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How active is a passive margin? Paleoseismicity in northeastern Brazil

¹ Departamento de Geologia, CCET, Universidade Federal do Rio Grande do Norte, Natal-RN, 59072-970, Brazil
² Department of Geological Sciences, University College, London WC1E 6BT, UK

The seismicity of intraplate areas reflects both far-field plate-boundary stresses and local effects, including dormant structures. In northeastern Brazil, within the passive margin of the South American plate, a short instrumental seismic record yields a pattern consistent with east-west plate push via events that do not exceed m_b = 5.2. Paleoseismic evidence from remote sensing, well records, earthquake-induced liquefaction, and radiocarbon dating of beachrock, coastal peats, and fault-filling sediment indicates the occurrence of larger prehistoric Holocene earthquakes, including some of Ms 6.8. The finding is of obvious significance for seismic hazard assessment, but because the azimuth of the maximum horizontal stress (S_Hmax) may be a poor guide to the present stress field, intraplate events on reactivated structures are of limited value for testing deformation models.

Key Words: Brazil • intraplate • paleoseismicity • neotectonics • radiocarbon