Evaluation of site effects by means of Joint Analysis of Sonogram and Standard Spectral Ratio (JASSSR)

In order to evaluate thefrequency dependence of site effects, thejoint analysis of the standard spectralratio technique with a sonogram approach isproposed (Joint Analysis of Sonogram andStandard Spectral Ratio). Two tools aresuggested: the enhanced sonograms andthe differential sonograms. Inparticular, the local geology effects areinvestigated by considering both theamplification of the seismic signals andtheir prolongation. Tests carried out onsynthetic signals show the capability ofthe sonogram approach to evaluate thefrequency-dependent lengthening of theseismic signal. The JASSSR is applied todata recorded by a dense local networkdeployed during the Umbria-Marche seismicsequence of 1997 and to data recorded inthe Volvi Test site (Euroseistest, Greece).The results show that using the JASSSR gives amore complete overview of the effects oflocal geology on seismic ground motion than maybe obtained by using the StandardSpectral Ratio alone. In particular, theeffect of secondary arrivals, both on theamplitude and on the lengthening of theseismic ground motion, can be pointed out.In addition, the characteristic frequencyof these arrivals can be evaluated.     

Prediction of strong ground motion at EURO-SEISTEST site using the empirical Green's function method

This paper presents some tests on the empirical Green's function method, performed in order to check its effectiveness to predict strong ground motion during future large earthquakes. These investigations basically consist of blind or pseudo-blind tests using part of the data set obtained at the Volvi (Greece) test-site within the framework of the EURO-SEISTEST project. In a first step, a moderate event (M_L=4.1) is simulated by using a small event (M_L=2.5), and taking into account the a priori knowledge of the relevant source parameters (magnitude and stress-drop) for both the base and target events. This check emphasizes the sensitivity of the prediction to the stress-drop values. In a second step, a pseudo-blind prediction is carried out in which the information on the target event is only the magnitude (M_L=5.3) besides the geometrical parameters such as source location, strike and dip. The other important parameters (seismic moment, fault area, stress-drop) are determined on the basis of specific empirical scaling laws derived from several small and moderate events occurring in the area. The synthetic motions are computed for two nearby sites, which are located on the southern edge and in the center of the Mygdonian graben, respectively, and correspond to much different site conditions: weathered rock and thick sediments. They are found in good agreement with the observations, which were unveiled after the simulation. In particular, the amplitude and the phase of the late, local surface waves generated on the southern edge of the graben, are very well reproduced at valley center. Finally, the last step is an attempt to predict strong ground motion for a hypothetical large earthquake of magnitude M_L=6.5. The results are shown to depend very significantly on the scaling laws.     

Seismic Microzonation: A Comparison between Geotechnical and Seismological Approaches in Pointe-à-Pitre (French West Indies)

Pointe-à-Pitre, the main city of Guadeloupe in the French West Indies, has on several occasions been partially destroyed by major historical earthquakes. Moreover, a post-seismic assessment of the damage from the 1985 Montserrat earthquake indicates that the town is prone to site effects. Consequently, from 1996 to 1998, BRGM conducted a seismic microzonation study based on geotechnical and geological data. At the same time, three seismological studies were being conducted – two based on earthquake recordings using a time-series analysis and the classical spectral ratio (CSR) method (CETE/LCPC and BRGM), and the third based on noise measurement at 400 points using the horizontal-to-vertical noise ratio (HVNR) method (CETE/LCPC). The objective of this paper is not to carry out a new microzonation study by taking into account all the results, but rather to show in what respects the results of these different methods are in agreement or not. A comparison of the results of the seismological studies with the geotechnical microzonation shows that they are in fairly good agreement, albeit with some discrepancies. The results indicate that the seismological methods and the geotechnical data are highly complementary and should be used together in compiling seismic transfer-function microzonation maps. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparisons among the five ground-motion models developed using RESORCE for the prediction of response spectral accelerations due to earthquakes in Europe and the Middle East

This article presents comparisons among the five ground-motion models described in other articles within this special issue, in terms of data selection criteria, characteristics of the models and predicted peak ground and response spectral accelerations. Comparisons are also made with predictions from the Next Generation Attenuation (NGA) models to which the models presented here have similarities (e.g. a common master database has been used) but also differences (e.g. some models in this issue are nonparametric). As a result of the differing data selection criteria and derivation techniques the predicted median ground motions show considerable differences (up to a factor of two for certain scenarios), particularly for magnitudes and distances close to or beyond the range of the available observations. The predicted influence of style-of-faulting shows much variation among models whereas site amplification factors are more similar, with peak amplification at around 1s. These differences are greater than those among predictions from the NGA models. The models for aleatory variability (sigma), however, are similar and suggest that ground-motion variability from this region is slightly higher than that predicted by the NGA models, based primarily on data from California and Taiwan.     

