Evaluation Of Site Effects Research Articles

Evaluation of site effects using numerical and experimental analyses in Città di Castello (Italy)

In the paper the results of the numerical and experimental analyses, at a site in the Umbria Region (Città di Castello—PG), are finalized and the evaluations of site effects are shown. The aim of the work was to compare the two types of analyses and give some methodologies that may be used at the level of urban planning. Therefore a series of geological, geomorphological (1:5.000 scale), geotechnical and seismic analyses have been carried out, to characterize the lithological units and to identify the areas affected by site effects. The expected seismic inputs were provided and two-dimensional numerical analyses have been done. An experimental analysis, using the registrations of small events, has been done. The results for the two approaches were performed in terms of elastic pseudo-acceleration spectra and amplification factors. The results are given as a ratio between spectral intensity, calculated using the pseudo-velocity spectra, in the periods of 0.1–0.5 s and 0.1–2.5 s of output and input. The results have been analyzed and compared, to provide a methodology that may be comprehensive and precise.

Site Effect Evaluation In Sellano (Italy) By1-D And 2-D Numerical Analyses

The 1997–1998 Umbria-Marche seismic sequence induced severe damage to the historical centre of Sellano (Perugia, Italy). During the long aftershock sequence two temporary accelerometric stations, about 300m apart, provided several ground motion records. These data allowed us to investigate the amplification of surface motion which can essentially be ascribed to local seismic effects. A site response study of the historical centre of the village was carried out by means of 1-D and 2-D numerical analyses to clarify the role played by the stratigraphic and topographic effects. The applicability of 1-D and 2-D models was examined by comparing the recorded and calculated acceleration response spectra. 1-D response analyses seem to indicate a significant stratigraphic effect on the ground response. Conversely, 2-D response analyses have shown only moderate amplification effects due to surface topography, which somewhat improved the predictions of site response spectra.

Evaluation of site effects by means of Joint Analysis of Sonogram and Standard Spectral Ratio (JASSSR)

In order to evaluate thefrequency dependence of site effects, thejoint analysis of the standard spectralratio technique with a sonogram approach isproposed (Joint Analysis of Sonogram andStandard Spectral Ratio). Two tools aresuggested: the enhanced sonograms andthe differential sonograms. Inparticular, the local geology effects areinvestigated by considering both theamplification of the seismic signals andtheir prolongation. Tests carried out onsynthetic signals show the capability ofthe sonogram approach to evaluate thefrequency-dependent lengthening of theseismic signal. The JASSSR is applied todata recorded by a dense local networkdeployed during the Umbria-Marche seismicsequence of 1997 and to data recorded inthe Volvi Test site (Euroseistest, Greece).The results show that using the JASSSR gives amore complete overview of the effects oflocal geology on seismic ground motion than maybe obtained by using the StandardSpectral Ratio alone. In particular, theeffect of secondary arrivals, both on theamplitude and on the lengthening of theseismic ground motion, can be pointed out.In addition, the characteristic frequencyof these arrivals can be evaluated.

Evaluation of site effects in some localities of ‘Alta Val Tiberina Umbra’ (Italy) by numerical analysis

In the paper the application of a methodology for the evaluation of site effects in an area of Umbria Region (Italy) is shown. In the project 20 localities were analyzed. In this paper the results of some representative localities are presented. The objective of the project was to develop a methodology that may be easily repeated, in order to take into account site effects in urban planning. Therefore a series of geologic, geomorphologic (1:5.000 scale) and geotechnical analyses have been carried out, to identify the areas affected by site effects and to characterize the lithotechnical units. The expected seismic inputs have been calculated and the numerical analyses 1D and 2D have been done. The results, in terms of elastic pseudo-acceleration spectra and amplification coefficients, as a ratio between spectral intensity of output and input, give elements for urban planning, both for urban general choice and for buildings design.

