To study the characteristics of long period microtremor and applicability of microtremor survey,we have made microtremor observations using long period seismometers of the China's mainland from coastal cities like...To study the characteristics of long period microtremor and applicability of microtremor survey,we have made microtremor observations using long period seismometers of the China's mainland from coastal cities like Shanghai and Tianjin through Beijing,Xi'an,to the far inland cities of Lanzhou and Tianshui.The observation shows that the level of power spectrum of long period microtremors reduced rapidly from the coast to the inland area.However,the energy of long period microtremors in Beijing,Xi'an,Lanzhou and Tianshui area are only approximately ten-thousandth to thousandth of that in Shanghai.Aiming at the complexity of the underground structure in the far inland,a series of underground structure models with different distributions were assessed using three-dimensional,dynamic finite element method(FEM) analyses.The results were used to evaluate microtremor survey methods and their limitations with regard to aggregate variability and thickness determinations.Multiple-wave reflections between layers with the change of underground structure distribution occurred,which have significant effect on the performance of the different field approaches.Information over a broad spectrum from which velocity-depth profiles were produced via inversion approaches.Neither the thickness nor the shear waveVelocity V of the subsurface layer inversion results appeared over a large evaluation with increasing slope angle.In particular,when the angle of the model reached 45°,it could not be accurately evaluated using one-dimensional inversion methods.Finally,the array microtremor survey(AMS) was carried out in Shanghai City,China.Although AMS techniques do not have the layer sensitivity or accuracy(velocity and layer thickness) of borehole techniques,the obtained shear wave velocity structure is especially useful for earthquake disaster prevention and seismic analysis.展开更多
In this paper we discuss the use of the Hilbert-Huang transform（HHT） to enhance the time-frequency analysis of microtremor measurements. HHT is a powerful algorithm that combines the process of empirical mode decomp...In this paper we discuss the use of the Hilbert-Huang transform（HHT） to enhance the time-frequency analysis of microtremor measurements. HHT is a powerful algorithm that combines the process of empirical mode decomposition（EMD） and the Hilbert transform to compose the HilbertHuang spectrum that contains the time-frequency-energy information of the recorded signals. HHT is an adaptive algorithm and does not require the signals to be linear or stationary. HHT is advantageous for analyzing microtremor data, since observed microtremors are commonly contaminated by nonstationary transient noises close to the recording instruments. This is especially true when microtremors are measured in an urban environment. In our data processing HHT was used to（1） eliminate the unwanted short-duration transient constituents from microtremor data and use only the coherent portion of the data to carry out the widely used horizontal to vertical spectral ratio（H/V） method;（2） identify and eliminate the continuous industrial noise in certain frequency band; and（3） enhance the H/V analysis by using the Hilbert-Huang spectrum（HHS）. The efficacy of this proposed approach is demonstrated by the examples of applying it to microtremor data acquired in the metropolitan Beijing area.展开更多
Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future.This may cause serious damage in the Kathmandu Valley,which sits on an ancient lake bed zone,with lacu...Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future.This may cause serious damage in the Kathmandu Valley,which sits on an ancient lake bed zone,with lacustrine sediments of more than 500 m depth.In this study,the predominant frequency of ground motion is evaluated using the Horizontal-to-Vertical (H/V) spectral ratio technique and recordings of ambient noise.The results of the H/V ratio show two peaks in about 20 percent of the locations,which are distributed mainly in and around the center and northern part of the Kathmandu Valley.The predominant frequencies vary from 0.5 Hz to 8.9 Hz in the study area,whereas the second resonance fiequency varies from 4 Hz to 6 Hz in the center and northern part of the valley.This indicates that the center and northern part of the valley have a wide range of resonance frequency due to two levels of impedance contrast- one may be from the surface layer and the other may be from the layer undemeath.These two levels of resonance indicate the importance of considering the effects of surface and lower layers during the planning and designing of infrastructures in the Kathmandu Valley.展开更多
It is widely known that the seismic response characteristics of a soil site depends heavily on several key dynamic properties of the soil stratum,such as predominant frequency and damping ratio.A widely used method fo...It is widely known that the seismic response characteristics of a soil site depends heavily on several key dynamic properties of the soil stratum,such as predominant frequency and damping ratio.A widely used method for estimating the predominant frequency of a soil site by using microtremor records,proposed by Nakamura,is investigated to determine its effectiveness in estimating the damping ratio.The authors conducted some microtremor measurements of soil sites in Hong Kong and found that Nakamura's method might also be used to estimate the damping ratio of a soil site.Damping ratio data from several typical soil sites were obtained from both Nakamura's ratio curves using the half power point method and resonant column tests.Regression analysis indicates that there is a strong correlation between the damping ratios derived from these two different approaches.展开更多
