Soil Dynamics and Earthquake Engineering

Published by Elsevier
Online ISSN: 0267-7261
Publications
Article
Safety against earthquake hazards presents two aspects: structural safety against potentially destructive dynamic forces and site safety related to geotechnical phenomena, such as amplification, landsliding and soil liquefaction. The correct evaluation of seismic hazard is, therefore, highly affected by risk factors due to geological nature and geotechnical properties of soils. In response to these new developments, several attempts have been made to identify and appraise geotechnical hazards and to represent them in the form of zoning maps, in which locations or zones with different levels of hazard potential are identified. The geotechnical zonation of the subsoil of the city of Catania (Italy) suggests a high vulnerability of the physical environment added to site amplification of the ground motion phenomena. The ground response analysis at the surface, in terms of time history and response spectra, has been obtained by some 1D equivalent linear models and by a 2D linear model, using a design scenario earthquake as input at the conventional bedrock. In particular, the study has regarded the evaluation of site effects in correspondence of the database of about 1200 boreholes and water-wells available in the data-bank of the Catania area. According to the response spectra obtained through the application of the 1D and 2D models, the city of Catania has been divided into some zones with different peak ground acceleration at the surface, to which corresponds a different value of the Seismic Geotechnical Hazard. A seismic microzoning map of the urban area of the city of Catania has been obtained. The map represents an important tool for the seismic improvement of the buildings, indispensable for the mitigation of the seismic risk.
 
Article
In this study, two different earthquake input models are introduced, i.e. massless foundation model and viscous-spring boundary input model considering radiation damping. Linear elastic and nonlinear contraction joint opening analyses of the 210 m high Dagangshan arch dam under construction in China are performed using the two different earthquake input models. First, the responses of the three-dimensional (3-D) canyon without the dam are analyzed, respectively, with massless-truncated foundation and with viscous-spring boundary; second, linear and nonlinear analyses of the dam–foundation system are performed and compared by using the two input models. Hydrodynamic effects are considered using finite element discretization for incompressible reservoir fluid. It is concluded that stresses and displacements and contraction joint opening in the dam are significantly reduced both in linear and nonlinear analyses when using viscous-spring boundary model. Interestingly, in the case of linear analysis of the Dagangshan, the massless foundation input model with a relatively higher damping ratio of 10% leads to a comparable response of the dam to that using viscous-spring boundary model. In addition, the maximum tensile stresses from nonlinear analysis are 10–25% larger than that of the corresponding linear cases due to a partial release of the arch action.
 
Article
Three studies of site amplification factors, based on the recorded aftershocks, and one study based on strong motion data, are compared one with another and with the observed distribution of damage from the Northridge, CA, earthquake of 17 January 1994 (ML=6.4). In the epicentral area, when the peak ground velocities are larger than nonlinear response of soil begins to distort the amplification factors determined from small amplitude (linear) wave motion. Moving into the area of near-field and strong ground motion the site response becomes progressively more affected by the nonlinear soil response. Based on the published results, it is concluded that site amplification factors determined from small amplitude waves (aftershocks, small earthquakes, coda waves) and their transfer-function representation may be useful for small and distant earthquake motions, where soils and structures respond to earthquake waves in a linear manner. However in San Fernando Valley, during the Northridge earthquake, the observed distribution of damage did not correlate with site amplification determined from spectra of recorded weak motions. Mapping geographical distribution of site amplification using other than very strong motion data, therefore appears to be of little use for seismic hazard analyses.
 
Article
The objective of this study is to investigate the effects of local site conditions in the Adapazarı plain crossing the severely damaged central part of Sakarya during the 17th August earthquake. A two-dimensional model has been used to obtain the free-field motions in the valley and on surface formations. A preliminary geotechnical profile model developed from available but limited soil data was checked using recordings of some specific locations where a strong ground motion accelerometer array was in operation after the main earthquake. The range of validity of the model was assessed and modifications were made to compare with the actual recorded motions. The purpose of building such a model, which was used to study the aftershocks in Adapazarı, is to provide an improved database to be used for the design of structures in the city. This study also attempts to provide greater insight into the local site response phenomena through the use of a two-dimensional nonlinear analysis. Simulated site responses are in global agreement with the recorded data. In general agreement between the amplification functions of the computed and recorded data is satisfactory for the frequencies of engineering interest. Alluvial sites show amplification factors in the range of 4–6 in the frequencies between 0.6 and 2 Hz. A relatively shallow alluvial station, HASTAHANE, experienced considerable amplification for small aftershocks and deamplification for the strongest aftershock in frequencies between 0.5 and 2 Hz. Another station, TOYOTA, with significant deamplification characteristics and a clearly observed peak response frequency shift at the soil resonant frequency may have experienced soil shear failure during the strongest aftershock. Other alluvial stations, SEKER and GENC, show deamplification characteristics at 0.55 and at around 1 Hz. with the peak frequency shifts during the magnitude 5.8 aftershock. In general, site responses are larger in the alluvial basin for all aftershocks except the magnitude 5.8 event.
 
Article
We present contours of the largest horizontal and vertical recorded peak velocities of strong ground motion during the Northridge, California, earthquake. Above the fault, the horizontal peak velocities exceeded 100 cm/s. The vertical velocities were larger than 20 cm/s. We also present contours of peak horizontal and vertical strain factors. Through most of the San Fernando Valley and the Santa Susana Mountains, the horizontal surface strain factor was larger than 10−3. The largest horizontal strain factor computed was for the Rinaldi Receiving Station ∼10−2·2. The corresponding vertical strains were >10−3·25 and 10−13, respectively. Through most of the Los Angeles Basin the horizontal peak surface strain factors were between 10−3·75 and 10−3.
 
Article
Contour maps of PSV (Pseudo Relative Velocity Spectrum) amplitudes during the Northridge, California, earthquake of 17 January 1994 are presented, based on strong motion recordings throughout the Los Angeles metropolitan area. These maps indicate that the PSV amplitudes do not attenuate uniformly with distance, but may be locally amplified or deamplified by interference of waves reflected from discontinuities and irregularities in the geological structure (boundaries of sedimentary basins, hills and mountains and vertical offsets of basements along faults). The contour maps in this paper represent one interpretation of the distribution of PSV amplitudes based on a limited number of unequally spaced data points, and thus do not capture all details of the actual ground motion (this would require a much denser distribution of strong motion stations). Yet, at locations where there were no strong motion recordings, based on these maps, one can estimate the ground motion more accurately than based on one or few close by recordings. These maps can be used by earthquake engineers to ‘construct’ a PSV spectrum at any site of interest within the area covered. They can also be used for validation of computer codes for simulation of ground motion in basins using simplified geologic models of the area covered by the maps in this paper.
 
