Local Site Effects in the Town of Benevento (Italy) from Noise Measurements

— The study of ground motion amplification produced by surface geology is extremely interesting in the Benevento area, Southern Italy, as it is characterized by high seismic hazard. The present moderate-to-low seismicity makes the noise method appropriate to estimate the seismic site response in the area. The three components of seismic noise have been recorded in five sites in the Benevento metropolitan area characterized by different surface geology, in order to estimate the seismic site response. In evaluating site amplification effects we used the direct interpretation of amplitude spectra and standard spectral ratio techniques, evaluating sediment-to-bedrock, sediment-to-average and H/V spectral ratios. The temporal evolution of the noise spectra is analysed within one day, in order to assess the stationarity of the noise signal. The noise wavefield properties have been studied through polarization analyses in selected bands of frequency, where spectral peaks are observed to dominate, to better understand the real nature of those peaks. Results give evidence of low amplification levels, missing any correlation between spectral amplitudes and sediment thickness over the basement. We interpret this result as due to the poor impedance contrast between sediments and basement, which is characterized by low values of shear waves velocity. Moreover, sharp amplitude peaks are observed in the raw spectra of the sediment-sites, in the 2–4 Hz frequency band; a numerical simulation interprets this effect as possibly associated with a wide-scale structure, invoking the presence of a sharper impedance contrast at greater depth. At high frequencies the action of ambient noise sources, mainly active on horizontal components of motion, is retained dominant to generate the prominent peaks observed in the H/V spectral ratios; in some cases the presence of a near-surface low-velocity layer can contribute to amplify the seismic motion generated at these frequencies.     

汶川地震远场地震动场地相关性与分析方法评价

为考查远场地震动的场地相关性并评价一些场地特性分析方法的适用性,采用不同方法对汶川地震山东省12个远场台站的强震记录进行了分析.选取台站分别位于按建筑抗震设计规范(CBC)场地划分中的Ⅰ—Ⅲ类场地上.地震动记录的分析方法包括傅里叶幅值谱法,地震反应谱法,水平与竖向谱比率法,参考点谱比率法,以及尾波分析等.结果表明,按傅里叶幅值谱法,地震反应谱法,水平与竖向谱比法计算得到的卓越周期均远大于台站场地的卓越周期,不同方法得到的结果之间也有较大差别,且主要反映长周期地震动的卓越频率;参考点谱比率法的结果未反映地震动的卓越周期,也与场地的卓越周期差别较大;对完整记录尾波分析所得的结果比较接近场地的卓越周期.希望本文能为考虑远场地震作用时设计谱的建立,以及场地特性估计时地震动分析方法的选取提供参考依据.     

Earthquake Induced Site Effect in the Algiers–Boumerdes Region: Relation Between Spectral Ratios Higher Peaks and Observed Damage During the May 21st M w 6.8 Boumerdes Earthquake (Algeria)

The Algiers–Boumerdes regions were hit by an M _w 6.8 destructive earthquake on May 21, 2003. The accelerometric and seismometric networks successfully recorded the main shock and many aftershocks at some locations where the damage was most extensive. A microtremor measurement was performed in the same locations; some of them are localized on the Mitidja basin. In this paper, we propose to analyze earthquake-induced site effect derived from horizontal to vertical spectral ratio from ambient noise (noise horizontal to vertical—NHV), or from very weak, weak, moderate and strong ( peak ground acceleration—PGA >10 % g) seismic motions (EHV), and transfer function evaluation from soil velocity profile data at four sites. H/V spectral ratios are computed by using both Fourier and response spectra. Compared to the transfer function, the obtained results show that in the case of soft soils, NHV as well as EHV give a good estimation of the soil's fundamental frequency, whereas the NHV underestimate the H/V amplitude and the EHV amplitude increase with the seismic motion intensity. In the case of firm soils, whereas the NHV gives flat curves synonymous for a rock site or a bump, the EHV is more appropriate as seen by identifying clear peaks with non negligible amplitude. In the case of soft sites as well as in the case of firm sites, strong peaks at frequencies higher than the fundamental one are found from EHV curves. Those peaks would not be found when looking at NHV ratios alone, are evidenced by the computed transfer function as well as by an analytical formulation, and are in agreement with the observed distribution of damage during the M _w 6.8 2003 Boumerdes earthquake. Finally, the same analysis is performed by using response spectra rather than Fourier spectra and leads to the same conclusions. Moreover, the calculation of the response spectra is more easily compared with the smoothing operation of the Fourier spectra.     

