Response Spectral Ratio Research Articles

Topographic gradient based site characterization in India complemented by strong ground-motion spectral attributes

We appraise topographic-gradient approach for site classification that employs correlations between 30m column averaged shear-wave velocity and topographic gradients. Assessments based on site classifications reported from cities across India indicate that the approach is reasonably viable at regional level. Additionally, we experiment three techniques for site classification based on strong ground-motion recordings, namely Horizontal-to-Vertical Spectral Ratio (HVSR), Response Spectra Shape (RSS), and Horizontal-to-Vertical Response Spectral Ratio (HVRSR) at the strong motion stations located across the Himalayas and northeast India. Statistical tests on the results indicate that these three techniques broadly differentiate soil and rock sites while RSS and HVRSR yield better signatures. The results also support the implemented site classification in the light of strong ground-motion spectral attributes observed in different parts of the globe.

Influence of epicentral distance on local seismic response in Kolkata City, India

The influence of source and epicentral distance on the local seismic response in the Kolkata city is investigated by computing the seismic ground motion along 2-D geological cross-sections in the Kolkata city for the earthquake that occurred on 12 June 1897 (M w = 8.1; focal mechanism: dip = 57°, strike = 110° and rake = 76°; focal depth = 9 km) in Shillong plateau. For the estimation of ground motion parameters, a hybrid technique is used, which is the combination of modal summation and finite difference method. This technique allows the estimation of site specific ground motion for various events located at different distances from Kolkata city, taking into account simultaneously the position and geometry of the seismic source, the mechanical properties of the propagation medium and the geotechnical properties of the site. The epicenter of the Shillong earthquake is about 460 km away from Kolkata. The estimated peak ground acceleration (PGA) varies in the range of 0.11–0.18 g and this range corresponds to the intensity of IX to X on the Mercalli-Cancani-Sieberg (MCS) scale and VIII on the Modified Mercalli (MM) scale. The maximum amplification in terms of response spectral ratio (RSR) varies from 10 to 12 in the frequency range 1.0–1.5 Hz. These amplifications occur in correspondence to low-velocity shallow, loose soil deposit. The comparison of these results with earlier ones obtained considering the Calcutta earthquake that occurred on 15 April 1964 (M w = 6.5; focal mechanism: dip = 32°, strike = 232° and rake = 56°; focal depth = 36 km) shows that the source parameters (magnitude and focal mechanism) and epicentral distance play an important role on site response but the variation in the frequency of the peak values (RSR) is negligible. The obtained results match with observed reported intensities in Kolkata region.

A nonparametric characterization of vertical ground motion effects

SUMMARYDesign guidelines have traditionally oversimplified the vertical ground motion effects by defining a constant vertical‐to‐horizontal response spectral ratio (V/H). With the recognition that such practice is not always conservative, recent studies have proposed improvements to the representation of vertical seismic effects in design codes, based on empirical ground motion relations.Conventional empirical modeling requires selecting the functional form of the predictive model. Because of the complicated nature of ground motions, identification of the underlying function is a challenge. A related drawback to this approach is its high susceptibility to overfitting, especially with today's highly complex models.To address these issues, this paper proposes a nonparametric approach to characterize the vertical seismic effects. Using support vector machines, the V/H ratio is determined without an assumed functional form. The accuracy of the model is measured by adopting an epsilon‐insensitive residual function with a regularization term added to prevent overfitting.An example application using ground motion records from strike‐slip and normal faulting earthquakes is presented, and the results are compared with a current empirical model, for different magnitude, distance, and local soil conditions. The median V/H estimates from the two models are shown to be in good general agreement. The standard deviation estimates from the proposed model are consistently larger than the estimates from the empirical model.The results from this study show that the proposed method is a viable alternative and offers the opportunity to characterize vertical seismic effects without an assumed functional form. Copyright © 2011 John Wiley & Sons, Ltd.