Article
Contour maps of site amplification at long periods (3–10 s), estimated from recorded ambient noise in San Fernando Valley, are compared with simple indicators of the strong motion amplitudes and damage from the 1994 Northridge, CA, earthquake. The results show that there is no simple correlation between these two sets of observations. Ambient noise measurements have been successful in identifying the model parameters of full-scale structures, and may become successful in identifying parameters of soil and sedimentary layers that are related to strong motion site amplification in the near field, but new methods of measurement and analysis are needed to achieve that.
 
Article
Three-dimensional finite-difference (3D-FD) simulations of earthquake wave propagations in the Yanhuai area were performed for the 1720 Shacheng earthquake (Ms 7.0) using a stochastic finite-fault model, running on a parallel supercomputer Hitachi-SR8000 (http://www.lrz-muenchen.de). A stochastic finite-fault model was implemented into the 3D-FD program. The basic idea of the stochastic finite-fault model is that the fault plane can be subdivided into several subfaults (or elements, sources). Radiation from a large earthquake is the sum of contributions from all subfaults with proper time delays, each of which acts as a small independent double-couple point source. Heterogeneity of the fault rupture process was modeled by randomizing the location of initial rupture (hypocenter), slip vectors (slip, rake), and rise-times of subfaults in this study. A 3D velocity model of the Yanhuai area was constructed based on studies that analyzed available geological and geophysical information. A grid increment of 75 m in three directions was used in the 3D-FD simulation, which made it possible to capture the short period information with a resolution as low as 0.5 s in sediment regions. The uncertainties of simulated results caused by the stochastic finite-fault model were studied with a homogeneous 3D model. We found that the effects of the randomness of source on simulated ground motions are only limited in near-fault-region including the surface exposure of the fault and its vicinities, which occupies about 5% of the whole study area. This article presents an integrated approach for simulating the strong ground motions for engineering purpose using the 3D-FD method. Such simulations would be useful for hazard mapping, land using planning, insurance rate assessment, particularly in planning, preparedness, and coordinating emergency response, which must be based on realistic situations induced by concrete (historic or scenario) earthquakes.
 
Article
This paper presents a comprehensive ground response study for the municipality of Villa Collemandina in Northern Tuscany (Italy). This site was selected following a macroseismic analysis of the 1920 Garfagnana earthquake (Mw=6.5), which, at Villa Collemandina, produced damage and losses that were larger than at any other site near the earthquake epicentre, thus indicating the presence of possible site effects. Hence, both experimental and numerical methods are applied in order to investigate the ground response at different locations within the Villa Collemandina municipality. Results obtained from the spectral analysis of earthquake recordings using the reference site method and those from a 2-dimensional dynamic simulation reveal the presence of site effects due to the buried geomorphology (basin-like effects), allowing us to explain the severe damage and losses produced by the 1920 Garfagnana earthquake. As a further result, horizontal to vertical spectral ratio techniques and 1D soil modelling are proved to be inadequate for an effective characterization of the ground response at sites that, like Villa Collemandina, present a complex local geology.
 
Article
This paper describes a kinematic model of tsunami generated by submarine slides and slumps spreading in two orthogonal directions. This model is a generalization of our previously studied models spreading in one direction. We show that focusing and amplification of tsunami amplitudes can occur in an arbitrary direction, determined by the velocities of spreading. This kinematic model is used to interpret the asymmetric distribution of observed tsunami amplitudes following the Grand Banks earthquake—slump of 1929.
 
Article
The 1996 Duvall earthquake in Washington State triggered ground motion stations in the Fraser Delta, British Columbia, located on varying depths of Holocene and Pleistocene soils to a maximum depth of 800 m and one station on bedrock. Recorded ground motions were used to examine the applicability of 1-D and 2-D site response analyses for amplification studies in the Delta. 1-D response analysis gave a good indication of the period of peak response. Response spectra computed by 1-D analysis did not compare well with recorded spectra except at very deep sites. The use of 2-D analyses to include buried topography generally improved the predictions of site response spectra at the shallow sites. However, for the shallow earth sites, the recorded motions showed a strong spectral response at short periods in the range of 0.1–0.25 s. Neither 1-D nor 2-D analysis predicted this response.
 
Article
In the near-field of an earthquake the effects of the rotational components of ground motion may not be negligible compared to the effects of translational motions. Analyses of the equations of motion of horizontal and vertical pendulums show that horizontal sensors are sensitive not only to translational motion but also to tilts. Ignoring this tilt sensitivity may produce unreliable results, especially in calculations of permanent displacements and long-period calculations. In contrast to horizontal sensors, vertical sensors do not have these limitations, since they are less sensitive to tilts. In general, only six-component systems measuring rotations and accelerations, or three-component systems similar to systems used in inertial navigation assuring purely translational motion of accelerometers can be used to calculate residual displacements.
 
Article
The region stricken by the Irpinia earthquake (Ms=6.9) on 1980, along the Southern Appenninc chain, is one of the highest seismic hazard areas of Italian peninsula.This event produced vast damaging and strong amplitude shaking on a wide area. This is mostly related to the occurrence of a multiple fracture process during which three different segments of a sub-parallel normal fault system have been activated. The principal aim of this study is to investigate how the source complexity can have influenced the ground motion recorded at the earth surface and the areal distribution of strong motion parameters and observed building damage. We use a deterministic approach to simulate the ground motion scenario in the whole Campania region generated by the Irpinia event, based on the present knowledge of its rupture history and multiple faulting geometry. The kinematic source model has been calibrated by comparing the observed and synthetic data in frequency and time domain. Maps of the main strong motion parameters and damage maps for different class of buildings and levels of damage have been computed and quantitatively compared to the observations of damages and macroseismic intensity data.
 
