Why does a seism happen? This is a frequently-asked question. The outermost part of the Earth, the lithospere, which is approximately 100km thick, is rigid and is divided into many parts or tectonics plates, which move in relation to one another. In the contact zones between these plates occur the strongest and most frequent earthquakes. The cause of this movement is the existence of geologic forces within the Earth, whose origin is still not well known and determined (Assumpção, 1983).

However, it is known that the state of tension, strain or stress in the lithospere can be due to a great variety of forces: of local origin, caused by structural heterogeneity, loading and unloading of the crust and thermal anomalies in the asthenosphere; and of regional origin, related to the movement of tectonic plates, such as a “push” of the oceanic dorsal, due to its higher elevation (compressing the interior of the plate); thrusting forces, for example, exerted by the Nazca Plate, which dives to underside of the the South American Plate; and viscous loading forces, which are caused by convection currents that may be putting some “load” on the South American Plate (Mendiguren & Richter, 1978; Assumpção & Suarez, 1988).

Brazil, due to its being situated above the South American Tectonic Plate, a stable continental region, displays a far lower seismicity than the one which may be seen at the plates' edge, as is the case in the contact zones between the Nazca and the South American Plates, where the seisms are more frequent and of stronger magnitudes. Moreover, the seismicity observed in Brazil is lesser than the one seen in other similar intraplate regions, as in eastern North America, India, Africa and Australia, where great earthquakes have already been observed, as, for example, the ones of New Madrid (U.S.A.), in 1811 and 1812, with 8,2 and 8.0 magnitudes, respectively (Johnston, 1989).

Brazilian natural seismicity map

The data presented in the map derive from the SIS/UnB data base (SISBRA), which was formed out of the compilation of Berrocal et al. (1984). It contains the historical and instrument-related registers of the seisms with an epicenter in Brazil and neighboring regions, from colonization to 1981. From then on, it has been updated with the Brazilian Seismic Bulletin data, published until 1995 in the Brazilian Geophysics Magazine and its data base has been kept in conjunction with UFRN and USP.

The first seism from the catalogue of Berrocal et al. (1984) dates back to the era of colonization, in 1560, in the São Vicente Captaincy. However, it is a recording which we must give little credit to. Not until early early in the 18^th century do more trustworthy catalog historical recordings come up, at which point the macroseismic descriptions are more realiable.

The instrumental data were compiled from two sources: the first one was obtained from international seismic bulletins of the World-wide Seismographs' Networks, which, according to Berrocal et al., (1984), include seisms greater than 5,5, since 1919 and 4,5-magnitude seisms, since 1965. The second one, derives from the data of Brazilian seismographic stations, which, as of 1967, with the installation of the South America Seismographic Array (SAAS), in Brasília, started to detect small seisms of 3,5 maginitude or less, depending on the epicentral region.

For the seismic catalog it was used the teleseimics body wave (p) magnitude, which was adapted to Brazil by Assumpção (1983), so that it may contemplate those magnitudes not detectable at teleseimics distances, that is, magnitudes inferior to 4,5 (Assumpção, 1998). This scale was named Regional Magnitude Scale (mR), valid for Brazil-located seism epicenters, whose epicentral distance ranges between 200 km and 1,500 km. Also, the Magnitude's Duration Scale was used (represented by the generic expression mD = log (D) + B log (R) + C), where the coefficients' values A, B and C depend on the region. D is the duration (in seconds) for the recording in the seismogram and R is the epicentral distance.

Brazilian natural seismicity's characteristics

By observing the map above, we notice the absence of seismicity in some areas, especially in the northern and middle-western regions, something that is not necessarily related to the absence of seisms, since it may also depend on the process of the Brazilian territorial occupation and on the late installation of seismographic stations. The middle and the souther parts of the Parana River basin seem to be the most non-seismic, whereas the seismicity at its edges is far more expressive, (both the natural and water reservoir-induced ones), with many attested cases.

Seismicity data are obtained through three types of registers: geologic, historical and instrumental (Reiter, 1991), the two last ones being more important, since the geologic recording is performed only when there are strong earthquakes, which leave marks on the land's surface, a correlation that is not always followed in Brazil.

The historical recording is performed through testimonies registered by newspapers, magazines and books, of those who felt the seism. These registers in Brazil, apart from being very recent, are intrinsically related to the Brazilian process of territorial occupation, which, as it is known, was carried out very irregularly, with the late occupation of the northern and middle-western regions, compared to the rest of the country.

The instrumental register, which is more recent in Brazil, is closely related to the seismographic monitoring of areas of great hydroelectric reservoirs, whose constructions took place especially in the end of 70's and from the beginning of 80's on. Nowadays, Brazil has three institutions which contribute directly to Brazil's seismicty map. Also, Brazil has further five stations of the worldwide network, creating a trustworthy map of Brazilian seismicity. (France, 2006)

References

Assumpção,M.S. 1983. Terremotos No Brasil. Ciência Hoje- SBPC(1983), 1(6):13 - 20. Risco Sismico, Sismos do Brasil

Assumpção,M.S.; Suarez,G. 1988. Source Mechanism of Moderate Size Earthquakes and Stress Orientation in Mid-Plate South America. Geophysical Journal,92: 253 - 267.

Assumpção, M. . Seismicity and stresses in the Brazilian passive margin.. Bulletin of the Seismological Society of America, Estados Unidos, v. 88, n. 1, p. 160-169, 1998.

Berrocal,J.; Assumpção,M.S.; Antezana,R.; Dias Neto,C.M.; Ortega,R.; Franca,H.; Veloso,J.A.V. 1984. Sismicidade do Brasil. Instituto Astronómico e Geofísico, São Paulo, Brasil, 320 p

França, G. S. . Brazil Seismicity. Bulletin of the International Institute of Seismology and Earthquake Engineering, Japão, v. 40, p. 23-36, 2006.

Johnston, A. C. (1989) The Seismicity of "Stable Continental Interiors". Proceedings of the NATO Advanced Research Workshop on Causes and Effects of Earthquakes at Passive Margins and in Areas of Postglacial Rebound on both Sides of the North Atlantic, Vordingborg, Denmark, (9–13) May 1988,pp. (299–327).

Mendiguren,J.A.; Richter,F.M. 1978. On the origin of compressional intraplate stress in South America. Revista Brasileira de Geociências(1978), 8(2): 90-103.

Reiter, L. 1991. Earthquake Hazard Analysis ,Columbia Univ. Press, New York