This study conducted microtremor testing along six survey lines that cross three typical earth fissures in the Datong basin to determine the dynamic response characteristics of earth fissure sites with regard to the F...This study conducted microtremor testing along six survey lines that cross three typical earth fissures in the Datong basin to determine the dynamic response characteristics of earth fissure sites with regard to the Fourier amplitude spectrum,response spectrum,and Arias intensity.The results show the following.(1)The predominant frequency of an earth fissure site is mainly affected by the thickness and the shear wave velocity of the soil layer and is minimally effected by the presence of an earth fissure.(2)Earth fissures have a pronounced amplification effect on dynamic response.Fourier amplitude,response acceleration,and Arias intensity are high near an earth fissure and decrease with an increase in distance from the earth fissure,tending toward stability at a distance of 20 m.(3)The area that is seriously affected by this amplification is within 6-8 m of an earth fissure,and the general affected area is farther out than this,to a distance of 25 m.(4)New construction should be avoided in an area affected by the amplification,and existing buildings in general and seriously affected areas need to be reinforced to increase their seismic fortification intensity.展开更多
Earthquake has left much life and property damages. The occurrence of such events necessitates the execution of plans for combating the earthquakes. One of the most important methods for combating earthquakes includes...Earthquake has left much life and property damages. The occurrence of such events necessitates the execution of plans for combating the earthquakes. One of the most important methods for combating earthquakes includes assessing dynamic characteristics of soil and site effect. One of the methods by which one can state dynamic characteristics of the soil of an area is the measurement of microtremors. Microtremors are small-scale vibrations that occur in the ground and have an amplitude range of about 0.1-1 microns. Microtremor measurement is fast, applica- ble, cost-effective. Microtremor measurements were taken at 15 stations in the Babol, north of Iran. Regarding H/V spectral ratio method, peak frequency and amplifica- tion factor were calculated for all microtremor stations. According to the analysis, the peak frequency varies from 0.67 to 8.10 Hz within the study area. Also, the authors investigated the validity of the results by comparing them with SESAME guidelines and geotechnical conditions of study area. The microtremor analysis results are consistent with SESAME guidelines and geotechnical condition of study area. The results show that the microtremor obser- vations are acceptable methods for assessing dynamic characteristics of soil and site effect in the Babol City.展开更多
The faults and fractures are known as two of the most important parameters in earthquake occurrence.During the construction in urban areas, faults and fractures may be covered in depth and thus are not visible at the ...The faults and fractures are known as two of the most important parameters in earthquake occurrence.During the construction in urban areas, faults and fractures may be covered in depth and thus are not visible at the ground surface. In this context, non-invasive geophysical prospecting methods(microtremor and geoelectrical methods) and borehole data were used to detect subsurface geological structures(hidden faults) in a suburb of Shiraz in Iran. The horizontal to vertical spectral ratio(HVSR) method was used to obtain the dynamic parameters(predominant frequency and resonance amplitude) of the soil, to detect hidden faults. The results show that the abrupt changes in the sediment thickness and predominant frequencies at a specific direction(NW-SE) can be related to the displacement of a nearly vertical fault with NW-SE trend. In addition, the electrical resistivity method using continuous resistivity profiling(CRP) and Schlumberger arrays was employed to detect a hidden fault and the results were compared with previous data. The obtained results of both arrays illustrate the presence of a nearly vertical fault with NW-SE trend in the region. Comparison of all results shows that the detected faults by both methods are consistent with each other. Therefore, it can be conclusive that combination of the two methods is a useful and reliable approach to study and detect hidden faults.展开更多
Microtremors array observation for estimating S-wave velocity structure from phase velocities of Rayleigh and Love wave on two practical sites in Tangshan area by a China-US joint group are researched.The phase veloci...Microtremors array observation for estimating S-wave velocity structure from phase velocities of Rayleigh and Love wave on two practical sites in Tangshan area by a China-US joint group are researched.The phase velocities of Rayleigh wave are estimated from vertical component records and those of Love wave are estimated from three-component records of microtremors array using modified spatial auto-correlation method.Haskell matrix method is used in calculating Rayleigh and Love wave phase velocities,and the shallow S-wave velocity structure of two practical sites are estimated by means of a hybrid approach of Genetic Algorithm and Simplex.The results are compared with the PS logging data of the two sites,showing it is feasible to estimate the shallow S-wave velocity structure of practical site from the observation of microtremor array.展开更多
In this study,the Beibu earth fissure site in the northeastern part of Weihe Basin,which contains four nearly parallel earth fissures,was studied.A long straight microtremor measuring line,containing 49 measuring poin...In this study,the Beibu earth fissure site in the northeastern part of Weihe Basin,which contains four nearly parallel earth fissures,was studied.A long straight microtremor measuring line,containing 49 measuring points across four earth fissures,was established to investigate the dynamic response of this site using Fourier spectrum,response acceleration spectrum,Arias intensity,and HVSR analyses.The main results are as follows:(1)The fundamental frequencies of 44 measuring points obtained from HVSR analysis are concentrated within 1.67 Hz-2.25 Hz,and the existence of the earth fissures has little effect on the fundamental frequency changes.(2)There is an amplification effect near a single earth fissure.The dynamic responses are large at the measuring points near the earth fissure,and the values decrease with increasing distance from the earth fissure.In areas between two adjacent earth fissures,these values decrease and are even lower than those in sites without amplification effects.(3)In this earth fissure site,the general area(or less affected area)and affected areas were delineated based on the amplification effect.In engineering applications,construction design should avoid these affected areas and existing structures should be reinforced to satisfy the seismic fortification requirements.展开更多