Site response in Tecoman,Colima, Mexico—I: Comparison of results from different instruments and analysis techniques

This paper presents a study of site effects in the urban area of Tecoman in Colima, Mexico. A variety of instruments (both accelerometers and seismometers) were used to record earthquakes and ambient vibration throughout the city. Earthquake records were analysed using several techniques to estimate site effects: spectral ratios relative to a reference station, spectral ratios of the horizontal components relative to the vertical recorded at the same site, and a parametric inversion of Fourier spectra. Ambient noise records were used to estimate a local transfer function using horizontal to vertical spectral ratios. The results show that local amplification at Tecoman is significant. Dominant frequency varies between 0.5 and 0.7 Hz, suggesting a large thickness of the soft sedimentary deposits. We did not observe systematic variations throughout the city. Our more reliable estimates indicate that maximum amplification is comprised between a factor 6 and 8. Comparisons among different sensors and recorders show that all combinations between velocimeters, accelerometers, and recorders provide reliable results provided that the electronic noise is smaller than the noise being recorded. This is notably not the case for accelerometers at quiet sites and for frequencies smaller than 2 Hz. This explains why previous studies disagree as to the usefulness of accelerometers to record ambient noise for site effect studies. This factor is, however, a function of noise amplitude at each site.     

New relationship between fundamental site frequency and thickness of soft sediments from seismic ambient noise

In this contribution, new relationship between the fundamental site frequency and the thickness of soft sediments is obtained for many sites in Egypt. The Horizontal-to-Vertical Spectral Ratio (“H/V”) technique (known as Nakamura technique) can be used as a robust tool to determine the thickness of soft sediments layers overlaying bedrock from observations and measurements of seismic ambient noise data. In Egypt, numerous seismic ambient noise measurements have been conducted in several areas to determine the dynamic properties of soft soil for engineering purposes. At each site in each studied area, the fundamental site frequency was accurately estimated from the main peak in the spectral ratio between the horizontal and vertical component. Consequently, an extensive database of microtremor measurements, well logging data, and shallow seismic refraction data have been configured and assembled for the studied areas. New formula between fundamental site frequency (f₀) and thickness of soft sediments (h) is established. The new formula has been validated and compared with other formulas of earlier scientists, and the results indicate that the calculated depth and geometry of the bedrock surface using new formula are in a good agreement with well logs data and previously published seismic refraction surveys in the investigated sites.     

Site characterizations for the Tehran network (TDMMO) in Tehran Region Using Micro-Earthquake,Microtremor and Quarry Blast Data

We did a Study of Horizontal-to-Vertical Component Spectral Ratio in the Tehran seismic zone. Micro-earthquakes, microtremors and quarry blasts data were used as an estimation of the site response in the Tehran zone. Site effects were studied based on horizontal to vertical ratios by the Nakamura׳s technique. Also, we used the spectra of signals for three components with the lowest noise levels for spectral slope studies. The analysis used seismic events from a network of 13 seismic stations by the permanent local seismological network of the Tehran Disaster Mitigation and Management Organization (TDMMO) from 2004 to 2007. The number of events used were different for each station. Quarry blast events were with 1.2≤M_L≤2.2 and micro-earthquakes were with 1.1≤M_L≤4.1.By comparing results for earthquake, microtremor and quarry blast, we could see that there is a significant difference between them. The data showed clear observations, especially in high-frequencies. The H/V spectral ratios indicate dominant frequency for rock/soft site with a higher ratio level for quarry blast ratios, which are comparable to the earthquake results due to their difference sources. The results derived by spectral H/V ratios and spectral analysis may be used to distinguish between local earthquakes and quarry blasts.     

