Variation Of Seismic Response Research Articles

Reliability based multi-objective robust design optimization of steel moment resisting frame considering spatial variability of connection parameters

The focus of this paper is on achieving robustness in the seismic design against uncertainties in the design process. In this regard, we present a reliability based robust design optimization approach to achieve designs that are not only safe and cost efficient, but also robust against uncertainties. The aim here is to adjust easy-to-control design parameters to reduce the variability of the seismic response to hard-to-control noise parameters while also considering design safety and budget. The proposed approach is demonstrated with a steel moment resisting frame design, considering uncertainties and spatial variability in connection parameters. Here, the sizes of the steel sections constitute the design parameters, while the uncertain and spatially variable parameters of the Ibarra–Krawinkler connection model are treated as noise parameters. The design objectives include the collapse prevention reliability conditional on a maximum earthquake intensity level, as well as the initial cost of the structure. To reduce computation demands in calculating the seismic response variation, design parameters that have a negligible effect on seismic response are first eliminated through sensitivity analysis. Furthermore, the seismic response variation is evaluated parametrically to investigate the effect of connection parameter spatial variability. Finally, the authors demonstrate the use of proposed robust design optimization approach to obtain a Pareto Front, a collection of optimal designs that are optimized for both reliability and cost.

Impact of uncertainty in remote sensing DEMs on topographic amplification of seismic response and Vs 30

Impact of topography on spatial variation of seismic response is well-observed synthetically, experimentally, and visually during seismic events. Numerical and experimental investigations for predicting topographic impact on seismic response are often limited to isolated and/or synthetic hills and ridges. Furthermore, most of these studies only focus on one of the many terrain parameters necessary for evaluating the impact of topographic features on amplification or de-amplification of seismic response. Seismic events located in rough terrain, like the 2005 Kashmir earthquake in northern Pakistan, exhibit intensified seismic response and associated devastation at hill ridges and on inclined slopes. Satellite remote sensing-derived digital elevation models (DEMs) are frequently and effectively used to compute topographic attributes and seismic parameters to evaluate topographic seismic response. However, the influence of DEM random errors on computed topographic attributes and seismic response is often overlooked. This study uses the Shuttle Radar Topography Mission (SRTM) DEM (90 m) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) GDEM (30 m) to quantify uncertainties in the computed topographic attributes and seismic parameters and ultimately the impact of these uncertainties on topographic seismic response. Accuracy assessment of the DEMs shows root mean square error (RMSE) of 13.78 m in the ASTER DEM and 23.71 m in the SRTM DEM. The influence of DEM errors on derived topographic attributes quantified through Monte Carlo simulations shows higher uncertainty in slope and aspect computed from ASTER DEM than from SRTM DEM. The influence of uncertainty in the SRTM and ASTER DEMs shows significant impact on the computed seismic parameters of slope, relative height, and V s 30 and ultimately derived topographic seismic response.

Effects of three-dimensionality on seismic response of Gaussian-shaped hills for simple incident pulses

In this paper the boundary element algorithm which uses the time-convoluted traction kernels is applied to a numerical parametric study on the seismic behavior of three-dimensional Gaussian-shaped hills subjected to vertically propagating incident waves. All calculations were executed in the time-domain and the medium was assumed to have a linear elastic constitutive behavior. Results are discussed in both time and frequency domain with respect to the dimensionless parameters. It was shown that wave length and site geometry, including shape and dimension ratios and, to some extent, wave type are the key independent parameters governing hill amplification behavior. Comparing two- and three-dimensional hills with similar shape ratios, two-dimensional hill had greater characteristic periods, where the three-dimensional hill had greater maximum amplification potential. Three-dimensionality has a strong effect on the seismic responses of the hill; however the rate of seismic response variation with the three-dimensionality factor depends on the shape ratio. It was shown that two-dimensional behavior was dominant in low height three-dimensional hills, however, as the shape ratio increased, three-dimensionality effects appeared and the seismic response of the hill tends toward the axisymmetric three-dimensional hill.

