A site effect study of the Adapazari basin,Turkey, from strong- and weak-motion data

Approximately 4000 people were killed due to collapse of buildings in downtown Adapazari during the 1999 Izmit, Turkey earthquake (Mw = 7.4). The downtown is located on a deep sedimentary basin, so-called Adapazari basin. We study site effects of the Adapazari basin based on strong- and weak-motion data obtained by a temporary array observation deployed in and around the Adapazari basin after the earthquake. Four moderate-size aftershocks (M4.6–M5.8) are selected in our study. We evaluate the S-wave amplifications in the basin by using the traditional spectral ratio method. The spectral ratios show that the S waves are considerably amplified in the frequency range of 0.5 to about 5 Hz at the basin sites, but are apparently de-amplified at frequencies higher than about 10 Hz. We make a quantitative interpretation of the empirical amplifications based on the S-wave velocity structures at the stiff-soil reference site as well as at the basin sites; these structures were estimated by the microtremor array measurements. Through the interpretation, we confirm that the amplifications at low frequencies are attributed to the thick sedimentary layers in the Adapazari basin and that the apparent de-amplifications at high frequencies are partly due to the reference site response. In addition to the considerable S-wave amplifications, the basin site records show long-period (about 2 sec) later phases after the S-wave arrival; these later phases are basin-transduced surface waves that are originated from the source and transmitted into the basin. The predominant period of these waves apparently depends on the earthquake magnitude. We conclude that heavy damage in downtown Adapazari during the 1999 Izmit earthquake was caused not only by strongly amplified S-waves but also by long-period basin surface waves of long duration.     

P- and S-Wave Site Response of the Seismic Network RESNOM Determined from Earthquakes of Northern Baja California, Mexico

—We determined the response to P- and S-wave incidence of the permanent stations of the seismic network of Baja California (RESNOM) using two independent methods. We selected 65 events with magnitudes between 2.2 and 4.8 and hypocentral distances ranging between 5 and 330 km. The site response of the ten stations analyzed was first estimated using average spectral ratios between the horizontal and the vertical components of motion (H/V ratios). As a second approach we performed a simultaneous inversion for source and site. In order to invert the spectral records to determine the site response, we made an independent estimate of the attenuation for two different source-station path regions. Then we corrected the spectral records for the attenuation effect before we made the inversion. Although the average H/V ratio of many sites is inside the error bars of the site response estimated with the spectral inversion, the spectral inversion tends to give higher values. For the S wave some sites show similar frequency of predominant peak when comparing the responses obtained with both methods. In contrast, for the P waves the H/V ratios disagree with the results of the inversion. In general, the site response of the stations is strongly frequency dependent for both P and S waves. We also found that the natural frequency of resonance of the sites is near 0.5 Hz for P and near 0.8 Hz for the S waves.     

Estimation of site amplification,attenuation and source spectra of S-waves in the East-Central Iran

Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in the East-Central Iran has been used to estimate simultaneously source parameters, site response and the S-wave attenuation (Q_s). In this regard, 190 three-component records were used corresponded to 40 earthquakes with the magnitudes M3.5–M7.3. These earthquakes were recorded at 42 stations in the hypocentral distance range from 9 to 200 km. The inverse problem was solved in 20 logarithmically equally spaced points in the frequency band from 0.4 to 15 Hz. The frequency-dependent site amplification was imposed, as a constraint, on two reference site responses in order to remove the undetermined degree of freedom in the inversion and obtain a unique inverse solution. Also, a geometrical spreading factor was assumed for removing the trade-off between geometrical spreading and anelastic attenuation. Different source parameters, such as seismic moment (M₀), seismic energy (E_s), corner frequency (f_c) and Brune stress drop (Δσ), were estimated for each event by fitting an ω² model to the spectra obtained from the inversion. The stress drop values of earthquakes, obtained in this research, are in good agreement with those of other studies. Also average site response values were correlated to the average shear wave velocities in the uppermost 30 m, in high and low frequency bands. The peak frequencies of site amplifications, estimated by the generalized inversion method, where in good agreement with those of horizontal to vertical (H/V) spectral ratios for the S-wave portion of records. However, no perfect matching in amplitude was obtained due to the deficiencies of the H/V ratio technique. By supposing a free shape for Q factor, a frequency dependent function was found, the logarithm of which could be approximated by a linear function, Q(f)=151f^0.75. The uncertainties of model parameters have been evaluated by covariance matrix of least-square fit. The residuals were also analyzed in order to assess the validity of the model. The analysis of residuals with respect to magnitude and distance indicates that they are distributed normally with approximately zero mean. The robustness of the results has been studied concerning their sensitivities to the omission of different datasets, selected randomly from original database. The results obtained here can be used in predicting ground-motion parameters applying stochastic methods.     