Comparative Analysis of the Influence of Slope Terrain on Characteristics of Horizontal and Vertical Ground Motion Response Spectrum

All previous earthquake damage show earthquake damage of the buildings on the slope is serious than the buildings on flat ground, so it is important to analyze of the influence of slope terrain on characteristics of horizontal and vertical ground motion response spectrum. The horizontal and vertical seismic elastic responses of 28 viscoelastic rocky slopes by white noises excited, with height of 10m to70m and slope of 15°to 60°, are calculated by numerical finite element analysis method. The influence laws of height and degree of slope on horizontal and vertical ground motion response spectrum and response spectral ratio are analyzed. The distribution laws of the corresponding period of the maximum of response spectral ratios are contrasted. The maximum of amplification factor of design horizontal and vertical ground motion for structures on rock slopes are given to be the references to code for seismic design of building structures.

Comparative Analysis of the Influence of Slope Terrain on Characteristics of Horizontal and Vertical Ground Motion Response Spectrum

All previous earthquake damage show earthquake damage of the buildings on the slope is serious than the buildings on flat ground, so it is important to analyze of the influence of slope terrain on characteristics of horizontal and vertical ground motion response spectrum. The horizontal and vertical seismic elastic responses of 28 viscoelastic rocky slopes by white noises excited, with height of 10m to70m and slope of 15°to 60°, are calculated by numerical finite element analysis method. The influence laws of height and degree of slope on horizontal and vertical ground motion response spectrum and response spectral ratio are analyzed. The distribution laws of the corresponding period of the maximum of response spectral ratios are contrasted. The maximum of amplification factor of design horizontal and vertical ground motion for structures on rock slopes are given to be the references to code for seismic design of building structures.

Vertical Ground Motion Characteristics of Wenchuan Earthquake

The Wenchuan earthquake of May 12, 2008(Mw=8.0) was the most significant earthquake that had affected Chinese Mainland since the Tangshan earthquake of 1976. The dense network of seismographs deployed in this region recorded ground motion acceleration data with good quality. In this paper, vertical ground motion characteristics of Wenchuan Earthquake are investigated from the database including 282 records at 94 stations. Special attention is focused on the behavior of the recorded vertical ground motion and its relation to the horizontal ground motion counterparts. The results indicate that the average value of the ratios of vertical to horizontal peak ground acceleration (av/ah) is 0.58, but the ratios of over 30% stations exceed the rule-of-thumb value of 2/3. This av/ah is much discrete in the near-fault region and tends to decrease with the increase of epicentral distance. Furthermore, site condition also influences this ratio. The curve of vertical to horizontal acceleration response spectral ratio (Sav/Sah) with period ranging from 0 to 6s appears to be saddle-shaped overall and Sav/Sah value varies within different period range. It is also can be seen that the vertical acceleration response spectrum (Sav) looks thin compared with the horizontal acceleration response spectrum (Sah). Except several stations, the ratio of vertical to horizontal ground motion predominant period (Tpv/ Tph) is less than 1.0, and it increases with the increase of epicentral distance and becomes smooth gradually.

A Model for Vertical-to-Horizontal Response Spectral Ratios for Europe and the Middle East

In the framework of probabilistic seismic hazard analysis, the preferred approach for obtaining the response spectrum of the vertical component of motion is to scale the horizontal spectrum by vertical-to-horizontal (V/H) spectral ratios. In order to apply these ratios to scenario or conditional mean spectra, the V/H ratios need to be defined as a function of variables such as magnitude, distance, and site classification. A new model for the prediction of V/H ratios for peak ground acceleration and spectral accelerations from 0.02 to 3.0 s is developed from the database of strong-motion accelerograms from Europe and the Middle East. A simple functional form, expres- sing the V/H ratios as a function of magnitude, style of faulting, distance, and site class, is found to be appropriate, and the associated aleatory variability is found to be at least as low as that obtained in other studies using more complex models. The predicted ratios from the new European model are found to be in broad agreement with recent models derived from predominantly western North America data.