3D near-surface soil response from H/V ambient-noise ratios

The applicability of the horizontal-to-vertical (H/V) ambient-noise spectral ratio for characterizing earthquake site effects caused by near-surface topography and velocity structures was evaluated at sites underlain by thick (i.e. >100 m) sediment deposits near the southern-end of the New Madrid seismic zone in the central United States. Three-component ambient-noise and velocity models derived from seismic (shear-wave) refraction/reflection surveys showed that a relatively horizontal, sharp shear-wave velocity interface in the soil column resulted in an H/V spectral ratio with a single well-defined peak. Observations at sites with more than one sharp shear-wave velocity contrast and horizontally arranged soil layers resulted in at least two well-defined H/V spectral ratio peaks. Furthermore, at sites where there were sharp shear-wave velocity contrasts in nonhorizontal, near-surface soil layers, the H/V spectra exhibited a broad-bandwidth, relatively low-amplitude signal instead of a single well-defined peak.     

南北地震带川滇甘陕地区竖向与水平向加速度反应谱比的统计分析

本文收集了从南北地震带川滇甘陕地区263个有详细场地资料的强震台站获得的802组强震动记录,按照场地、震级和震中距等因素统计分析了竖向与水平向加速度反应谱比的形状以及不同场地、震级和震中距下不同周期段内的谱比平均值,并与建筑抗震规范GB50011—2010规定的0.65进行了比较。结果表明:竖向与水平向加速度反应谱比受场地、震级和震中距等因素的影响; Ⅰ类和Ⅱ类场地在全周期段的谱比均值基本上大于0.65;无论何种分组情况谱比均值在0.1—1.0 s周期范围内基本低于定值0.65,周期大于1.0 s的谱比均值基本上远高于0.65。因此对于大部分抗震规范直接把竖向地震作用取为水平向地震作用的0.65倍,有待商榷。本文建议根据水平向反应谱的标定方式来确定现有竖向强震动记录的竖向地震作用。      

2020年7月12日唐山古冶5.1级地震揭示的北京城区地震动场地效应分析

局部场地条件对地震动特性影响显著,深厚的软弱覆盖层引起的地震动场地效应会显著放大中长周期反应谱。采用谱比法,对2020年7月12日唐山古冶5.1级地震中获得的部分强震动记录进行统计,发现在本次地震中北京城区的地震动场地效应显著,深厚覆盖层明显放大了加速度反应谱,在T=1.2 s左右反应谱放大倍数可达4.0,说明北京地区的场地和盆地效应使得远场地震动的中长周期成分显著放大。此外,发现参考基岩场地记录是否与土层场地处遭受的基岩地震动一致,仍然是制约统计结果可靠性的关键因素。      

Site response study in Abruzzo (Central Italy): underground array versus surface stations

In this site response study we examined local earthquakes recorded at surface stations of a local seismic network and at a temporary underground seismic array installed in a tunnel underneath the Gran Sasso Massif in Abruzzo (central Italy). This allowed us to compare the seismic site response beneath the mountain and on the surface in similar geological environment (soft rock sites). We applied spectral ratios method on different segments of the seismograms and used different reference spectra in the 1–20 Hz frequency band. We found little or no amplification effects at most of the surface stations whereas site transfer functions evaluated with respect to underground sites show an amplification factor up to 6 in the 1–8 Hz frequency range. Coda spectral ratios estimated at soft rock sites are confirmed as good estimates of shear wave transfer function.     

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.     

Experimental study of seismic behavior of two hilly sites in Tehran and comparison with 2D and 3D numerical modeling

Two hilly sites were selected to study seismic site response due to topography effects. The sites were selected in a manner to be as much as possible homogenous and free of the soft soil layers effects. The hills were instrumented by nine velocimetric stations to record microtremors and the obtained data were analyzed using horizontal to vertical spectral ratios. Some standard spectral ratio tests were performed on noise as well. Then the instrumented hills were modeled (both 2D and 3D) assuming a linear elastic constitutive behavior subjected to vertically propagating SV and P Ricker wavelets. All calculations were performed in time domain using direct boundary element method. Different transfer function components, amplification patterns and spectral ratios were calculated in frequency domain. The frequency of vibration, obtained by experimental studies, is between 4 and 5 Hz for both of the hills. The spectral ratios derived by numerical simulations were compared with the observed spectral ratios. They show relatively good similarities between the results of these two methods. The frequencies of vibration derived from different methods seem to be nearly identical. The agreement in term of resonance frequency between microtremors and numerical modeling suggests that noise measurements could represent a simple, even if preliminary, tool in order to identify possible topographic amplification.     

