Contribution à la connaissance de l'aléa sismique dans les Antilles françaises: analyses des données sismologiques et accélérométriques régionales.

The Lesser Antilles is an area of high volcanic and earthquake activity, characterized by a 1000 km convergence zone resulting from the Atlantic plate subduction under the Caribbean plate with a slow convergent motion (2 cm/year). With five years of available data, the CDSA data base presents a more homogeneous vision of Lesser Antilles arc seismicity and allows detecting low seismic activity zones, in the north near the Virgin Islands, and in the south between St-Lucia and Grenada. The accuracy improvements of location is used to better study the relationship between tectonic structures and seismicity, to better define the subduction slab geometry, and to analyse the spatial variations of the Gutenberg-Richter law b-value. Concerning the 21 november 2004 Les Saintes earthquake, EMS98 intensities in Les Saintes islands were evaluated from an individual request and hypocenter relocalisations of the Les Saintes earthquake and the biggest aftershock of 14 february 2005 were made using a method of master/slave in order to contrain the seismic source of the mainchock. As seismic hazard assessment depends from strong ground motion generated by earthquakes, the last part of this work concerns modeling accelerometric records of the Les Saintes earthquakes, combining a composite source model with an empirical Green Function technique. In our study, we used Les Saintes aftershocks as Empirical Green Function.L'arc des Petites Antilles situé sur la bordure nord-est de la plaque Caraïbe, résulte d'un phénomène de subduction, les plaques américaines plongeant sous celle de la Caraïbe avec une vitesse de 2 cm/an. Les Antilles Françaises représentent la région française où le risque sismique est le plus important. Ce travail dont l'objectif est d'améliorer la connaissance de l'aléa sismique dans les Antilles Françaises, repose sur les données du Réseau Accélérométrique Permanent (RAP) et sur celles du « Centre de Données des Antilles Françaises ». L'analyse de ces données permet de mieux contraindre la sismicité de l'arc. Deux zones de très faible sismicité sont mises en évidence : au nord près des Iles Vierges et au sud entre Sainte-Lucie et Grenade. D'autres points sont aussi résolus : imagerie de la subduction le long de l'arc, relations entre la sismicité superficielle et la tectonique active, variations de la pente (b-value) de la loi de Gutenberg-Richter. Le séisme majeur du 21 novembre 2004 Mw=6.3 est au cœur des deux derniers chapitres. Plusieurs aspects y sont présentés : macrosismicité avec l'évaluation d'intensités EMS98 obtenues du dépouillement de formulaires individuels recueillis pour les Iles des Saintes, et relocalisation par la méthode de maître/esclave du choc principal et de sa plus forte réplique du 14 février 2005, de façon à mieux contraindre l'imagerie de la source du choc principal, étape primordiale pour la dernière partie portant sur la modélisation des signaux du choc principal des Saintes. Nous tentons dans cette dernière partie de modéliser les enregistrements de mouvements forts avec une approche semi-empirique en se basant sur un modèle stochastique large-bande et proposons l'utilisation de plusieurs Fonctions de Green Empiriques (FGE) sélectionnées parmi les répliques du séisme des Saintes

MATRIX Reference Reports

The Multi-HAzard and MulTi-RIsK Assessment MethodS for Europe or MATRIX project (01.10.2010 to 31.12.2013), coordinated by the GFZ, set out to tackle some of the issues associated with multi-hazard and risk assessment. Disaster risk reduction (DDR) activities generally treat different natural hazards and their associated risks separately within what may be termed a "single-type" approach. However, this ignores the spatial and temporal interactions that often arise along the disaster risk chain. For instance, one hazardous event may trigger others, e.g., earthquakes causing tsunamis, or several different types may occur concurrently, e.g., severe weather and earthquakes. Considering vulnerability, an initial event would leave a community more susceptible to future, possibly different, hazards, e.g., an earthquake weakening buildings which are damaged further by windstorms. The temporal dimension may include changes in exposure, e.g., increased urbanisation, altering the total risk to an area, while repeated events lessen a community's resilience. Meanwhile, although losses are estimated by usually only considering direct economic losses or casualties, this ignores less tangible losses such as reduced business activity or the loss of cultural heritage. In short, the total risk estimated when incorporating interactions between multiple hazards and risks is likely to be greater than the sum of the individual parts. Hence, for a more comprehensive risk assessment paradigm, these, and other, interactions need to be considered. Therefore, MATRIX set out to develop concepts, methods, frameworks and tools for dealing with risk assessment within a multi-hazard and risk environment. The focus was on the hazards that most affect Europe, namely earthquakes, landslides, volcanos, tsunamis, wild fires, storms and fluvial and coastal flooding. Interactions at all the different levels were considered, such as cascading events and time dependency in vulnerability. The resulting products were applied at three test cases: Naples, Italy, the French West Indies, and Cologne, Germany. Considerable interaction with end-users was also undertaken, including identifying biases at the individual and institutional level which may hinder employing a multi-type framework for risk governance. This Scientific Technical Report presents two so-called "Reference reports" produced during the MATRIX project. These reports were provided to the European Commission as deliverables, namely D8.4 "MATRIX Results I and Reference Report" and D8.5 "MATRIX Results II and Reference Report". D8.4 presented a series of specific reports outlining the results of the project, written in a manner accessible not only to the specialist but with a broader audience in mind. D8.5 deals with the risk governance within a multi-hazard and risk context and has since been published. We therefore divide with document in two, where part 1 represented the outcomes presented in D8.4 while D8.5 forms part 2. We believe the MATRIX project was a very important step towards the goal of establishing the multi-hazard and risk environment as the norm within a European context, and we hope that the reader will benefit from the results presented here

