THE SEISMICITY OF THE ALGECIRAS FAULT SYSTEM, SOUTHWESTERN COLOMBIA

The Algeciras Fault System (AFS) is located in the Eastern Cordillera, south western Colombia. In this fault system has been allocated at least four big earthquakes in the last 230 years. All these earthquakes has affected mainly to Bogotá, and all these earthquakes has been magnitudes (M) > 7.0.

This paper describes the seismicity registered by the Colombian National Seismological Network. Also, it shows the regional geotectonic skecht. The AFS is shaped by three thrust faults. From north to south these are: Guayuriba Fault (160 km long), the Algeciras Fault (149 km long), and the Garzon - Pitalito Fault (128 km long). The AFS is very near to the Colombian - Ecuadorian trench that is related with the subduction of the Nazca plate under South America. Nazca plate converges with South America to a speed of 7 cm/year. From the Colombian trench to the Eastern Cordillera from 2°N to 4°N there are less than 200 km away. In this región a retro arc basin located between Central and Eastern cordilleras. This basin is known as Alto Magdalena Valley (AMV). In this región the lithosphere is formed by three plates that from the west to east are: the old oceanic Caribbean plate that limits with Romeral Fault System with old continental Mesoproterozoic Grevillian (or Mexican Oaxaquian terrane) toward east, and the old continental crust of the Amazon Craton that limits by the AFS and NE trending subparallels faults with Mesoproterozoc Grenvillian terrane toward west.

The geotectonic analysis realized here help to improve the knowledge of the scope of seismic hazard for this fault system which not only affects its area of influence but also the center and west of the country, a region inhabited by more than 65% of the population of Colombia.