The intraplate Maranhão earthquake of 2017 January 3, northern Brazil: evidence for uniform regional stresses along the Brazilian equatorial margin

Lithospheric stresses in intraplate regions can be characterized by many different wavelengths. In some areas, stresses vary over short distances of less than ∼100 km, but in other regions uniform stresses can be recognized for more than ∼1000 km or so. However, not all intraplate regions are well sampled with stress measurements to allow a good characterization of the lithospheric stresses. On 2017 January 3, a magnitude mb 4 earthquake occurred near the equatorial coast of the Maranhão State, an aseismic area of northern Brazil. Despite the few permanent stations in northern Brazil, a well-constrained strike-slip mechanism was obtained from regional moment-tensor inversion. A detailed analysis of the backazimuths of aftershocks recorded by the closest station (∼40 km away) allowed the identification of the fault plane to be the NNW–SSE trending nodal plane. An estimate of the rupture length, about 2 km, was also possible. The strike-slip mechanism has coast-parallel P axis and coast-perpendicular T axis, in agreement with most of the focal mechanisms found further to the east. The coast parallel P axis is also similar to the SHmax orientations from breakouts measurements further along the coast. The Maranhão earthquake fills an important gap of stress indicators in northern Brazil and suggests that the intraplate stress field is uniform along the 2000 km long northern coast.