[S04-3] Historical and macroseismic studies of earthquakes III

[S04-3-04] The newly discovered 1885 earthquake in the French Guiana - Brazil border, 6.0 mb, the largest historical mid-plate event in South America

^○Marcelo Assumpcao¹, Alberto Veloso² (1.University of Sao Paulo, Sao Paulo, Brazil, 2.University of Brasilia, Brasilia, Brazil)

On August 4, 1885, a strong tremor was felt along 1400 km of the Amazon river, from Belem near the coast to Urucurituba near Manaus, with intensities reaching IV-V MM, as reported by all newspapers of Belem and Monte Alegre, Parah state. The French Seismic catalogue (http://www.sisfrance.net/Antilles/) reports a strong shaking (intensities V to VI MM) in Cayenne and Guisanbourgh, French Guiana. Other, independent accounts of this earthquake were reported by newspapers in Surinam (V in Paramaribo) and Guyana (IV in Georgetown). A survey of all available macroseismic data indicates the event was felt up to 1200km distance. We used an intensity = f (magnitude, distance) attenuation relation for Brazilian intraplate earthquakes to locate the best epicenter and magnitude of the 1885 earthquake. The epicenter was located in the border between French Guyana and Brazil and a magnitude of 6.0 (+- 0.3) mb was estimated. This event is one of the largest to have occurred in mid-plate South America. Previously known large events in northern Brazil were 1955.01.31 in Mato Grosso state (6.2 mb) with an average felt radius of about 600 km, and the 1985.08.03 Amazonas (5.5 mb) felt up to 420 km, on average. Other historical accounts of earthquakes felt near the border between French Guiana and Brazil (1949, ~5.3 mb; 1951, ~4.8 mb) as well as the recent 2006, 5.2 mb, event near Caienne, suggest that this region could be regarded as a seismic zone, not previously considered in seismotectonic studies in South America.