Lithospheric stress state in South America as inferred from tidal triggering of earthquakes

Abstract

We present a model for the lithospheric stress state in South America inferred from tidal triggering of earthquakes. The South American plate is attractive for study b~cause the slab pull component should be small and the other components are reasonably isolated geographically. A catalogue of South America earthquakes which contains 23,399 events with M>3.5 has been used for the study of the Earth tide triggering effect. The normalized difference, t, between the number of shocks occurring during positive and negative phases of tidal vector components is used to estimate the tidal triggering effect. This value t has a mosaic spatial distribution and changes with time. The tidal triggering effect is also connected with a few harmonic components of the Earth tide that differ from area to area. We present maps of the Lode-Nadai coefficient, that characterizes the lithosphere stress state. These maps show a dominant state of tension along the Cordilleras and a rather chaotic distribution in the inner-continental area. The detailed features have a complex character and are not distinctly tied with tectonic elements. The maps of the orientation of compression and tension axes are also obtained using a tidal triggering effect.