We used the receiver function technique to deduce crustal thickness beneath the northwestern Andean system, using data from the permanent seismic network of Colombia, combined with some of the IRIS and CTBTO stations in Colombia and Ecuador. The estimation of crustal thickness was made using the primary P to s conversion and crustal reverberations. The bulk crustal VP/VS ratio was constrained using a crustal thickness versus VP/VS stacking method, in addition to estimations using a time to depth conversion technique based on results of a modified Wadati diagram analysis. We observed a wide range of crustal thicknesses, including values around 17 km beneath the Malpelo Island on the Pacific Ocean, 20 to 30 km at the coastal Pacific and Caribbean plains of Colombia, 25 to 40 km beneath the eastern plains and foothills, 35 km beneath the Western Cordillera, 45 km at the Magdalena River intermountain valley, 52 to 58 km under the northern Central Cordillera, and reaching almost 60 km beneath some of the volcanoes of the Southern Cordilleran system of Colombia; crustal thickness can be slightly greater than 60 km beneath the plateau of the Eastern Cordillera. The values of VP/VS are particularly high for some of the stations on the volcanic centers, reaching values above 1.79, probably related to the addition of mafic materials to the lower crust, and in the plateau of the Eastern Cordillera near Bogota, where we speculate about the possibility of crustal seismic anisotropy associated with shear zones.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014JB011304

Receiver functions and crustal structure of the northwestern Andean region, Colombia

Moho estimates for the Yellow Sea and Bohai Basin were developed from the EGM2008 gravity, ETOPO1 topography/bathymetry, and NOAA’s sedimentary thickness databases. Spectral correlation analysis between the free-air gravity anomalies (FAGA) and the terrain’s gravity effects (TGE) at 20 km altitude was made to estimate the terrain-correlated FAGA, which can provide information on the isostatic status of the study area. This study included the additional terrain gravity effects due to density contrasts from sedimentary rock thickness variations over the Yellow Sea. Spectral correlation filtering of the FAGA with the TGE estimated the terrain correlated FAGA, which were subtracted from the TGE to infer isostatically compensated gravity effects for inversion constraints on the Moho modeling. The derived Moho estimates show considerable sensitivity to the sedimentary thickness variations and tectonic features of the Yellow Sea region. In addition, the gradients in the dynamic crustal topography inferred from the terrain-correlated FAGA show apparent associations with the distribution of the Yellow Sea’s earthquakes with predominantly strike-slip focal mechanisms.

Moho modeling of the Yellow Sea (West Sea) from spectrally correlated free-air and terrain gravity data

Global observations show that the crustal thickness varies through the tectonic regions. While the continental crust is 30–70 km thick, the oceanic crustal thickness is 6–12 km. The oceanic crust is also denser (2.8–3.0 g/cm 3 ) than the continental crust (2.6–2.7 g/cm 3 ). The average Archean crust is ~35 km thick, whereas the Proterozoic crust is significantly thicker (~45 km). The nature of the Moho is also not quite consistent; at a few places it is a sharp boundary, whereas at most of the other places it is diffused. A number of models exist to describe the global crust in diverse geological terrains. Since the Indian shield is a mosaic of diverse terrane bearing the imprints of various tectonic episodes in geological history, from Archean to the late Proterozoic eon, the crust is heterogeneous in nature.

Global and Indian Scenario of Crustal Thickness

En la zona axial de la Cordillera Oriental, al suroeste de Bogota, se encuentran varios lineamientos, asociados con actividad neotectonica segun algunos autores. Entre estos se destaca el conocido como Falla La Cajita, localizada sobre el flanco occidental del Macizo de Sumapaz, que ha sido senalada como falla activa, con sismo historico asociado y, por lo tanto, de gran importancia para la amenaza sismica de Bogota. En contraste, los registros de sismicidad de la Red Sismologica Nacional muestran muy escasa actividad sismica en el area del lineamiento. Mediante un estudio geomorfologico a partir de fotografias aereas y observaciones de campo y revision de los registros de actividad sismica historica e instrumental en el area, en este trabajo se analizan los indicios morfotectonicos y sismologicos del Lineamiento La Cajita con el fin de caracterizar su actividad reciente. La fotointerpretacion geomorfologica destaca una deformacion notoria -localizada solo en el sector de la Laguna La Cajita y la Laguna Cajones- y la ausencia de evidencias morfotectonicas en el tramo norte de la Laguna del Alar. La interpretacion de las observaciones geomorfologicas apoya la hipotesis de un sistema masivo de deslizamiento con componente rotacional en el sector La Cajita - Cajones. Este proceso de remocion en masa estaria favorecido por la fuerte pendiente del terreno, los cambios litologicos y la presencia de una zona de alta precipitacion que ha generado aportes masivos hacia el cono torrencial de Pasca – Fusagasuga durante el Cuaternario. El analisis de los datos sismologicos sugiere que los eventos mas significativos en la zona del Paramo de Sumapaz estan asociados con focos localizados mas al sureste de la capital y no con el Lineamiento de La Cajita. De acuerdo con este analisis, se concluye que los indicios de deformacion en algunos sectores del Lineamiento de La Cajita no corresponden a una actividad neotectonica relacionada con un falla activa sino con un sistema masivo de deslizamientos.

/pdf/fallamiento-activo-en-la-cordillera-oriental-al-suroeste-de-32gwtjmosg.pdf

¿Fallamiento activo en la Cordillera Oriental al suroeste de Bogotá, Colombia? Is there active faulting in the Eastern Cordillera southwest of Bogotá, Colombia?