Empirical model for rapid macroseismic intensities prediction in Guadeloupe and Martinique Modèle empirique pour la prédiction rapide des intensités macrosismiques en Guadeloupe et Martinique

International audienceWe describe a simple model for prediction of macroseismic intensities adapted to Guadeloupe and Martinique (Lesser Antilles), based on a combination of peak ground acceleration (PGA) predictive equation and a forward relation between acceleration and intensity. The PGA predictive equation is built from a 3-parameter functional form constrained by measurements from permanent accelerometer stations, mostly associated with Les Saintes crustal earthquake (21/11/2004, Mw = 6:3) and its many aftershocks. The forward intensity model is checked on a database of recent instrumental events of various origins with magnitudes 1.6 to 7.4, distances from 4 to 300 km, and observed intensities from I to VIII. Global sigma residual equals 0.8 in the MSK scale, suggesting a larger applicability range than the intermediate PGA predictive equation. The model is presently used by the French Lesser Antilles observatories to produce automatic reports for earthquakes potentially felt

The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes,

International audienceOn November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (Mw = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock

Bootstrap determination of the reliability of b-values: an assessment of statistical estimators with synthetic magnitude series

International audienceWe consider some practical issues of the determination of the b-value of sequences of magnitudes with the bootstrap method for short series of length L and various quantization levels Dm of the magnitude. Preliminary Monte Carlo tests performed with Dm ¼ 0 demonstrate the superiority of the maximum likelihood estimator bMLE, and the inconsistency of the, yet often used, bLR estimator defined as the least-squares slope of the experimental Gutenberg-Richter curve. The Monte Carlo tests are also applied to an estimator, bKS, which minimizes the Kolmogorov-Smirnov distance between the cumulative distribution of magnitudes and a power-law model. Monte Carlo tests of discrete versions of the bMLE and bKS estimators are done for Dm ¼ f0:1; 0:2; 0:3g and used as reference to evaluate the performance of the bootstrap determination of b. We show that all estimators provide b estimates within 10 % error for L C 100 and if a large number, n = 2 9 105, of bootstrapped sample series is used. A resolution test done with Dm ¼ 0:1 reveals that a clear distinction between b = 0.8, 1.0, and 1.2 is obtained if L= 200

Automatic picking and earthquake relocation for the Antilles subduction zone (1972-2013)

International audienc

Seismic microzonations of municipalities in French West Indies

Spatial variability of ground motion may be explained by local soil conditions; it is the so-called "site effects". Site effects are responsible for increasing duration and important amplification of strong ground motion during earthquakes and must be considered as a key parameter in local seismic hazard assessment. French West Indies are prone to strong site effects because of a particular geology, characterized by soft soil deposits and strong alteration of volcanic deposits. In this framework, and considering that French West Indies are considered as a high seismicity level zone in the French building code, part of seismic hazard mitigation policy consists in technical studies to perform identification and mapping of zones characterized by homogeneous seismic response and quantification of associated ground motion. It is the aim of seismic microzonations. In this presentation, we will expose the methodology used on the scale of municipalities for mapping homogeneous seismic areas, based on a study of geological and geotechnical conditions and some geophysical testings. Then, a study of vulnerability of buildings allows to estimate the probability of damage on various sectors for a scenario of risk corresponding to the seismic microzonation

CDSA: A New Seismological Data Center for the French Lesser Antilles

International audienc