Site effect assessment using KiK-net data: Part 1. A simple correction procedure for surface/downhole spectral ratios

When data is available, the estimation of site effects is usually performed using the “standard spectral ratio” (SSR) technique with respect to an outcropping, reference rock site. This study uses the Japanese KiK-net network, which has more than 600 pairs of surface-downhole stations allowing the computation of empirical borehole transfer functions, consisting of mean spectral ratios of surface over downhole recordings. The borehole transfer function deviates from the SSR in two respects: the reference is located at depth, and the downhole velocity varies from one site to another. These differences bias the estimation of the transfer function with reference to a standard outcrop rock site. The goal of this paper is to develop a simple and robust methodology to correct for such bias. The proposed correction procedure consists of two steps: a depth correction designed to account, in a simplified and physically acceptable way, for the existence at depth of destructive interferences and the absence of free-surface effects in the high-frequency range; and an impedance correction designed to normalize the shear wave velocity at depth. The depth correction involves a simple, frequency-dependent curve to be adapted for each site as a function of the first destructive interference frequency at depth. The impedance normalization combines the use of “generic” rock velocity profiles and a quarter-wavelength approach, resulting in a smooth frequency-dependent amplitude correction. The proposed methodology is applied on a large subset of KiK-net data in view of analysing the correlation between site amplification factors and site parameters in a companion paper.     

Composition of hornblendes formed during the hercynian progressive metamorphism of the Aracena metamorphic belt (SW Spain)

This paper attempts to illustrate the chemical variations of metamorphic hornblendes regarding host rocks and prograde variations. Changes related to bulk chemistry (orthoamphibolites) mainly concern Si, Al, Mg, Fe_tot and Ca. The Mg, Fe²⁺ and Fe³⁺ contents of hornblendes are, however, not strictly related to host rook compositions and Mg enrichments are correlated with increasing Fe³⁺ contents in the amphiboles. Thus, variations of oxygen fugacity may control the Mg contents of the Ca amphiboles studied but this does not show clear relations with the prograde metamorphism. The most sensitive but irregular variation related to the metamorphic conditions is the prograde enrichment of the alkalis into the A vacant position and an increase of the (Na+K)_tot/Na+K+Ca ratios of the amphiboles. Increasing Ti and Al^IV contents as well as decreasing Al^VI concentrations are also, but much less evidently, related to increasing T and P. A variation trend from tschermakitic to edenitic hornblendes may be drawn using Shido's end members calculation; this tendency and the relative deficiency of Al^VI contents in the low-grade members suggests that the amphiboles studied were subjected to conditions of a low-pressure metamorphism type. Such a conclusion is in agreement with the occurrence of andalusite-cordierite/sillimanite-cordierite associations in the metapelitic rocks, and the absence of Fe-rich garnet and epidote from the orthoamphibolites of the amphibolite facies at Aracena. Comparisons with Ca amphiboles from other metamorphic areas show, in agreement with various authors, that Abukuma hornblendes are similar to those encountered in high-grade thermal aureoles and tonalitic intrusives but different from the hornblendes of Barrovian metamorphism types.     

Characterization of site conditions (soil class,V<Subscript>S30</Subscript>, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

Data provided by accelerometric networks are important for seismic hazard assessment. The correct use of accelerometric signals is conditioned by the station site metadata quality (i.e., soil class, V_S30, velocity profiles, and other relevant information that can help to quantify site effects). In France, the permanent accelerometric network consists of about 150 stations. Thirty-three of these stations in the southern half of France have been characterized, using surface-wave-based methods that allow derivation of velocity profiles from dispersion curves of surface waves. The computation of dispersion curves and their subsequent inversion in terms of shear-wave velocity profiles has allowed estimation of V_S30 values and designation of soil classes, which include the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) perform both active (multi-analysis surface waves) and passive (ambient vibration arrays) measurements to derive dispersion curves in a broadband frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities. Even when the logistic contexts are sometimes difficult, the use of surface-wave-based methods is suitable for station-site characterization, even on rock sites. In comparison with previous studies that have mainly estimated V_S30 indirectly, the new values here are globally lower, but the EC8-A class sites remain numerous. However, even on rock sites, high frequency amplifications may affect accelerometric records, due to the shallow relatively softer layers.     

Seismic response of Beirut (Lebanon) buildings: instrumental results from ambient vibrations

Resonance period is a key parameter in the seismic design of a structure, thus dynamic parameters of buildings in Beirut (Lebanon) were investigated based on ambient vibration method for risk and vulnerability assessment. Lebanon is facing high seismic hazard due to its major faults, combined to a high seismic risk caused by dense urbanization in addition to the lack of a seismic design code implementation. For this study, ambient vibration recordings have been performed on 330 RC buildings, period parameters extracted and statistically analyzed to identify correlations with physical building parameters (height, horizontal dimensions, age) and site characteristics (rock sites or soft sites). The study shows that (1) the building height or number of floors (N) is the primary statistically robust parameter for the estimation of the fundamental period T; (2) the correlation between T and N is linear and site dependent: T ≈ N/23 for rock sites and N/18 for soft sites; (3) the measured damping is inversely proportional to the period: the taller the building the lower is the damping; (4) a significant overestimation of the period exists in current building codes. However part of the large discrepancy with building code recommendations may be due to the very low level of loading.     

Phases tectoniques superposées dans les métasédiments Précambriens du domaine côtier occidental de la Cote d'Ivoire

Microstructural analysis of the coastal high-grade metasediments of the South Western Ivory Coast show at least four folding phases. The oldest phase does not seem to affect the lower Birrimian mesometamorphic series and it is concluded that this phase affected a pre-Eburnean crystalline and granitic basement. The name Liberian has been applied in the Ivory Coast to this basement. The Birrimian mesometamorphic rocks record the three youngest folding phases: two of them may correspond to two major north north east to south south west synmetamorphic phases of the Eburnean orogeny. The last phase is clearly post-Eburnean and may be synchronous with the north west south east trending chevron-folds (with crenulated schistosity) and the main north north east strike slip faults of the West African shield in the Ivory Coast.