Cécile Cornou

Effects of site geometry on short-distance spatial coherency in Argostoli, Greece

The spatial variation of the earthquake ground motion over short distances can significantly affect the dynamic response of large and extended engineered structures, especially on sites with inhomogeneity in surface geology and geometry. In current practices, such variation is taken into account in terms of coherency, a function of frequency and distance, established on an essentially empirical basis and difficult to extrapolate at different sites. Hence, a better understanding of its physical significance and its relationship with the underlying ground structure is indispensable. A two-dimensional dense array, deployed at the small and shallow Koutavos-Argostoli valley in Cephalonia, Greece, provided an abundance of data to study the stochastic characteristics of seismic ground motions over very short distances. A set of 46 magnitude 2–5 events at epicentral distances 0–200xa0km has been selected for the analysis. The lagged coherency of the S-wave dominating seismogram was computed for each station-pair within the array and was averaged over various distance intervals for the whole data set. The results indicate a lack of a clear dependence of the average coherency on the magnitude, back azimuth or site-to-source distance of the event. The most striking result concerns the influence of the site geometry: the coherency is systematically lower for the pairs aligned perpendicular to the axis of the valley (2D) compared to those aligned in the parallel direction. This finding is consistent with the dominance of valley-edge generated surface waves propagating from one edge to the other. The averaged coherency estimates are only weakly represented by the existing parametric models, indicating its strong site dependent nature.

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

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.

Sensitivity of ground motion coherency to the choice of time windows from a dense seismic array in Argostoli, Greece

The stochastic estimation of coherency is perceived, in some studies, to be significantly influenced by the time window length under consideration. For the engineering purposes, usually, coherency is estimated from the strong motion, i.e., the shear (S-) wave segment of the recorded time series. However, the identification of a purely S-wave dominated window could be challenging in case of complex wave mixing on the seismograms. Moreover, there has been relatively little research on how much variation, in reality, is introduced in the coherency estimates owing to the choice of signal length. Therefore, the debate about the procedure for selecting a representative time window has remained inconclusive: different authors keep their different practices. The current article is an effort to shed light on the sensitivity of the coherency estimates to the choice of various time windows. The research draws on the dataset gathered from a dense seismic-array deployed at the small size, shallow alluvial valley of Koutavos-Argostoli, situated in Cephalonia Island, Greece. The lagged coherency over interstation distance ranges from 5 to 80xa0m has been estimated from a set of 46 earthquakes having magnitude 2–5 and epicentral distance 3–200xa0km, considering different lengths of time windows. The findings revealed that the statistics of coherency estimates derived from many events is only weakly sensitive to the selection of time window lengths provided that the segments include the same energetic pulse.

Characterization of site conditions (soil class, VS30, 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, VS30, 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 VS30 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 VS30 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.

Spatial Variability of the Directivity Pulse Periods Observed during an Earthquake

The ground velocity pulses generated by rupture directivity effects in the near‐fault region can cause a large amount of damage to structures. Proper estimation of the period of such velocity pulses is of particular importance in characterizing near‐fault seismic hazard and mitigating potential damage. We propose a simple equation to determine the pulse period as a function of the site location with respect to the fault rupture (defined by the hypocentral distance hypD , the closest distance to the rupture area clsD , and the length of the rupture area that breaks toward the site D ) and some basic rupture properties (average rupture speed and average rise time). Our equation is first validated from a dataset of synthetic velocity time histories, deploying simulations of various strike‐slip extended ruptures in a homogeneous medium. The analysis of the synthetic dataset confirms that the pulse period does not depend on the whole rupture area, but only on the parameter D . It also reveals that the pulse period is not sensitive to the level of slip heterogeneity on the fault plane. Our model is tested next on a real dataset build from the Next Generation Attenuation‐West2 Project database, compiling 110 observations of velocity pulse periods from 10 strike‐slip events and 6 non‐strike‐slip events. The standard deviation of the natural logarithm residuals between observations and predictions is ∼0.5. Furthermore, the correlation coefficient between observations and predictions equals ∼0.8, indicating that despite its simplicity, our model explains fairly well the spatial variability of the pulse periods.

