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Estela Minaya, Observatorio San Calixto (Bolivia)
Victor I. Ramirez, Servicio Nacional de Geologia y Tecnico de Minas (Bolivia)
Reginald L. Hermanns, Norges Geologiske undersøkelse (Norway)
John J Clague, Simon Fraser University (Canada)
Magaly Gonzalez, Observatorio San Calixto (Bolivia)
Javier Valencia, Servicio Nacional de Geologia y Tecnico de Minas (Bolivia)
Oscar Cerritos, Geological Survey of Canada (Canada)
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El Alto is a city of 1 million inhabitants at the edge of the Bolivian Altiplano, adjacent to the valley in which La Paz is located. No significant historical earthquakes have occurred in this region. However, several normal faults with significant morphologic expression form scarps on The Altiplano plateau. Two of them form a horst and offset strata on a part of the plateau which is today covered by the city of El Alto. The faults are called the El Alto fault system and converge towards the west and continue as a stepped en-echelon normal fault dipping towards the southwest. The fault can be traced on early aerial photographs for a distance of 11.4 km, but today it is obscured by buildings and the La Paz airport.
Mapping of the horst structure with differential GPS has shown that the northern and southern bounding faults have maximum offsets of, respectively, 12 m and 5 m. The faults intersect the edge of the Altiplano, below which are deposits of giant mass movements ranging in area from several square kilometers (Llojeta and Allpacoma earth flows) to about 60 km2 (Achocaya earth flow). The landslides have been interpreted to have been seismically triggered. Based on radiocarbon dating, the Achocaya earth flow is about 6000 cal yr old and the Allpacoma earth flow about 1600 cal yr old. We excavated two trenches across the fault to test the hypothesis that the large landslides at the edge of the Altiplano were seismically triggered. Both trenches were opened in outwash gravels that directly underlie the Altiplano surface. One of these trenches across the fault, west of the horst, showed a total vertical offset of 4.89 m during four inferred events, with a maximum event offset of about 2 m. Slickensides indicate purely normal displacement. Offset events are defined by colluvial wedges adjacent to the footwall of the fault.
Preliminary OSL dating of silt layers interbedded with colluvium indicates that these four events occurred between 54 and 15 ka. A second trench across the southern branch of the horst was limited in length by the presence of buildings close to the fault. It revealed three discrete slip events two of them offsetting a 30 cm thick silt layer with a total vertical offset of 2.24 m. Offset strata indicate extension; one event involved horizontal displacement of about 2 m and vertical displacement of about 1.5 m. An earlier event is defined by a third offset near one end of the trench, which resulted in the deposition of the silt layer. This layer provided a preliminary OSL date of 53 ka, nearly identical to the oldest event in the first trench. These data suggest that the Achocaya and Allpacoma landslides are not related to offsets along the El Alto fault system. However, several other faults with morphologic expression occur in the vicinity of these landslides and have not yet been studied.
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