Article
This study aims to determine the influence of torsional coupling on the inelastic response of a series of models representing typical structural configurations in real buildings. The lake bed (SCT) east-west component of the 1985 Mexico City earthquake was employed in the analysis, and is representative of a severe ground motion known to have induced large inelastic structural deformations in a high proportion of those buildings having asymmetrical distributions of stiffness and/or strength. Material non-linearity in lateral load-resisting elements has been defined using a hysteretic Ramberg-Osgood model. Structural eccentricities have been introduced into the building models by (i) asymmetrical distributions of stiffness and/or strength, (ii) asymmetrical configuration of lateral load-resisting elements, or (iii) varying post-elastic material behaviour in the resisting elements. The dynamic inelastic response of these models has been obtained by a numerical integration of the relevant equations of motion, expressed in a non-dimensional incremental form.In the elastic range, the results correlate well with those of previous studies. In the inelastic range, it is concluded that the peak ductility demand of the worst-affected element increases with the ground excitation level across the range of building periods considered, and that the influence of torsional coupling on the key response parameters is model dependent. Most significantly, the strength eccentricity relative to the centre of mass has been shown to influence the peak edge displacement response more than conventionally employed stiffness eccentricity.
 
Article
For a moderate earthquake (Ms = 6·8), the Spitak, Armenia earthquake of 7 December 1988 caused unprecedented destruction and fatalities. Unlike other destructive earthquakes, a coordinated research to understand the circumstances and conditions that led to the extensive damage from this earthquake has not yet been adequately carried out. This paper pieces together the little and uncoordinated research that has been accomplished to-date to form a comprehensive view of why so much destruction occurred. The main factors appear to be the unusual ground motion where, within a span of 40s, two significant events followed each other; the lack of adequate seismic design considerations such as ductulity in precast joints and vertical accelerations in the absence of adequate factors of safety for dead and live load design; unique microzonation practices that could change the design basis by a factor of four within a distance of only 1–2 km; and a host of other issues in structural analysis, concrete technology and city planning.
 
Article
Stochastic finite-fault simulation is used to simulate the acceleration time histories of two intermediate depth (h∼90 km) earthquakes, which occurred on 30 May 1990 in Vrancea (Romania) and on 22 January 2002 near Karpathos island in Greece. Site amplification functions were used for all stations included in the simulation process. For the Romanian earthquake, peak acceleration values were observed at the more distant from the epicentre stations. In Romania, strong anisotropy in Q-values was observed along the NE–SW direction (along strike of the fault) compared to the normal to the strike of the fault direction. Specific site corrections were required for the station in Bucharest, employing the H/V ratio technique, to refine the site-specific amplification function and achieve better match of the simulated spectra. In the Aegean Sea region, average values for attenuation and site amplification functions were used for all stations with overall satisfactory results. Any unmatched discrepancies between observed and simulated spectra would require further accurate knowledge of the site characteristics.
 
Article
Plots of smoothed contours of peak amplitudes and of areas with the same peak sign are presented for the radial, transverse and vertical components of acceleration, velocity and displacement. These were drawn by hand based on strong motion recordings, and represent direct observational evidence of the nature of the attenuation of strong motion with distance at high, intermediate and low frequencies. The contours of peak amplitudes indicate that, close to the source, those are affected by the source radiation pattern, and away from the source, by the geological structure. Slower attenuation of peak amplitudes with distance is observed for waves travelling through the sediments of the Los Angeles basin. Large areas with consistent peak polarity are observed, often tens of kilometers in size, indicating that the sign of the peak is not random. The time of the peak amplitude, relative to first arrivals of S-waves, was also calculated; the areas where this time was greater than 7 s were contoured and shaded, indicating peak occurrence later than the direct arrivals from the source. These plots show that, at distances larger than about 20–30 km, for acceleration, the largest peak occurs mostly before, and, for displacement, mostly after the arrival of surface waves. This indicates that the attenuation of strong ground motion is governed by body waves at short periods and by surface waves at long periods. The presented plots will be useful in refinement of attenuation laws for ground motion peak amplitudes, and for frequency dependent response spectrum ordinates.
 
Article
The spatial distributions of severely damaged buildings (red-tagged) and of breaks in the water distribution system following the 1994 Northridge, California, earthquake (ML = 6·4) are investigated relative to the local characteristics of surficial geology. The pipe breaks are used as an indicator of nonlinear soil response, and the red-tagged buildings as indicator of severe shaking. The surficial geology is described by several generalized categories based on age, textural character and thickness of the near surface layer. Two regions are studied: the San Fernando Valley and Los Angeles-Santa Monica. The analysis shows that there is no simple correlation between damage patterns and surficial geology. Single family wood-frame buildings were damaged less when built on fine silt and clay (0–3 m thick) from the late Holocene.
 
Article
Results of two detailed ambient vibration surveys of a 7-story reinforced concrete building in Van Nuys, California, are presented. Both surveys were conducted after the building was severely damaged by the 17 January 1994, Northridge earthquake (ML=5.3, epicenter 1.5 km west from the building site) and its early aftershocks. The first survey was conducted on 4 and 5 February 1994, and the second one on 19 and 20 April 1994, about one month after the 20 March aftershock (ML=5.3, epicenter 1.2 km north–west from the building site). The apparent frequencies and two- and three-dimensional mode shapes for longitudinal, transverse and vertical vibrations were calculated. The attempts to detect the highly localized damage by simple spectral analyses of the ambient noise data were not successful. It is suggested that very high spatial resolution of recording points is required to identify localized column and beam damage, due to the complex building behavior, with many interacting structural components. The loss of the axial capacity of the damaged columns could be seen in the vertical response of the columns, but similar moderate or weak damage typically would not be noticed in ambient vibration surveys. Previous analysis of the recorded response of this building to 12 earthquakes suggests that, during large response of the foundation and piles, the soil is pushed sideways and gaps form between the foundation and the soil. These gaps appear to be closing during “dynamic compaction” when the building site is shaken by many small aftershocks. The apparent frequencies of the soil–foundation–structure system appear to be influenced significantly by variations in the effective soil–foundation stiffness. These variations can be monitored by a sequence of specialized ambient vibration tests.
 