Application of the Spatial Auto-Correlation Method for Shear-Wave Velocity Studies Using Ambient Noise

Ambient seismic noise or microtremor observations used in spatial auto-correlation (SPAC) array methods consist of a wide frequency range of surface waves from the frequency of about 0.1 Hz to several tens of Hz. The wavelengths (and hence depth sensitivity of such surface waves) allow determination of the site S-wave velocity model from a depth of 1 or 2 m down to a maximum of several kilometres; it is a passive seismic method using only ambient noise as the energy source. Application usually uses a 2D seismic array with a small number of seismometers (generally between 2 and 15) to estimate the phase velocity dispersion curve and hence the S-wave velocity depth profile for the site. A large number of methods have been proposed and used to estimate the dispersion curve; SPAC is the one of the oldest and the most commonly used methods due to its versatility and minimal instrumentation requirements. We show that direct fitting of observed and model SPAC spectra generally gives a superior bandwidth of useable data than does the more common approach of inversion after the intermediate step of constructing an observed dispersion curve. Current case histories demonstrate the method with a range of array types including two-station arrays, L-shaped multi-station arrays, triangular and circular arrays. Array sizes from a few metres to several-km in diameter have been successfully deployed in sites ranging from downtown urban settings to rural and remote desert sites. A fundamental requirement of the method is the ability to average wave propagation over a range of azimuths; this can be achieved with either or both of the wave sources being widely distributed in azimuth, and the use of a 2D array sampling the wave field over a range of azimuths. Several variants of the method extend its applicability to under-sampled data from sparse arrays, the complexity of multiple-mode propagation of energy, and the problem of precise estimation where array geometry departs from an ideal regular array. We find that sparse nested triangular arrays are generally sufficient, and the use of high-density circular arrays is unlikely to be cost-effective in routine applications. We recommend that passive seismic arrays should be the method of first choice when characterizing average S-wave velocity to a depth of 30 m (V_s30) and deeper, with active seismic methods such as multichannel analysis of surface waves (MASW) being a complementary method for use if and when conditions so require. The use of computer inversion methodology allows estimation of not only the S-wave velocity profile but also parameter uncertainties in terms of layer thickness and velocity. The coupling of SPAC methods with horizontal/vertical particle motion spectral ratio analysis generally allows use of lower frequency data, with consequent resolution of deeper layers than is possible with SPAC alone. Considering its non-invasive methodology, logistical flexibility, simplicity, applicability, and stability, the SPAC method and its various modified extensions will play an increasingly important role in site effect evaluation. The paper summarizes the fundamental theory of the SPAC method, reviews recent developments, and offers recommendations for future blind studies.     

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)?=?87.4f^0.78. 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.     

Crustal attenuation characteristics of S-waves beneath the Eastern Tohoku region,Japan

An inversion method was applied to crustal earthquakes dataset to find S-wave attenuation characteristics beneath the Eastern Tohoku region of Japan.Accelerograms from 85 shallow crustal earthquakes up to25 km depth and magnitude range between 3.5 and 5.5were analyzed to estimate the seismic quality factor Q_s. A homogeneous attenuation model Q_s for the wave propagation path was evaluated from spectral amplitudes, at 24 different frequencies between 0.5 and 20 Hz by using generalized inversion technique. To do this, non-parametric attenuation functions were calculated to observe spectral amplitude decay with hypocentral distance. Then, these functions were parameterized to estimate Q_s. It was found that in Eastern Tohoku region, the Q_s frequency dependence can be approximated with the function 33 f~(1.22) within a frequency range between 0.5 and 20 Hz. However,the frequency dependence of Q_s in the frequency range between 0.5 and 6 Hz is best approximated by Q_s(f) = 36f~(0.94) showing relatively weaker frequency dependence as compared to the relation Q_s(f) = 6 f~(2.09) for the frequency range between 6 and 15 Hz. These results could be used to estimate source and site parameters for seismic hazard assessment in the region.     