Behavior of a RC Frame Under Differential Seismic Excitation

ABSTRACT Two very dense seismographic arrays were deployed in a seismically active area in Greece to incorporate the difference in amplitude and phase between two stations located within the dimension of a structure. The spatial variability in seismic ground motion is generally attributed to the wave passage effect, the incoherence effect, and the local site effect. It can cause severe damage on lifeline structures. This article studies the behavior of a reinforced concrete 2D frame structure subjected to differential seismic excitation at the supports. Both linear and nonlinear finite multifiber element models of the seismic behavior of this structure are used. The nonlinear behavior of the structure, under these different cases, displays different damage patterns and maximum displacements. This study allows evaluating the uncertainty that can be propagated through the finite element model, aiming at reducing variability for structural design purposes.Two very dense seismographic arrays were deployed in a seismically active area in Greece to incorporate the difference in amplitude and phase between two stations located within the dimension of a structure. The spatial variability in seismic ground motion is generally attributed to the wave passage effect, the incoherence effect, and the local site effect. It can cause severe damage on lifeline structures. This article studies the behavior of a reinforced concrete 2D frame structure subjected to differential seismic excitation at the supports. Both linear and nonlinear finite multifiber element models of the seismic behavior of this structure are used. The nonlinear behavior of the structure, under these different cases, displays different damage patterns and maximum displacements. This study allows evaluating the uncertainty that can be propagated through the finite element model, aiming at reducing variability for structural design purposes.

Wavefield Characteristics and Spatial Incoherency: A Comparative Study from Argostoli Rock‐ and Soil‐Site Dense Seismic ArraysWavefield Characteristics and Spatial Incoherency

Recordings from two dense arrays deployed at Argostoli, Cephalonia Island, Greece, are analyzed with three objectives: (1) exploring to what extent the diffracted surface waves influence the seismic wavefield at a rock site, (2) investigating the loss of coherency of ground motions, and (3) comparing the results for two nearby sites with different soil conditions. The two dense arrays under consideration consist of 21 velocimeters encompassing a central station in four concentric circles with diameters 10–180 m at the soft‐soil site and 20–360 m at the rock site. The datasets include 40 or more events with relatively homogeneous distributions in epicentral distances (10–200 km), magnitudes (2–5), and back azimuths. The wavefields are analyzed using the MUSIQUE algorithm: back azimuth and slowness of dominant incoming waves are extracted, and Love and Rayleigh waves are identified. Average lagged coherency estimates are provided for interstation distances 10–20, 20–30, 30–40, and 80–90 m. Coherency is observed to be generally larger on the rock site compared with the soft‐soil site, especially at frequencies below 5 Hz. At the soil site, lower coherency is observed for pairs along the valley‐perpendicular direction while no such directional dependence is observed at the rock. Although about 40%–60% of the seismogram energy at the soft‐soil site could be associated with diffracted surface waves (Love and Rayleigh) propagating along the valley‐perpendicular direction, only about 20% of energy at the rock site could be characterized as diffracted surface waves. Comparison with the widely quoted parametric models reveals little correlation with the observed decay of coherency at both sites. These significant differences between rock and soft‐soil array results indicate that the spatial incoherency is largely site dependent and is likely to be closely related to the formation of locally generated wavefield.

Towards a microzonation of the Greater Beirut area: an instrumental approach combining earthquake and ambient vibration recordings

Lebanon is situated on the 1000xa0km long Levant transform fault that separates the Arabic from the African tectonic plates. In Lebanon, the Levant fault splits up into a set of ramifications that had, in the past, generated major destructive earthquakes causing a lot of destruction and thousands of casualties. The most devastating one was the 551 A.D. offshore earthquake that destroyed Beirut, the capital of Lebanon. This paper presents a site effect study in Beirut, aimed at proposing a framework for future microzonation works in the city. It includes two complementary parts. A 6-month, temporary seismological experiment was first conducted to estimate the site response at 10 sites sampling the main geological units of Beirut on the basis of local and regional earthquake recordings. This spatially sparse information was then complemented by a large number (615) of microtremor measurements covering the Beirut municipality and part of its suburbs with a 400xa0m dense grid. The recordings were analysed with the standard site-to-reference and horizontal-to-vertical spectral ratio methods for earthquake recordings, and the horizontal-to-vertical ratio for ambient noise recordings. Significant ground motion amplification effects (up to a factor of 8) are found in a few areas corresponding to recent deposits. The consistency between results from earthquake and microtremor recordings allows proposing a map of the resonance frequencies within the city and its suburbs, with frequencies ranging from 0.5 to 5xa0Hz for the deepest deposits, and 5–10xa0Hz for shallow areas. Finally, the results are discussed and a way to combine the results obtained from the temporary stations to the great number of recordings coming from the permanent Lebanese network is proposed.