The Refraction Microtremor (ReMi) method is being used around the world by the geotechnical and geophysical community to determine shear-wave velocities. This is due to its faster, less expensive and accurate determin...The Refraction Microtremor (ReMi) method is being used around the world by the geotechnical and geophysical community to determine shear-wave velocities. This is due to its faster, less expensive and accurate determination of shear wave velocities, when compared to other methods used. Unlike standard crosshole and downhole techniques, ReMi does not require any drilling. It eliminates the problem of shear-wave source and quiet site that are pre-requisites for good seismic refraction surveys. In this paper we present refraction microtremors (ReMi) measurements done at sites underlain by different soil types in Egypt. The ReMi data were collected using standard refraction equipment employing 12, 24 or 48 channels. We used deep oceanographic noise and ambient noise including energy from power generators, pile drivers and traffic. The data were processed using the SeisOpt? ReMi? (? Optim, Inc.) software to reveal one-dimensional shear-wave velocity structures beneath the arrays. To access the validity of the method for the Egyptian soils, the shear-wave profiles obtained from the ReMi measurements were compared to downhole and crosshole data for different soils. Comparisons demonstrate the robustness of the ReMi technique for obtaining shear-wave velocities for different soil types in Egypt.展开更多
The present study aims mainly at investigat</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:...The present study aims mainly at investigat</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">ing</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> the site effect at the new Luxor city, Egypt, using microtremor measurements. Accordingly, 41</span></span></span></span><span><span><span><span style="font-family:""> </span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">ambient noise recording</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">s</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> were implemented covering the investigated area in a reasonable manner with spacing nearly 1 km between each measurement station. The data were analyzed using Nakamura</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">’</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">s technique. The results show that;the investigated area exhibits low fundamental frequency values ranging between 0.28 - 0.47 Hz and its corresponding amplification factor values var</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">y</span></span></span></span><span><span><span><span style="font-family:""><span style="font-family:Verdana;"> between 2.3 and 6.8. Consequently, the studied site is characterized by the presence of thick soft sediments overlying a hard bed rock. The seismic vulnerability index (</span><i><span style="font-family:Verdana;">Kg</span></i><span style="font-family:Verdana;">) was estimated, relying on the estimated fundamental frequency (</span><i><span style="font-family:Verdana;">F</span></i><sub><span style="font-family:Verdana;">0</span></sub><span style="font-family:Verdana;">) and amplification factor (</span><i><span style="font-family:Verdana;">A</span></i><sub><span style="font-family:Verdana;">0</span></sub><span style="font-family:Verdana;">), through the range between 11 and 95 across the mapped area. The obtained </span><i><span style="font-family:Verdana;">Kg</span></i><span style="font-family:Verdana;"> values were noticed to be compatible with amplification level results. The central part of the investigated area reflects the higher </span><i><span style="font-family:Verdana;">Kg</span></i><span style="font-family:Verdana;"> and </span><i><span style="font-family:Verdana;">A</span></i><sub><span style="font-family:Verdana;">0</span></sub><span style="font-family:Verdana;"> values.展开更多
The single-station microtremor method is one of the fastest,most reliable,and cheapest methods used to identify dynamic soil properties.This study utilizes 49 single-station microtremor measurements to identify the dy...The single-station microtremor method is one of the fastest,most reliable,and cheapest methods used to identify dynamic soil properties.This study utilizes 49 single-station microtremor measurements to identify the dynamic soil properties of the Hilalkent quarter of the Yakutiye district in Erzurum.Soil dominant frequency and the amplification factor were calculated by using the Nakamura horizontal/vertical spectral ratio(H/V)method.While the soil dominant frequency values varied between 0.4 Hz and 10 Hz,the soil amplification factor changed between 1 and 10.Higher H/V values were acquired with lower frequency values.The vulnerability index(K_(g))and shear strain parameters that are utilized to estimate the damage that may be caused by an earthquake were mapped.Especially in the west side of the study area,higher K_(g) values were observed.The shear strain map was created with 0.25 g,0.50 g and 0.75 g bedrock accelerations,and soil types that lost elasticity during an earthquake were identified.The average shear wave velocity for the first 30 m(V_(s30))was calculated.Finally,it was observed that the western part of the study area,which resulted in a higher period and higher H/V,higher K_(g) and lower V_(s30) values,presents a higher risk of damage during an earthquake.展开更多
It is of crucial importance to investigate the spatial structures of ancient landslides in the eastern Tibetan Plateau’s alpine canyons as they could provide valuable insights into the evolutionary history of the lan...It is of crucial importance to investigate the spatial structures of ancient landslides in the eastern Tibetan Plateau’s alpine canyons as they could provide valuable insights into the evolutionary history of the landslides and indicate the potential for future reactivation.This study examines the Deda ancient landslide,situated in the Chalong-ranbu fault zone,where creep deformation suggests a complex underground structure.By integrating remote sensing,field surveys,Audio-frequency Magnetotellurics(AMT),and Microtremor Survey Method(MSM)techniques,along with engineering geological drilling for validation,to uncover the landslide’s spatial feature s.The research indicates that a fault is developed in the upper part of the Deda ancient landslide,and the gully divides it into Deda landslide accumulation zoneⅠand Deda landslide accumulation zoneⅡin space.The distinctive geological characteristics detectable by MSM in the shallow subsurface and by AMT in deeper layers.The findings include the identification of two sliding zones in the Deda I landslide,the shallow sliding zone(DD-I-S1)depth is approximately 20 m,and the deep sliding zone(DD-I-S2)depth is 36.2-49.9 m.The sliding zone(DD-Ⅱ-S1)depth of the DedaⅡlandslide is 37.6-43.1 m.A novel MSM-based method for sliding zone identification is proposed,achieving less than 5%discrepancy in depth determination when compared with drilling data.These results provide a valuable reference for the spatial structural analysis of large-deepseated landslides in geologically complex regions like the eastern Tibetan Plateau.展开更多