Article
The 1994 Northridge earthquake occurred underneath a densely populated metropolitan area, and was recorded by over 200 strong motion stations in the metropolitan area and vicinity. This rare coincidence made it an ideal case to study, in statistical sense, the correlation of damage to structures with the level of strong shaking, in particular with respect to (1) instrumental characteristics of shaking and (2) the reported site intensity scale. In this paper, statistics for the incidence of red-tagged building in 1 × 1 km2 blocks in San Fernando Valley and Los Angeles is presented and analyzed, as function of the observed peak ground velocity or the local intensity of shaking. The ‘observed’ peak velocity is estimated from contour maps based on the recorded strong motion. The intensity of shaking is estimated from the published intensity map and from our modification of this map to make it more consistent with observed high damage to buildings in some localized areas. Finally, empirical scaling equations are derived which predict the average density of red-tagged buildings (per km2) as a function of peak ground velocity or site intensity of shaking. These scaling equations are specific to the region studied, and apply to Wooden Frame Construction, typical of post World War II period, which is the prevailing building type in the sample studied. These can be used to predict the density of red-tagged buildings per km2 in San Fernando Valley and in Los Angeles for a scenario earthquake or for an ensemble of earthquakes during specified exposure, within the framework of probabilistic seismic hazard analysis. Such predictions will be useful to government officials for emergency planning, to the insurance industry for realistic assessment of insured losses, and to structural engineers for assessment of the overall performance of this type of buildings.
 
Article
The spatial relationship between areas with severely damaged (red-tagged) buildings and areas with large strains in the soil (indicated by reported breaks in the water distribution system), observed during the 1994 Northridge earthquake, is analysed. It is shown that these areas can be separated almost everywhere. Minimal overlapping is observed only in the regions with very large amplitudes of shaking (peak ground velocity exceeding about 150 cm s−1). One explanation for this remarkable separation is that the buildings on ‘soft’ soils, which experienced nonlinear strain levels, were damaged to a lesser degree, possibly because the soil absorbed a significant portion of the incident seismic wave energy. As a result, the total number of severely damaged (red-tagged) buildings in San Fernando Valley, Los Angeles and Santa Monica may have been reduced by a factor of two or more. This interpretation is consistent with the recorded peak accelerations of strong motion in the same area. It is concluded that significant reduction in the potential damage to wood frame single family dwellings may be expected in areas where the soil experiences ‘large’ strains (beyond the linear range) during strong earthquake shaking, but not significant differential motions, settlement or lateral spreading, near the surface.
 
Article
An extended Kalman filter algorithm with local iteration is presented for the identification of non-linear and non-stationary soil properties. Borehole-array strong motions were recorded at a liquefied site during the 1995 Hyogoken-nanbu earthquake. In this study, a modified Kalman filtering method in which the extended Kalman filter is iteratively used at every local time-step to track rapid parameter changes is proposed. The method is then applied to the instrumented soil layer, which is modeled by an equivalent linear model. An identification of non-linear and non-stationary soil properties was conducted successfully; and non-linear restoring force–displacement relationships including progression with time were obtained.
 
Article
The Greek coastal town of Egion on 15 June 1995 was shaken by a strong, small epicentral distance, earthquake that caused heavy damages to buildings and loss of life. The damages were concentrated in the central elevated part of the town whereas the flat coastal region remained almost intact. This non-uniform distribution of damage is studied in this article in terms of surface topography effects by conducting seismic response analyses of a simplified 2-D profile of the town. A dynamic finite element code implementing the equivalent-linear soil behavior (FLUSHPLUS) was used for the analyses and it was found that the step-like topography amplified greatly the intensity of motion without affecting its frequency content. The analyses showed that the motion recorded by an accelerograph installed at the center of the town is in agreement with the computed values; they also indicated a particularly intense amplification close to the crest of the steep slope, where a multi-story RC residential building partially collapsed. In contrast, the level of motion was found to be low at the flat coastal zone of the town where the earthquake damages were insignificant. It is concluded that the characteristic surface topography of the town played an important role in modifying the intensity of base motion.
 
Article
A novel method is developed, motivated by one of the mysteries of the Hyogoken-Nambu (Kobe): the Nikawa landslide. To this end, prompted by the hypothesis of “sliding-surface liquefaction” advocated by Sassa [Development of a new cyclic loading ring shear apparatus to study earthquake-induced-landslides. Report for grant-in-aid for development of scientific research by the Ministry on Education, Science and Culture, Japan (project no. 03556021), 1994, p. 1–106; Keynote lecture: access to the dynamics of landslides during earthquakes by a new cyclic loading high-speed ring shear apparatus. In: Proceedings of the sixth international symposium on landslides, 1992. In: Landslides, vol. 3. Balkema: Rotterdam; 1995. p. 1919–39], a dynamic analysis of the early stages of an earth slide is presented considering two mechanically coupled sub-structures: (a) the rapidly deforming shear band at the base of the slide and (b) the accelerating sliding mass modeled as a rigid body. The proposed model for sliding is based on: (i) the concept of high pore-water pressure generation by grain crushing along the sliding surface (proposed by Sassa et al. in 1995), (ii) an experimental model developed by Hardin [Crushing of soil particles. J Geotech Eng 1985;111(10):1177–92] for crushing of soil particles under compression and shear, expressed with a set of developed equations governing the mechanism of breakage, and (iii) the hysteretic stress–strain Bouc–Wen-type constitutive model coupled with the Coulomb friction law. An attempt is made to adjust the model parameters to Sassa's experimental data in ring-shear tests. The method leads to a reasonable prediction of the large displacement of the Nikawa landslide. A sensitivity analysis is also carried out for the influence of key model parameters (e.g. shape, crushing hardness, void ratio, grain size distribution, effective normal stress) on the pore-pressure rise due to particle breakage.
 
Article
The 28 March 1996 earthquake (Mw=5.7) produced extensive damage in Pujili, a small town located in the central part of the Inter-Andean valley of Ecuador. Variations in the damage rate of adobe constructions throughout the city let us suppose a ground motion amplification related to site effects. A site effect study using the H/V spectral ratio confirmed the good agreement with geological formations and showed that a second peak in some soil response appears at sites that are located over a zone of alluvial deposits, in the most damaged area of the city. This second peak is in the frequency range 5–7 Hz, close to the natural response of adobe building frequency. Thus, the second peak seems to increase the damage rate of adobe buildings and may be related to superficial alluvial deposits of a river, acting independently of the rest of the soil column.
 
