The São Vicente earthquake of 2008 April and seismicity in the continental shelf off SE Brazil: further evidence for flexural stresses

Abstract

S U M M A R YThe continental margin and shelf of most stable intraplate regions tend to be relatively more seismically active than the continental interior. In the southeast continental margin of Brazil, a seismic zone extends from Rio Grande do Sul to Espírito Santo, with seismic activity occurring mainly along the continental slope and suggesting a close relationship with flexural stresses caused by the weight of the sediments. In this region, earthquakes with magnitudes larger than 5 m b occur every 20-25 yr, on average. The focal mechanism solutions of previous earthquakes in this zone indicated reverse faulting on planes dipping approximately 45 • with horizontal P-axes. The recent 5.2 m b earthquake of 2008 April 23 occurred 125 km south of São Vicente and was well recorded by many stations in SE Brazil, as well as at teleseismic distances in North America and Africa. Its focal depth was 17 km, locating the hypocentre in the lower crust. A well-determined focal mechanism solution shows one vertical nodal plane and one subhorizontal nodal plane. The P-and T-axes exhibit large dips, which were confirmed by a regional moment tensor inversion. This unusual orientation of the fault mechanism can be attributed to a rotation of the principal stress directions in the lower crust caused by flexural effects due to the load of recent sedimentation.

Some remarks on the earthquakes of Fennoscandia: A conceptual seismological model drawn from the perspective of hyperextension

“…Although strong spatial and geophysical arguments have been made for post-glacial rebound as the main perpetrator of Fennoscandia's earthquakes (e.g., Gudmundsson, 1999), less-glaciated (e.g., North America's southeastern seaboard) and never-glaciated (e.g., South America's southeast Brasilian coast) rifted margins also experience minor, medium, and sometimes major tremors (Sykes, 1978;Johnston & Kanter, 1990;Assumpção et al, 2011). Moreover, some rifted margins are more seismic than others (op.…”

“…Were COB GPE the sole cause of the intraplate stress field, all rifted margins should exhibit similar frequencies and patterns of earthquakes. However, such likeness does not seem to be the case (Assumpção et al, 2011). Sykes (1978) suggested that the occurrence of intraplate earthquakes is related to preconditioning factors that created zones of crustal weakness.…”

“…We thus cast our eyes briefly towards the southeast margin of Brasil, where both the concepts of hyperextension (Zalán et al, 2011) and a strikingly similar seismological model (Assumpção et al, 2011) have recently been proposed. Lima et al (1997) suggested that offshore sediment loading on the continental shelf can produce the coast-parallel Sh max observed in borehole breakouts from the onshore parts of many marginal basins (see Reis et al, 2013).…”

“…Although the Brasilian escarpments were not incised by post-breakup glacial erosion, Neogene sedimentary thicknesses in parts of Pelotas, Santos, and Campos basins range between 0.5 to perhaps more than 2 km (Modica & Brush, 2004;Assumpção et al, 2011;Contreras, 2011). The load is applied atop very thin crust (Mohriak et al, 2010;Assumpção et al, 2011;Zalán et al, 2011;Mohriak & Leroy, 2012).…”

“…This is particularly so in the light of current concepts of how extended margins evolve. Other newly reinterpreted, non-glaciated, hyperextended margins such as Southeast Brasil also display seismicity in association with onshore escarpments and offshore sedimentary basins (Riccomini et al, 1989;Assumpção, 1992Assumpção, , 1998Assumpção et al, 1997Assumpção et al, , 2011Riccomini & Assumpção, 1999;Bezerra & Vita-Finzi, 2000;Bezerra et al, 2008Bezerra et al, , 2014. In this paper we focus on how patterns of Scandinavian seismicity can be interpreted within the context of a specific geologic model that describes the evolution of the Scandinavian rifted margin from Mesozoic extension to the present-day.…”