Estimation of local site effects by a generalized inversion scheme using observed records of ‘Small-Titan’

A simple-extended array of observation for strong motions, named ‘Small-Titan’, was installed in Sendai City, Japan in 1997. This array system consists of 20 observation sites, which have an average of about 4 km for the station-to-station distance. Each observation station was located at various kinds of soils to effectively obtain local site effects. Since its completion, the system has obtained about 160 motion records. As is well known, ground motions are influenced by the three main factors such as source, path and local site effects. An accurate prediction of ground motions should thus be performed based on the three factors. In this context, theoretical methods used commonly lack enough information related with source and path effects. A generalized inversion scheme to separate the effects of source, propagation path and local site from S-wave spectra was applied to the records of Small-Titan obtained in the Sendai region, Japan. The objective of this paper is to estimate local site effects in Sendai City and to obtain meaningful information of input motions available to earthquake-resistant design, aimed at setting in terms of local soil conditions. This paper also describes the applicability of the generalized inversion scheme to seismic zoning.

Site amplification factors estimated from the generalized inversion scheme vary significantly, reflecting the effects of soil characteristics at each site. Strong site-dependency was found in the site amplification factors. In particular, the local site amplification factors in the western side are smaller than those in the eastern side, across the Nagamachi–Rifu Fault that exists in the central part of the Sendai area. Results from the generalized inversion scheme are compared with those obtained by the direct spectral ratio scheme, the H/V method and the bootstrap inversion. There is a good agreement in the amplification factors between the generalized inversion scheme, the direct spectral ratio scheme and the bootstrap inversion scheme. The H/V method for estimating site amplification is found to extract the same dominant periods as the other methods. This paper especially emphasizes a good agreement in not only dominant period, but also amplification factor.

The generalized inversion scheme is a useful method in separating the effects of source, propagation path and local site and in revealing the site amplification. In addition, the generalized inversion scheme provides meaningful information of input motions available to earthquake-resistant design, aimed at setting in terms of local soil conditions. Moreover, this paper indicates that the generalized inversion scheme is applicable to the seismic zoning toward disaster mitigation.     

Separation of source and site effects by generalized inversion technique using the aftershock recordings of the 2009 L��Aquila earthquake

We exploit S-wave spectral amplitudes from 112 aftershocks (3.0 ≤ M_L ≤ 5.3) of the L’Aquila 2009 seismic sequence recorded at 23 temporary stations in the epicentral area to estimate the source parameters of these events, the seismic attenuation characteristics and the site amplification effects at the recording sites. The spectral attenuation curves exhibit a very fast decay in the first few kilometers that could be attributed to the large attenuation of waves traveling trough the highly heterogeneous and fractured crust in the fault zone of the L’Aquila mainshock. The S-waves total attenuation in the first 30 km can be parameterized by a quality factor Q_S(f) = 23f ^0.58 obtained by fixing the geometrical spreading to 1/R. The source spectra can be satisfactorily modeled using the omega-square model that provides stress drops between 0.3 and 60 MPa with a mean value of 3.3±2.8 MPa. The site responses show a large variability over the study area and significant amplification peaks are visible in the frequency range from 1 to more than 10 Hz. Finally, the vertical component of the motion is amplified at a number of sites where, as a consequence, the horizontal-to-vertical spectral ratios (HVSR) method fails in detecting the amplitude levels and in few cases the resonance frequencies.     

