Luis G. Pujades

Intrinsic and scattering seismic wave attenuation in the Canary Islands

The Canary Islands volcanic complex is studied in terms of coda wave attenuation. A multiple lapse time window method, based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers, is applied to the available seismic data in order to obtain the intrinsic absorption (Qi−1) and the scattering attenuation (Qs−1) in the Canarian lithosphere. The analysis is performed for two hypocentral distance ranges: from 0 to 80 km and from 0 to 140 km. Results show that in both cases and for all the studied frequency bands (1–2, 2–4,6–8 and 8–10 Hz ) intrinsic absorption dominates. The low albedos found in the region indicate the low degree of heterogeneity in the Canarian lithosphere at the scale length of the studied frequencies. On the other hand the coda attenuation (Qc−1) calculated on the basis of the single-scattering theory gives values near (Qi−1) for low frequencies and near the total attenuation (Qt−1) for high frequencies. The degree of frequency dependence of the attenuation parameters is strong in all cases. A correlation of the observed attenuation parameters with the geological evidence for a hotspot-type archipelago and with other geophysical studies is suggested.

Coda-Q distribution in the Canary Islands

Abstract The Canary Archipelago, near the African plate between 27° and 30° N and 13° and 19° W, is included in the oldest region (> 135 m.y.) of the eastern Atlantic Ocean. The tectonic seismicity is moderate, with low magnitudes, and it is almost totally related to an active fault located between the islands of Tenerife and Gran Canaria. The purpose of this paper is to provide a clear idea of the anelasticity of coda waves in the region. In order to compute master dispersion curves as a function of the quality factor, Q, we have applied inversion methods to obtain analytical magnification curves for all the seismographic stations used. fp-t data, corresponding to the coda wave dispersion of 52 local and regional seismic events recorded at 6 seismographic stations, have been obtained. The fit of the data to the master curves determined yields estimates of coda-Q values and their frequency dependence. Most of the Q0 values determined, at 1 Hz, vary between 100 and 300, showing a strong frequency dependence in the range 0.2–2 Hz. The characteristic v value (degree of frequency dependence of Q) in the region, given by the law Q = Q0 fv is v = 0.9. By applying regionalization techniques an iso-Q0 map, for a reference frequency of 1 Hz, has been obtained for the Canary Archipelago. This map shows important lateral variations in Q0, reflecting the tectonics and the volcanism of the region. The lowest Q0 values are found along the trend of the fault and beneath the islands.

Shear wave velocity structure below the Iberian Peninsula as obtained by a detailed analysis of surface waves

Abstract A tentative tomography of Iberia based on broadband surface wave data is performed for the first time. In this paper we have complemented a previous study of the northern half of the Iberian Peninsula with a study of the southern half of the peninsula based on a detailed analysis of Rayleigh wave dispersion. Now we present the first shear velocity contour maps at different subcrustal depths. Quality digital records provided by the NARS array have been used. Standard techniques, as multiple filtering and time-variable filtering, are employed to efficiently remove higher mode interference and improve isolation of the fundamental mode Rayleigh wave from the seismograms, and thus to calculate the interstation Rayleigh wave group velocity. Frequency-domain Wiener deconvolution is used to determine the interstation phase velocity. We perform simultaneous inversion of phase and group velocity dispersion data by means of the stochastic inverse operator. The inversion results previously reported for the northern Iberian region together with the results obtained here are used for 3-D mapping of shear wave velocity over the Iberian Peninsula. Lateral variation in velocity at five depth intervals chosen in agreement with the theoretical earth models determined by joint inversion, is displayed for the lithosphere and asthenosphere of Iberia.

Capacity, fragility and damage in reinforced concrete buildings: a probabilistic approach

The main goals of this article are to analyze the use of simplified deterministic nonlinear static procedures for assessing the seismic response of buildings and to evaluate the influence that the uncertainties in the mechanical properties of the materials and in the features of the seismic actions have in the uncertainties of the structural response. A reinforced concrete building is used as a guiding case study. In the calculation of the expected spectral displacement, deterministic nonlinear static methods are simple and straightforward. For not severe earthquakes these approaches lead to somewhat conservative but adequate results when compared to more sophisticated procedures involving nonlinear dynamic analyses. Concerning the probabilistic assessment, the strength properties of the materials, concrete and steel, and the seismic action are considered as random variables. The Monte Carlo method is then used to analyze the structural response of the building. The obtained results show that significant uncertainties are expected; uncertainties in the structural response increase with the severity of the seismic actions. The major influence in the randomness of the structural response comes from the randomness of the seismic action. A useful example for selected earthquake scenarios is used to illustrate the applicability of the probabilistic approach for assessing expected damage and risk. An important conclusion of this work is the need of broaching the fragility of the buildings and expected damage assessment issues from a probabilistic perspective.

