducing seismic risk and the vulnerability of the city of Mobarakeh to earthquakes, a site effect microzonation Study was conducted. Seismic hazard analysis for a return period of 475 years was carried out. Data from 10 borings was collected and analyzed, geophysical surveys were conducted and seismology and geoelectric measurements taken in more than 17 stations through out the city. The study area was divided into a grid of 500×500 m2 elements and the sub-surface ground conditions were classified into 5 representative geotechnical profiles. Electric resistivity was measured in close to 17 geotechnical boreholes and surface and sub-surface sediments were collected and analyzed. Site response analyses were carried out on each representative profile using 30 different base rock input motions. Distribution maps of site periods and peak ground acceleration and old and new texture buildings through out the city were developed, providing a useful basis for land-use planning in the city.