Article
The recent 1997 Umbria-Marche, Central Italy, earthquake sequence allowed us to model recorded ground motions using a method developed by Beresnev and Atkinson [Bull Seism Soc Am 87 (1997) 67–84; Seism Res Lett, 69 (1998) 27–32; Bull Seism Soc Am 88 (1998) 1392–1401]. The method generalizes the stochastic ground-motion simulation technique, developed for point sources, to the case of finite faults. It subdivides the fault plane into subfaults and assumes each subfault to be a point source with a ω2 spectrum. Geometric spreading and regional anelastic attenuation are included in the model. The data include horizontal acceleration recordings from the SSN and ENEL databases of the 1997 Umbria-Marche events on 26 September, at 00:33 GMT, with Mw=5.7, and at 09:40 GMT, with Mw=6.0; and on 14 October at 15:23 GMT, with Mw=5.6. The strong motion simulations are performed using model parameters based on the results of previous studies, and adjusting the subfault size to calibrate the simulation model against recorded ground motions. Local site response is considered to account for observed amplification effects at specific recording sites (e.g. Nocera Umbra). A good agreement is found between the simulated response spectra and the recorded data, concluding that this method reproduces the salient ground-motion characteristics at different distances and azimuths.
 
Article
The Ms 5.9 earthquake of 1999 produced valuable records in three underground structures, as follows: (a) in the just completed cut-and-cover station of Sepolia two accelerographs recorded the free-field and the station-base motion; (b) in the still under-construction tunnelled station of Monastiraki an accelerograph recorded the ground surface motion, and (c) in the nearby Kerameikos station, abandoned for non-technical reasons, the temporary prestressed-anchor piled (PAP) wall was still in place and produced a record of total seismic displacement at its top. Directly or indirectly utilising these records, the article outlines the results of numerical analyses aimed at ‘recovering’ the complete seismic response of the three underground structures. Particular emphasis is given to Sepolia station, where the developed accelerations (with PGA of about 0.17 g at the station base and 0.43 g at the station roof) are shown to have been almost exactly equal to the design accelerations according to the seismic code under the assumption that the station responds as an aboveground structure. The successful performance of the two temporary structures, in Monastiraki and Kerameikos (which had been designed against minimal acceleration levels but experienced ground-surface high-frequency accelerations of the order of 0.50 g) is explained through dynamic response analyses.
 
Article
A great number of free-field ground motion records are obtained during the 1999 Chi-Chi, Taiwan, earthquake. Records from 130 near fault free-field stations within 55 km to the causative fault surface are used as database, and characteristics of earthquake peak ground acceleration, velocity, displacement and duration are analyzed. According to this study, near fault ground motions are strongly affected by distance to fault, fault rupture directivity, site condition, as well as thrust of hanging wall. Compared with empirical strong ground motion attenuation relations used in China, US and Japan, the PGAs and PGVs recorded in this earthquake are not as large as what we have expected for a large earthquake as magnitude 7.6. However, the largest PGV and PGD worldwide were recorded in this event, which are 292 cm/s and 867 cm, respectively. Caused by nonlinear site effects of soil, peaks and corresponding ratios on E-class site were markedly different from those on other sites. Just as observed in historic earthquakes, fault rupture directivity effects caused significant differences between peaks of ground motion of two horizontal components, but took very slight effects on the duration of ground motion. The significant velocity pulses associated with large PGVs and PGDs, as well as large permanent displacements, which may result from the large thrust of the hanging wall, became the outstanding character of this event. Based on this study, we point out that 3D waveform modeling is needed to understand and predict near fault ground motion of large earthquakes.
 
Article
The Athens, Greece, earthquake of 7 September 1999 provided a number of reliable strong motion recordings and well-defined patterns of damage at sites with known geological and geotechnical conditions. Joint evaluation of this evidence shows that the very stiff soils of the Athens basin, compared to the nearby outcropping soft rocks, have amplified the peak horizontal acceleration by an average of 40% or more and have shifted elastic response spectra to higher periods. US and the European seismic code provisions (NEHRP-97 and EC-8), place stiff soils and soft rocks at the same site category and consequently fail to predict these adverse effects. A larger number of site categories and new site coefficients that depend on the seismic excitation frequency appear necessary in order to overcome this deficit of the codes.
 
Article
An embankment slope on a major highway totally collapsed during 12 November 1999 Düzce earthquake (Mw=7.1) due to the intense near field ground motion. The slope had performed satisfactorily, without even minor deformations or cracks, during the İzmit earthquake (Mw=7.4), another major event that occurred 3 months before and was far field. Predominantly coarse-grained fill of the embankment exhibited typical non-brittle behavior during laboratory tests, implying that the prefailure shear strength would remain relatively unchanged. Stability and failure mechanism of the embankment slope were investigated through a series of static, pseudostatic and fully coupled non-linear dynamic analyses. While respective performances were correctly predicted by the pseudostatic method on a failure—no failure basis during the two earthquakes, the computed failure surface geometry did not conform to that observed. Fully coupled dynamic response analysis, on the other hand, predicted the failure mechanism and failure surface configuration in conformance with the post-failure observations. Computed displacements were generally less than those observed and critically dependent on the potential uncertainties.
 
Article
This paper describes the process of construction of the 2D model of Volvi’s geological structure and results of empirical and theoretical approaches to the evaluation of site response at Euroseistest. The construction of the 2D model is based on a re-interpretation of the available geophysical and geotechnical data in an effort to improve the definition of the subsoil structure at Euroseistest in terms of the most important parameters needed to model site response. The results of this re-interpretation are compared with a previous published 2D model of the same alluvial valley. Different analysis of the measurements and different criteria in the synthesis of data have led to a different model, even if both studies had access to the same field measurements. This underscores the fact that a model results of an interpretation and is not uniquely determined by the data, no matter how detailed they are. The well known subsoil structure opened the possibility to correlate the geometry and the dynamic properties of the 2D model with the results of site response determined from a detailed analysis of two events in frequency and time domains and 1D numerical modeling. The study of site response shows the important effect of the lateral variations on the ground motion and suggests that the contribution of locally generated surface waves to the resonant peak may be important. In the case of Volvi’s graben, the limitations of the 1D approximation to simulate ground motion under complex soil conditions in both frequency and time domains are also shown. This paper lays the ground for a companion article dealing with 2D site effects in this basin.
 
