The Loess Plateau is an earthquake prone region of China, where the effects of loess deposit on ground motion were discovered during the 2008 Wenchuan earthquake(Ms8.0) and the 2013 Minxian-Zhangxian earthquake(Ms6.6)...The Loess Plateau is an earthquake prone region of China, where the effects of loess deposit on ground motion were discovered during the 2008 Wenchuan earthquake(Ms8.0) and the 2013 Minxian-Zhangxian earthquake(Ms6.6). The field investigations, observations, and analyses indicated that large number of casualties and tremendous economic losses were caused not only by collapse and damage of houses with poor seismic performance, landslides, but also amplification effects of site conditions, topography and thickness of loess deposit, on ground motion. In this paper, we chose Dazhai Village and Majiagou Village as the typical loess site affected by the two earthquakes for intensity evaluation, borehole exploration, temporary strong motion array, micro tremor survey, and numerical analysis. The aim is to explore the relations between amplification factors and site conditions in terms of topography and thickness of loess deposit. We also developed site amplification factors of ground motion for engineering design consideration at loess sites. The results showed that the amplification effects are more predominant with increase in thickness of loess deposit and slope height. The amplification mayincrease seismic intensity by 1 degree, PGA and predominant period by 2 times, respectively.展开更多
The available models for eff ective periods of site and structure are reviewed in context of frequency tuning in the inelastic seismic response of soil-structure system. The eff ect of seismic intensity and ductility ...The available models for eff ective periods of site and structure are reviewed in context of frequency tuning in the inelastic seismic response of soil-structure system. The eff ect of seismic intensity and ductility demand, on the eff ective periods, is investigated, and inelastic site amplifi cation is shown to be strongly correlated to the normalized eff ective period. Two non-dimensional parameters, analogous to the conventional site amplifi cation factors in codes, are defi ned to quantify the inelastic site amplifi cation. It is shown that the inelastic site amplifi cation factor (i.e. ratio of constant ductility spectral ordinates at soil site to those at rock outcrop) is able to represent the site eff ects more clearly, as compared to the inelastic site amplifi cation ratio (i.e. ratio of inelastic spectral ordinates at soil site to the corresponding elastic spectral ordinates at rock outcrop). Further, the peak in the amplifi cation factor corresponding to the eff ective site period diminishes rapidly with increasing ductility demand.展开更多
Topography can have signifi cant eff ects on seismic ground response during an earthquake because topographic irregularities cause considerable diff erences between the seismic waves emitted by the source and the wave...Topography can have signifi cant eff ects on seismic ground response during an earthquake because topographic irregularities cause considerable diff erences between the seismic waves emitted by the source and the waves reaching the ground surface. When a seismic motion happens in a topographically irregular area, seismic waves are trapped and refl ected between the topographic features. Therefore, the interaction between topographies can amplify seismic ground response. In order to reveal how interaction between topographies infl uences seismic response, several numerical fi nite element studies have been performed by using the ABAQUS program. The results show that topographic features a greater distance between the seismic source and the site would cause greater seismic motion amplifi cation and is perceptible for the hills far away from the source and the ridges. Also, site acceleration response is impacted by surrounding topography further than site velocity and displacement response.展开更多
The role played by the diffraction field on the problem of seismic site effects is studied. For that purpose we solve and analyze simple scattering problems under P and SV in-plane wave assumptions, using two well kno...The role played by the diffraction field on the problem of seismic site effects is studied. For that purpose we solve and analyze simple scattering problems under P and SV in-plane wave assumptions, using two well known direct boundary-element-based numerical methods. After establishing the difference between scattered and diffracted motions, and introducing the concept of artificious and physically based incoming fields, we obtain the amplitude of the Fourier spectra for the diffracted part of the response： this is achieved after establishing the connection between the spatial distribution of the transfer function over the studied simple topographies and the diffracted field. From the numerical simulations it is observed that this diffracted part of the response is responsible for the amplification of the surface ground motions due to the geometric effect. Furthermore, it is also found that the diffraction field sets in a fingerprint of the topographic effect in the total ground motions. These conclusions are further supported by observations in the time-domain in terms of snapshots of the propagation patterns over the complete computational model. In this sense the geometric singularities are clearly identified as sources of diffraction and for the considered range of dimensionless frequencies it is evident that larger amplifications are obtained for the geometries containing a larger number of diffraction sources thus resulting in a stronger topographic effect. The need for closed-form solutions of canonical problems to construct a robust analysis method based on the diffraction field is identified.展开更多