Evaluation of the measured seismic response of the Mexico City Cathedral

AbstractThe seismic response of the Mexico City Cathedral built of very soft soil deposits is evaluated by using motions recorded in various parts of the structure during several moderate earthquakes. This unique set of records provides significant insight into the seismic response of this and other similar historic stone masonry structures.Free‐field ground motions are carefully compared in time and frequency domains with motions recorded at building basement. The dynamic characteristics of the structure are inferred from the earthquake records by using system identification techniques. Variation of seismic response for different seismic intensities is discussed. It is shown that, due to the soil–structure interaction, due to large differences between dominant frequencies of earthquake ground motions at the site and modal frequencies of vibration of the structure, and due to a particularly high viscous damping, seismic amplifications of ground motion in this and similar historic buildings erected on soft soil deposits are much smaller than that induced in most modern constructions. Nevertheless, earthquake records and analytical results show that several components of the structure such as its central dome and the bell towers may be subjected to local vibrations that significantly amplify ground motions. Overall, results indicate that in its present state the structure has an acceptable level of seismic safety. Copyright © 2008 John Wiley & Sons, Ltd.

A study of the seismic response of the city of Benevento (Southern Italy) through a combined analysis of seismological and geological data

A previous analysis [Improta, L., G. Di Giulio, and A. Rovelli (2005). Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings, J. Seism. 9, 191–210.] of small magnitude earthquakes recorded at 12 sites within the city of Benevento has stressed the significant role played by near-surface geology in causing variability of the ground motion. In this paper, we extend the study of the seismic response from 12 sites to the entire urban area. Based on inferences from the comparison at the 12 sites between earthquake and ambient vibration results, we have collected ambient noise at about 100 sites within the city, intensifying measurements across the main shallow geological variations. We use borehole data to interpret ambient noise H/V spectral ratios in terms of near-surface geology comparing H/V curves to theoretical transfer functions of 1D models along five well-constrained profiles. On the basis of geological, geotechnical, and seismic data, we identify three main typologies of seismic response in the city. Each type of response is associated to zones sharing common soil conditions and similar soil classes according to building codes for seismic design. Moreover, we find that the spatial variation of the seismic response in the ancient town area is consistent with the damage pattern produced by a very destructive, well-documented historical earthquake that struck the city in 1688, causing MCS intensity of IX–X in Benevento. Finally, we use ground motions recorded during the experiment by Improta et al. [Improta, L., G. Di Giulio, and A. Rovelli (2005). Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings, J. Seism. 9, 191–210.] to generate synthetic seismograms of moderate to strong (Mw 5.7, Molise 2002 and Ms 6.9, 1980 Irpinia) earthquakes. We calibrate the random summation technique by Ordaz et al. [Ordaz, M., J. Arboleda, and S.K. Singh (1995). A scheme of random summation of an Empirical Green's Function to estimate ground motions for future large earthquakes, Bull. Seism. Soc. Am. 85, 1635–1647.] using recordings of these earthquakes available in Benevento. After a satisfactory fit between observed and synthetic seismograms, we compute response spectra at different sites and speculate on effects of the geology class at large level of shaking, including soil nonlinearity. We find that large discrepancies from design spectra prescribed by seismic codes can occur for a wide sector of Benevento, especially for periods < 0.5 s.

Fluid substitution in shaley sediment using effective porosity

The traditional method of fluid substitution in porous rock requires the total porosity and the elastic modulus of the mineral phase as input and assumes that the fluid reaches instantaneous hydraulic equilibrium throughout the pore space. This assumption may not be appropriate for shaley sediment because of the low permeability of shale and the resulting immobility of the water in it. To address this problem, we propose an alternative method that uses effective porosity instead of total porosity. Effective porosity is lower than total porosity if porous shale is present in the system. A new, composite mineral phase is introduced, which includes the porous water-saturated shale together with the nonporous minerals and whose elastic modulus is an average of those of its components, including the porous shale. This alternative method increases the sensitivity of the elastic properties of sediment-to-pore-fluid changes and therefore may be used as a physics-based theoretical tool to better explain and interpret seismic data during exploration as well as variations in seismic response as hydrocarbon production progresses.

