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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