Paleoseismologic trenching confirms recent Holocene activity of the major Algeciras fault system in southern Colombia

Abstract

Abstract The Algeciras Fault System (AFS) in Colombia, which is characterized by its predominant dextral strike-slip movement, is part of the major transcurrent fault system known in Ecuador, Colombia and Venezuela as the Eastern Frontal Fault System, that disconnects the North Andean Block (NAB) from the South American plate in a process of tectonic escape. It is responsible for some of the most important seismic events in the seismic history of Colombia that occurred in 1827 with Mw 7.1 that destroyed large part of all settlements in the Huila Department and caused considerable damage and loss of life in the capital of Bogota. More recently another event of Mw 7.0 took place in 1967 that also caused damage in the capital, and in December 2019 an event of Mw 6.0 caused minor damage in various settlements in the Eastern foothills of the Eastern Cordillera. So far, mapping studies based on data obtained from satellite imagery, airphotos and structural field data have outlined the complexity of the AFS and have identified the existence of at least six tectonic basins occurring along the main fault of the system. In 2016, within the framework of activities of the Space Geodesy Research Group of the Geohazards Directorate of the Geological Survey of Colombia (SGC) a program of morphotectonic field studies was carried out in and around the Algeciras pull-apart basin in the Huila Department that led to the opening of two exploratory paleoseismologic trenches. This paper presents the results of the first of these two trenches, located in the center of the Algeciras valley at a place named Lagunillas. It is situated at the rim of a shallow fault-dammed lake formed by an uphill facing scarp generated by the dextral strike-slip displacement of a shortcut fault that traverses the alluvial fill of the basin along its longitudinal axis. The results of the paleoseismologic study prove the occurrence of seven seismic events with magnitudes Mw 7.0 during the Holocene with an average recurrence interval of 1300–1400 years. This constitutes a significant contribution to the study of seismic hazard assessment of the AFS that complements, in a quantitative way, the evidence provided by the data of historic seismicity of this major fault and points out the considerable danger that the fault presents to the country as a whole and in particular to the metropolitan area of Bogota.