Imaging global chemical and thermal heterogeneity in the subcontinental lithospheric mantle with garnets and xenoliths: Geophysical implications

Suites of mantle-derived xenoliths in volcanic rocks provide estimates of the geothermal gradient and composition of the subcontinental lithospheric mantle (SCLM) at the time of the volcanic eruption. The development of single-grain thermometry and barometry, applied to xenocryst minerals in volcanic rocks, has greatly expanded the number of localities for which such data can be obtained and made it feasible to map the geology of the SCLM on a broader scale, both vertically and laterally. From garnet xenocrysts, it is possible to derive profiles showing mean values of olivine composition, bulk-rock composition, density and seismic velocities, as well as geotherm parameters and constraints on the thickness of the SCLM. Geochemical profiles, coupled with Re–Os dating of peridotites and their enclosed sulfide minerals, show that Archean or Proterozoic SCLM is preserved at shallow levels beneath many areas of younger tectonothermal age; this implies rapid vertical variations in Vs and Vp with depth, which may affect seismic interpretations. Data from several hundred localities worldwide define a secular evolution in the composition of the SCLM, related to the tectonothermal age of the overlying crust. Archean SCLM is typically strongly depleted in basaltic components, highly magnesian and thick (160–250 km), and has low geotherms; Phanerozoic SCLM is typically fertile (rich in basaltic components), Fe-rich, thin (50–100 km) and has a range of high geotherms; Proterozoic SCLM (much of which may be reworked Archean mantle) tends to be intermediate in all respects. The correlated variations in SCLM fertility, lithospheric thickness and geotherm reinforce the effects of each on seismic velocity, and produce more rapid lateral variations in seismic response than would result from thermal effects alone. These correlations are the key to using seismic tomography images to map the lateral extent of different types of SCLM.

Sensitivity analysis of Gassmann's fluid substitution equations: Some implications in feasibility studies of time-lapse seismic reservoir monitoring

Here, we discuss the sensitivity of the seismic response to uncertainties in the physical parameters of the reservoir rock. For this purpose, a probabilistic sensitivity analysis of Gassmann's fluid substitution equations using a Monte Carlo approach was carried out. We represented uncertainties related to each parameter as probability density functions to evaluate the contribution of each parameter uncertainty to the variance of the seismic response ( V p), calculated by means of the Monte Carlo approach. We show that uncertainties related to grain density ( ρ gr), dry shear modulus ( G d) and dry bulk modulus ( K d) contribute more significantly on the variance of V p, if all parameters are uncorrelated. This outcome changes, when physical dependencies are represented as correlations in the Monte Carlo sampling of some of the parameters. In this sense, correlations distribute more evenly the contributions to uncertainty in V p. On the other hand, we also evaluated scenarios of fluid substitution, in which fluid 1 is replaced by fluid 2, with the corresponding variations in seismic response. In this case, V p2 is the P-wave velocity of rock saturated with a fluid 2. If V p2 were forecasted from an initial set of parameters of the rock saturated with fluid 1 ( V p1, V s1, etc.) the uncertainties related to V p1, V s1 and K gr would contribute more significantly to the variance of V p2. From these three initial parameters, the most important contributions come form V p1 and V s1. Concomitantly, we evaluated the contribution of possible variations in fluid phase density and bulk modulus and of a pore pressure perturbation (4MPa) for several scenarios of connate and injection fluids on the variance of V p. We did this for several values of initial differential pressure. Results indicate that the contribution of the elastic piezosensitivity and possible changes in the fluid phase properties depend not only on the initial differential pressure, but also on the type of fluids involved in substitution process. We conclude that sensitivity information, limited in this case to Gassman's equations, can be used as a tool to improve feasibility studies in time-lapse seismic reservoir monitoring and as a priori qualitative knowledge. The latter can guide the inversion process or help to diminish the uncertainties due to poorly constrained inversion schemes.

