Abstract

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 mb = 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 (SHmax) may be a poor guide to the present stress field, intraplate events on reactivated structures are of limited value for testing deformation models.

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