Article
This paper presents analytical solutions for computing the 3D displacements in a flat solid elastic stratum bounded by a rigid base, when it is subjected to spatially sinusoidal harmonic line loads. These functions are also used as Greens functions in a boundary element method code that simulates the seismic wave propagation in a confined or semi-confined 2D valley, avoiding the discretization of the free and rigid horizontal boundaries.The models developed are then used to simulate wave propagation within a rigid stratum and valleys with different dimensions and geometries, when struck by a spatially sinusoidal harmonic vertical line load. Simulations are performed in the frequency domain, for varying spatial wave numbers in the axial direction of the valley. Time results are obtained by means of inverse Fourier transforms, to help understand how the geometry of the valley may affect the variation of the displacement field.
 
Article
Ground vibrations induced by railway traffic at grade and in tunnels are often studied by means of two-and-half dimensional (2.5D) models that are based on a Fourier transform of the coordinate in the longitudinal direction of the track. In this paper, the need for 2.5D coupled finite element-boundary element models is demonstrated in two cases where the prediction of railway induced vibrations is considered. A recently proposed novel 2.5D methodology is used where the finite element method is combined with a boundary element method, based on a regularized boundary integral equation. In the formulation of the boundary integral equation, Green's functions of a layered elastic halfspace are used, so that no discretization of the free surface or the layer interfaces is required. In the first case, two alternative models for a ballasted track on an embankment are compared. In the first model, the ballast and the embankment are modelled as a continuum using 2.5D solid elements, whereas a simplified beam representation is adopted in the second model. The free field vibrations predicted by both models are compared to those measured during a passage of the TGVA at a site in Reugny (France). A very large difference is found for the free field response of both models that is due to the fact that the deformation of the cross section of the embankment is disregarded in the simplified representation. In the second case, the track and free field response due to a harmonic load in a tunnel embedded in a layered halfspace are considered. A simplified methodology based on the use of the full space Green's function in the tunnel–soil interaction problem is investigated. It is shown that the rigorous finite element-boundary element method is required when the distance between the tunnel and the free surface and the layer interfaces of the halfspace is small compared to the wavelength in the soil.
 
Article
The small Central American republic of El Salvador has experienced, on average, one destructive earthquake per decade during the last hundred years. The latest events occurred on 13 January and 13 February 2001, with magnitudes Mw 7.7 and 6.6, respectively. The two events, which were of different tectonic origin, follow the patterns of the seismicity of the region although neither event has a known precedent in the earthquake catalogue in terms of size and location. The earthquakes caused damage to thousands of traditionally built houses and triggered hundreds of landslides, which were the main causes of fatalities. The earthquakes have clearly demonstrated trends of increasing seismic risk in El Salvador due to rapid population expansion in areas of high shaking and landslide hazard, exacerbated by deforestation and uncontrolled urbanisation. The institutional mechanisms required for the control of land use and building practice are very weak and present a major obstacle to risk mitigation.
 
Finite element analysis results: static behavior-maximum values Bending moment (N m) Stress (MPa)
Article
In this study we address the question that concerns the shape of the original dome of Hagia Sophia in Istanbul, built by the two great architects, through two approaches. The first is through tests performed on the structural model. By means of this computer model we can study the structural integrity of the various shapes suggested for the first dome and compare their predicted behaviors. Second, we look at literary descriptions of Hagia Sophia's vaulting by Procopius, Agathias, Paul the Silentiary, and Malalas, paying particular attention to the evidence of what collapsed, what was taken down, and finally what was rebuilt. Based on both the technical studies and a careful reinterpretation of the available sources, we propose a new reconstruction of the first dome of Hagia Sophia, which is compatible both with the results of our scientific studies and with the original sources.
 
Article
An Mw 6.25 earthquake occurred on April 6, 2009 at 03:33 a.m. local time, in the Abruzzo region (Central Italy), close to the city of L’Aquila. The earthquake ruptured a North-West (NW)–South-East (SE) oriented normal fault dipping toward the South-West (SW), with the city of L’Aquila lying a few kilometers away on the hanging wall.The main shock has been recorded by fifty-eight accelerometric stations: the highest number of digital recordings ever obtained in Italy for a single earthquake, one of the best-recorded earthquakes with a normal fault mechanism. Very high values of peak ground acceleration (0.3–0.65 g) were observed close to the center of L’Aquila (6 stations at zero JB distance from the fault). The earthquake caused severe loss of lives (299 victims and 1500 injured) and damage (about 18000 unusable buildings) in the epicentral area.In this study we analyze the ground motion characteristics of both the main shock in terms of peak ground acceleration (PGA), peak ground velocity (PGV), and pseudo-acceleration response spectra (5% of damping ratio). In particular, we compare the pseudo-acceleration response spectra for horizontal directions with the EC8 design spectrum and the new Italian building code (NTC08). In order to understand the characteristics of the ground motions induced by L’Aquila earthquake, we also study the source-related effects and site response of the strong motion stations that recorded the seismic sequence. A novel method is used for the analysis of inter-station and site-specific H/V spectral ratios for the main event and for 12 aftershocks.
 
Article
The response of an elastic circular wedge on a flexible foundation embedded into a half-space is investigated in the frequency domain for incident pane SH-waves. The problem is solved by expansion of the motion in all three media (wedge, foundation and half-space) in cylindrical wave functions (Fourier-Bessel series). The structural model is simple, but accounts for both differential motions of the base and for the effects of soil-structure interaction. Usually, structural models in earthquake engineering consider either differential ground motion, but ignore soil-structure interaction, or consider soil-structure interaction, but for a rigid foundation, thus ignoring differential ground motion. The purpose of the study is to find how stiff the foundation should be relative to the soil so that the rigid foundation assumption in soil-structure interaction models is valid. The shortest wavelength of the incident waves considered in this study is one equal to the width of the base of the wedge. It is concluded that, for this model, a foundation with same mass density as the soil but 50 times larger shear modulus behaves as ‘rigid’. For ratio of shear moduli less than 16, the rigid foundation assumption is not valid. Considering differential motions is important because of additional stresses in structures that are not predicted by fixed-base and rigid foundation models.
 