场地放大效应的估计

本利用唐山地区9个台站得到的13次近场地震记录分析了场地放大效应，其中一些站位于废弃的地下煤矿坑道内，最深的一个台站在地下822m处，首先检验确定了地面反射波对地下场地的入射波没有很大影响，在后利用地下基岩场地台站（－822m)作参考场地，用线性反演法来同时分离震源，传播途径和局部场地效应，结果发现：在1至10Hz的频段上S波品质因子Qs基本上与频率成正比；与以地面基岩场地作参考场地所得的结果相比较，地面基岩场地的反应并不是一常数，它在大于6Hz的高频段上有明显的放大作用，在本的研究事件中，在6至10Hz的频段上这个放大纱数约为2至4。这表明由于地面基岩场地本身的放大作用，将导致在作场地震动预测时会低估地震作用，最后采用一种新的非参考场地法，即遗传算法，用w^2震源模型来估计绝对场地效应。在本的研究结果中，对大多数台站场地来说，所得的绝对场地应应与以地下基岩台站作参考场地所得的相对场地效应十分一致。这表明这作场地震动分析时，当仔细考虑了反射波对入射波的影响后，地下或井下基岩场地是比地面基岩场地更合适的参考场地。     

Inversion of Scholte wave dispersion and waveform modeling for shallow structure of the Ninetyeast Ridge

The construction of S-wave velocity models of marine sediments down to hundreds of meters below the seafloor is important in a number of disciplines. One of the most significant trends in marine geophysics is to use interface waves to estimate shallow shear velocities which play an important role in determining the shallow crustal structure. In marine settings, the waves trapped near the fluid–solid interface are called Scholte waves, and this is the subject of the study. In 1998, there were experiments on the Ninetyeast Ridge (Central Indian Ocean) to study the shallow seismic structure at the drilled site. The data were acquired by both ocean bottom seismometer and ocean bottom hydrophone. A new type of seafloor implosion sources has been used in this experiment, which successfully excited fast and high frequency (>500 Hz) body waves and slow, intermediate frequency (<20 Hz) Scholte waves. The fundamental and first higher mode Scholte waves have both been excited by the implosion source. Here, the Scholte waves are investigated with a full waveform modeling and a group velocity inversion approach. Shear wave velocities for the uppermost layers of the region are inferred and results from the different methods are compared. We find that the full waveform modeling is important to understand the intrinsic attenuation of the Scholte waves between 1 and 20 Hz. The modeling shows that the S-wave velocity varies from 195 to 350 m/s in the first 16 m of the uppermost layer. Depths levels of high S-wave impedance contrasts compare well to the layer depth derived from a P-wave analysis as well as from drilling data. As expected, the P- to S-wave velocity ratio is very high in the uppermost 16 m of the seafloor and the Poisson ratio is nearly 0.5. Depth levels of high S-wave impedance contrasts are comparable to the layer depth derived from drilling data.     

Inversion of local S-wave velocity structures from average H/V ratios, and their use for the estimation of site-effects

H/V spectral ratios from microtremors areused to retrieve the S-velocity structurefrom a single ambient vibration record, byusing its relation to the ellipticity ofthe fundamental mode Rayleigh wave and theamplitude of observed H/V ratio.Constraints are needed in order to restrictthe possible range of solutions, and theinversion is applied to sites where thethickness of the unconsolidated sedimentsis approximately known from boreholeinformation. Within the uncertainty, theinverted structures agree well with theresults from other S-wave measuringtechniques such as downhole and cross-holemeasurements, and the analysis of ambientvibrations measured on an array.The influence of the inversion uncertaintyon site-amplification estimates forearthquakes is then investigated. For allinverted models, site response is computedfor a large number of events, which allowsto define the uncertainty by the aprioriunknown source position and mechanism of afuture earthquake. In most cases thevariability between the results obtainedfor the different models is much smallerthan the variability introduced by theunknown source position. The accuracy withwhich S-wave velocity structures can beretrieved from observed H/V ratios istherefore sufficient for an application ofthe method in seismic hazard analysis for aspecific site.     

