BANJO

In a collaborative effort with the Carnegie Institution of Washington, and Lawerence Livermore National Lab, the University of Arizona deployed a passive source, broadband, seismic experiment in the central Andean Cordillera of Bolivia and northern Chile. This was an international [multi-institutional] project with other participants including, San Calixto Observatory and the University of Bolivia in La Paz, Bolivia, the University of Chile in Santiago, and ORSTROM, a French research organization.

Our scientific objectives are to extend our understanding of the deep structure and tectonics of the central Andean Cordillera and in particular the high Altiplano plateau. Our passive source experiment consisted of an east-west transect called the BANJO (Broadband ANdean JOint) Experiment and a north-south transect called the SEDA (Seismic Exploration of the Deep Altiplano) Experiment. The equipment for the east-west transect was provided by the PASSCAL program of IRIS (Incorporated Research Institutions for Seismology) and the Carnegie Institution of Washington (USA). The equipment for the north-south transect was provided by Lawrence Livermore National Laboratory (USA).

The BANJO experiment consisted of 16 stations that were deployed from April 1994 to September 1995, along an east-west transect at 19Sto 20S and extended from northern Chile across Bolivia to the Chaco Plain. The sixteen stations consisted of Streckeisen STS-2 broadband sensors and Reftek (24 and 16 bit) digital recorders. The highest station (red cubes) had an elevation of 14,000 feet and the lowest an elevation of 2500 feet.

The SEDA experiment consisted of seven stations deployed in a north-south transect along the the spine of the Altiplano, between La Paz and Uyuni, Bolivia. The north-south transect was about 300 km long with a station spacing of approximately 50 km. These stations consisted of Guralp CMB-3ESP and 40T sensors and Reftek (16 bit) digital recorders and were deployed from April 1994 to June 1995.

The transects are spectacular both in terms of geology and scenery. The east-west transect crossed over the entire Central Andes perpendicular to the strike of the structural elements. It spaned the Western Cordillera, Altiplano plateau, Eastern Cordillera, Sub- Andean zone, and Chaco Plain, a length of nearly 1000 km. The north-south transect, along the length of the Altiplano plateau, crossed the hingeline of the Bolivian Orocline. The Western Cordillera is an active volcanic arc associated with the subduction of the Nazca Plate beneath the South American plate. The Altiplano, in southern Peru, western Bolivia, and northern Chile forms one of the worlds highest and largest plateaus, second only to Tibet, with an average elevation of nearly 4 km (12,000 feet), a crustal thickness of 70-80 km, and covers an area of over 600,000 km2. The Eastern Cordillera is a highly folded and faulted terrane with large ignimbrite fields. The Sub-Andean zone is an active fold and thrust belt. The Chaco Plain is relatively undeformed and underlain by the Brazilian shield.

The largest (Mw 8.3) deep focus earthquake in modern recorded history occurred on June 9, 1994. This event ruptured the down-going Nazca slab at a depth of 636 km under Boliva (blue circle) and was felt as far away as Canada. Having our portable array directly over the hypocenter was literally like winning the seismc lottery. During this earthquake, a seismic crew was in Bolivia. Although they were only 700 km away from the epicenter, they slept through the event.

More information regarding this experiment can be found here.

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