Site dependence of far-source ground motions during the Wenchuan earthquake

This paper aimed to examine the site dependence and evaluate the methods for site analysis of far-source ground motions. This was achieved through the examination of frequency content estimated by different methods based on strong ground motions recorded at twelve far-source stations in Shandong province during the Wenchuan earthquake. The stations were located in sites with soil profiles ranging from code classes Ⅰ to Ⅲ. Approaches used included the Fourier amplitude spectrum (FAS), the earthquake response spectrum (ERS), the spectral ratio between the horizontal and the vertical components (H/V), the spectral ratio between the spectra at the site and at a reference site (SRRS), and coda wave analysis (CWA). Results showed that major periods of these ground motions obtained by FAS, ERS and H/V ratio methods were all evidently larger than site dominant periods; the periods were also different from each other and mainly reflected the frequency content of long period components. Prominent periods obtained by the SRRS approach neither illuminated the long period aspect nor efficiently determined site features of the motions. The CWA resulted in a period close to site period for stations with good quality recordings. The results obtained in this study will be useful for the evaluation of far-source effect in constructing seismic design spectra and in selecting methods for ground motion site analysis.     

Applicability of the H/V spectral ratio of microtremors in assessing site effects on seismic motion

The authors examine the reliability of site response estimations obtained by the horizontal to vertical (H/V) spectral ratios of microtremors by means of cross‐validation with the ratio of the horizontal spectra of earthquake motion with respect to reference sites. The data comprise microtremor and ground motion records recorded at 150 sites of Yokohama strong motion array. The use of non‐supervised pattern recognition techniques aims to group the sites with more objectivity. Attributes defining the overall shape of the amplification spectra serve as input in the computation of Euclidean distance similarity coefficients amongst sites. The implementation of the Ward clustering scheme leads to the attainment of a meaningful tree diagram. Its analysis shows the possibility of summarizing the results into six general patterns. A good coincidence of site effects estimates at 80 per cent of the sites becomes apparent. However, this coincidence appears poor for sites characterized by H/V amplification ratios around 2 or smaller and predominant periods longer than 0.5 s. In such cases, the presence of stiff, sandy sediments in the soil profile proves common. To proscribe H/V estimations, relying solely on the small spectral ratios criterion seems inadequate. Copyright © 2001 John Wiley & Sons, Ltd.     

Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings

The city of Benevento (Southern Italy) has been repeatedly struck by large historical earthquakes. A heterogeneous geologic structure and widespread soft soil conditions make the estimation of site effects crucial for the seismic hazard assessment of the city. From 2000 until 2004, we installed seismic stations to collect earthquake data over zones with different geological conditions. Despite the high level of urban noise, we recorded more than 150 earthquakes at twelve sites. This data set yields the first, well documented experimental evidence for weak to moderate local amplifications. We investigated site effects primarily by the classical spectral ratio technique (CSR) using a rock station placed on the Benevento hill as reference. All sites in the Calore river valley and in the eastern part of the Benevento hill show a moderate high-frequency (f > 4 Hz) amplification peak. Conversely, sites in the Sabato river valley share weak-to-moderate amplification in a wide frequency band (from 1–2 to 7–10 Hz), without evident frequency peaks. Application of no-reference-site techniques to earthquake and noise data confirms the results of the CSRs in the sites of the Calore river valley and of the eastern part of the Benevento hill, but fails in providing indications for site effects in the Sabato river valley, being the H/V ratios nearly flat. One-dimensional modeling indicates that the ground motion amplification can be essentially explained in terms of a vertically varying geologic structure. High-frequency narrow peaks are caused by the strong impedance contrast existing between near-surface soft deposits and stiff cemented conglomerates. Conversely, broad-band amplifications in the Sabato river valley are likely due to a more complex layering with weak impedance contrasts both in the shallow and deep structure of the valley.     

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.     

An Evaluation on H/V Ratio vs. Spectral Ratio for Site-response Estimation Using the 1994 Northridge Earthquake Sequences

—The applicability of the single-station H/V method, based on the spectral ratio between the horizontal and the vertical components of strong ground motions, is examined for site-response estimation using the high quality data from the 1994 Northridge earthquake sequence. Instead of using Rayleigh-wave data from microtremors, the large amplitude-wave part of the S-wave data is used and based on the 1994 Northridge mainshock and aftershock recordings. We have found that upon averaging over a number of recordings for a given station, the station site responses, derived both from the single-station H/V ratio and from the standard spectral ratio (with respect to a reference rock-site station) are sufficiently close for practical purposes. We therefore conclude that the H/V ratio can reasonably predict the resonant frequency and the amplification level of a site response, especially for sites in the neighborhood of the epicenters. In the absence of a reference rock-site station, the H/V ratio provides a practical alternative to the standard site-response estimation.     