Site amplification studies towards seismic microzonation in Jabalpur urban area, central India

Site amplifications at over 100 sites in the frequency range of 0.1–10 Hz were investigated in and around Jabalpur city of central India using the conventional Nakamura technique of computing H/ V spectral ratios, to estimate the predominant frequencies and corresponding amplifications at each site. To validate these results, Multichannel Analysis of Surface Waves (MASW) technique was adopted, coupled with refraction measurements over short profiles (∼100 m) resulting in detailed images of shear wave and primary wave velocity structure of the shallow subsurface as well as the depth to basement, which are crucial for estimation of the amplification of seismic signals propagating through the medium. Synthetic seismograms generated based on these detailed structural models using a hybrid method comprising normal mode summation for the regional 1D structure and finite difference approach for site specific 2D structure of the medium are used to compare with site response results from the Nakamura approach. The local site amplifications expressed as the response spectral ratios – the ratio of 2D to 1D ground acceleration response-compare fairly well with the H/ V results enabling proper interpretation of the composition and thickness of the upper soil cover and its hazard potential. The most significant amplification values are seen over the alluvial filled region in the north-western part of Jabalpur having soil thickness up to 30–50 m, with amplifications ranging from 4.0 to 6.0, predominantly in the frequency range of 4–5 Hz. Also, the areas covered with Lameta sediments in the west and northeast have thicker soil cover and indicate amplifications twice that of the adjoining Deccan trap areas. The Gondwana sediments are an exception where, in spite of thick soil covers, lower amplification values are observed which are about half of that in the alluvial regions with similar thicknesses. This is probably due to the compactness of the sedimentary formations in the former as compared to the loose, low velocity and low density alluvium in the latter. The study indicates the importance of a multi-disciplinary approach for better understanding the site amplification characteristics in any seismically active region.

A Comparison of Recorded Response Spectra from the 2008 Wenchuan, China, Earthquake with Modern Ground-Motion Prediction Models

We compared response spectra from the M w 7.9 2008 Wenchuan earthquake with five modern ground-motion prediction equations (GMPEs). Ninety-three strong-motion records within 300 km of the fault plane were selected for comparison with the GMPE models of Zhao, Zhang et al. (2006), Abrahamson and Silva (2008), Boore and Atkinson (2008), Campbell and Bozorgnia (2008) and Chiou and Youngs (2008) for spectral periods up to 5.0 s. The site class of the recording stations used for the Zhao, Zhang et al. (2006) model was inferred from response spectral ratios of the horizontal and vertical components (H/V) computed from the strong-motion records in moving and overlapping time windows. The average shear-wave velocity of the top 30 m ( V S 30) was only available for two stations. V S 30 was extrapolated from the average of the top 20 m ( V S 20) when possible and inferred from the H/V response spectral ratios when necessary. The average predictions of all models were acceptable. The Zhao, Zhang et al. (2006) model gave the best predictions for peak ground acceleration and short spectral periods, especially up to 100 km of the source distance. All Next Generation Attenuation (NGA) models predicted the recorded spectra very well for periods of 0.5–1.0 s and at 5.0 s. The Chiou and Youngs (2008) model gave the best overall predictions. The standard deviations of all attenuation models were similar at a 5% significance level. However, differences between spectra estimated by various NGA models were statistically and practically significant, with the largest difference between the average predictions being nearly a factor of 1.4 at the 0.1-s period and 2.3 at the 5.0-s period for data within a source distance of 100 km. Although one earthquake did not produce median ground motions that the GMPEs are designed to predict, such a large difference represents a challenge for empirical models when estimating spectra from very large crustal earthquakes.

