Contribution of ambient vibration recordings (free-field and buildings) for post-seismic analysis: The case of the Mw 7.3 Martinique (French Lesser Antilles) earthquake, November 29, 2007

Highlights

• We performed ambient vibration recordings in both soil and structure after a damaging earthquake.

• We investigate the sources of damage to buildings.

• We compare pre-and post earthquake recordings to evaluate damage grade.

• We propose recommendations for the use of such recordings in post seismic survey.

Abstract

Following the Mw 7.3 Martinique earthquake, November 29^th, 2007, a post-seismic survey was conducted by the Bureau Central Sismologique Français (BCSF) for macroseismic intensities assessment. In addition to the inventories, ambient vibration recordings were performed close to the particularly damaged zones in the free-field and the buildings. The objective of the paper is to show the relevancy of performing ambient vibration recordings for post-earthquake surveys. The analyses of the recordings aim at explaining the variability of the damages through site effects, structure vulnerability or resonance phenomena and to help the characterization of the post-seismic building integrity. In three sites prone to site effects, we suspect damage to be related to a concordance between soil fundamental frequency and building resonance frequency. Besides, the recordings of ambient vibrations at La Trinité hospital before and after the earthquake allow us to quantify the damage due to earthquake in terms of stiffness loss.

Introduction

A large part of knowledge in the fields of earthquake engineering and engineering seismology has been accumulated during post-seismic surveys all around the world. These surveys have many different objectives: (1) estimate the buildings safety right after the earthquake, (2) characterize the ground motion by establishing macroseismic maps, (3) provide feedback for earthquake engineering by studying damage features and, eventually (4) help urban planning in defining zones with ground motion amplification and induced effects (liquefaction, landslides…). However, the knowledge of the structural damage causes is prior information necessary to relevantly reach these objectives. For a given deformation capacity, e.g. associated to a building class, damage will only depend on the building response to the ground motion. The building response depends on the incident seismic motion (that can be largely affected by the site response) and its representing parameters (maximal amplitude, frequency content…) with regard to the structure and its dynamic properties (e.g. [7]). Thus, two key parameters among those influencing the seismic demand can be considered: (1) the resonance frequencies of the site and (2) the building resonance frequencies.

Seismic noise recordings in free-field and ambient vibration recordings in buildings are robust and low cost methods for estimating the soil and structure low-strain resonance frequencies. Since the 1990s and the widespread studies for site effects based upon the Horizontal to Vertical Noise Spectral Ratio (HVNSR), several papers have shown the relevancy of HVSNR to partially explain damage locations and/or grades (e.g. [1], [9], [14], [29]). However, other studies show that HVNSR alone cannot be directly linked to damage distribution ([20], [21], [27], [28],) and the damage variability can also be related to the building capacity rather than the site characteristics ([6]).

Besides, ambient vibration recordings in buildings have gained more and more interest for last decades, for earthquake engineering and civil engineering applications. The elastic fundamental frequency is a key-parameter in earthquake engineering for building response assessment (e.g. [18], [19]) and structural health monitoring (e.g. [4], [8]).

The joint approach (i.e. free-field and building investigation) can be relevant for post-seismic evaluation of the origin of the damage variability and building integrity. Gallipoli et al. [10], Gosar and Martinec [11] and Mucciarelli et al. [21], [22] showed by ambient vibrations applications that soil–structure resonance could play a major role in damage location.

Following the 29th November 2007 Mw=7.3 Martinique earthquake, a post-seismic survey was set up to collect macroseismic data by the Bureau Central Sismologique Français in charge of the definition of the macroseismic intensities after earthquakes [26]. During this survey, the authors performed ambient vibration recordings in highly damaged zones.

The scope of this paper is to show a case study of the usefulness of the joint utilization of ambient vibration recordings in free-field and building to (1) improve the evaluation of the damages, (2) understand the origins of the damage variability by understanding the low-strain response of the soil and building and (3) show the relevancy of these information to complete a macroseismic study such as the one led by the BCSF.