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Laffaille J.,University of Los Andes, Venezuela | Audemard F.M.,Fundacion Venezolana de Investigaciones Sismologicas | Alvarado M.,University of Los Andes, Venezuela
Special Paper of the Geological Society of America | Year: 2010

The prime cause of the relocation of one of the first villages founded in Venezuela by Spaniards in the early seventeenth century was likely motivated by earthquakes. San Antonio de Mucuñó, located in the Merida Andes ∼200 km south-southeast of Maracaibo, was subjected to the effects of landslides triggered by a series of seismic events that took place in and around the year 1674. Historical documents, the geological and seismo-tectonic setting, and paleoseismic data support the conclusion that the earthquakes of 1674 occurred on the nearby, seismically active Bocono fault. © 2010 The Geological Society of America. All rights reserved.

Carcaillet J.,Joseph Fourier University | Angel I.,Central University of Venezuela | Carrillo E.,Central University of Venezuela | Audemard F.A.,Fundacion Venezolana de Investigaciones Sismologicas | Beck C.,University of Savoy
Quaternary Research (United States) | Year: 2013

In the tropical Mérida Andes (northwestern Venezuela), glacial landforms were found at altitudes between 2600 and 5000m, corresponding to 600km2 of ice cover during the maximum glacial extension. However, the lack of sufficient absolute age data prevents detailed reconstruction of the timing of the last deglaciation. On the northwestern flank of the Mucuñuque Massif, successive moraines and striated eroded basement surfaces were sampled for cosmogenic 10Be investigation. Their compilation with published data allows the establishment of a detailed chronology of the post-LGM glacier history. The oldest moraines (18.1 and 16.8ka) correspond to the Oldest Dryas. Successive moraine ridges indicate stops in the overall retreat between the LGM and the Younger Dryas. The cold and short Older Dryas stadial has been identified. Results indicate that most of the ice withdrew during the Pleistocene. The dataset supports an intensification of the vertical retreat rate from ~25m/ka during the late Pleistocene to ~310m/ka during the Pleistocene/Holocene. Afterwards, the glacier was confined and located in the higher altitude zones. The altitude difference of the Younger Dryas moraines in the Mucubají, La Victoria and Los Zerpa valleys indicates a strong effect of valley orientation on the altitude of moraine development. © 2013 University of Washington.

Meng L.,California Institute of Technology | Ampuero J.-P.,California Institute of Technology | Sladen A.,University of Nice Sophia Antipolis | Rendon H.,Fundacion Venezolana de Investigaciones Sismologicas
Journal of Geophysical Research: Solid Earth | Year: 2012

A catastrophic Mw7 earthquake ruptured on 12 January 2010 on a complex fault system near Port-au-Prince, Haiti. Offshore rupture is suggested by aftershock locations and marine geophysics studies, but its extent remains difficult to define using geodetic and teleseismic observations. Here we perform the multitaper multiple signal classification (MUSIC) analysis, a high-resolution array technique, at regional distance with recordings from the Venezuela National Seismic Network to resolve high-frequency (about 0.4 Hz) aspects of the earthquake process. Our results indicate westward rupture with two subevents, roughly 35 km apart. In comparison, a lower-frequency finite source inversion with fault geometry based on new geologic and aftershock data shows two slip patches with centroids 21 km apart. Apparent source time functions from USArray further constrain the intersubevent time delay, implying a rupture speed of 3.3 km/s. The tips of the slip zones coincide with subevents imaged by backprojections. The different subevent locations found by backprojection and source inversion suggest spatial complementarity between high- and low-frequency source radiation consistent with high-frequency radiation originating from rupture arrest phases at the edges of main slip areas. The centroid moment tensor (CMT) solution and a geodetic-only inversion have similar moment, indicating most of the moment released is captured by geodetic observations and no additional rupture is required beyond where it is imaged in our preferred model. Our results demonstrate the contribution of backprojections of regional seismic array data for earthquakes down to M ≈ 7, especially when incomplete coverage of seismic and geodetic data implies large uncertainties in source inversions. © 2012 by the American Geophysical Union.

Rengifo M.,University of Los Andes, Colombia | Aranguren R.,University of Los Andes, Colombia | Laffaille J.,University of Los Andes, Colombia | Ferrer C.,University of Los Andes, Colombia | Audemard F.,Fundacion Venezolana de Investigaciones Sismologicas
Boletin de Geologia | Year: 2015

A survey of total radon soil gas have been accomplished on the Boconó fault in the Merida Andes, where a segment of this active fault of ~170 km long, between the city of La Grita, in the Táchira state, and the Santo Domingo village, in Mérida state, was considered. Measurements of soil gas were taken along traverse lines perpendicular to the trace using a portable radiation monitor. The aim of this work is to characterize the diverse provinces traversed by this active fault, as well as, to evaluate the response of the method in different geological - geomorphological site conditions such as fault scarps, shutter ridges, saddles, trenches, sag ponds, depressions, landslides and an offset moraine. Along the fault trace, 36 profiles were taken and the results indicate that in 40 percent of the transects the trace was weakly detected, or not detected; the best results are in saddles and hillsides, the worst in scarps and shutter ridges. This suggests that, leaving aside the meteorological conditions, geology and geomorphology are important factors for keeping in mind when this method is applied. The relative concentration of total radon found is highest in the Mocotíes zone (Tovar - Estánques) and it is lowest in the páramo de Mucuchíes.

Morales C.,Fundacion Venezolana de Investigaciones Sismologicas | Schmitz M.,Fundacion Venezolana de Investigaciones Sismologicas | Pullammanappallil S.,Optim Software Reno Nv
Boletin de Geologia | Year: 2015

In the frame of Barquisimeto and Cabudare seismic microzoning project, we have carried out the revision of geological, geophysical and geotechnical data, with the purpose to obtain the characteristics of shallow and deep sedimentes of both Barquisimeto alluvial terrace and the Cabudare pull-a part basin. For this study, we have used data from seven refraction seismic profiles acquired previously in the area. A reinterpretation applying refraction microtremor (ReMi) method was done in Barquisimeto, which helped to define the shear wave velocity profile. In order to define a geological/geophysical model, these results were integrated with available geological and geotechnical information. The geological/geophysical model was used to calculate the surface response spectrum, useful in engineering design, taking into account the soil depth influence and its characteristics. With this information, the surface spectral response at Barquisimeto and Cabudare was calculated according to the sites conditions. On the other hand, with the acquisition of real earthquake data (Tucacas Mw = 6.4 - September 29, 2009), acquired through an accelerograph observatory installed in the frame of the microzonig project, it was possible to perform a calibration of the soil profile and spectra. Using the accelerograms from the observatory at bedrock, we calculated the spectral response at the soil surface by dynamic modeling, and the result was compared with the response spectrum acquired by the observatory at the surface, from the same earthquake, showing a good fit between both spectra form.

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