Shallow structure of part of northwestern Iberia from short-period Rayleigh-wave observations

Propagation of 0.2–2.5 s short-period Rayleigh-waves across a part of northwestern Iberia is investigated. Analysis of a seismic profile line with recorded seismograms up to distances of 70 km from the shot-point, provides group-velocities and attenuation coefficients of the fundamental mode. First higher-mode group-velocity has been obtained for only one of the studied cases. The studied region has been divided into two subrogions, the northwestern and the southeastern side from the shot-point. Inversion procedures applied to the velocity and attenuation data yield the shear velocity and the Q−1β models up to a depth of about 1.5 km for the NW, and 2.4 for the SE side. From the shear-wave obtained models corresponding to the NW and to the SE sides, it is possible to infer the existence of some degree of lateral variation of S velocity. The NW side presents velocities varying between 1.8 km · s−1 (at the surface) and 3.3 km · s−1 (at 1.2 km depth). The velocities in the SE model, for the same depths, are 2.2 and 3.4 km · s−1, respectively. Using only the fist six traces of the SE side, it is possible to infer a low-velocity region located between 0.4 km and 0.7 km depth. It seems that the velocities are higher in the complex geological region of the SE side than in the NW one, which is in agreement with P velocity models found by others. From the tentative anelastic attenuation coefficients, Q−1β models have been inferred for both sides of the seismic refraction profile. Although the obtained Q−1β models are somehow different in shape, the maximum Q−1β value is about 0.05 in both cases.

Anelastic structure of the Iberian Peninsula obtained from an automated regionalization algorithm and stochastic inversion

Abstract An automated regionalization of a reduced domain of the Earths surface, without a-priori seismotectonic information, and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis, a clustering procedure and the stochastic inversion algorithm to anelastic attenuation coefficients of Rayleigh waves. Our database is formed by path-averaged attenuation coefficients derived from fundamental mode Rayleigh waves crossing the Iberian Peninsula. The wavetrains were recorded at the broad-band stations installed some years ago in the Iberian Peninsula for the ILIHA project. Before the sequential application of the algorithms, the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10–120 s period range. All these maps were obtained by applying Yanovskayas formulation for laterally heterogeneous media to the set of path-averaged attenuation coefficients mentioned above. After that, we divided the Iberian Peninsula into six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis and the clustering algorithm. We then obtained, by stochastic inversion, the respective anelastic structure down to a depth of 225 km for the six homogeneous regions. The coefficients vary from 1.0 to 2.0×10−3 km−1 and the Q−1β structures, ranging from 110×10−3 to 40×10−3, suggest for five of the regions an asthenosphere with its upper and lower boundaries close to 80 and 180 km depth, respectively. A shallow beginning of the asthenosphere is detected for the sixth homogeneous region. This range is very similar to the depths deduced from previous tomographic studies of the Iberian Peninsula based on Rayleigh wave phase and group velocities. Finally, correlations obtained between the regions and some seismotectonic characteristics of the Peninsula are discussed. It is noteworthy that we cannot associate each of the six homogeneous regions with a single Hercynian, Alpine or Neogene domain.

Shear velocity structure beneath the Iberian Massif from broadband Rayleigh wave data

Abstract Up to a few years ago, dispersion analyses of surface waves across the Iberian Peninsula and adjacent zones were based on analog data recorded at the long-period Iberian stations. The installation of the NARS array on Iberian territory for a period of one year, has provided a greater station density than was previously available with the very few permanent long-period seismological stations installed on the peninsula. The NARS array also provided quality digital records, and increased the path coverage for two-station surface wave velocity measurements. Fundamental mode Rayleigh waves recorded at broadband stations belonging to this array have been analyzed to produce phase and group velocity dispersion curves for the period range 10–90 s. With the dataset now available, the elastic structure beneath the Iberian Massif has been investigated in terms of the shear velocity distribution as a function of depth. Time-variable filtering is employed to remove higher mode interference efficiently and to improve isolation of the fundamental mode Rayleigh wave from the seismograms. Multiple filtering is then used to compute group velocities at each station. The interstation Rayleigh wave group velocity can thus be easily calculated. Frequency-domain Wiener deconvolution is used to determine the interstation phase velocity. We perform inversion of velocity dispersion data containing both Rayleigh wave phase velocities and group velocities according to the generalized inversion theory by means of the stochastic inverse operator. The theoretical models for the lower crust and uppermost mantle beneath the Iberian Massif obtained by joint inversion, show a continental lithosphere with a thickness of 81 km. The crustal and subcrustal velocities are greater than in other areas of the Iberian Peninsula. The asthenosphere appears as a layer 100 km thick defined by very low velocities when compared with the rest of the peninsular area. Both the lithosphere and the asthenosphere exhibit a low velocity channel. In the lithosphere a subcrustal low velocity channel has velocity constrained between 4.33 and 4.62 km s−1. In the asthenosphere the low velocity channel is constrained between 4.13 and 4.36 km s−1.

