Relocalizing a historical earthquake using recent methods: The 10 November 1935 Earthquake near Montserrat, Lesser Antilles

International audienceThis study investigates the hypothesis of Feuillet et al. (2011) that the hypocenter of the seismic event on November 10, 1935 near Montserrat, Lesser Antilles (MS 6 1/4) (Gutenberg and Richter, 1954) was mislocated by other authors and is actually located in the Montserrat-Havers fault zone. While this proposal was based both on a Ground Motion Prediction Equation and on the assumption that earthquakes in this region are bound to prominent fault systems, our study relies on earthquake localization methods using arrival times of the International Seismological Summary (ISS). Results of our methodology suggest that the hypocenter was really located at 16.90° N, 62.53° W. This solution is about 25 km north-west of the location proposed by Feuillet et al. (2011) within the Redonda fault system, northward of the Montserrat-Havers fault zone. As depth phases that contribute valuable insights to the focal depth are not included in the ISS data set and the reassociation of these phases is difficult, the error in depth is high. Taking into account tectonic constraints and the vertical extend of NonLinLoc's uncertainty area of the preferred solution we assume that the focus is most probably in the lower crust between 20 km and the Moho. Our approach shows that the information of the ISS can lead to a reliable solution even without an exhaustive search for seismograms and station bulletins. This is encouraging for a better assessment of seismic and tsunami hazard in the Caribbean, Mexico, South and Central America, where many moderate to large earthquakes occurred in the first half of the 20th century. The limitations during this early phase of seismology which complicate such relocations are described in detail in this study

The largest aftershock: How strong, how far away, how delayed?

International audienceProposed in the 1950's, Båth's law states that the largest aftershock has a magnitude that is typically 1.2 less than that of the mainshock. Thirty years of the global earthquake catalog allow us to extend Båth's law in time, space and focal mechanism. On average, reverse faults have a smaller magnitude and distance from the mainshock to largest aftershock than strike-slip faults. The distribution of the time intervals between mainshocks and their largest aftershocks obeys power law, but with a somewhat faster rate of decay than for aftershocks, in general. This implies that the largest aftershocks are more likely to occur earlier rather than later in a given sequence of aftershocks

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

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Development of slope instabilities on experimental permafrost: topography, warming and water effects

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Accurate analysis of the distribution of epicenters in Western Provence and Eastern Languedoc (Southern France)

International audienceThe present seismicity in Western Provence and Eastern Languedoc (Southern France) is weak. However, when the historical seismicity is considered, these regions are certainly among the most seismic areas of southern France. The tectonic setting of both regions is one of an active intraplate zone. In comparatively “stable” areas like these, the study of small instrumental earthquakes (M<5) is an indispensable source of information. Unfortunately, in these regions, the instrumental seismicity is, as a rule, rare and diffuse. Therefore, interpreting the spatial pattern of this seismicity through a visual inspection is a difficult and subjective process. This paper presents a quantifiable analysis of the seismicity of the study regions. Earthquakes are associated with fault zones by examining the number of epicenters per unit area. The analysis is performed through the blade method applied on collapsed epicenters. The analysed data are extracted from the Laboratoire de Détection et de Géophysique Catalog. Our analysis highlights several significant epicenter alignments associated with known tectonic features

The structural evolution of the English Channel area

International audienceThe structural evolution of the English Channel area is controlled by structure and particularly by the pre-existing Cadomian and Variscan crustal discontinuities, which have been reactivated repeatedly in post-Variscan times. They controlled the crustal subsidence that produced basin development in the Mesozoic, prior to the sea-floor spreading in the North Atlantic region. They were then reactivated during the Cenozoic compression and basin inversion. The English Channel development is ascribed to mid-Tertiary differential uplift (Oligocene to Miocene). During late Tertiary to Quaternary times the Channel displays characteristics of a tectonically controlled fluvial basin periodically invaded by the sea. At the lithospheric scale, the Channel can be considered as an active intraplate area influenced by the NW–SE ‘Alpine push', the NW–SE ‘Atlantic ridge push' and glacial rebound stresses