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Lopez M.C.C.,Conjunto Arboletes Casa 38 | Audemard F.A.M.,Fundacion Venezolana de Investigaciones Sismologicas FUNVISIS
Special Paper of the Geological Society of America | Year: 2011

Morphotectonic and paleoseismic studies carried out in the surrounds of Tuluá (4°N, 76°W) provide strong supporting evidence for ongoing E-W compression in the Cauca Valley, Colombia, during the late Pleistocene and Holocene. This local tectonic regime is kinematically and mechanically connected with the ENE-striking, rightlateral, strike-slip Ibagué fault system, which crosscuts and offsets the Central Cordillera of Colombia. Morphologic, stratigraphic, kinematic, and chronologic evidence obtained on fl exural scarps, which are currently shaping the foothills of the Central Cordillera, attests to the recent activity of a compressional fault system. This includes both hinterland-propagating back-thrust faults and foreland-verging thrust faults that reutilize a fold-and-thrust belt, previously considered to be active only during Tertiary times. Kinematic measurements on the back-thrust faults further support an ongoing E-W-oriented maximum horizontal stress at the latitude of Tuluá. In terms of seismic hazard for this region, these investigations provide evidence for Ms 7 earthquakes with recurrence in the order of 6 k.y. on this frontal thrust system. In addition, should the A.D. 1766 earthquake have not taken place on these active thrust faults, the probability of occurrence of a forthcoming event with such characteristics would be high. © 2011 The Geological Society of America.

Audemard F.A.M.,Fundacion Venezolana de Investigaciones Sismologicas FUNVISIS | Audemard F.A.M.,Central University of Venezuela | Michetti A.M.,University of Insubria
Special Paper of the Geological Society of America | Year: 2011

The identifi cation 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 fi rst critical overview in the fi eld 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's 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 fi rst 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-which necessarily conditions the paleoseismic approach employed-is 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.© 2011 The Geological Society of America. All rights reserved.

This work aims at preparing evacuation routes in the areas affected by subsidence in the east coast of Lake Maracaibo, in case of rupture of the coastal protection dike by a seismic shock, using a geographic information systems -GIS-. The study area is below mean sea level, being its lowest part at -4.5 m as to 1996 year. So an eventual breach of the protecting dam would flood this depressed area, thus requiring the creation of the most effective routes to evacuate people to refuge areas in the shortest time possible. At present, a contingency plan for the area exists in case of dam breaching, but it does not have evacuation routes defined, to move people between affected areas and refuge centers in the most efficient manner. These route needs to be integrated in the urban planning in order to ensure their preservation through time during urban development. For the creation of the fastest and shortest evacuation routes, a GIS raster format of the study area was generated, which was based on the weighted values of speed of movement according to land use.

Oropeza J.,Fundacion Venezolana de Investigaciones Sismologicas FUNVISIS | Singer A.,Central University of Venezuela
Revista de la Facultad de Ingenieria | Year: 2011

Due to the lack of subsoil geological information of the alluvial sediments, has been created a urban geology database for the Caracas Valley. These geological data base supports a geological proposal of 11 microzones considered homogeneous for seismic microzoning, which have been reflected on a quaternary map of geological formations and faults. The characterization of each microzone is based on the geological evaluation of 7 parameters considered as significant.

Alvarado L.,Fundacion Venezolana de Investigaciones Sismologicas FUNVISIS | Schmitz M.,Fundacion Venezolana de Investigaciones Sismologicas FUNVISIS | Rendon H.,Fundacion Venezolana de Investigaciones Sismologicas FUNVISIS
Boletin de Geologia | Year: 2015

The preliminary results of the 2D superficial seismic wave modeling along four cross sections through the Barquisimeto and Cabudare cities are presented. The 2D modeling was done using a technique that combines an analytical method (modal summation) with a numerical method (finite difference) in order to obtain the seismic response in terms of synthetic seismograms generation. We used two seismic scenarios with magnitude 6.0 and 7.0 Mw. The 2D/1D spectra ratio shows the vertical component of the acceleration with an amplification factor 6, around 2 Hz; whereas the horizontal components have amplification of factor 2, around 1 Hz. We also observed an increase in amplitude and duration of the seismic signal at the edge of the sedimentary basin.

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