Three‐dimensional finite difference method and hybrid modeling of earthquake ground motion

A new two‐step hybrid technique (hybrid) has been developed to estimate the earthquake ground motion in three‐dimensional (3‐D) local models excited by external sources. The main use of the technique is fast evaluation of site effects and seismic hazard because our technique needs less computer memory and time than all‐in‐one source‐path‐site computational methods. The first step of the hybrid requires an arbitrary 3‐D method (e.g., finite difference (FD), discrete wave number, ray, or analytical solution) to compute three‐component excitation on a grid box surrounding a site of interest. A particular excitation contains information on the source and path effects and is saved on disk as a time history. Depending on the method used, the first step may also contain nonplanar topography. The second step (3‐D FD) employs local structure and topography inside the excitation box and a procedure that exactly couples the first step results with the second step. The speedup of the hybrid method is made possible through the computation of the excitation (saved on disk) in the first step and the use of FD on irregular grids in the second step of the hybrid. The second step computational model can be just a small spatial fraction of the original source‐path‐site model. A practical parameter study example of a point double‐couple source and regional topography model (applied in the first step) with a local low‐velocity basin structure and ridge (both applied in the second step only) is presented.

Empirical evaluation of site effects in the metropolitan area of San José, Costa Rica

Site effects for 11 selected locations were determined in the capital city of Costa Rica. We used a strong motion network made of eight K2 and three SSA accelerographs. The network recorded more than 60 earthquakes in the magnitude range from 2 to 5 during a period of nine months. The site effects were determined using the sediment-to-bedrock spectral ratio (SBSR) and the horizontal-to-vertical spectral ratio (HVSR) techniques and a time window 4s beginning from the S-wave arrival. The result suggests that the amplification in the capital city is to be in the range from 2.0 to 3.0. The fundamental frequencies were found to be high in the southern and eastern part of the study area and low in the northern and western part. A possible topographic effect was also observed for one of the stations located nearby a river canyon. The results from earthquake data were compared with the ones obtained from noise data. The horizontal-to-vertical noise ratio (HVNR) technique was used to estimate the site effects using ambient noise. The fundamental frequencies were found to correlate very well between both sets of data; on the other hand, the amplitude given by the noise was observed to be always lower than the one derived from the earthquake data.

遠距離大規模地震記録を用いた足柄平野の広周波数帯域地震応答

遠距離大規模地震記録を用いた足柄平野の広周波数帯域地震応答

小田原市内での強震観測とそれに基づく地盤特性の検討

A dense strong-motion array in the central part of Odawara city, Japan has been installed for understanding the detailed spatial distribution of ground motion characteristics. To evaluate the local site effect on the observed earthquake motions, the spectral ratios at soil sites with respect to two reference sites are calculated for several events. When the reference site KNB located about 3km from the soil sites is used, the spectral ratios are scattered in different events. For the reference site SRY which is very close to the soil sites, however, the spectral ratios are stable. One possible interpretation for the scatter in the ratios with respect to KNB may be large attenuation of seismic waves in the propagation path between KNB and the other sites. The spectral ratios with respect to the reference site SRY are compared to the horizontal/vertical ratios of microtremor in order to clarify the applicability of microtremor to site effect evaluation. Both ratios show spectral peaks at the same periods. The correlation is also found in levels of both ratios at the peaks.

Empirical site response evaluations: case studies in Israel

A closed-loop, time-efficient technique for site effect evaluation for seismic microzonation in urban areas is developed and implemented. It includes the following successive steps: microtremor measurements using a triangular array of three-component seismometers; estimation of the Nakamura horizontal-vertical spectral ratio, calculation of Rayleigh wave dispersion using the Aki spatial correlation method; inversion of the dispersion curve to determine a shear wave velocity model of the shallow subsurface; analytical computation of Rayleigh wave spectral ellipticity and 1-D SH transfer function for this model; and a comparison of empirical and analytical ratios and correction of the model if required. The technique has been tested at several locations in a number of towns near the active Jordan Rift valley. A reasonable agreement is obtained when comparing empirical spectral ratios and analytical transfer functions.

Site effects in Mexico City urban zone. A complementary study

Site effects are a major factor affecting seismic risk at Mexico City conurbation and have been carefully studied. However, most of the studies and instrumentation concentrate in Mexico City. We present an evaluation of site effects in the counties of Estado de Mexico, to the North of Mexico City, that have been incorporated into Mexico City conurbation. We measured microtremors at 67 points and operated a temporal digital seismograph network. Our results allow us to extend the dominant period map of Mexico City to the heavily populated and industrialized counties of Estado de Mexico. Additionally, we propose a map of maximum relative amplification. Both maps are consistent with local geology, and could be useful for microzonation of this region.

