SEISMIC ACTIVITY IN MONSUABA (STATE OF RIO-DE-JANEIRO), BRAZIL, BETWEEN 1988 DECEMBER AND 1989 FEBRUARY

Between 1988 December and 1989 February, a sequence of microearthquakes occurred in Monsuaba, district of Angra dos Reis, state of Rio de Janeiro, Brazil, 24 km away from the Angra dos Reis nuclear power plant. The activity was recorded initially by BMA, 35 km away from the epicentral area, and later by a local network of five portable seismographs installed a few days after the activity began. The hypocentral distribution and the composite fault plane solution suggest that the activity may have been related to a relatively small structure (2 x 1.5 km) produced by a probable reverse fault (25-degrees of azimuth and 35-degrees of dip in the SE direction), representing a compressional stress regime with a NW-SE oriented P axis. The temporal distribution of the Monsuaba earthquakes shows a typical seismic sequence with foreshocks, main shock (m(b) = 3.0) and aftershocks. The frequency/magnitude relations indicate a b-value of 0.9 for the single frequency distribution and 1.0 for the cumulative one. The main shock, maximum intensity of VMM at Monsuaba, was felt up to a distance of 10 km.