The authors performed single mobile microtremor measurements at 218 sites at KMA （Kingston Metropolitan Area） with the objective of estimating the amplification effects due to the earthquake ground motion on the sur...The authors performed single mobile microtremor measurements at 218 sites at KMA （Kingston Metropolitan Area） with the objective of estimating the amplification effects due to the earthquake ground motion on the surface geology. The Fourier transform was applied to the most stationary parts of the triaxial wave motion recordings for each individual site and applied the traditional Nakamura technique, namely, the horizontal to vertical spectral ratio （H/V） to retrieve the predominant shear wave period of vibration of the soil profiles above the bedrock. The results yield predominant long periods of about 3.0-4.0 s in the port area and the waterfront, 1.0-2.0 s in the central part of Kingston, 0.3-1.0 s in Portmore and very stiff soil conditions in the surrounding area of the city. The results coincide fairly well with previous geological studies in the region, geotechnical data in boreholes, gravimetric measurements and strong motion recordings, suggesting a high degree of amplification of ground motion in the whole period range of engineering interest. Additionally, the authors obtained the liquefaction vulnerability factor Kg proposed by Nakamura based on the H/V ratio of microtremors. The results suggest that the port area, the waterfront and the Port Royal are highly susceptible to liquefaction. Finally, the authors obtained fundamental periods of vibration based on microtremor measurements on the roof and the basement of four important buildings in the KMA and indicated future lines of research employing ambient noise measurements on structures.展开更多
Recent researchers have discovered microtremor applications for evaluating the liquefaction potential. Microtremor measurement is a fast, applicable and cost-effective method with extensive applications. In the presen...Recent researchers have discovered microtremor applications for evaluating the liquefaction potential. Microtremor measurement is a fast, applicable and cost-effective method with extensive applications. In the present research the liquefaction potential has been reviewed by utilization of microtremor measurement results in Babol city. For this purpose microtremor measurements were performed at 60 measurement stations and the data were analyzed by suing Nakmaura＇s method. By using the fundamental frequency and amplification factor, the value of vulnerability index （Kg） was calculated and the liquefaction potential has been evaluated. To control the accuracy of this method, its output has been compared with the results of Seed and Idriss [1] method in 30 excavated boreholes within the study area. Also, the results obtained by the artificial neural network （ANN） were compared with microtremor measurement. Regarding the results of these three methods, it was concluded that the threshold value of liquefaction potential is Kg = 5. On the basis of the analysis performed in this research it is concluded that microtremors have the capability of assessing the liquefaction potential with desirable accuracy.展开更多
Many tunnels all over the world have been in service for several decades,which require effective inspection methods to assess their health conditions.Microtremor,as a type of ambient vibration originating from natural...Many tunnels all over the world have been in service for several decades,which require effective inspection methods to assess their health conditions.Microtremor,as a type of ambient vibration originating from natural or artificial oscillations without specific sources,has attracted more and more attentions in the recent study of the microtremor dynamic properties of concrete structures.In this study,the microtremors of the tunnel lining were simulated numerically based on the Distinct Element Method(DEM).The Power Spectra Density(PSD)of signals obtained from numerical simulations were calculated and the nature frequencies were identified using the peak-picking method.The influences of the rock-concrete joint,the rock type and the concrete type on the nature frequencies were also evaluated.The results of a comprehensive numerical analysis show that the nature frequencies lower than 100 Hz can be identified.As the bonding condition becomes worse,the nature frequencies decrease.The nature frequencies change proportionally with the normal stiffness of the rock-concrete joint.As the concrete grade decreases,the third mode of frequency also decreases gradually while the variation of the first two modes of frequencies can hardly be identified.Additionally,the field microtremor measurements of tunnel lining were also carried out to verify the numerical results.展开更多
A campaign of microtremor measurements was conducted in summer 2007 for assessing local site effects of seismic strong ground motion in metropolitan Beijing. Using the measurements from over 600 sites with approximate...A campaign of microtremor measurements was conducted in summer 2007 for assessing local site effects of seismic strong ground motion in metropolitan Beijing. Using the measurements from over 600 sites with approximately 1―2 km spacing covering the entire area inside Beijing’s 5th Beltway, we present the analysis results in the form of contours of the predominant resonant frequency, the thick- ness of the uppermost soft sediments, and the ground motion amplification factor. The microtremor- derived soft sediment thickness is generally in agreement with previous results based on much sparser borehole data, with the revealing of more short-wavelength undulations, which coincide with major geomorphological and neotectonic expressions in metropolitan Beijing. This study provides additional valuable information to the earthquake-resistant design of civil infrastructures and seismic hazard response in metropolitan Beijing. It also provides a feasible geophysical approaching to explore the 3-D structure beneath metropolitan cities.展开更多
