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Central Cordillera of Colombia near to the Nevado del Ruiz volcano, is a seismically active area above the subducting slab of the Nazca plate and deforming upper crust of the Andes. Buildings in the region require strengthening against the effects of both types of hazards: earthquakes and volcanoes. During these trips, we will discuss the 13November, 1985, Nevado del Ruiz Volcano eruption that destroyed the Armero city with about 22,000 deceases, as well the 25 January, 1999, Armenia earthquake (M 6.2) that killed about 2000 people and injured 4000 largely as the result of older, poorly 3 constructed buildings. The economic impact of the quake on the region was significant with about 8000 coffee farms either completely or partially destroyed and over 13,000 structures either partially damaged or completely destroyed.Both phenomena are associated to a large-transversal structure termed Caldas tear, which is controlling strong motion earthquakes and the Quaternary volcanism of the northern Andes.

In order to characterize the Caballos formation reservoir in the Orito field in the Putumayo basin - Colombia, a multiattribute analysis was applied to a 50 km2 seismic volume along with 16 boreholes. Some properties of the reservoir were reliably estimated and very accurate when compared with well data. The porosity, permeability and volume of shale were calculated in the seismic volume by at least second order multivariate polynomial. A good correlation between porosity and acoustic impedance was observed by means of crossplot analysis performed on properties measured and estimated in cores or borehole logs as well as on properties calculated in the seismic volume. The estimated property values were well behaved according to the rocks physics analysis. With the property maps generated and the geological environments of the reservoir a new interpretation of the Caballos formation was established. High correlation coefficients and low estimated errors point out competence to calculate these three reservoir properties in places far from the influence of the wells. The multiple equation system was established through weighted hierarchical grouping of attributes and their coefficients calculated applying the inverse generalized matrix method.

We applied the NMO and CRS (Common Reflector Surface) approaches to a complex geological area in order to compare their performances for obtaining enhanced images. Unlike NMO, CRS does not depend on a previous time velocity model and uses a hyperbolic equation to estimate 2D travel times through three parameters (Normal ray emergence angle, NIP and N wavefront curvatures). To obtain the image a solution provided by coherence analysis algorithm was used.A low quality Colombian seismic line acquired in Middle Magdalena basin was used, where a foothill geological area is characterizedby a thrusting fault. The CRS provided an enhanced image which allowed a new geological interpretation that is best constrained with other regional observations.

A geometric model for the subduction of the Nazca plate beneath the South American plate in southwestern of Colombia is proposed based on the relocation of hypocenters of local and distantearthquakes. By means of the simultaneous inversion of teleseismic P and SH body waves, the depths of the 15 events with Mw ≥ 5.8 were constrained, and the hypocenters of the 250 earthquakes recordedbetween 1990 and 2005 by the International Seismological Centre (ISC) and U.S. Geological Survey, National Earthquake Information Center (NEIC) were constrained and relocated. A model is proposedfor the hypocentral sections taking into account the trench along of the Earth and Colombia-Ecuador.Three different possible shapes of subduction of the Nazca plate in the Colombia-Ecuador trench were obtained: The first configuration, in the Cali A segment, the dip angle changes from17º to 45º down to a maximum depth of 100km; the second configuration, in the Popayán B and Nariño C segments, the dip angle holds approximately constant at 30º down to a maximum depth of 200 km; and the third configuration, in the Quito D segment, the dip angle changes of 9º to 50º to a maximum depth 220 km. The maximum depth of seismicity along the Colombia-Ecuador trench shows two increases, the first between latitudes 4.5ºN-5ºN and the second between the latitudes 1ºS-2ºS, which suggest that the presence of the Malpelo and Carnegie Ridges may generate a differential blockage at the Pacific Colombia-Ecuador basin.

Ha sido interpretada la sección geológica entre las ciudades de Ibagué y Armenia basada en observaciones geofísicas y estructurales en busca de establecer el estilo estructural dominante y la configuración del basamento en profundidad. Inicialmente se realizaron mediciones gravimétricas y magnetométricas levantadas cada 500 m entre estas ciudades, las cuales permitieron soportar el desarrollo del experimento sísmico "Armenia-Quake". El alcance del experimento sísmico permitió detectar reflectores hasta aproximadamente 17 km de profundidad en un área de influencia de aproximadamente 25 km. La interpretación integral de la información geológica y geofísica permite concluir que el Complejo Cajamarca define una estructura sinclinal amplia cerca al Municipio de Cajamarca. Partiendo de dicha información se pudo reconocer la presencia de un basamento compuesto por cuñas de rocas basálticas y sedimentarias asociadas a las formaciones Quebradagrande y Arquía. Igualmente se reconoce la existencia de una gran flexión de la paleo-placa oceánica relacionada a la formación Barroso, la cual se encuentra limitada al occidente por la falla Cauca-Almaguer. La estructura en flexión y la evolución compresiva del estado de esfuerzos en esta región de América del Sur podrían ser las responsables del depósito y posterior plegamiento de la formación Cinta de Piedra que constituye en gran medida la Serranía de Santa Bárbara.