Evaluation of local site effects and their removal from borehole records observed in the Sendai region, Japan

AbstractWe present results from a borehole observation network composed of 12 borehole sites deployed in a 20 × 20 km area in the Sendai region, Japan. The subsurface Quaternary layers vary with each site, but a Pliocene layer is commonly underlying at a depth of 0 to 80 m throughout the Sendai region. We define this Pliocene layer, whose S-wave velocity is larger than 500 m/sec and whose N value of the standard penetration test is greater than 50, as an engineering bedrock in the region. Once we characterize ground motions at the engineering bedrock, we can either use these motions directly in the response analysis of whole soil-building systems or use them in the nonlinear analysis of soft soil layers as an input. The purpose of this study is to evaluate the local site effects due to surface layers overlying the engineering bedrock and to remove them by using one-dimensional (1D) soil models whose properties are determined by weak-motion records of 18 events. First we identify S-wave velocities and frequency-dependent quality factors Q from amplification factors between surface records and borehole records observed at 10 sites at depths of several tens of meters. The identified S-wave velocities are very close to S-wave logging values. The identified Q values show strong frequency dependence, proportional to f(0.46 to 1.16), where f is frequency. We confirm that the observed amplification factors in the frequency range from about 0.1 to 20 Hz can be explained well by the theoretical ones based on the 1D wave propagation theory with the identified S-wave velocities and Q. Then we estimate the so-called engineering bedrock waves, which are supposed to be observed on the outcrop of the engineering bedrock, from borehole records by using the 1D models with these identified soil constants. We confirm that local site effects due to surface layers overlying the engineering bedrock are properly removed so that we have similar characteristics in the resultant engineering bedrock waves from records at different sites.

Site effect evaluation at Mexico City: dominant period and relative amplification from strong motion and microtremor records

Site effect evaluation at Mexico City: dominant period and relative amplification from strong motion and microtremor records

Site effect evaluation at Mexico City: Dominant period and relative amplification from strong motion and microtremor records

Site effects in Mexico City are discussed in terms of simple 1D, one-layer, linear models. The analysis is focussed on two parameters: dominant period and maximum amplification relative to a firm site within the city. The data used is a compilation of strong motion data and microtremor measurements. Strong motion data consist of digital acceleration records for nine events recorded by the Accelerographic Network of Mexico City. The authors analyzed spectral ratios of horizontal components of soft soil sites relative to an average of firm site observations for this data set. Dominant period, maximum relative amplification and an estimate of material damping were computed from the empirical transfer functions thus obtained. Microtremor data were compiled from measurement of different groups during the period 1985–1992. In all, 409 measurement points were analyzed. Values of dominant period obtained from microtremor measurements are in excellent agreement with those obtained from empirical transfer functions for strong motion data. The synthesis of results allows us to draw a detailed and robust map of dominant period for Mexico City. Based on this map, the authors propose some modifications to the current microzonation of Mexico City and evaluate a proposed model to account for site effects in this city.

Site effect evaluation using spectral ratios with only one station

AbstractThe spectral ratio technique is a common useful way to estimate empirical transfer function to evaluates site effects in regions of moderate to high seismicity. The purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station. The technique, originally proposed by Nakamura to analyze Rayleigh waves in the microtremor records, is presented briefly and it is discussed why it may be applicable to study the intense S-wave part in earthquake records. Results are presented for three different cities in Mexico: Oaxaca, Oax., Acapulco, Gro., and Mexico City. These cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city. Each time we compare the results of Nakamura's technique with standard spectral ratios. In all three cases the results are very encouraging. We conclude that, if site effects are caused by simple geology, a first estimate of dominant period and local amplification level can be obtained using records of only one station.

