Mediterranean Sea and anthropogenic influences on ambient vibration amplitudes in the low-frequency and high-frequency domains in the Algiers region

Ambient vibrations have been continuously recorded at Dar El Beida, about 20 km from Algiers (Algeria). This data set allows determining that, in the low-frequency domain (<1 Hz), ambient vibration sources are mainly linked to Mediterranean Sea effects, while in the high-frequency domain, they are closely related to anthropogenic activity. Climatic conditions have an influence on the ambient vibration spectral amplitudes in the low-frequency domain, which is not the case in the high-frequency domain. The limit between the low-frequency and high-frequency domain, based on natural versus anthropogenic activity, is not clear cut and lies between 1.25 and 1.50 Hz. Variations of H/V peak amplitudes in the low-frequency domain are clearly linked to the climatic conditions. In the high-frequency domain, H/V peaks are not related to climatic conditions and cannot be clearly related to anthropogenic source changes

Smooth bumps in H/V curves over a broad area from single-station ambient noise recordings are meaningful and reveal the importance of Q in array processing: The Boumerdes (Algeria) case

International audienceSingle-station H/V curves from ambient noise recordings in Boumerdes (Algeria) show smooth bumps around 1 and 3 Hz. A complementary microtremor study, based on two 34 and 134-meter aperture arrays, evidences that these bumps are indeed real peaks produced by two strong VS contrasts at 37 and 118 meters depth, strongly smoothed by very high S-wave attenuation in the two sedimentary layers. These two H/V bumps, observed over a broad area, are meaningful and reveal the importance of Q in S-wave velocity modeling from microtremor array data processing. It also appears that Tertiary rocks should be, at least in some cases, taken into account, together with the Quaternary sediments, to explain single-station H/V frequency peaks, and therefore that considering only the first 30 m of soil for VS amplification evaluation, as usually recommended, sometimes leads to flaky results by artificially eliminating non-explained low-frequency peaks from the analysis

Using ambient vibration measurements for risk assessment at an urban scale: from numerical proof of concept to Beirut case study (Lebanon)

Abstract Post-seismic investigations repeatedly indicate that structures having frequencies close to foundation soil frequencies exhibit significantly heavier damages (Caracas 1967; Mexico 1985; Pujili, Ecuador 1996; L’Aquila 2009). However, observations of modal frequencies of soils and buildings in a region or within a current seismic risk analysis are not fully considered together, even when past earthquakes have demonstrated that coinciding soil and building frequencies leads to greater damage. The present paper thus focuses on a comprehensive numerical analysis to investigate the effect of coincidence between site and building frequencies. A total of 887 realistic soil profiles are coupled with a set of 141 single-degree-of-freedom elastoplastic oscillators, and their combined (nonlinear) response is computed for both linear and nonlinear soil behaviors, for a large number (60) of synthetic input signals with various PGA levels and frequency contents. The associated damage is quantified on the basis of the maximum displacement as compared to both yield and ultimate post-elastic displacements, according to the RISK-UE project recommendations (Lagomarsino and Giovinazzi in Bull Earthq Eng 4(4):415–443, 2006), and compared with the damage obtained in the case of a similar building located on rock. The correlation between this soil/rock damage increment and a number of simplified mechanical and loading parameters is then analyzed using a neural network approach. The results emphasize the key role played by the building/soil frequency ratio even when both soil and building behave nonlinearly; other important parameters are the PGA level, the soil/rock velocity contrast and the building ductility. A numerical investigation based on simulation of ambient noise for the whole set of 887 profiles also indicates that the amplitude of H/V ratio may be considered as a satisfactory proxy for site amplification when applied to measurements at urban scale. A very easy implementation of this method, using ambient vibration measurements both at ground level and within buildings, is illustrated with an example application for the city of Beirut (Lebanon). Graphical abstract

Implementing effects of site conditions in damage estimated at urban scale

International audienceLocal site effects due to geotechnical conditions modify seismic motions on surface. This implies that during a given earthquake, buildings located on soft sites may experience a higher damage than similar buildings resting on nearby rock sites. The aim of this study is to provide an estimation of the influence of site conditions on the buildings damage distribution. We combine an approach adapted from the Hazus methodology for the assessment of building damage, with the Borcherdt non linear site amplification factors, that enable to characterize the high and low frequency amplification as a function of VS30 (the average shear wave velocity in the upper 30 m) and ground motion levels. Analysis of obtained results indicates that, seismic damage expressed by the normalized mean damage index depends not only on seismic shaking level and building typology but also on site conditions through the shear wave velocity proxy. A regression relationship is established between the seismic damage and both shaking levels and site conditions, aiming at presenting a simple, rapid tool for estimating this damage at urban areas. An index, the “damage increase ratio”, is proposed to quantify the increase of damage resulting from site effects, and its dependence on loading level and site conditions are quantified and discussed for the main building typologies present in Algeria. Depending on the building typology, the overall damage may vary within a range of 2–5 for moderate shaking (0.1 g) between hard rock and very soft soil, and within a range 1–1.5 for strong shaking (0.5 g). The reduction of the impact of site conditions with increasing shaking level is directly linked with the nonlinear soil behavior