Brazil

Brazil is one of South America's least seismically active countries on a per-unit-area basis, sitting on the stable South American Platform — one of the world's oldest and most rigid continental cratons — far from any active plate boundary. The country does experience intraplate seismicity, generated by the reactivation of ancient basement faults under the diffuse stress field transmitted eastward from the Andean collision zone and the passive Atlantic margin, but the frequency and magnitude of events is low compared to the Andean nations. The most seismically active regions are the northeastern portion of the country (centred on the states of Rio Grande do Norte, Ceará, and Paraíba), where a complex pattern of ancient Precambrian shear zones and fracture zones has been reactivated; the Amazon region, where Paleozoic intracratonic basins provide structural discontinuities; and the southeastern states of Minas Gerais and Goiás, where ancient basement faults generate occasional moderate events. The Pantanal Basin in central-western Brazil generates rare but occasionally felt events from basin-related faulting. Brazil's most economically significant seismic risk may be from induced seismicity associated with the filling of large hydroelectric reservoirs, several of which sit on old basement fault structures that respond to changes in water loading.

The earthquake history of Brazil, while sparse, includes some significant events. The 24 October 1955 São Roque earthquake (magnitude 5.8) near São Paulo caused minor damage. The 20 November 1986 João Câmara earthquake (magnitude 5.1) in Rio Grande do Norte state triggered an extended swarm of earthquakes that lasted for years and included thousands of events, causing widespread public alarm and some building damage in the small town of João Câmara, which sits on a reactivated Precambrian shear zone. The 1955 Amapá earthquake (magnitude 7.1) struck near the Amazon delta and remains one of the largest instrumentally recorded Brazilian earthquakes; it occurred within the Guiana craton near the French Guiana border. The Araçuaí earthquake swarm in Minas Gerais state has produced multiple M4–5 events since the 1980s. The Caraíbas earthquake of 2007 (magnitude 4.9) in Bahia caused minor structural damage in rural communities. Induced seismicity from the filling of the Sobradinho Reservoir in Bahia and other large hydro projects in the Amazon basin has been documented scientifically, including events of up to M4.

Brazil's deep geological structure is dominated by the São Francisco Craton in the east and the Amazon Craton in the west, both Archaean to Proterozoic nuclei that formed the core of the continent. Around these ancient nuclei, younger Proterozoic mobile belts — zones of intensely deformed metamorphic rocks formed during the assembly of the supercontinent Gondwana approximately 600 million years ago — have been reactivated by modern stress fields to produce the country's intraplate seismicity. The Northeast region's earthquake swarms are spatially correlated with these ancient shear zones, which represent pre-existing weaknesses in the crust susceptible to reactivation when the ambient stress field rotates or intensifies. The passive Atlantic margin of Brazil has not produced a major ocean-floor spreading-related earthquake in historical time, though small events on offshore transform faults occasionally produce minor felt shaking in coastal cities. [[seismic-wave]] propagation through the cold, old Brazilian craton is highly efficient, explaining why even moderate M4–5 events in the interior can be felt hundreds of kilometres away. Brazil's national seismological network (RSBR) is operated by multiple research universities and the national geological survey (CPRM).