Side-effect of using response spectral amplification ratios for soft soil sites—Earthquake source-type dependent amplification ratios

Our previous studies show that site effects (amplification of rock motions), source and path effects are coupled when response spectra are used to characterize the amplification ratios for a soil site modelled as nonlinear or elastic. The coupling is referred to as a “side effect” of using response spectral amplification ratios. In the present study we use a suite of rock site records, well distributed with respect to magnitude and source distance, from crustal, subduction interface and slab earthquakes to evaluate the response spectral amplification ratio for soft soil sites. We compare these side-effects for ground motions generated by three types of earthquakes, and we find that, at periods much shorter or much longer than the natural period of a soil site modelled as elastic, the average amplification ratios with respect to rock site ground motions from three types of earthquakes are moderately different and are very similar for other spectral periods. These differences are not statistically significant because of the moderately large scatter of the amplification ratios. However, the extent of magnitude- and source-distance-dependence of amplification ratios differs significantly. After the effects of magnitude and source distance on the amplification ratios are accounted for, the differences in amplification ratios between crustal and subduction earthquake records are very large in some particular combinations of source distance and magnitude range. These findings may have potential impact in establishing design spectra for soft soil sites using strong motion attenuation models or numerical modelling.

Site Effects at Long Periods from Digital Strong Motion Records of the KiK-net, Japan

The high-quality digital records of the Japanese KiK-net were examined, with the aim of studying the influence of local site conditions on the displacement spectral ordinates at long periods. The attention was limited to those records for which the velocity profiles up to 100–200 m depth were known, and corresponding surface and borehole accelerograms were available. Based on the available records and with the support of 1D numerical simulations, different aspects that may have an influence on the amplification of long period spectral ordinates were studied, including the bedrock velocity profile, the site classification using Vs,30, and the earthquake magnitude. Small amplification factors at long periods were found, ranging from 1 to 1.3, with median value from 1.05 to 1.1, for Eurocode 8 site classes B and C, respectively. Only for two records on soft soils (at the boundary between Eurocode 8 classes C and D), from small magnitude earthquakes, large amplification factors were obtained, up to about 4. A good correlation was found of the amplification levels with the response spectral ratio D(T0)/D(10), where D(T0) and D(10) are displacement spectral ordinates of the input signal at bedrock, at the fundamental period T0 of the soil profile and at T = 10 s, respectively.

Factors affecting site response to multi-directional earthquake loading

This paper presents an investigation into various factors that may affect the ground response to multi-directional earthquake loading, focusing mainly on the behavior of vertical ground motion and its relation with the horizontal counterpart. The factors investigated herein include the intensity of input motion and the associated soil nonlinearity, the location of input motion (rock outcrop versus bedrock), the variation of water table, and the damping property of soil. Influence of these factors is studied on the characteristics of site amplification in both vertical and horizontal directions, the response spectra of vertical and horizontal ground surface motions, the spectral ratio between the two components (V/H) at the ground surface, and the distributions of stresses and strains in the ground. One of the main results is that varying water table can bring about a significant impact on vertical motion and the relationship between vertical and horizontal motions. The surface response spectral ratio (V/H) can largely exceed the rule-of-thumb value of 2/3 at low periods with lowering the water table, but does not appear to be substantially affected at long periods.

Side effect of using response spectral amplification ratios for soil sites—variability and earthquake-magnitude and source-distance dependent amplification ratios for soil sites

We investigate a special type of variability in response spectral amplification ratios computed from numerical “engineering” models for a soft soil site. The engineering models are defined by shallow soil layers over “engineering” bedrock with a shear-wave velocity over 600–700 m/s and the model is subjected to vertical propagating shear waves. The variability, perhaps unique in earthquake engineering, is a result of the “perfectly accurate” computational procedure. For example, an engineering soil site model, subjected to two rock site records or the two horizontal components of a rock site record, produces different response spectral amplification ratios. We use a large number of strong-motion records from “engineering” rock sites, with a reasonably balanced distribution with respect to magnitude and source distance, generated by subduction earthquakes in Japan, to investigate the nature of the variability. In order to avoid any approximation in removing the effect of soil nonlinear response, we use a simple model, a single horizontal soil layer over a bedrock, modelled as elastic. We then demonstrate that a similar type of variability observed in the one- or two-dimensional nonlinear soil models is caused by the nature of response spectral amplification ratios, not a direct result of soil nonlinear response. Examination of variability reveals that the average of response spectral amplification ratios systematically depends on both earthquake magnitude and source distance. We find that, at periods much longer than the site natural periods of the soil sites, the scatter of the amplification ratios decreases with increasing magnitude and source distance. These findings may have a potential impact in establishing design spectra for soft soil sites using strong-motion attenuation models or dynamic numerical modelling.