The Temporal variation of coda-Q was analyzed for Galeras volcano, located in the Andes of southwestern Colombia. The Q -1 value was calculated using the Single Back Scattering (SBS) model working with seismograms filtered in frequency bands centered in 1.5, 3.0, 6.0 and 12.0 Hz. The analysis was done in 1990-2002 period, using the URCR station and the frequency dependence Q = Qo f -n was calculated before and after some eruptions. We used volcano tectonic events with magnitude lower than 1.5 and RMS, GAP, vertical and horizontal errors lower than 0.1s, 180 and 1km.The temporal change of attenuation occurred in two stages, the first in 1989-1992 period, when the Q -1 value increases from 0.015 to 0.070. The second is the 1992-2002 period that is characterized by a decreasing Q -1 value. Before July 1992 eruption the attenuation increase is associated with the extrusion of an andesitic dome, after this eruption the dome was destroyed and the Q -1 value drops. Starting from 1994 low attenuation values was associated with low Galera's activity. The analysis shows that Q value increases before an eruption and decreases after it.

The space-time variation of the parameter of attenuation Q-Coda in a sector of the Piedemonte L1anero was studied using the Coda waves. The estimation ofthe attenuation of the amplitude of coda is calculated by means of the Model of simple Isotropic. Dispersion of Sa to, from earthquakes seismograms happened in the region from September 1993 to June 2002. Following the methodology used by Aki and Chouet (1975), they analyzed seismograms in four centered bands to 1.5,3,6 and 12 Hertz with its corresponding bandwidths of 1, 2, 4 and 8 Hertz. And by regression to linear calculated the value of Qc to I Hertz. From the expression proposed by Puli to model the amplitude of the waves coda by means of ellipses, this expression was extended to calculate the space variation ofthe Qc-I, (anelastic tomography). And by inverse square minimums, the value of attenuation for each cell of the three-dimensional model was calculated. Their results were correlated with the regional tectonic frame showing that when the attenuation increases by depth, it corresponds to zones with Tectonics of thick skin and, on the contrary, where the attenuation diminishes by depth, it corresponds to zones with Tectonics of thin skin.On the other hand, the temporary variation of minimum values ofQc (maximum attenuations) was correlated to qualitatively earthquakes of magnitude greater to 3,5 and, in addition, the temporary distribution ofQc was quantified by dimension fractal showing that the grouping of the minimum values ofQc increases before ocurring a main event and its grouping decreases after occurring the main event.La variación espacial y temporal del parámetro de atenuación Q-Coda en un sector del Piedemonte Llanero es estudiada usando las ondas Coda. La estirnación de la atenuación de la amplitud de la coda se calcula por medio del Modelo de Dispersión Isotropica Simple de Sato, a partir de sismogramas de sismos que ocurrieron en la región entre septiembre de 1993 a junio 2002. Siguiendo la metodología empleada por Aki y Chouet (1975), se analizaron los sismogramas en cuatro bandas centradas a 1.5, 3, 6 y 12 Hz con sus respectivos anchos de banda de 1, 2, 4 y 8 Hz. Por regresión linear se cálculo el valor de Qc a 1 Hz. A partir de la expresión propuesta por Puli para modelar las amplitudes de las ondas coda por medio de elipses, se amplio esta expresión para calcular la variación espacial del Qc-l, para un medio tridimensional (tomografía anelástica). Y por medio de inversión por mínimos cuadrados se calculó el valor de atenuación para cada celda del modelo tridimensional. Los resultados se correlacionaron con el marco tectónico regional encontrando que cuando la atenuación aumenta con la profundidad, corresponde a zonas con tectónica de piel gruesa y por el contrario, cuando la atenuación disminuye con la profundidad, corresponde a zonas con tectónica de piel delgada.Por otro lado, la variación temporal de los mínimos valores de Qc (máximas atenuaciones) se correlacionó cualitativamente con sismos de magnitud mayor de 3.5 y, además, se cuantificó la distribución temporal de Qc por medio de la dimensión fractal, mostrando que la agrupación de los valores mínimos de Qc aumenta antes de la ocurrencia de un evento principal y decrece su agrupación después de la ocurrencia del evento principal.

In this study, the authors worked on identifying a possible relationship between electromagnetic signals (EM) and seismic events in the lithospheric system in the central region of Colombia. The data, both seismic records and electromagnetic signals, were taken from the catalog of the Seismological Network of the National University of Colombia (RSUNAL) and the catalog of the National Seismological Network of Colombia (RSNC). The project included the design and instrument testing phases for recording seismic signals, electrical potential variations, and magnetic field variations to try to identify possible relationships between these signals. Possible electromagnetic precursors for seismic events were observed, mainly magnetic disturbances, but it was not possible to locate evident electrical anomalies (Seismic Electric Signals - SES). Thus, although the results are not conclusive, the magnetic disturbances identified deserve further long-term analysis. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented about the lithospheric structure of the Latin American and Caribbean territory.