Modernist Unreinforced Masonry (URM) Buildings of Barcelona: Seismic Vulnerability and Risk Assessment

The main objective of this work is to assess the vulnerability and seismic risk of typical existing modernist unreinforced masonry (URM) modernist buildings and aggregates situated in the Eixample district of Barcelona, part of the architectural heritage of the city. The context of the analysis is the methodology proposed by the Risk-UE project. The buildings are characterized by their capacity spectrum and the earthquake demand is defined by the 5% damped elastic response spectrum, considering deterministic and probabilistic earthquake scenarios. A discussion addresses the basis of the seismic damage states probabilities and the calculated damage index. An important research effort has been focused on the buildings modeling. All the architectural elements and their mechanical properties have been studied and evaluated accurately. It has been evidenced that a detailed and complete knowledge of all the structural elements existing in this type of buildings influence directly their behavior and hence the calculations and the results. The analysis of the isolated buildings and of the aggregate building has been performed for both mentioned seismic scenarios. Finally, a complete discussion of the results is included.

Parametric model for capacity curves

A parametric model for capacity curves and capacity spectra is proposed. The capacity curve is considered to be composed of a linear part and a nonlinear part. The normalized nonlinear part is modelled by means of a cumulative lognormal function. Instead, the cumulative Beta function can be used. Moreover, this new conceptualization of the capacity curves allows defining stiffness and energy functions relative to the total energy loss and stiffness degradation at the ultimate capacity point. Based on these functions, a new damage index is proposed and it is shown that this index, obtained from nonlinear static analysis, is compatible with the Park and Ang index obtained from dynamic analysis. This capacity based damage index allows setting up a fragility model. Specific reinforced concrete buildings are used to illustrate the adequacy of the capacity, damage and fragility models. The usefulness of the models here proposed is highlighted showing how the parametric model is representative for a family of capacity curves having the same normalized nonlinear part and how important variables can be tabulated as empirical functions of the two main parameters defining the capacity model. The availability of this new mathematical model may be a powerful tool for current earthquake engineering research, especially in seismic risk assessments at regional scale and in probabilistic approaches where massive computations are needed.

Nanozonation in Dense Cities: Testing a Combined Methodology in Barcelona City (Spain)

Microzonation is widely used in seismic risk evaluations to define the predominant period values, which are usually associated with extended areas of a few hundred meters. However, the representative values corresponding to these areas are obtained from few measurements in each area. Thereby, results are accurate only in the case of depth-dependent soils. However, not detected narrow and sharp lateral changes in soil are potentially the cause of imprecision and could be a source of specific errors. This article aims to present several tests conducted in order to emphasise the importance of accurate selection of points, to underscore the necessity of more precise and detailed evaluations, and to suggest a possible methodology to select the most appropriate data acquisition points. Results highlight the need to divide microzonation areas into smaller zones for a precise evaluation in locations where sudden changes in soil characteristics exist. Therefore, in such sites the requirement of nanozonation appears; defining zones with the same soil response. Distance between vibration measurements could be the main problem for nanozonation; data acquisition in areas with irregular geology can be time consuming when a precise analysis is required. In the most complicated environments or in dense cities, it could even be unfeasible. Consequently, it is necessary to establish a functional methodology to adequately distribute the measurement points throughout the area. On this occasion, three sites in Barcelona city were studied. This city is surrounded by mountains at NW, W, and S, and by the Mediterranean Sea at N and E. As a consequence, the shallow geology is characterized by many paleochannels and streams that are currently buried. These geological structures most likely affect the soil response. Several tests were carried out to determine this dependence. The tests were based on Ground Penetrating Radar (GPR) surveys to define the paleochannels position and on vibration measurements in order to define properly the soil response. The results from both methods were compared to the known geology to accurately define the effect of the shallow geological structures in the predominant period and in the GPR images. Areas with the same geological unit but different materials were identified in the GPR images, allowing the selection of the most appropriate distance between vibration measurements in each place. As a final result, predominant periods that were measured over the same geological unit but over different material showed changes higher than the 40% in short distances. This procedure could improve the soil response maps, including nanozonation.