微动HVSR(horizontal-to-vertical spectral ratio)法是一种高效、非侵入式的地球物理探测手段,广泛用于城市地质调查与工程勘探。然而,行人、车辆等瞬态干扰会导致HVSR曲线畸变。现有瞬态干扰剔除方法存在局限:STA/LTA(short-term-aver...微动HVSR(horizontal-to-vertical spectral ratio)法是一种高效、非侵入式的地球物理探测手段,广泛用于城市地质调查与工程勘探。然而,行人、车辆等瞬态干扰会导致HVSR曲线畸变。现有瞬态干扰剔除方法存在局限:STA/LTA(short-term-average over long-term-average)法易误判且调参复杂,手动剔除法效率低,频域窗剔除法仅关注峰值频率信息。为此,本文提出一种基于机器学习的微动HVSR数据干扰信号压制方法。首先通过提取曲线形态特征训练曲线剔除模型,用于剔除HVSR数据中显著偏离平均趋势的离群HVSR曲线;随后提取峰值特征训练峰值识别模型,用于识别曲线中的有效共振峰值;最后结合DBSCAN(density-based spatial clustering of applications with noise)聚类算法对识别出的有效峰值进行聚类与二次剔除,去除频率和振幅异常的峰值所在曲线。曲线剔除模型和峰值识别模在测试集上的F_(1)分数分别为0.967和0.985,均表现出优异的分类性能。实际算例结果表明,本文方法在异常曲线剔除方面相较于STA/LTA法和频域窗剔除法具有更高的稳定性与准确性,处理后的曲线频谱分布更加集中,标准差曲线更加收敛,峰值更加清晰稳定。本文方法在实现高效自动处理的同时,剔除效果与手动剔除法高度一致。展开更多
基金financially supported by the National Natural Science Foundation of China(Grant No.11372180)
文摘To study the characteristics of long period microtremor and applicability of microtremor survey,we have made microtremor observations using long period seismometers of the China's mainland from coastal cities like Shanghai and Tianjin through Beijing,Xi'an,to the far inland cities of Lanzhou and Tianshui.The observation shows that the level of power spectrum of long period microtremors reduced rapidly from the coast to the inland area.However,the energy of long period microtremors in Beijing,Xi'an,Lanzhou and Tianshui area are only approximately ten-thousandth to thousandth of that in Shanghai.Aiming at the complexity of the underground structure in the far inland,a series of underground structure models with different distributions were assessed using three-dimensional,dynamic finite element method(FEM) analyses.The results were used to evaluate microtremor survey methods and their limitations with regard to aggregate variability and thickness determinations.Multiple-wave reflections between layers with the change of underground structure distribution occurred,which have significant effect on the performance of the different field approaches.Information over a broad spectrum from which velocity-depth profiles were produced via inversion approaches.Neither the thickness nor the shear waveVelocity V of the subsurface layer inversion results appeared over a large evaluation with increasing slope angle.In particular,when the angle of the model reached 45°,it could not be accurately evaluated using one-dimensional inversion methods.Finally,the array microtremor survey(AMS) was carried out in Shanghai City,China.Although AMS techniques do not have the layer sensitivity or accuracy(velocity and layer thickness) of borehole techniques,the obtained shear wave velocity structure is especially useful for earthquake disaster prevention and seismic analysis.
基金supported by the Ministry of Science and Technology of China (No. 2006DFA21650)the Institute of Earthquake Science, China Earthquake Administration (No. 0207690229)
文摘In this paper we discuss the use of the Hilbert-Huang transform（HHT） to enhance the time-frequency analysis of microtremor measurements. HHT is a powerful algorithm that combines the process of empirical mode decomposition（EMD） and the Hilbert transform to compose the HilbertHuang spectrum that contains the time-frequency-energy information of the recorded signals. HHT is an adaptive algorithm and does not require the signals to be linear or stationary. HHT is advantageous for analyzing microtremor data, since observed microtremors are commonly contaminated by nonstationary transient noises close to the recording instruments. This is especially true when microtremors are measured in an urban environment. In our data processing HHT was used to（1） eliminate the unwanted short-duration transient constituents from microtremor data and use only the coherent portion of the data to carry out the widely used horizontal to vertical spectral ratio（H/V） method;（2） identify and eliminate the continuous industrial noise in certain frequency band; and（3） enhance the H/V analysis by using the Hilbert-Huang spectrum（HHS）. The efficacy of this proposed approach is demonstrated by the examples of applying it to microtremor data acquired in the metropolitan Beijing area.
文摘Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future.This may cause serious damage in the Kathmandu Valley,which sits on an ancient lake bed zone,with lacustrine sediments of more than 500 m depth.In this study,the predominant frequency of ground motion is evaluated using the Horizontal-to-Vertical (H/V) spectral ratio technique and recordings of ambient noise.The results of the H/V ratio show two peaks in about 20 percent of the locations,which are distributed mainly in and around the center and northern part of the Kathmandu Valley.The predominant frequencies vary from 0.5 Hz to 8.9 Hz in the study area,whereas the second resonance fiequency varies from 4 Hz to 6 Hz in the center and northern part of the valley.This indicates that the center and northern part of the valley have a wide range of resonance frequency due to two levels of impedance contrast- one may be from the surface layer and the other may be from the layer undemeath.These two levels of resonance indicate the importance of considering the effects of surface and lower layers during the planning and designing of infrastructures in the Kathmandu Valley.
基金Research project (PolyU 5076/97E),of the Department of Civil and Structural Engineering,The Hong Kong Polytechnic University
文摘It is widely known that the seismic response characteristics of a soil site depends heavily on several key dynamic properties of the soil stratum,such as predominant frequency and damping ratio.A widely used method for estimating the predominant frequency of a soil site by using microtremor records,proposed by Nakamura,is investigated to determine its effectiveness in estimating the damping ratio.The authors conducted some microtremor measurements of soil sites in Hong Kong and found that Nakamura's method might also be used to estimate the damping ratio of a soil site.Damping ratio data from several typical soil sites were obtained from both Nakamura's ratio curves using the half power point method and resonant column tests.Regression analysis indicates that there is a strong correlation between the damping ratios derived from these two different approaches.