Attenuation of S wave in the crust of Ordos massif

Introduction Located between Qingzang (Qinghai-Xizang) Plateau that highly uplifts due to compression and North China plain that badly subsides due to crustal extension, the Ordos massif has both the basic attributes of stretching tectonic region of North China marginal basin and the features of marginal shear-compression zone of Qingzang Plateau. It has a length of about 600 km in the N-S direction and a width of about 400 km in the W-E direction. Its geological structure is very simple…     

用H/V谱比法计算云南区域数字地震台站的场地响应

用H/V谱比法,计算给出了云南区域数字地震台网23个子台S波随频率变化的台站场地响应特征。结果表明:在1～10Hz频段内,23个子台S波的场地响应相对较平坦,在1.41～2.91之间变化,平均2.08;在大于10Hz的高频段,部分台站的场地响应有较明显的放大。     

2000年姚安MS65地震震源及近邻区的低QS

利用震源距23 km范围内观测的2000年姚安MS65地震余震记录，计算了震源及近邻区域的尾波规一直达S波在频率15～20 Hz之间的傅里叶谱振幅.结果显示谱振幅随震源距增大而增大， 在对谱振幅进行了震源辐射方向性校正之后, 才出现谱振幅随震源距衰减的现象.由此获得了震源及近邻区域S波的Q(f)值,可表示为QS(f)=89f098其值比由尾波得出的姚安地区的平均QC(f)值低得多，表明了震源破裂带的强烈非均匀性对QS(f)的重大影响.     

Amplification effects from earthquakes and ambient noise in the Dead Sea rift (Israel)

Seven sites were instrumented in the Parsa area located in the seismically active Dead Sea rift system. Moderate and weak motions generated by earthquakes and ambient noise were used to identify amplifications due to geological and topographic effects.Three observation methods were applied to estimate site effects: (1) conventional soil–bedrock station-pair spectral ratios for earthquake motions and microtremors; (2) horizontal-to-vertical component spectral ratios for shear-waves observed simultaneously at a site (receiver function estimates) and (3) horizontal-to-vertical spectral ratios of microtremor measurements (Nakamura estimate). The site response spectra of soil sites exhibited significant peaks between 1 and 3 Hz with amplification factors typically within the range of 2.5–4.0. A bedrock site on the high plateau near the escarpment top showed a peak between 2 and 3 Hz, mainly due to an EW oscillation of the NS topographic feature. Our observations indicated that seismograms recorded in the tunnel were either enriched or depleted at certain frequencies owing to interference of incident and surface-reflected waves.     

Site effects observed in alluvial basins: the case of Norcia (Central Italy)

In this work, we investigate the site amplification effects observed in the Norcia plain, Central Italy. Data from 30 selected local earthquakes (2 ≤ Ml ≤ 4.1) recorded by a temporary seismic network composed by 15 stations, are analyzed to determine the spatial variability of site effects. Both the Horizontal-to-Vertical spectral ratio and the Standard Spectral Ratio techniques are applied to estimate the site amplification effects. The results show that most of the sites in the valley are affected by strong amplifications (up to a factor of 20) in the frequency range 0.5–5 Hz. The value of the fundamental frequency of resonance is strictly dependent on the location within the basin and on the sediment thickness. Strong amplifications also affect the vertical components. The time-frequency analysis performed on a station located inside the basin shows the presence of a large spectral amplitudes after the S-wave phase, not observed on a station located on the bedrock, suggesting the presence of locally generated wave trains. Then, in agreement with earlier observations for other alluvial basins in Central Italy, 2D–3D effects play an important role in determining the site amplification effects in Norcia.     

Application of numerical wave-propagation techniques to study local soil effects: The case of Benevento (Italy)

Many of the numerical techniques used for seismic zonation studies treat one-dimensional structural models and/or the incidence of plane polarized body waves. These techniques are often not adequate for laterally heterogeneous structures and for sources that are not located beneath the site of interest. In such cases a more rigorous treatment of the combined effects of the source, the path and the site response is needed. This can be accomplished with a hybrid approach combining modal summation and the finite-difference technique. To demonstrate the differences between these techniques, the ground motion in the city of Benevento (Italy) is modelled. We first compare the results obtained with one-and two-dimensional structural models for vertical incidence of plane polarized body waves. These results are then compared with those obtained with the hybrid approach for two-dimensional structural models.The comparisons have allowed us to find important differences in the response obtained with the different modelling techniques. For the same site, these differences consist of strong variations in amplitude and in the shape of the spectral amplifications. For a seismic source which is not located beneath the site, vertical incidence of waves significantly overestimates the local hazard in a laterally homogeneous structure. For a laterally heterogeneous area, we can conclude that one-dimensional modelling fails to estimate the seismic hazard, whereas for a seismic source which is not located beneath the site of interest, two-dimensional modelling with vertical incidence of plane polarized body waves may not allow reliable estimates to be made of the frequency bands at which amplifications occur. The results obtained for two-dimensional structural models are used for a zonation of the city of Benevento.     

