GPS-derived Deformation of the Central Andes Including the 1995 Antofagasta M w = 8.0 Earthquake

Abstract

In order to study both the interplate seismic loading cycle and the distribution of intraplate deformation of the Andes, a 215 site GPS network covering Chile and the western part of Argentina was selected, monumented and observed in 1993 and 1994. A dense part of the network in northern Chile and northwest Argentina, comprising some 70 sites, was re-observed after two years in October/November, 1995. The M w = 8.0 Antofagasta (North Chile) earthquake of 30th July, 1995 took place between the two observations. The city of Antofagasta shifted 80 cm westwards by this event and the displacement still reached 10 cm at locations 300 km from the trench. Three different deformation processes have been considered for modeling the measured displacements: (1) interseismic accumulation of elastic strain due to subduction coupling, (2) coseismic strain release during the Antofagasta earthquake and (3) crustal shortening in the Sub-Andes.Eastward displacement of the sites to the north and to the south of the area affected by the earthquake is due to the interseismic accumulation of elastic deformation. Assuming a uniform slip model of interseismic coupling, the observed displacements at the coast require a fully locked subduction interface and a depth of seismic coupling of 50 km. The geodetically derived fault plane parameters of the Antofagasta earthquake are consistent with results derived from wave-form modeling of seismological data. The coseismic slip predicted by the variable slip model reaches values of 3.2 m in the dip-slip and 1.4 m in the strike-slip directions. The derived rake is 66°. Our geodetic results suggest that the oblique Nazca-South American plate convergence is accommodated by oblique earthquake slip with no slip partitioning. The observed displacements in the back-arc indicate a present-day crustal shortening rate of 3-4 mm/year which is significantly slower than the average of 10 mm/year experienced during the evolution of the Andean plateau.

Asperity generating upper crustal sources revealed by b value and isostatic residual anomaly grids in the area of Antofagasta, Chile

“…Nevertheless, the area of largest coseismic slip in the 1995 Antofagasta earthquake deduced from GPS data [Klotz et al, 1999;Pritchard et al, 2002;Xia et al, 2003;Chlieh et al, 2004] is almost identical with the slip area identified by local seismological parameters from investigating the aftershock sequence [Sobiesiak, 2000]. However, this local study showed a more detailed picture of the coseismic slip area separated in two patches of large slip values.…”

Tectonics, climate, and mountain topography

“…They found that by subtracting 3.4 mm/yr from the Nazca-South America convergence rate, they could fit the rest of the GPS data assuming elastic strain along the plate margin, and allowing for post-seismic deformation following the 1960 Chilean earthquake. Khazaradze and Klotz [2003] later reported the result of Klotz et al [1999] as 3.4 AE 2 mm/yr. These rates seem consistent with the summary by Kendrick et al [2006].…”

“…[66] Chile (Northern, 23°S-30°S) (CN): We rely on Klotz et al [1999] who inferred 3-4 mm/yr of shortening across the northern part of this region and on Brooks et al [2003] who estimated $4.5 mm/yr across the southern part. They found that by subtracting 3.4 mm/yr from the Nazca-South America convergence rate, they could fit the rest of the GPS data assuming elastic strain along the plate margin, and allowing for post-seismic deformation following the 1960 Chilean earthquake.…”

Alzamiento litoral Pleistoceno del norte de Chile: edades 21Ne de la terraza costera mas alta del area deCaldera-Bahia Inglesa.

“…De este modo este alzamiento estaría vinculado a algún proceso responsable de la convergencia de la placa de Nazca bajo la Sudamericana como lo son los terremotos de subducción. En la zona de la península de Mejillones, 500 km al norte del área de estudio, también se reconocen líneas de paleocostas y la parte sur de esta península, fue afectada por el terremoto de Antofagasta Mw=8,1 de mecanismo inverso, que produjo un alzamiento centimétrico en la parte sur de esta península (Ruegg et al, 1996;Klotz et al, 1999), y gran parte de ese alzamiento se recuperó en los tres años siguientes a ese terremoto (Chlieh et al, 2004 de 1995 (Beck et al, 1998). Un modelado preliminar de este terremoto (Quezada et al, 2005), indica que el área de Caldera Bahía Inglesa debiera experimentar un solevantamiento que fluctúa entre 1-1,5 m durante un terremoto como el de 1922 y que gran parte de ese alzamiento cosísmico, se recupera intersísmicamente y el alzamiento remanente durante un ciclo sísmico, explica el alzamiento de carácter regional que experimentan las terrazas marinas en el litoral de Caldera-Bahía Inglesa en particular y del Norte de Chile en general.…”