Response spectral amplification ratios from 1- and 2-dimensional nonlinear soil site models

In order to determine the effect of geometry on the ground response of 2-dimensional (2-D) basins filled with soils that can develop nonlinear response, we use three basin models with width/depth ratios 3, 6 and 10. The three basins are subjected to a suite of rock site records with various magnitudes and source distances. We compute response spectral amplification ratios at four locations on the surface of the 2-D basins, and determine the average variation of the amplification ratios with respect to excitation spectra, for peak ground acceleration (PGA) and 3 spectral periods of 0.2, 0.5, 1 s. Similarly, we compute the average response spectral amplification ratios for two 1-dimensional (1-D) nonlinear models, one having the soil profile at the basin centre and the other having a soil profile at half the depth of the basin. From the relationship between the average amplification ratios and excitation spectra, we determine the cross-over point in terms of excitation spectral values that separate the amplification range from the deamplification range. Our results show that the cross-over point varies significantly from one location to another on the ground surface and from one basin to another, in a range of 0.3–1.1 g for PGA. The effects of basin geometry are very strong at weak and moderate excitation, but decrease with increasing excitation spectra in a significant portion around the basin centre. Our results provide some justification for using 1-D models for 2-D basins with a width/depth ratio ⩾6 if the soil site is subjected to strong ground shaking.

Applying empirical methods in site classification, using response spectral ratio (H/V): A case study on Iranian strong motion network (ISMN)

Shear wave velocity, measured recently at 107 strong motion stations, is a new empirical basis in the applicability investigation of empirical classification techniques. These stations are classified considering Iranian Practice Code criteria (Standard 2800). To check the applicability of empirical methods, three different empirical techniques are applied to re-classify the stations using previously determined site classes. The first method is based only on the determination of peak periods at each station. It is found that the fundamental periods in different site categories are within the ranges proposed by Japanese Road Association. The second one is upon the site classification index (SI), suggested by Zhao et al. In this study, a new site index term is proposed for quantitative site classification using the empirical H/V spectral ratio (here after HVRS) method. It is shown that the results from this scheme are comparable with those obtained by applying the method of Zhao et al. and are more reliable than the results from using only peak periods. A large number of strong motion stations are classified in Iran for more control of proposed SI applicability. The mean response spectral ratio curves for all data of ISMN stations are found to be fairly consistent with those obtained by Zhao et al. The results show the practicability and efficiency of the proposed method in site classification. However, more shear wave measurements and further information, like surface geology, borehole data etc., are still needed to clarify the uncertainties of such empirical schemes.

Site Classification Using Horizontal-to-vertical Response Spectral Ratios and its Impact when Deriving Empirical Ground-motion Prediction Equations

We classify sites based on their predominant period computed using average horizontal-to-vertical (H/V) response spectral ratios and examine the impact of this classification scheme on empirical ground-motion models. One advantage of this classification is that deep geological profiles and high shear-wave velocities are mapped to the resonance frequency of the site. We apply this classification scheme to the database of Fukushima et al. [2003], for which stations were originally classified as simply rock or soil. The calculation of average H/V response spectral ratios permits the majority of sites in the database to be unambiguously classified. Soft soil conditions are clearly apparent using this technique. Ground-motion prediction equations are then computed using this alternative classification scheme. The aleatoric variability of these equations (measured by their standard deviations) is slightly lower than those derived using only soil and rock classes. However, perhaps more importantly, predicted response spectra are radically different to those predicted using the soil/rock classification. In addition, since the H/V response spectral ratios were used to classify stations the predicted spectra for different sites show clear separation. Thus, site classification using the predominant period appears to be partially mapped into the site coefficients of the ground-motion model.