基金National Natural Science Foundation of China under Grant No.41772275the Fundamental Research Funds for the Central Universities under Grant No.300102268203。
文摘This study conducted microtremor testing along six survey lines that cross three typical earth fissures in the Datong basin to determine the dynamic response characteristics of earth fissure sites with regard to the Fourier amplitude spectrum,response spectrum,and Arias intensity.The results show the following.(1)The predominant frequency of an earth fissure site is mainly affected by the thickness and the shear wave velocity of the soil layer and is minimally effected by the presence of an earth fissure.(2)Earth fissures have a pronounced amplification effect on dynamic response.Fourier amplitude,response acceleration,and Arias intensity are high near an earth fissure and decrease with an increase in distance from the earth fissure,tending toward stability at a distance of 20 m.(3)The area that is seriously affected by this amplification is within 6-8 m of an earth fissure,and the general affected area is farther out than this,to a distance of 25 m.(4)New construction should be avoided in an area affected by the amplification,and existing buildings in general and seriously affected areas need to be reinforced to increase their seismic fortification intensity.
文摘Earthquake has left much life and property damages. The occurrence of such events necessitates the execution of plans for combating the earthquakes. One of the most important methods for combating earthquakes includes assessing dynamic characteristics of soil and site effect. One of the methods by which one can state dynamic characteristics of the soil of an area is the measurement of microtremors. Microtremors are small-scale vibrations that occur in the ground and have an amplitude range of about 0.1-1 microns. Microtremor measurement is fast, applica- ble, cost-effective. Microtremor measurements were taken at 15 stations in the Babol, north of Iran. Regarding H/V spectral ratio method, peak frequency and amplifica- tion factor were calculated for all microtremor stations. According to the analysis, the peak frequency varies from 0.67 to 8.10 Hz within the study area. Also, the authors investigated the validity of the results by comparing them with SESAME guidelines and geotechnical conditions of study area. The microtremor analysis results are consistent with SESAME guidelines and geotechnical condition of study area. The results show that the microtremor obser- vations are acceptable methods for assessing dynamic characteristics of soil and site effect in the Babol City.
基金supported by the Center of Excellence for Environmental Geohazards and the Research Council of Shiraz University
文摘The faults and fractures are known as two of the most important parameters in earthquake occurrence.During the construction in urban areas, faults and fractures may be covered in depth and thus are not visible at the ground surface. In this context, non-invasive geophysical prospecting methods(microtremor and geoelectrical methods) and borehole data were used to detect subsurface geological structures(hidden faults) in a suburb of Shiraz in Iran. The horizontal to vertical spectral ratio(HVSR) method was used to obtain the dynamic parameters(predominant frequency and resonance amplitude) of the soil, to detect hidden faults. The results show that the abrupt changes in the sediment thickness and predominant frequencies at a specific direction(NW-SE) can be related to the displacement of a nearly vertical fault with NW-SE trend. In addition, the electrical resistivity method using continuous resistivity profiling(CRP) and Schlumberger arrays was employed to detect a hidden fault and the results were compared with previous data. The obtained results of both arrays illustrate the presence of a nearly vertical fault with NW-SE trend in the region. Comparison of all results shows that the detected faults by both methods are consistent with each other. Therefore, it can be conclusive that combination of the two methods is a useful and reliable approach to study and detect hidden faults.
基金Supported by National Natural Science Foundation of China(No.50378032and No.50538030)Associated Foundation of Earthquake Science(No.201009)Foundation of Heilongjiang Institute of Science and Technology(No.04-15).
文摘Microtremors array observation for estimating S-wave velocity structure from phase velocities of Rayleigh and Love wave on two practical sites in Tangshan area by a China-US joint group are researched.The phase velocities of Rayleigh wave are estimated from vertical component records and those of Love wave are estimated from three-component records of microtremors array using modified spatial auto-correlation method.Haskell matrix method is used in calculating Rayleigh and Love wave phase velocities,and the shallow S-wave velocity structure of two practical sites are estimated by means of a hybrid approach of Genetic Algorithm and Simplex.The results are compared with the PS logging data of the two sites,showing it is feasible to estimate the shallow S-wave velocity structure of practical site from the observation of microtremor array.
基金National Natural Science Foundation of China under Grant No.41772275the Fundamental Research Funds for the Central Universities under Grant No.CHD300102268203。
文摘In this study,the Beibu earth fissure site in the northeastern part of Weihe Basin,which contains four nearly parallel earth fissures,was studied.A long straight microtremor measuring line,containing 49 measuring points across four earth fissures,was established to investigate the dynamic response of this site using Fourier spectrum,response acceleration spectrum,Arias intensity,and HVSR analyses.The main results are as follows:(1)The fundamental frequencies of 44 measuring points obtained from HVSR analysis are concentrated within 1.67 Hz-2.25 Hz,and the existence of the earth fissures has little effect on the fundamental frequency changes.(2)There is an amplification effect near a single earth fissure.The dynamic responses are large at the measuring points near the earth fissure,and the values decrease with increasing distance from the earth fissure.In areas between two adjacent earth fissures,these values decrease and are even lower than those in sites without amplification effects.(3)In this earth fissure site,the general area(or less affected area)and affected areas were delineated based on the amplification effect.In engineering applications,construction design should avoid these affected areas and existing structures should be reinforced to satisfy the seismic fortification requirements.
