An Albian–Cenomanian unconformity in the northern Andes: Evidence and tectonic significance

The Villeta Group of Colombia and equivalent stratigraphic units of Venezuela and Ecuador comprise marine sequences ranging from Albian to Santonian in age. Deposition of the Villeta Group was presumed to take place entirely in quiet tectonic conditions in a passive margin setting that occupied NW South America. From a large database of 2D/3D seismic, well, surface geology, and biostratigraphic data, we present evidence for intra-Villeta (mostly late Albian–Cenomanian) deformation in parts of the Upper Magdalena Valley and Eastern Cordillera of Colombia, controlled by transpressional fault reactivation, produced by transpressional fault reactivation and thrusting that resulted in an angular unconformity. This event has been largely unnoticed in the literature, but previously scattered evidence supports our observations, suggesting regionally extensive tectonism. Published fission-track age determinations and other geologic evidence from Colombia and Venezuela suggest significant uplifts around 80–100 m.y., which may reflect changes in the subduction regime, with compressional deformation in certain regions and extensional deformation in others. A late Albian onset of compressional deformation along the Colombian and Peruvian segments of the Andes may be related to the opening of the South Atlantic Ocean at equatorial latitudes. Identification of tectonic activity with development of an unconformity in intra-Villeta times provides new insights into understanding the evolution of the Upper Magdalena Valley and adjacent areas of Colombia and western Venezuela and creates new possibilities for hydrocarbon exploration, with additional trapping phases, better reservoir preservation by early migration and secondary porosity, and ultimately facies changes with stratigraphic potential.     

Ultrasonic Spectral and Complexity Measurements on Brine and Oil Saturated Rocks

The effects of the rheological properties of fluids on the acoustic behavior of heterogeneous media like rocks are usually very difficult to characterize. We show here that, in ultrasonic tests and their spectral characterizations, signal attenuation is the most evident difference between water and oil saturated rock samples. Density effects, as explained by the inertial component of the Biot’s poroelastic theory, along with entropic analysis seem to provide a good understanding of the pore-fluid effects on the acoustic response of saturated rocks.     

Damages observed in locations of Oaxaca due to the Tehuantepec Mw8.2 earthquake,Mexico

On September 7, 2017, at 23:49 h (local time), a Mw8.2 intermediate-depth normal-fault earthquake occurred in the Gulf of Tehuantepec, 133 km away from Pijijiapan, Chiapas, and about 700 km away from Mexico City. This event caused 95 fatalities and severe damage to different types of structures located close to the epicenter. The main objective of this work is to present observed damages caused in the state of Oaxaca by this earthquake, which were mainly concentrated in self-built houses and historical and ancient buildings. The locations visited by the reconnaissance team of the Institute of Engineering from UNAM in Oaxaca included Salina Cruz, Tehuantepec, Ixtaltepec, Juchitán, Huatulco and La Ventosa.

     

The Risk Atlas of Mexico City,Mexico: a tool for decision-making and disaster prevention

Natural Hazards - We present a Risk Atlas of Mexico City based on a Geographical Information System (RA-GIS). We identified the prevalent social risk to the more relevant hazards in Mexico City...     

The Tesistan,Mexico earthquake (Mw 4.9) of 11 May 2016: seismic-tectonic environment and resonance vulnerability on buildings

To highlight the importance of small earthquakes in seismic hazard, a study of the 11 May 2016, M_w 4.9, Tesistan, Mexico earthquake is presented. Due to the close proximity of the event to the city, accelerations were considerably higher than those caused by historical severe earthquakes(6.0 M_w 8.2). This paper addresses two objectives related to the Tesistan event: the first is to estimate the focal mechanism solution in order to place the event in the context of the tectonic environment of this area. The second is focused on a vulnerability evaluation of buildings that suffered resonance. Several building′s typologies with variations in construction system and height are assessed in terms of resonance with the structural and soil periods. The results show that around the Zapopan station, strong damage is expected in intermediate to high-rise buildings(12-30 m) with moment resistant frame systems and in reinforced concrete shear walls. Masonry structures around this station may not present resonance. In contrast, in the surroundings of the Guadalajara station, all intermediate height buildings from 9 to 21 m may present resonance.