PREHISTORICAL EARTHQUAKES RECORD IN THE WESTERN PORTION OF THE STATE OF SÃO PAULO, BRAZIL.

Abstract

The Brazilian territory is characterized by a low magnitude and a low frequency seismic activity, due to, mainly, its position in the South-American tectonic plate. In spite of this, our territory is not avoided of seismic risk. The seismic knowledge about Brazil is very poor yet, especially because the interior territorial occupation dates no more than a few decades. Fortunately, strong earthquakes often print geologic impressions in the Cenozoic sedimentary record, and these evidences, herein referred as seismites, might be investigated. The study of this past ground effects from “fossil” earthquakes is pertinent to a branch of the Geosciences know as Paleoseismology. Among the seismites, one can includes features derived from seismic shake liquefaction, that encompasses dykes, sills and sand volcanoes. Their size, type, frequency and distribution provide good clues to define ancient earthquakes, including the meizoseismic zone delimitation and magnitude estimations. This paper presents some example of seismite occurrences in the Rio do Peixe valley, western portion of the State of São Paulo. In that area, the paleoseismologic analysis allowed the definition of three expressive past earthquakes during the period comprising 34 ka. and 10 ka. BP, all of them with magnitudes greater than 5,5. The magnitude of that biggest one has been estimated in 6.4, similar to the greatest historic earthquakes in Brazil. It means, without no doubt, that seismic risks should be evaluated using the paleoseismologic approach, in order to avoid or minimize injuries and economic losses to the Society.