Article
The dynamic response of a poroelastic half-plane soil medium subjected to moving loads is studied analytically/numerically under conditions of plane strain. The material is idealized as a uniform, fully saturated poroelastic layer of soil obeying Biot's dynamic poroelastic theory. Hysteretic damping in the soil skeleton may also be present. The loading function is presented by a Fourier series expansion allowing the analysis to handle different types of load such as uniform pressures and multiple loads. The parameters examined include mainly the poroelastic material properties such as porosity, permeability, shear modulus, compressibility of fluid and the velocity of the moving load. The response quantities include the vertical displacement of the solid skeleton, the fluid pressure and the solid vertical effective stress. It is concluded that the effect of porosity and permeability on response is more pronounced in soft materials, especially at high load speeds. Some of the results of the parametric study are qualitatively well compared with theoretical results obtained by other investigations.
 
Article
During the 1999 Chi-Chi, Taiwan earthquake, many sand boiling phenomena were observed in central Taiwan, which caused severe ground settlement and structure damages. According to the installed accelerograms, the peak ground surface horizontal accelerations in the liquefaction-affected areas range from 774.42 to 121.3 gal. The writers carried out an extensive investigation of soil liquefaction in this earthquake. In this paper, we present results of the CPT exploration and post-earthquake liquefaction analysis. Two hundred and seventy five (275) cone penetration test data were collected from the liquefaction-affected areas, and 46 liquefaction case histories and 88 non-liquefaction case histories were derived that can be used to evaluate the accuracy of existing liquefaction evaluation models. In addition, the strength of the liquefied soils after earthquake and the implication of its liquefaction potential in the future event are discussed.
 
Article
Results from a benchmark test on full-scale piles are used to investigate the response of piles to lateral spreading. In the experiment, two single piles, a relatively flexible pile that moves together with the surrounding soil and a relatively stiff pile that does not follow the ground movement have been subjected to large post-liquefaction ground displacement simulating piles in laterally spreading soils. The observed response of the piles is first presented and then the results are used to examine the lateral loads on the pile from a non-liquefied soil at the ground surface and to evaluate the stiffness characteristics of the spreading soils. The measured ultimate lateral pressure from the crust soil on the stiff pile was about 4.5 times the Rankine passive pressure. The back-calculated stiffness of the liquefied soil was found to be in the range between 1/30 and 1/80 of the initial stiffness of the soil showing gradual decrease in the course of lateral spreading.
 
Article
This paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed.
 
Article
This paper deals with the evaluation of seismic site effects due to the local topographical and geotechnical characteristics. The amplification of surface motions is calculated by a numerical method combining finite elements in the near field and boundary elements in the far field (FEM/BEM). The numerical technique is improved by time truncation. In the first part of this article, the accuracy and the relevance of this optimized method are presented. Moreover, parametric studies are done on slopes, ridges and canyons to characterize topographical site effects. The second part deals with sedimentary valleys. The complexity of the combination of geometrical and sedimentary effects is underlined. Extensive parametrical studies are done to discriminate the topographical and geotechnical effects on seismic ground movement amplifications in two-dimensional irregular configurations. Characteristic coefficients are defined to predict the amplifications of horizontal displacements. The accuracy of this quantitative evaluation technique is tested and discussed.
 
Article
The Direct Boundary Element Method (DBEM) is presented to solve the elastodynamic field equations in 2D, and a complete comprehensive implementation is given. The DBEM is a useful approach to obtain reliable numerical estimates of site effects on seismic ground motion due to irregular geological configurations, both of layering and topography.The method is based on the discretization of the classical Somigliana's elastodynamic representation equation which stems from the reciprocity theorem. This equation is given in terms of the Green's function which is the full-space harmonic steady-state fundamental solution. The formulation permits the treatment of viscoelastic media, therefore site models with intrinsic attenuation can be examined. By means of this approach, the calculation of 2D scattering of seismic waves, due to the incidence of P and SV waves on irregular topographical profiles is performed. Sites such as, canyons, mountains and valleys in irregular multilayered media are computed to test the technique. The obtained transfer functions show excellent agreement with already published results.
 
Article
The present study aims to demonstrate how the dynamic behaviour of structures is affected by local inhomogeneities inside the soil. Particularly, the influence of block-shaped elastic inclusions directly beneath a vertically vibrating block foundation is considered. Comparisons between a 2D and a 3D modelling of the soil—structure system are made. It can be observed that, depending on the stiffness of the inclusion and especially on the excitation frequency, the dynamic response of the foundation may either increase or decrease.
 
Article
The purpose of this work is to present three methods of analysis for elastic waves propagating in two dimensional, elastic nonhomogeneous media. The first step, common to all methods, is a transformation of the governing equations of motion so that derivatives with respect to the material parameters no longer appear in the differential operator. This procedure, however, restricts analysis to a very specific class of nonhomogeneous media, namely those for which Poisson's ratio is equal to 0.25 and the elastic parameters are quadratic functions of position. Subsequently, fundamental solutions are evaluated by: (i) conformal mapping in conjunction with wave decomposition, which in principle allows for both vertical and lateral heterogeneities; (ii) wave decomposition into pseudo-dilatational and pseudo-rotational components, which results in an Euler-type equation for the transformed solution if medium heterogeneity is a function of one coordinate only; and (iii) Fourier transformation followed by a first order differential equation system solution, where the final step involving inverse transformation from the wavenumber domain is accomplished numerically. Finally, in the companion paper numerical examples serve to illustrate the above methodologies and to delineate their range of applicability.
 
Article
The response of underground structures subjected to subsurface blast is an important topic in protective engineering. Due to various constraints, pertinent experimental data are extremely scarce. Adequately detailed numerical simulation thus becomes a desirable alternative. However, the physical processes involved in the explosion and blast wave propagation are very complex, hence a realistic and detailed reproduction of the phenomena would require sophisticated numerical models for the loading and material responses. In this paper, a fully coupled numerical model is used to simulate the response of a buried concrete structure under subsurface blast, with emphasis on the comparative performance of 2D and 3D modeling schemes. The explosive charge, soil medium and the RC structure are all incorporated in a single model system. The SPH (smooth particle hydrodynamics) technique is employed to model the explosive charge and the close-in zones where large deformation takes place, while the normal FEM is used to model the remaining soil region and the buried structure. Results show that the 2D model can provide reasonably accurate results concerning the crater size, blast loading on the structure, and the critical response in the front wall. The response in the remaining part of the structure shows noticeable differences between the 2D and 3D models. Based on the simulation results, the characteristics of the in-structure shock environment are also discussed in terms of the shock response spectra.
 
