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): 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).

Deriving amplification factors from simple site parameters using generalized regression neural networks: implications for relevant site proxies

“…It was originally compiled by C. Cornou (Salameh 2016;Salameh et al 2017) and consists of about 600 Japanese KiK-net sites, more than 200 sites from the USA, made available by D. Boore (http://quake.usgs.gov/~boore), and 22 European sites measured during the NERIES project (Di Giulio et al 2012). The main characteristics of this set of site profiles are presented in Salameh (2016), Almakari et al (2016) and Salameh et al (2017): They are primarily usual (i.e., normally soft, with S-wave velocities generally >200 m/s) and stiff soils, with shallow to intermediate thicknesses, <200 m in most cases, with only few sites-about 50-with fundamental frequency below 1 Hz. They generally have "normally hard" to very hard underlying bedrock; the "bedrock" velocity, i.e., the velocity of the underlying half-space, varies from <500 m/s to >3 km/s.…”

Estimating urban seismic damages and debris at the building-level: Application to the city of Beirut, Lebanon

“…A similar approach as in Salameh et al (2017) was adopted for the estimation of urban seismic damages at the building-level. We use the same dataset of Salameh et al (2017) composed of 887 1-D multi-layered soil profiles from worldwide available profiles, 60 synthetic base rock accelerations ranging from 0.02 to 8.6 m/s 2 mimicking accelerations produced by earthquakes with magnitude ranging from 3 to 7 and epicentral distances from 5 to 100 km.…”

“…A similar approach as in Salameh et al (2017) was adopted for the estimation of urban seismic damages at the building-level. We use the same dataset of Salameh et al (2017) composed of 887 1-D multi-layered soil profiles from worldwide available profiles, 60 synthetic base rock accelerations ranging from 0.02 to 8.6 m/s 2 mimicking accelerations produced by earthquakes with magnitude ranging from 3 to 7 and epicentral distances from 5 to 100 km. Finally, 141 Single-Degree Of Freedom (SDOF) oscillators representing different building typologies, namely masonry buildings (Class 1), non-designed reinforced concrete buildings (Class 2) and DCL reinforced concrete buildings (Class 3).…”

Joint analysis of Rayleigh-wave dispersion curves and diffuse-field HVSR for site characterization: The case of El Ejido town (SE Spain)

“…Nevertheless, such classifications of soil based solely on the average shear-wave velocities for the upper 30 m (VS30) ignore velocity contrasts between soil layers below 30 m, especially in cases of deep sites as Campo de Dalías [4][5][6] which could result in a less accurate determination of elastic response spectra. Moreover, the ratio between the resonant periods and the natural period of the building structures has been demonstrated as an explanatory parameter of damage increment in affected buildings [7].…”