HISTORICAL SEISMICITY OF THE ALGECIRAS FAULT SYSTEM, SOUTHWESTERN COLOMBIA

Abstract:

The Algeciras Fault System (AFS) is located in the Eastern Cordillera south western Colombia. This fault system has been allocated at least four big earthquakes in the last 230 years. In this work we describe the macroseismic intensities of these earthquakes not only to its epicentral zone but also in others places as Bogotá metropolitan area far from AFS more of 200 km. The AFS is shaped by three thrust faults. From north to south these are Guayuriba Fault with with 160 km of lengh, the Algeciras Fault with 149 km of lengh, and the Garzon - Pitalito Fault with 128 km of lengh.

The big earthquakes, whose macroseismic data are analyzed here, its that of the 1785 (M=6.8) event, for which the Guayuriba Fault was related; it caused heavy damage in Bogotá and Neiva. This fault also produced the 1917 (6.9Ms) earthquake which significantly affected to Bogotá and Villavicencio. The 1967 earthquake (7.2Mw) is related to the Algeciras Fault; this event was very destructive in rural villages of Huila Department and caused significant damage in Bogotá and Neiva. With the latter earthquake high vulnerability was evident in the Bogota metropolitan area front to a large event ocurred by this fault system. The 16 November 1827 (M=7.3) earthquake ocurred on the Garzon - Pitalito Fault and was felt throughout the whole Andean region of Colombia. This event produced high intensities both in Bogota like in Popayan, Neiva, Pasto and where today are located the cities of Armenia, Manizales and Pereira toward west of Colombia. These lattest cities were founded in the second half of nineteen century after happened this earthquake.

From historical seismicity review, we can determine 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 región inhabited by more than 65% of the population of Colombia.