地震動強さの距離減衰式 (経験式) に関する最近の研究動向

Recent studies on the attenuation relations of earthquake ground motion, which have been derived mainly by multiple regression analyses of observed data, are surveyed. Since the studies on the attenuation relations are involved with not only derivation of attenuation characteristics but also evaluation of source and site effects, they have wide variety of theme. In this report, these studies are reviewed by dividing them into four aspects of the study: parameters used in regression models, observed data, regression models and methods of statistical analyses.Main topics of recent studies are as follows. (a) Ordinary regression models that assume linear relationship with respect to magnitude will fail to represent the response spectra in wider magnitude range, since magnitude dependency of the ω-2 source spectra is not linear around corner frequencies. (b) Distance coefficients of attenuation relations obtained from the regression analyses can be converted to the Q values, which agree well to those derived by different approaches. (c) The phenomena of amplitude saturation in the near source region, which is prominent in high-frequency components of strong ground motion, can be expressed by substituting the hypocentral distance with the minimum distance to the fault plus the source region size. Another approach is to sum up the contributions from small elements located on a large fault plane. (d) Significantly smaller distance coefficients for large numbers of earthquake than for an individual earthquake will be derived by the ordinary one-step regression analysis, because usually strong correlation between magnitude and distance exists in such a large database. The two-step stratified regression analysis using dummy variables is a very effective method to correct these unreasonable results. Thus care should be taken for the choice of the regression procedures and models, especially in case that the observed data have a bias.Since attenuation relations are derived based on observed records, the ground motion predicted by these relations will be a good approximation of the actual value as long as the model parameters for prediction fall within the range of the data for regression. In near future, the attenuation relations applicable to wider frequency, magnitude, and distance ranges are anticipated to be developed with the help of increasing strong motion data and appropriate physical modeling.

Preliminary Analysis for Evaluation of Local Site Effects from Strong Motion Spectra by an Inversion Method.

Simultaneous separation of source, propagation path, and local site effects from observed strong motion records is carried out by an inversion method in the frequency range from 1 to 10 Hz for the purpose of empirical evaluation of the local site effects in different geological conditions. The analyzed data are S-wave portions of 167 accelerograms from 20 events observed at 7 stations along the Pacific coast of southern Tohoku and Kanto districts, Japan. These events are shallow earthquakes with magnitude from 4.0 to 6.7 and with hypocentral distance from 43 to 243 km.

Strong-motion accelerograph array in Santiago, Chile, and preliminary evaluation of site effects

A strong-motion accelerograph array in Santiago, Chile has been installed. One of the sites is located on rock and the other six sites are on soil ground with different surface geology, so that local site effects on ground motions can be studied. As a preliminary evaluation of the site effects, the spectral ratios of weak-motion records at soil sites with respect to the rock site are calculated. The spectral ratios show that the amplification of ground motions with respect to the rock site is approximately 1.25 on dense gravel deposits, 2.5 on stiff pumice ground and 3.5 on soft silt ground.

The influence of highly stratified dissipative soil on ground motion

AbstractThe influence of the depth and material properties of a sedimentary deposit on earthquake ground motion is examie through theoretical modelling. The strong stratification of a typical deposit is represented by a number of homogene layers of dissipative viscoelastic material. The motion is assumed to be produced by a line source. The calculations carried out by means of a recently developed theoretical technique. The near surface low velocity layers are found to has a very strong amplification effect on the ground motion at some frequencies. The widely used one dimensional approae for site effect evaluation is shown to be valid for smaller epicentral distances only. The deamplification effect of mate dissipation is shown to be extremely important in most cases.

Observation of 1- to 5-second microtremors and their application to earthquake engineering. Part II. Evaluation of site effect upon seismic wave amplification due to extremely deep soil deposits

abstractThe usefulness of long-period microtremor observation for earthquake engineering problems at extremely deep soil deposit site was examined in this study. Microtremor observations were made in the Niigata Plain, Japan, and in Los Angeles, California, where depths to the basement rock are several kilometers. These two locations were selected because strong-motion records obtained during the Niigata earthquake of 1964 and the San Fernando earthquake of 1971 contain large long-period amplitudes. Understanding why these predominate long-period motions were recorded is important for the evaluation of seismic input motions for large-scale structures.Long-period microtremor observations were carried out in both areas in order to evaluate the characteristics of deep soil. Observations were performed simultaneously at a number of stations to distinguish the nature of deep soil deposit. The result of Fourier analysis showed that amplitude of microtremors in long-period range increases systematically from the point of basement rock outcrop to deep deposit site, with the increase corresponding to the depth-to-basement rock. This relationship coincides with that observed in the strong-motion earthquake records. These analyses indicated that simultaneous observation of long-period microtremors at a number of stations can provide insight to the characteristic of deep soil amplification and, therefore, permit estimation of seismic input motions for large-scale structures.