Seismicity, seismic input and site effects in the Sahel—Algiers region (North Algeria)

Algiers city is located in a seismogenic zone. To reduce the impact of seismic risk in this Capital city, a realistic modelling of the seismic ground motion (SGM) is conducted by using the hybrid method that combines the finite differences method and the modal summation. For this purpose, a complete database of geological, geophysical and earthquake data is constructed. A critical re-appraisal of the seismicity of the zone [2.25°E–3.50°E, 36.50°N–37.00°N] is performed and an earthquake list, for the period 1359–2002, is compiled. The analysis of existing and newly retrieved macroseismic information allowed the definition of earthquake parameters of macroseismic events for which a degree of reliability is assigned. Geological cross sections have been built up to model the SGM in the city, caused by the 1989 Mont-Chenoua and the 1924 Douéra earthquakes. Synthetic seismograms and response spectral ratio is produced for Algiers, and they show that the soft sediments in Algiers centre are responsible of the noticed amplification of the SGM.

COMPARISON OF METHODS TO PREDICT RESPONSE SPECTRA AT INSTRUMENTED SITES GIVEN THE MAGNITUDE AND DISTANCE OF AN EARTHQUAKE

We compare four methods, all based on Bayesian regressions, to predict response spectra at instrumented sites, located at the hill and lakebed zones in Mexico City, that have recorded several recent earthquakes. The regressions are built as functions of magnitude and closest distance to the rupture area of the earthquake, using more than 20 subduction earthquakes (thrust-faulting mechanism) recorded since 1964. The four methods are: (1) a regression on observed response spectra; (2) a regression on observed Fourier amplitude spectra and use of random vibration theory to estimate the response spectra, (3) a regression on observed response spectra at the reference hill-zone station multiplied by response spectral ratios; and (4) a regression on Fourier amplitude spectra at the reference hill zone, multiplied by Fourier spectral ratios and use of the random vibration theory to estimate response spectra.

An Empirical Site-Classification Method for Strong-Motion Stations in Japan Using H/V Response Spectral Ratio

Having a reliable site classification scheme is vital for the development of robust strong-motion attenuation models. We discuss a promising empirical site- classification scheme based on strong-motion data from Japan. We assigned site classes, according to the site classification defined in engineering design codes in Japan, for those K-net sites where boreholes reached either to rock or to stiff soils with shear-wave velocity of 600 m/sec or larger, using four site classes defined by dominant site period. The average response spectral ratios of the horizontal and vertical components (h/v) of earthquake records for all site classes were found not to be strongly affected by Japan Meteorological Agency (jma) magnitude, hypocentral distance, and focal depth for all site classes. We used h/v ratios for records from the classified K-net sites to establish a site classification index using the mean spectral ratios over a wide range of spectral period. Using the index, we were able to classify both K-net stations with soil layers thicker than 20 m and other strong-motion stations in Japan. The peak period of the h/v spectral ratio can also be used to identify soft soil sites. The site amplification factors calculated from the site class terms based on the new site classification are consistent with the period bands defined for these site classes.

Site-specific response spectra from the combination of microzonation with probabilistic seismic hazard assessment—An example for the Cologne (Germany) area

Seismic noise was measured at some 20 sites in the Cologne area (Germany) aligned nearly perpendicular to a graben structure. The H/V spectral noise ratio for each site was used to derive realistic S-wave velocity profiles down to the bedrock by means of a genetic algorithm inversion. Numerical simulations were performed for different combinations of source and propagation path parameters: focal depth, epicentral distance, attenuation and fault mechanism. Synthetic seismograms were produced and converted to Fourier and response spectra. Finally, the site-specific values from response spectral ratios, with their uncertainties, were used to modify attenuation functions entering the logic-tree algorithm of the probabilistic seismic hazard assessment (PSHA). The site-specific response spectra show the significance of taking into account the local S-wave velocity structure in PSHA.