基于台阵记录的土层山体场地效应分析

选取2008年5月25日至8月7日期间由甘肃省文县上城山地形效应台阵获取的12次汶川地震余震事件(M_S≥4.0),在分析其地震动基本参数的基础上,采用参考场地谱比(RSSR)法和水平-竖向谱比(HVSR)法,研究了不同地震作用下上城山地形台阵的场地效应.分析结果显示:随着高程和覆盖层的增加,记录台站地震动的PGA呈增大趋势,地震频谱形状由宽变窄;上城山台阵记录到的地震波在地形基阶频段(2—4 Hz)和浅部土层频段(7—9 Hz)的幅值明显放大,RSSR曲线显示山顶NS向的土层频段谱比大于山体地形频段谱比;由于土层山体竖向地震动在中高频段放大,使得HVSR方法谱比结果在中高频段较RSSR方法所得结果明显偏低,而在山体基阶频段附近两种方法的谱比值接近.松散土层山体的台阵记录特征体现了地形和土层对地震动的联合作用,揭示了强震区起伏地形场地震害加重及地震滑坡集中发生的原因所在.      

A comparison of surface and underground array measurements of ambient noise recorded in Naples (Italy)

In this study, we describe two experiments of seismic noise measurements carried out in Naples, Italy. The site allowed measurements to be obtained both at the surface and in a tunnel that is 120-m-deep. The main goal was to compare the seismic response evaluated at the surface to the in-tunnel response, through spectral, polarization, and resonance directivity analyses. In the 1 to 20 Hz frequency band, the noise level was up to 15 dB higher at the surface than in the tunnel. The polarization properties and horizontal-to-vertical spectral ratios appear not to be influenced by the tunnel geometry or by the topography. Some preferential alignments were observed in the polarization azimuths computed at the surface, which are likely to be due to local sources, rather than morphological features. The absence of directivity effects and the low noise levels in the tunnel make this site suitable for installing seismic stations. We also studied how the subsoil structure affects the seismic motion at the surface. The dispersive properties of the Rayleigh waves were investigated using the spatial autocorrelation method. A joint inversion of the dispersion data and the horizontal-to-vertical spectral ratios provided the subsurface Vs profile. The derived model has a low velocity contrast at depth, such as to generate moderate and broad H/V spectral ratio peak amplitude. The normalized spectral ratio appears more appropriate to identify the soil-resonance frequencies.     

Seismic characterization and monitoring of Fucino Basin (Central Italy)

The Fucino basin (Central Italy) is one of the largest intramountain alluvial plain in the Apennines range. It has a tectonic origin related to the presence of important systems of faults located in its northern and eastern edges. Some of these faults are still active and capable of generating strong seismic events. Site effects related to the soft soils filling the basin can be very important. In this paper we show the preliminary results of a seismic network installed in the Fucino area in order to collect information about site amplification effects and geometry of the basin. We analyze ambient seismic vibrations and recordings of about 150 local earthquakes mainly related to the seismic sequence of the April 6th 2009 Mw 6.3 L’Aquila event. Moreover the strongest events of L’Aquila sequence were analyzed at the three permanent strong-motion stations operating in the area. Using standard spectral techniques we investigate the variation of resonance frequencies within the basin. The ground motion recorded in the Fucino plain is mainly characterized by strong energy at low-frequencies (f < 1 Hz) affecting both horizontal and vertical components. This is particularly evident for stations deployed in correspondence of very thick deposits of sedimentary filling, where a significant increase of ground-motion amplitude and duration is likely caused by locally generated surface waves. The amplification at low-frequencies (<1 Hz) on the horizontal components can reach up a factor of 10 in comparison to nearby stiff sites. However, we found evidences of seismic amplification phenomena also for stiff sites surrounding the basin, including stations of the Italian strong motion network. The independent geological information and the shallow shear-velocity profiles available for the basin can be combined with resonance frequencies for deriving representative geological sections to be used as base for future numerical 2D–3D modeling of the basin.