HINTS ABOUT SITE AMPLIFICATION EFFECTS COMPARING MACROSEISMIC HAZARD ESTIMATE WITH MICROTREMOR MEASUREMENTS: THE AGRI VALLEY (ITALY) EXAMPLE

We approach from a new standpoint the problem of estimating seismic hazard for some towns and villages located in Val d'Agri area (Southern Italy) that in the past have been affected by several seismic events. The estimates are carried out using a method that is based on the analysis of site seismic history extracted from macroseismic catalogues. To study the influence of site effects two different procedures have been performed: in the first, seismic hazard estimates have been deduced from epicentral data only, in the second, intensity data actually observed at the site are also considered. The difference between the two estimates can be correlated with local variations of seismic response due to local geological features which are responsible for possible cases of amplification. In order to validate the presence of such correlation, seismic hazard estimates have been compared to site amplification measurements obtained by using the HVSR (Horizontal to Vertical Spectral Ratio) technique. Our findings reveal a good correlation between seismic hazard enhancements and the presence of site amplification effects. The application of this kind of analysis to the Val d'Agri area has pointed out that the joint estimates of site seismic hazard enhancement and HVSR measurements could be a helpful tool to identify problems related to seismic microzonation.

The integrated use of 3D seismic data, well information and seismic forward modelling – Lake Hope field area

The Lake Hope 3D survey was acquired in 1992 and covers five producing oil fields in the Lake Hope Block of the South Australian sector of the Cooper/Eromanga Basins. A total of 38 wells have been drilled within the survey limits, of which 24 are currently producing from eight different reservoir levels. The bulk of the production comes from the Early Cretaceous/Namur Sandstone and Jurassic Birkhead Formation, Hutton Sandstone and Poolowanna Formation. The reservoirs occur between 1.2 and 1.5 seconds two way time (1300?1800 m below MSL). The excellent quality of the Lake Hope 3D seismic data and the large number of wells in the area allowed extensive use of seismic forward modelling. The object was to better understand the quality and distribution of the various reservoirs in the area. Integration of well data, seismic amplitude information and production history made it possible to define the presence and extent of producing Birkhead Formation sands. Two dimensional forward modelling was utilised to interactively alter well-logs and observe the corresponding change in seismic response. By using these procedures and horizon slices from the 3D seismic volume it was possible to prognose additional areas where similar sands are likely to be present. The Hutton Sandstone reservoir contains over one quarter of the oil within the Lake Hope Fields and is also a very productive reservoir elsewhere in the Eromanga Basin. The Birkhead Formation is the top seal to this unit and the Birkhead/Hutton Interface marks the change from the high energy braided stream environment of the Hutton Sandstone to the lacustrine/low energy fluvial environment of the Birkhead Formation. This interface varies from an abrupt change to a transitional change over a 10 metre interval. Reservoir quality and pay thickness are heavily dependent on the nature of this transition. Well information was correlated with observed seismic response to establish the character of the Birkhead/Hutton Interface within the area of the 3D seismic survey. The above studies demonstrate the importance of integrating geological information, well production history and observed variations in seismic response to fully utilise 3D seismic data.

The prediction of high porosity chalks in the East Hod Field

The East Hod Field in the Norwegian Block 2/11 produces from the Lower Tertiary and Upper Cretaceous Chalk Group. Although at first sight the trapping mechanism of the East Hod Field appears to be structural, increased understanding of porosity preservation, combined with new seismic interpretation, demonstrates that there is also a strong stratigraphic component. Both autochthonous and allochthonous facies occur, the latter forming the principal reservoirs. The extremely homogeneous nature of the matrix means that variations in seismic response directly reflect changes in pore volume, and a good correlation between seismic impedance and porosity is observed. High quality 3D seismic data made it possible to undertake seismic inversion which, together with the correlation between seismic impedance and porosity, allows prediction of porosity variations within the inter-well volume. The results indicate that within the allochthonous reservoir units, porosity is maintained in down-flank areas at depths previously believed to be predominantly tight. Seismic inversion is particularly accurate for predicting porosity because of the exceptionally uniform nature of the chalk matrix.

Spatial variations in seismic motions

A method for evaluating the spatial variations in strong seismic motions for a linear, homogeneous, and horizontally stratified soil layer system is presented. The procedure accounts for the focal depth and the epicentral distance, the corresponding angle of incidence, and the relative contributions of both shear and Rayleigh waves. The inclined propagation of shear waves is studied using the multiple reflection refraction theory. The range of possible values of Rayleigh wave phase velocity in the soil layer system is determined, and using an averaging procedure the Rayleigh wave amplification factors are computed. The influences of various factors on the spatial variations in seismic response are discussed. The method is general so that it can be used for various problems involving spatial motion computations. The application of the method in computing the responses of a soil–pile system is described and some typical results given.