To highlight the importance of small earthquakes in seismic hazard, a study of the 11 May 2016, Mw 4.9, Tesistan, Mexico earthquake is presented. Due to the close proximity of the event to the city, accelerations were...To highlight the importance of small earthquakes in seismic hazard, a study of the 11 May 2016, Mw 4.9, Tesistan, Mexico earthquake is presented. Due to the close proximity of the event to the city, accelerations were considerably higher than those caused by historical severe earthquakes (6.0 < Mw < 8.2). This paper addresses two objectives related to the Tesistan event: the fi rst is to estimate the focal mechanism solution in order to place the event in the context of the tectonic environment of this area. The second is focused on a vulnerability evaluation of buildings that suff ered resonance. Several building′s typologies with variations in construction system and height are assessed in terms of resonance with the structural and soil periods. The results show that around the Zapopan station, strong damage is expected in intermediate to high-rise buildings (12-30 m) with moment resistant frame systems and in reinforced concrete shear walls. Masonry structures around this station may not present resonance. In contrast, in the surroundings of the Guadalajara station, all intermediate height buildings from 9 to 21 m may present resonance.展开更多
A seismic hazard was assessed related to site effects at Abbadia San Salvatore, central Italy, on the Mt. Amiata slopes, an ancient volcanic area characterized by residual soils(thick layers of loose to dense sands or...A seismic hazard was assessed related to site effects at Abbadia San Salvatore, central Italy, on the Mt. Amiata slopes, an ancient volcanic area characterized by residual soils(thick layers of loose to dense sands originated from weathering of the trachydacitic lavas). The seismic ground amplification and soil liquefaction related to these layers were recognized as the major seismic hazards for the area.Geological, geophysical, and geotechnical surveys were carried out on the volcanic rocks. The Horizontal-to-Vertical Spectral Ratio(HVSR) analysis of 252 noise measurements and 29 shear-wave velocity models of the subsoil allowed a seismic microzonation of the studied area, distinguished by thick weathered volcanic sands and shear-wave impedance contrast with respect to the seismic bedrock(volcanic bedrock). The differentiation of classified zones allowed recognition of areas characterized by residual(almost undisturbed) soils from those with soils probably affected by flowing water. The analysis of hazards revealed that peak acceleration by seismic amplification of ground motion exceeded the value set by the national rules(0.175 g) in a restricted area of the zone characterized by the most perturbed soils(Zone D);the potential occurrence of soil liquefaction was also greater in this zone. Finally, the study showed potential high hazards due to site effects of the volcanic mountainous area characterized by residual soils as opposed to an alluvial plain formed by volcanic debris where these effects have generally been more recognized.展开更多
This study presents a systematic analysis of double-frequency(DF) microseisms recorded on the unconsolidated sediments in the eastern and southeastern coasts of United States. For all recordings, the site effect param...This study presents a systematic analysis of double-frequency(DF) microseisms recorded on the unconsolidated sediments in the eastern and southeastern coasts of United States. For all recordings, the site effect parameters(predominant frequency(f_(0)), amplification factor and unconsolidated sediment thickness(UST)) are obtained by Nakamura method and the DF spectra are classified into five groups in terms of the DF peak patterns and the recording locations relative to the coastline. The frequencies and energy levels of the DF peaks in horizontal direction and the amplification factors are associated with the UST which is resulted from seismic site effect. By polarization analysis, the primary vibration directions of the DF peaks are identified and presented as great circles passing through the recording stations intersecting mainly along the continental slope. Correlation analyses of time histories of the DF energy and the ocean wave climate observed at buoys show that the low(<0.2 Hz) and high(>0.2 Hz) frequency DF microseisms are generated in the deep ocean and the continental shelf respectively. It is concluded that the continental slope plays a significant role in the generation of DF microseisms as it causes reflection of waves from the open ocean, initiating standing waves.展开更多