Site response and source spectra of S waves in the Zagros region, Iran

S wave amplitude spectra from shallow earthquakes with magnitudes ranging between 4.2 and 6.2 in the Zagros region of Iran that occurred between 1998 and 2008 are used to examine source parameters and site response of S waves. A generalized inversion scheme has been used to separate the source, propagation path, and local site effects from S wave spectra. For removing the trade-off between source and site terms and propagation effects (including geometric and anelastic attenuation), the spectral amplitudes of the records used were corrected for attenuation and geometrical spreading function using a path model proposed by Zafarani and Soghrat (Bull Seism Soc Am 102:2031–2045, 2012) for the region. We assume a Brune’s point source model to retrieve source parameters like corner frequency, moment magnitude, and high-frequency fall off coefficient, for each event. When the source spectra are interpreted in terms of Brune’s model, the average stress drops obtained are about 7.1 and 5.9 MPa (71 and 59 bars), respectively for the eastern and western Zagros regions. Stress drops range from 1.4 to 35.0 MPa (14 to 350 bars), with no clear dependence on magnitude. The results in terms of stress drop and S wave seismic energy indicate that the Zagros events are more similar to interplate earthquakes of western North America than to intraplate events of eastern North America. The method also provides us with site responses for all 40 stations individually and is an interesting alternative to other methods, such as the H/V method. A new empirical relationship between body-wave magnitudes and moment magnitude has been proposed for the Iranian plateau using derived seismic moment from the inversion.     

Ground motion modeling for site effects at L’Aquila and middle Aterno river valley (central Italy) for the MW 6.3, 2009 earthquake

Ground motion for the 6 April 2009 (M_w 6.3) earthquake is computed along 2-D cross-sections at L’Aquila by a hybrid method (modal summation plus finite differences) and validated with recordings at AQU, AQK, AQG, AQA and AQV stations. Parametric studies of S-wave velocities of the shallowest lithotypes allow to get a general agreement between synthetic and observed response spectra, despite the scaled point-source approach and the lack of detailed geological and seismic studies. It results that the megabreccia covering on lacustrine soils, characterizing the historical center of L’Aquila, is responsible of spectral amplifications along the vertical (2–7) and horizontal components (2–3) at a wide frequency range (0.6–7 Hz). The covering of alluvial soils in the middle Aterno river valley is responsible of amplifications at 2–7 Hz both in the horizontal and vertical planes of the motion. Such amplifications evidence that site effects might have been responsible of structural damages.     

考虑介质对地震波吸收衰减作用的场地放大效应估计

当参考场地震源距小于研究场地震源距时,传统谱比法低估了场地放大效应,且参考场地与研究场地震源距相差越大,低估程度越大;当参考场地震源距大于研究场地震源距时,传统谱比法高估了场地放大效应,且参考场地与研究场地震源距相差越大,高估程度越大,计算结果依赖参考场地的选择。上述问题存在的原因是传统谱比法估计场地放大效应时未考虑地震波传播过程中介质的吸收衰减作用,故传统谱比法适用于参考场地与研究场地地质构造类似且震源距相差较小的情况,为此本文通过考虑介质对地震波的吸收衰减作用,提出改进谱比法,可适用于地质构造类似、任意震源距的情况。本文分别采用未改进方法和改进方法估计汶川地震中渭河盆地多个台站场地反应,证实了传统谱比法存在的上述问题。采用改进谱比法估计的场地放大倍数均在2 Hz以下出现1个峰值,且这些峰值多为最大值;采用未改进谱比法估计的场地放大倍数峰值出现的频率多大于2 Hz;改进谱比法可信度较高。