文摘The Refraction Microtremor (ReMi) method is being used around the world by the geotechnical and geophysical community to determine shear-wave velocities. This is due to its faster, less expensive and accurate determination of shear wave velocities, when compared to other methods used. Unlike standard crosshole and downhole techniques, ReMi does not require any drilling. It eliminates the problem of shear-wave source and quiet site that are pre-requisites for good seismic refraction surveys. In this paper we present refraction microtremors (ReMi) measurements done at sites underlain by different soil types in Egypt. The ReMi data were collected using standard refraction equipment employing 12, 24 or 48 channels. We used deep oceanographic noise and ambient noise including energy from power generators, pile drivers and traffic. The data were processed using the SeisOpt? ReMi? (? Optim, Inc.) software to reveal one-dimensional shear-wave velocity structures beneath the arrays. To access the validity of the method for the Egyptian soils, the shear-wave profiles obtained from the ReMi measurements were compared to downhole and crosshole data for different soils. Comparisons demonstrate the robustness of the ReMi technique for obtaining shear-wave velocities for different soil types in Egypt.
文摘The present study aims mainly at investigat</span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">ing</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> the site effect at the new Luxor city, Egypt, using microtremor measurements. Accordingly, 41</span></span></span></span><span><span><span><span style="font-family:""> </span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">ambient noise recording</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">s</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"> were implemented covering the investigated area in a reasonable manner with spacing nearly 1 km between each measurement station. The data were analyzed using Nakamura</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">’</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">s technique. The results show that;the investigated area exhibits low fundamental frequency values ranging between 0.28 - 0.47 Hz and its corresponding amplification factor values var</span></span></span></span><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;"><span style="font-family:Verdana;">y</span></span></span></span><span><span><span><span style="font-family:""><span style="font-family:Verdana;"> between 2.3 and 6.8. Consequently, the studied site is characterized by the presence of thick soft sediments overlying a hard bed rock. The seismic vulnerability index (</span><i><span style="font-family:Verdana;">Kg</span></i><span style="font-family:Verdana;">) was estimated, relying on the estimated fundamental frequency (</span><i><span style="font-family:Verdana;">F</span></i><sub><span style="font-family:Verdana;">0</span></sub><span style="font-family:Verdana;">) and amplification factor (</span><i><span style="font-family:Verdana;">A</span></i><sub><span style="font-family:Verdana;">0</span></sub><span style="font-family:Verdana;">), through the range between 11 and 95 across the mapped area. The obtained </span><i><span style="font-family:Verdana;">Kg</span></i><span style="font-family:Verdana;"> values were noticed to be compatible with amplification level results. The central part of the investigated area reflects the higher </span><i><span style="font-family:Verdana;">Kg</span></i><span style="font-family:Verdana;"> and </span><i><span style="font-family:Verdana;">A</span></i><sub><span style="font-family:Verdana;">0</span></sub><span style="font-family:Verdana;"> values.
文摘The single-station microtremor method is one of the fastest,most reliable,and cheapest methods used to identify dynamic soil properties.This study utilizes 49 single-station microtremor measurements to identify the dynamic soil properties of the Hilalkent quarter of the Yakutiye district in Erzurum.Soil dominant frequency and the amplification factor were calculated by using the Nakamura horizontal/vertical spectral ratio(H/V)method.While the soil dominant frequency values varied between 0.4 Hz and 10 Hz,the soil amplification factor changed between 1 and 10.Higher H/V values were acquired with lower frequency values.The vulnerability index(K_(g))and shear strain parameters that are utilized to estimate the damage that may be caused by an earthquake were mapped.Especially in the west side of the study area,higher K_(g) values were observed.The shear strain map was created with 0.25 g,0.50 g and 0.75 g bedrock accelerations,and soil types that lost elasticity during an earthquake were identified.The average shear wave velocity for the first 30 m(V_(s30))was calculated.Finally,it was observed that the western part of the study area,which resulted in a higher period and higher H/V,higher K_(g) and lower V_(s30) values,presents a higher risk of damage during an earthquake.
基金supported by the National Natural Science Foundation of China(42372339)the China Geological Survey Project(DD20221816,DD20190319)。
文摘It is of crucial importance to investigate the spatial structures of ancient landslides in the eastern Tibetan Plateau’s alpine canyons as they could provide valuable insights into the evolutionary history of the landslides and indicate the potential for future reactivation.This study examines the Deda ancient landslide,situated in the Chalong-ranbu fault zone,where creep deformation suggests a complex underground structure.By integrating remote sensing,field surveys,Audio-frequency Magnetotellurics(AMT),and Microtremor Survey Method(MSM)techniques,along with engineering geological drilling for validation,to uncover the landslide’s spatial feature s.The research indicates that a fault is developed in the upper part of the Deda ancient landslide,and the gully divides it into Deda landslide accumulation zoneⅠand Deda landslide accumulation zoneⅡin space.The distinctive geological characteristics detectable by MSM in the shallow subsurface and by AMT in deeper layers.The findings include the identification of two sliding zones in the Deda I landslide,the shallow sliding zone(DD-I-S1)depth is approximately 20 m,and the deep sliding zone(DD-I-S2)depth is 36.2-49.9 m.The sliding zone(DD-Ⅱ-S1)depth of the DedaⅡlandslide is 37.6-43.1 m.A novel MSM-based method for sliding zone identification is proposed,achieving less than 5%discrepancy in depth determination when compared with drilling data.These results provide a valuable reference for the spatial structural analysis of large-deepseated landslides in geologically complex regions like the eastern Tibetan Plateau.