基金financially supported by National Natural Science Foundation of China (No.51478444 & No.41472297)
文摘The Loess Plateau is an earthquake prone region of China, where the effects of loess deposit on ground motion were discovered during the 2008 Wenchuan earthquake(Ms8.0) and the 2013 Minxian-Zhangxian earthquake(Ms6.6). The field investigations, observations, and analyses indicated that large number of casualties and tremendous economic losses were caused not only by collapse and damage of houses with poor seismic performance, landslides, but also amplification effects of site conditions, topography and thickness of loess deposit, on ground motion. In this paper, we chose Dazhai Village and Majiagou Village as the typical loess site affected by the two earthquakes for intensity evaluation, borehole exploration, temporary strong motion array, micro tremor survey, and numerical analysis. The aim is to explore the relations between amplification factors and site conditions in terms of topography and thickness of loess deposit. We also developed site amplification factors of ground motion for engineering design consideration at loess sites. The results showed that the amplification effects are more predominant with increase in thickness of loess deposit and slope height. The amplification mayincrease seismic intensity by 1 degree, PGA and predominant period by 2 times, respectively.
文摘The available models for eff ective periods of site and structure are reviewed in context of frequency tuning in the inelastic seismic response of soil-structure system. The eff ect of seismic intensity and ductility demand, on the eff ective periods, is investigated, and inelastic site amplifi cation is shown to be strongly correlated to the normalized eff ective period. Two non-dimensional parameters, analogous to the conventional site amplifi cation factors in codes, are defi ned to quantify the inelastic site amplifi cation. It is shown that the inelastic site amplifi cation factor (i.e. ratio of constant ductility spectral ordinates at soil site to those at rock outcrop) is able to represent the site eff ects more clearly, as compared to the inelastic site amplifi cation ratio (i.e. ratio of inelastic spectral ordinates at soil site to the corresponding elastic spectral ordinates at rock outcrop). Further, the peak in the amplifi cation factor corresponding to the eff ective site period diminishes rapidly with increasing ductility demand.
文摘Topography can have signifi cant eff ects on seismic ground response during an earthquake because topographic irregularities cause considerable diff erences between the seismic waves emitted by the source and the waves reaching the ground surface. When a seismic motion happens in a topographically irregular area, seismic waves are trapped and refl ected between the topographic features. Therefore, the interaction between topographies can amplify seismic ground response. In order to reveal how interaction between topographies infl uences seismic response, several numerical fi nite element studies have been performed by using the ABAQUS program. The results show that topographic features a greater distance between the seismic source and the site would cause greater seismic motion amplifi cation and is perceptible for the hills far away from the source and the ridges. Also, site acceleration response is impacted by surrounding topography further than site velocity and displacement response.
基金conducted with financial support from ‘‘Investigaciones Geote'cnicas Solingral S.A’’, Departamen to Administrativo de Ciencia, Tecnología e Innovación,COLCIENCIAS and from Universidad EAFIT through research Grant 1216-502-27173 under contract 509-2011.
文摘The role played by the diffraction field on the problem of seismic site effects is studied. For that purpose we solve and analyze simple scattering problems under P and SV in-plane wave assumptions, using two well known direct boundary-element-based numerical methods. After establishing the difference between scattered and diffracted motions, and introducing the concept of artificious and physically based incoming fields, we obtain the amplitude of the Fourier spectra for the diffracted part of the response： this is achieved after establishing the connection between the spatial distribution of the transfer function over the studied simple topographies and the diffracted field. From the numerical simulations it is observed that this diffracted part of the response is responsible for the amplification of the surface ground motions due to the geometric effect. Furthermore, it is also found that the diffraction field sets in a fingerprint of the topographic effect in the total ground motions. These conclusions are further supported by observations in the time-domain in terms of snapshots of the propagation patterns over the complete computational model. In this sense the geometric singularities are clearly identified as sources of diffraction and for the considered range of dimensionless frequencies it is evident that larger amplifications are obtained for the geometries containing a larger number of diffraction sources thus resulting in a stronger topographic effect. The need for closed-form solutions of canonical problems to construct a robust analysis method based on the diffraction field is identified.
