Abstract

VARGAS JIMENEZ, CARLOS A  and  DURAN TOVAR, JUAN PABLO. STATE OF STRAIN AND STRESS IN NORTHWESTERN OF SOUTH AMERICA . Earth Sci. Res. J. [online]. 2005, vol.9, n.1, pp.41-49. ISSN 1794-6190.

Analysis of stress inversion of 96 focal mechanisms (1976 - 2000, Mw > 5) and satellital geodesy (CASA 1991, 1994, 1996) allowed to establish a compressive tendency in the WE sense of the Panama-Costa Rica Block (BPC), that hit the North Andes Block (BNA). Some seismic evidences would confi rm the existence of the proposed Chocó Block (BC), with western limit Darién Range, and eastern limit in the zone of Murindó, demonstrated by the stress inversion of these two zones, with sub-parallel tendency NS. The southern limit, already 4º N, also shows stress with NS tendency. Some located superfi cial earthquakes in the Pacifi c Coast, apparently western limit of Chocó Block, shows stress with NW tendency. Additionally, the vectors of displacement and the main stress axis in the Colombian southwest, near to the zone of Tumaco, shows clear tendencies WE. The deep seismicity in the zone of Bucaramanga displays stress with tendency NNW - SSE, coherent with the Caribbean Plate subduction under Sudamerica.The results of shear wave analysis (Jan. 1994 to dec. 1997) allowed to estimate the direction of shear wave polarization in 6 regions that should be parallel or sub parallel to the maximum horizontal compressive stress (Kaneshima, 1990). The leading shear waves in the region seismicity in the zone of Bucaramanga from deep earthquakes (depth > 40 km) are polarized in E-SE direction. In the region 2, throughout the System of Faults of Borde Llanero, leading shear waves from shallow earthquakes (depth < 40 km) are polarized NE-SE direction. In the region 3 to the latitude 1ºN in the Colombian Pacifi c Coast, leading shear waves from shallow earthquakes are polarized NE direction. In the region 4 and 5 in the cordillera Western, leading shear waves from deep earthquakes (depth > 40 km) are polarized in direction NNE-SE. In the region 6 in the Pacifi c Coast, leading shear waves from shallow earthquakes are polarized SE, direction. The discrepancies in orientation between the leading shear wave and the maximum horizontal compression can be result of local crustal heterogeneities.

Keywords : stress; strain; shear wave splitting.

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