Mecanismo Focal do Terremoto Principal de 10 de Março de 1998, Porto dos Gaúchos, Mato Grosso, Brasil

The active earthquake area of Porto dos Gaúchos, Mato Grosso state, Brazil, hereinafter called the Seismogenic Zone of Porto dos Gaúchos (whose Portuguese acronym is ZSPG) is placed at junction of the southern boundary of the Amazonian Craton with the northern part of the Parecis Basin. This area represents one of the most outstanding active seismic areas of Brazil intraplate seismicity, where it occurred in 1955 the largest known Brazilian earthquake and where recently, on March 23, 2005, happened another significant earthquake with magnitude 5.0 mb (USGS). The ZSPG quake of March 10, 1998, reaching a magnitude of 5.3 mb(ISC) is the second largest event of the ZSPG and the largest seismic event during the recent times of digital seismology, hence supplying a great amount of useful data due to the large number of seismographic stations recording it. Using the polarities of the incident P waves at 12 world-wide seismographic stations it was possible to workout the focal mechanism of the 1998 mainshock. This mechanism shows a predominant strike slip fault plane with a small reverse component. The preferred slipping plane has a strike of 303°, coincident with the kilometer extension lineaments (faults and shear zones) observed in the Amazonian Craton and with the structure of the Parecis Basin and also matching early aftershock lineament of the 1998 mainshock. The inferred fault mechanism is compatible with a model presenting an approximately EW compression coinciding with the direction of the predominant regional tectonic stresses acting in the South America Tectonic Plate.