Article
A three-dimensional (3D) model for the soil–railway track system is proposed. It is based on a geometrical periodic hypothesis. The dynamic soil–structure interaction is taken into account. This representation is used for the case of a ballasted railway track subjected to high-speed moving loads and a new formulation of dynamic responses of the system is proposed. Moreover, recent in situ measurements performed in a high-speed line of the North of France are presented. Lateral and vertical accelerations in several locations of the railway track have been measured and the periodic model is confronted with these records.
 
Article
This paper presents a new practical modeling approach, based on the beam-on-a-nonlinear Winkler foundation (BNWF) model, to simulate the 3D rocking, vertical and horizontal responses of shallow foundations using structural elements that are readily available in the element library of commercially available structural analysis programs. An assemblage of a moment-rotation hinge, shear hinge connected in series with an elastic frame member attached to the bottom end of ground story columns was proposed to model the response of the footing under combined action of vertical, horizontal and moment loading. To couple the responses of these hinges, two bounding surfaces equations were introduced and derived mathematically: a surface that defines the interaction between the rocking and vertical capacities of the footing along its width and length; and a surface that defines the interaction between the horizontal capacities of the footing along its width and length. Simple calculation steps to evaluate the geometric and mechanical properties of the proposed assemblage of structural elements are provided. The proposed modeling approach was verified using experimental results from large scale model foundations subjected to cyclic loading. Based on this study, it was found that the proposed assemblage can be reliably used in modeling the rocking and horizontal responses of shallow foundations under cyclic loading.
 
Location map (upper figure) and geological map of the Mygdonian basin [19]. The geological map corresponds to the area enclosed in the black rectangle in the location map. The thick solid lines indicate the main fault (F-GNSP) and its two branches (F-VL and F-Sx). Possible faults are shown with thick dashed lines. Red squares indicate the locations of the stations of the Euroseistest permanent accelerograph network, currently in operation. The central recording system of the network is installed at TST site. The location of the villages referred to in the text is also shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 
Map showing the location of the measurements used to build the 3D model of the Mygdonian basin. Solid lines show the 17 2D cross sections. The continuation of some 2D sections outside the map is indicated by an arrow at the end. Circles with a cross inside indicate boreholes, open rhombuses indicate cone penetration tests. Squares represent microtremor array measurements and stars show the four sources used for the refraction experiments. Circles show the electrical tomographies and soundings at the edges of the basin. The outline of the villages is indicated with thick solid lines. Not all symbols are labelled to avoid crowding the figure. We labelled only those explicitly referred to in the text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 
Maps showing the distribution of dominant frequency (a) and maximum amplification (b) observed throughout the basin. The colours indicate the distribution of those values estimated from HVSR computed at 350 sites (black points). Solid lines indicate the main faults crossing the basin. Black polygons indicate the location of the villages in the basin. The details of the measurements and analyses are given in Manakou [34]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 
(a) Vertical components records written by explosion SP3 at 13 sites instrumented with Reftek CMG-40 seismometers (with natural period 30 s). (b) Observed and computed phase-velocity for Rayleigh waves. The symbols indicate the phase-velocity dispersion determined from the analysis of the Rayleigh waves recorded from the explosion. The solid red line shows the dispersion curve computed for the inverted model. (c) V s profile versus depth inverted from the observed phase velocity dispersion curve, together with the corresponding resolving kernels. The shape of the resolving kernels with depth shows that all layers in the V s profile are well constrained by the inversion process. 
(a) V s velocity profiles and geological data available in the vicinity of site DAB (Fig. 2). V s profiles at sites DAB2, SP3, and VOL correlate well with the lithological 
Article
The 3D structure of the Mygdonian sedimentary basin (N. Greece) is investigated. The aim of this study is to propose a 3D model of this sedimentary structure that can later be used to model the seismic records currently being obtained by the permanent accelerograph network operating in the area. This model builds on previous efforts and incorporates new data. The geometry and dynamic properties of the soil layers were inverted using data from microtremor array measurements, seismic refraction profiles, boreholes, and geotechnical investigations. Phase-velocity dispersion curves of Rayleigh waves were determined at 27 sites in the basin using the spatial autocorrelation method (SPAC) introduced by Aki [1]. S-wave velocity profiles were inverted from these dispersion curves and the whole valley structure was interpolated using our new results and all previously available data. The proposed 3D model describes the geometry and shear-wave velocities of the Mygdonian and pre-Mygdonian sedimentary systems, and the top bedrock surface. Our results indicate that this 3D model correctly reflects the geometry and dynamic properties of the sedimentary layers. The case of Euroseistest, where the subsoil structure is the result of bringing together many disparate data, could be used as an example for similar alluvial basins throughout the world, where usually only scarce data is available.
 
Article
A damage detection method based on harmonic structural vibrations has been applied to reconstruct typical damage patterns of reinforced concrete beams and frames vibrating in three dimensions. It has been shown that comparison of harmonic vibrations of damaged and intact structures makes it possible to formulate optimization criterion which can be solved effectively for large number of unknowns. The application of combined genetic algorithms and Levenberg–Marquardt local search enabled to reconstruct vectors of distinct stiffness distributions containing up to 32 elements with stiffness loss as low as 5% and with accuracy of 1%. To test the algorithm in more realistic environment of practical data acquisition, measuring noise with various levels has been added to the analyzed signal. Such a method of damage detection can be quite effective during post-earthquake assessment of civil engineering structures or if one wants to control the stiffness loss of a reinforced concrete structure after it was accidentally overloaded.
 
Top-cited authors
Mihailo Trifunac
  • University of Southern California
Maria I Todorovska
  • University of Southern California
Mihailo Trifunac
  • University of Southern California
Julian Bommer
  • Imperial College London
Subhamoy Bhattacharya
  • University of Surrey