文摘To highlight the importance of small earthquakes in seismic hazard, a study of the 11 May 2016, Mw 4.9, Tesistan, Mexico earthquake is presented. Due to the close proximity of the event to the city, accelerations were considerably higher than those caused by historical severe earthquakes (6.0 < Mw < 8.2). This paper addresses two objectives related to the Tesistan event: the fi rst is to estimate the focal mechanism solution in order to place the event in the context of the tectonic environment of this area. The second is focused on a vulnerability evaluation of buildings that suff ered resonance. Several building′s typologies with variations in construction system and height are assessed in terms of resonance with the structural and soil periods. The results show that around the Zapopan station, strong damage is expected in intermediate to high-rise buildings (12-30 m) with moment resistant frame systems and in reinforced concrete shear walls. Masonry structures around this station may not present resonance. In contrast, in the surroundings of the Guadalajara station, all intermediate height buildings from 9 to 21 m may present resonance.
基金funded by the Regione Toscana (Regional Government of Tuscany), on behalf of the Dipartimento di Protezione Civile (National Civil Protection Department)the Unione dei Comuni Amiata Val d’Orcia (Union of the Municipalities of Amiata Val d’Orcia)
文摘A seismic hazard was assessed related to site effects at Abbadia San Salvatore, central Italy, on the Mt. Amiata slopes, an ancient volcanic area characterized by residual soils(thick layers of loose to dense sands originated from weathering of the trachydacitic lavas). The seismic ground amplification and soil liquefaction related to these layers were recognized as the major seismic hazards for the area.Geological, geophysical, and geotechnical surveys were carried out on the volcanic rocks. The Horizontal-to-Vertical Spectral Ratio(HVSR) analysis of 252 noise measurements and 29 shear-wave velocity models of the subsoil allowed a seismic microzonation of the studied area, distinguished by thick weathered volcanic sands and shear-wave impedance contrast with respect to the seismic bedrock(volcanic bedrock). The differentiation of classified zones allowed recognition of areas characterized by residual(almost undisturbed) soils from those with soils probably affected by flowing water. The analysis of hazards revealed that peak acceleration by seismic amplification of ground motion exceeded the value set by the national rules(0.175 g) in a restricted area of the zone characterized by the most perturbed soils(Zone D);the potential occurrence of soil liquefaction was also greater in this zone. Finally, the study showed potential high hazards due to site effects of the volcanic mountainous area characterized by residual soils as opposed to an alluvial plain formed by volcanic debris where these effects have generally been more recognized.
文摘This study presents a systematic analysis of double-frequency(DF) microseisms recorded on the unconsolidated sediments in the eastern and southeastern coasts of United States. For all recordings, the site effect parameters(predominant frequency(f_(0)), amplification factor and unconsolidated sediment thickness(UST)) are obtained by Nakamura method and the DF spectra are classified into five groups in terms of the DF peak patterns and the recording locations relative to the coastline. The frequencies and energy levels of the DF peaks in horizontal direction and the amplification factors are associated with the UST which is resulted from seismic site effect. By polarization analysis, the primary vibration directions of the DF peaks are identified and presented as great circles passing through the recording stations intersecting mainly along the continental slope. Correlation analyses of time histories of the DF energy and the ocean wave climate observed at buoys show that the low(<0.2 Hz) and high(>0.2 Hz) frequency DF microseisms are generated in the deep ocean and the continental shelf respectively. It is concluded that the continental slope plays a significant role in the generation of DF microseisms as it causes reflection of waves from the open ocean, initiating standing waves.