文摘The authors performed single mobile microtremor measurements at 218 sites at KMA （Kingston Metropolitan Area） with the objective of estimating the amplification effects due to the earthquake ground motion on the surface geology. The Fourier transform was applied to the most stationary parts of the triaxial wave motion recordings for each individual site and applied the traditional Nakamura technique, namely, the horizontal to vertical spectral ratio （H/V） to retrieve the predominant shear wave period of vibration of the soil profiles above the bedrock. The results yield predominant long periods of about 3.0-4.0 s in the port area and the waterfront, 1.0-2.0 s in the central part of Kingston, 0.3-1.0 s in Portmore and very stiff soil conditions in the surrounding area of the city. The results coincide fairly well with previous geological studies in the region, geotechnical data in boreholes, gravimetric measurements and strong motion recordings, suggesting a high degree of amplification of ground motion in the whole period range of engineering interest. Additionally, the authors obtained the liquefaction vulnerability factor Kg proposed by Nakamura based on the H/V ratio of microtremors. The results suggest that the port area, the waterfront and the Port Royal are highly susceptible to liquefaction. Finally, the authors obtained fundamental periods of vibration based on microtremor measurements on the roof and the basement of four important buildings in the KMA and indicated future lines of research employing ambient noise measurements on structures.
文摘Recent researchers have discovered microtremor applications for evaluating the liquefaction potential. Microtremor measurement is a fast, applicable and cost-effective method with extensive applications. In the present research the liquefaction potential has been reviewed by utilization of microtremor measurement results in Babol city. For this purpose microtremor measurements were performed at 60 measurement stations and the data were analyzed by suing Nakmaura＇s method. By using the fundamental frequency and amplification factor, the value of vulnerability index （Kg） was calculated and the liquefaction potential has been evaluated. To control the accuracy of this method, its output has been compared with the results of Seed and Idriss [1] method in 30 excavated boreholes within the study area. Also, the results obtained by the artificial neural network （ANN） were compared with microtremor measurement. Regarding the results of these three methods, it was concluded that the threshold value of liquefaction potential is Kg = 5. On the basis of the analysis performed in this research it is concluded that microtremors have the capability of assessing the liquefaction potential with desirable accuracy.
基金Development of this study is supported by the Civil Engineering Division,Nagasaki Prefectural Government.
文摘Many tunnels all over the world have been in service for several decades,which require effective inspection methods to assess their health conditions.Microtremor,as a type of ambient vibration originating from natural or artificial oscillations without specific sources,has attracted more and more attentions in the recent study of the microtremor dynamic properties of concrete structures.In this study,the microtremors of the tunnel lining were simulated numerically based on the Distinct Element Method(DEM).The Power Spectra Density(PSD)of signals obtained from numerical simulations were calculated and the nature frequencies were identified using the peak-picking method.The influences of the rock-concrete joint,the rock type and the concrete type on the nature frequencies were also evaluated.The results of a comprehensive numerical analysis show that the nature frequencies lower than 100 Hz can be identified.As the bonding condition becomes worse,the nature frequencies decrease.The nature frequencies change proportionally with the normal stiffness of the rock-concrete joint.As the concrete grade decreases,the third mode of frequency also decreases gradually while the variation of the first two modes of frequencies can hardly be identified.Additionally,the field microtremor measurements of tunnel lining were also carried out to verify the numerical results.
基金Supported by the Ministry of Science and Technology of China (Grant No. 2006DFA21650)Special Scientific Research Projects on Earthquake (Grant No. 0207690229 and 200708008)
文摘A campaign of microtremor measurements was conducted in summer 2007 for assessing local site effects of seismic strong ground motion in metropolitan Beijing. Using the measurements from over 600 sites with approximately 1―2 km spacing covering the entire area inside Beijing’s 5th Beltway, we present the analysis results in the form of contours of the predominant resonant frequency, the thick- ness of the uppermost soft sediments, and the ground motion amplification factor. The microtremor- derived soft sediment thickness is generally in agreement with previous results based on much sparser borehole data, with the revealing of more short-wavelength undulations, which coincide with major geomorphological and neotectonic expressions in metropolitan Beijing. This study provides additional valuable information to the earthquake-resistant design of civil infrastructures and seismic hazard response in metropolitan Beijing. It also provides a feasible geophysical approaching to explore the 3-D structure beneath metropolitan cities.
文摘微动HVSR(horizontal-to-vertical spectral ratio)法是一种高效、非侵入式的地球物理探测手段,广泛用于城市地质调查与工程勘探。然而,行人、车辆等瞬态干扰会导致HVSR曲线畸变。现有瞬态干扰剔除方法存在局限:STA/LTA(short-term-average over long-term-average)法易误判且调参复杂,手动剔除法效率低,频域窗剔除法仅关注峰值频率信息。为此,本文提出一种基于机器学习的微动HVSR数据干扰信号压制方法。首先通过提取曲线形态特征训练曲线剔除模型,用于剔除HVSR数据中显著偏离平均趋势的离群HVSR曲线;随后提取峰值特征训练峰值识别模型,用于识别曲线中的有效共振峰值;最后结合DBSCAN(density-based spatial clustering of applications with noise)聚类算法对识别出的有效峰值进行聚类与二次剔除,去除频率和振幅异常的峰值所在曲线。曲线剔除模型和峰值识别模在测试集上的F_(1)分数分别为0.967和0.985,均表现出优异的分类性能。实际算例结果表明,本文方法在异常曲线剔除方面相较于STA/LTA法和频域窗剔除法具有更高的稳定性与准确性,处理后的曲线频谱分布更加集中,标准差曲线更加收敛,峰值更加清晰稳定。本文方法在实现高效自动处理的同时,剔除效果与手动剔除法高度一致。
