Intraplate seismicity in SE Brazil: stress concentration in lithospheric thin spots

Abstract

S U M M A R YIntraplate seismicity has generally poor correlation with surface geological patterns. Except for major extensional features, such as aborted continental rifts, which may act as weak zones, it is usually difficult to find simple geology based models to explain differences in seismic activity in stable continental regions. Seismicity in Brazil is clearly not uniform and a few areas of higher activity have been identified. However, the seismic areas show almost no correlation with the main geological provinces, which is typical of other intraplate settings. A recent uppermantle tomography study in SE and central Brazil, using approximately 8500 P-wave and 2000 PKP-wave arrivals recorded in 59 sites since 1992, has mapped P-wave velocity anomalies from lithospheric depths down to 1300 km. In this region, higher seismic activity occurs preferentially in areas with low P-wave velocities at 150-250 km depth. The low P-wave velocities are interpreted as shallower asthenosphere. In such areas, a hotter geotherm will reduce the strength of the lithospheric upper mantle causing most of the intraplate forces to be concentrated in the brittle upper crust. The low-velocity anomalies coincide with Late Cretaceous provinces of alkaline intrusions. The proposed ponding of the Trindade plume head beneath lithospheric thin spots is consistent with our tomography results, suggesting that plume effects may have helped to preserve lithosphere/asthenosphere topography. Although other factors are also important, the present data show that stress concentrations resulting from lithosphere/asthenosphere topography should play an important role in explaining the intraplate seismicity in the Brazilian platform.

Tomografia sísmica com ondas P e S para o estudo do manto superior no Brasil

“…High velocity anomalies in the western portion of the São Francisco Craton support the hypothesis that this craton was part of a major Neoproterozoic plate. Low velocity anomalies beneath the Tocantins Province (mainly fold belts between the Amazon and São Francisco cratons) are interpreted as due to lithospheric thinning, consistent with a good correlation between intraplate seismicity and low velocity anomalies in this region (Assumpção et al, 2004b). The resolution improvement of the high velocity anomalies beneath the Parana Basin (at ∼200 km) allows a discussion about the geometry of the cratonic nucleous of this Basin.…”

“…Foi interpretado que esta anomalia poderia também fazer parte do impacto inicial da Pluma de Trindade. Estas duas anomalias poderiam ainda estar relacionada ao Lineamento Trans-Brasiliano (LTB), indicando que nesta região existiria um afinamento na litosfera (Assumpção et al, 2004b). Feng et al (2004) observaram nesta região uma anomalia de baixa velocidade no manto superior (∼ 100 km) com uma tendência NE-SW, e propuseram que esta anomalia estaria relacionada ao LTB.…”

“…Também foram observadas anomalias de baixa velocidade nas ProvínciasÍgneas de Serra do Mar e Ponta Grossa (Schimmel et al, 2003;Escalante, 2002;Rocha, 2003). Assumpção et al (2004b) mostraram uma correlação positiva entre as anomalias de baixa velocidade para onda P, intrusões alcalinas do Cretáceo Superior (citadas acima), é areas com grande sismicidade no SE e Centro-Oeste do país. Neste trabalho foi proposto que a interface litosfera/astenosfera, obtida qualitativamente com a tomografia de ondas P, concorda muito bem com os pontos de afinamento da litosfera, propostos por Gibson et al (1995) e Gibson et al (1997), para a origem das provínciasígneas, e ainda, que o afinamento da litosfera induzido pela temperatura pode ser um importante mecanismo para ajudar a explicar a atividade sísmica no SE e Centro-Oeste do Brasil.…”

“…Anomalias de alta velocidade na porção oeste do Cráton do São Francisco apóiam a hipótese de que este cráton foi parte de uma placa Neoproterozóica maior. Anomalias de baixa velocidade sob a Província Tocantins (principalmente nas faixas móveis entre os Crátons Amazônico e do São Francisco) foram interpretadas como causadas por afinamento litosférico, consistente com a boa correlação entre a sismicidade intraplaca e as anomalias de baixa velocidade nesta região (Assumpção et al, 2004b). A melhora na resolução da anomalia de alta velocidade sob a Bacia do Paraná (∼200 km) permitiu uma discussão sobre a geometria do núcleo cratônico desta Bacia.…”

Intraplate seismicity in Canada: a graph theoretic approach to data analysis and interpretation

“…This class of seismicity has been extensively studied in recent decades (Gregersen, 1979;Adams and Basham, 1990;Johnston and Kanter, 1990;Denith and Featherstone, 2003;Sandiford and Egholm, 2008; Correspondence to: K. Vasudevan (vasudeva@ucalgary.ca) Liu, 2009) and continues to draw the interest of seismologists and seismic hazard analysts, including earthquakes in the continental interior of the US (Coppersmith et al, 1987;Dewey, 1988;Johnston and Kanter, 1990;Zoback, 1992;Li et al, 2009;Stein and Liu, 2009), several regions of Canada (Basham et al, 1977;Quinlan, 1984;Adams, 1989a;Bent, 1996;Eaton et al, 2005;Darbyshire et al, 2007;Wang et al, 2008;Woodgold, 2010), Australia (Bowman et al, 1990;Denith and Featherstone, 2003;Sandiford and Egholm, 2008), southeast Brazil (Assumpção et al, 2004), and northern Norway (Gregersen, 1979;Atakan et al, 1994;Bungum et al, 2010;Gregersen and Voss, 2009). One particularly intriguing aspect is the high spatial and temporal variability of seismic activity in continental interiors compared with plate boundaries .…”