Geological Criteria for Evaluating Seismicity Revisited: Forty Years of Paleoseismic Investigations and the Natural Record of Past Earthquakes

The identification of individual past earthquakes and their characterization in time and space, as well as in magnitude, can be approached in many different ways with a large variety of methods and techniques, using a wide spectrum of objects and features. We revise the stratigraphic and geomorphic evidence currently used in the study of paleoseismicity, after more than three decades since the work by Allen (1975), which was arguably the first critical overview in the field of earthquake geology. Natural objects or geomarkers suitable for paleoseismic analyses are essentially preserved in the sediments, and in a broader sense, in the geologic record. Therefore, the study of these features requires the involvement of geoscientists, but very frequently it is a multidisciplinary effort. The constructed environment and heritage, which typically are the focus of archaeoseismology and macroseismology, here are left aside. The geomarkers suitable to paleoseismic assessment can be grouped based on their physical relation to the earthquake\u2019s causative fault. If directly associated with the fault surface rupture, these objects are known as direct or on-fault features (primary effects in the Environmental Seismic Intensity [ESI] 2007 scale). Conversely, those indicators not in direct contact with the fault plane are known as indirect or off-fault evidence (secondary effects in the ESI 2007 scale). This second class of evidence can be subdivided into three types or subclasses: type A, which encompasses seismically induced effects, including soft-sediment deformation (soil liquefaction, mud diapirism), mass movements (including slumps), broken (disturbed) speleothems, fallen precarious rocks, shattered basement rocks, and marks of degassing (pockmarks, mud volcanoes); type B, which consists of remobilized and redeposited sediments (turbidites, homogenites, and tsunamites) and transported rock fragments (erratic blocks); and type C, entailing regional markers of uplift or subsidence (such as reef tracts, microatolls, terrace risers, river channels, and in some cases progressive unconformities). The first subclass of objects (type A) is generated by seismic shaking. The second subclass (type B) relates either to water bodies set in motion by the earthquake (for the sediments and erratic blocks) or to earthquake shaking; in a general way, they all relate to wave propagation through different materials. The third subclass (type C) is mostly related to the tectonic deformation itself and can range from local (next to the causative fault) to regional scale. The natural exposure of the paleoseismic objects\u2014which necessarily conditions the paleoseismic approach employed\u2014is largely controlled by the geodynamic setting. For instance, oceanic subduction zones are mostly submarine, while collisional settings tend to occur in continental environments. Divergent and wrenching margins may occur anywhere, in any marine, transitional, or continental environment. Despite the fact that most past subduction earthquakes have to be assessed through indirect evidence, paleoseismic analyses of this category of events have made dramatic progress recently, owing to the increasingly catastrophic impact that they have on human society

Géodynamique andine : résumés étendus = Andean geodynamics : extended abstracts

Situé dans le piémont de la cordillère orientale de la Colombie, l'anticlinal de La Florida, figurant également dans la bibliographie pétrolière sous le nom d'anticlinal de Cusiana, a été reconnu lors de l'exploration géophysique réalisée vers la fin des années 80. Nous présentons de nombreuses données géomorphologiques et géologiques de surface, permettant de démontrer, non seulement l'existence de cette structure, mais aussi son activité tectonique quaternaire. (Résumé d'auteur

Géodynamique andine : résumés étendus = Andean geodynamics : extended abstracts

Le système de failles de Guadalupe-Mina de Coro-Chuchure s'étend sur une soixantaine de kilomètres en direction ENE-WSW au nord de l'état de Falcon (Venezuela nord-occidental). La trace de ce système présente deux virgations qui décalent le front de quelques kilomètres. Nous avons modélisé analogiquement la plus orientale des deux virgations au moyen de modèles en sable. Cette virgation s'avère être une zone de transfert dûe à la présence d'un rejet au niveau du socle, introduit par la faille normale de Los Médanos. (Résumé d'auteur

Quaternary faults and stress regime of Venezuela

International audienc