The crustal thickness is a very important property of the crust which is used for different purposes like understanding the crustal evolution, in geodynamic models and in modeling wave-propagation in global and regional seismic studies. In this study, we analyze the reflections from the underside of the Moho for five intermediate earthquakes occurred in the northern Andes and recorded at teleseismic distances (40°85°) to estimate the crustal thickness at the bounce point. The reflected phases are identified as precursors of the pP phase and sS phase and are called pmP and smS phases respectively. We find that the crustal thickness in the study area varied from 25 km to 60 km for the northern Andes. The results obtained show a crustal thickness in accordance with previous works done in this area.

Crustal thickness in the northern Andes from teleseismic pP and sS precursors

Any uncompensated mass of the northern Andes Mountains is presumably under pressure to adjust within the Earth to its ideal state of isostatic equilibrium. Isostasy is the ideal state that any uncompensated mass seeks to achieve in time. These pressures interact with the relative motions between adjacent plates that give rise to earthquakes along the plate boundaries. By combining the gravity MOHO estimates and crustal discontinuities with historical and instrumental seismological catalogs the correlation between isostatically disturbed terrains and seismicity has been established. The thinner and thicker crustal regions were mapped from the zero horizontal curvature of the crustal thickness estimates. These boundaries or edges of crustal thickness variations were compared to crustal discontinuities inferred from gravity and magnetic anomalies and the patterns of seismicity that have been catalogued for the last 363 years. The seismicity is very intense along the Nazca-North Andes, Caribbean-North American and North Andes-South American collision zones and associated with regional tectonic compressional stresses that have locally increased and/or diminished by compressional and tensional stress, respectively, due to crustal thickness variations. High seismicity is also associated with the Nazca-Cocos diverging plate boundary whereas low seismicity is associated with the Panama-Nazca Transform Fault and the South American Plate.

/pdf/crustal-thickness-variations-and-seismicity-of-northwestern-5cvhkln2cz.pdf

Crustal thickness variations and seismicity of northwestern south america

Potential fields modeling for the Cayos Basin (Western Caribbean Plate): Implications in basin crustal structure

The central part of the Andean range, east of the Pasca town, presents a series of geomorphologic features that are typical of active tectonic areas. A morphological analysis has been carried out based on satelite images and areal photos. It was possible to outline two main lineament systems with N1OOE and N45°W strikes, respectively, along which a series of geoforms suggesting neotectonic events that had faulted and deformed quaternary deposits. Based on the above information and field control, each lineament was characterized, confirming the presence of the cajitas Fault ans its neotectonic activity.

/pdf/analisis-neotectonico-de-un-area-ubicada-al-oriente-del-3dp3eugy0w.pdf

Análisis Neotectónico de un área ubicada al Oriente del Municipio de Pasca, Departamento de Cundinamarca

espanolEl proposito principal de esta investigacion es esclarecer el estado isostatico del area de abanico del Rio Magdalena a partir de la interpretacion de anomalias de campos potenciales en la Cuenca Colombia con el fin de profundizar el conocimiento teorico sobre los procesos geodinamicos y el fenomeno de la isostasia. Las investigaciones sugieren que la condicion isostatica de la corteza esta fuertemente afectada por la correlacion entre la anomalia gravimetrica de aire libre (FAGA) y los efectos gravimetricos del terreno (TGE). El analisis e interpretacion incluyen el procesamiento y la elaboracion de mapas de las anomalias gravimetricas y magneticas. Las correlaciones e interpretaciones de las anomalias gravimetricas confirman perturbaciones isostaticas a lo largo del margen noroccidental colombiano y la Cuenca Colombia. La validacion de los resultados se ha realizado mediante el modelamiento directo e inverso de datos de gravimetria y magnetometria y posterior comparacion con modelos de espesor de corteza a partir de datos sismicos. Los modelos sugieren un bajo angulo de subduccion y la deformacion elastica de la placa Caribe, el cual se muestra afectada por la carga sedimentaria del paleo-delta del rio Magdalena, las provincias geotectonicas de los cinturones del Sinu - San Jacinto y la corteza continental. EnglishThe main purpose of this research is to clear up the isostatic state of the fan area of the Magdalena River from the interpretation of gravity and magnetic anomalies of the Colombia Basin in order to improve the theoretical knowledge on the geodynamic processes and the phenomenon of isostasy. Previous research suggests that the isostatic condition of the crust is strongly affected by the correlation between Free Air Gravity Anomalies (FAGA) and Terrain Gravity Effects (TGE). The analysis and interpretation include processing and mapping of gravity and magnetic anomalies. Correlations and interpretations of gravimetric anomalies confim isostatic perturbations along the Colombian northwestern margin and the Colombia Basin. The validation of the results was done through the forward and inverse modeling from gravity and magnetic data and subsequent comparison with models of crust thickness from seismic data. The models suggest a low subduction angle and elastic deformation of the Caribbean plate, which is affected by the sedimentary load of the paleo-delta of the Magdalena River, the geo-tectonic provinces of the Sinu-San Jacinto belt and the continental crust.

/pdf/estado-isostatico-del-abanico-del-rio-magdalena-a-partir-de-1yoh099cf6.pdf

Orlando Hernández-Pardo

Papers

Crustal thickness variations and seismicity of northwestern south america

Potential fields modeling for the Cayos Basin (Western Caribbean Plate): Implications in basin crustal structure

Análisis Neotectónico de un área ubicada al Oriente del Municipio de Pasca, Departamento de Cundinamarca

Estado isostático del abanico del rio Magdalena a partir de la interpretación de anomalías de campos potenciales, Cuenca Colombia