[S13-P] Poster

[S13-P-12] The intraplate Maranhao earthquake of 2017 Jan 03, northern Brazil: evidence of uniform regional stresses along the Brazilian equatorial margin

Fabio Dias¹, ^○Marcelo Assumpcao², Marcelo Bianchi², Lucas Barros³, Juraci Carvalho³ (1.National Observatory, Brazil, 2.Sao Paulo University, Brazil, 3.Brasilia University, Brazil)

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 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 January 3, 2017, a magnitude 4 mb earthquake occurred near the equatorial coast of the Maranhao 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 back-azimuths of tens 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 1 to 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 measurements from breakouts further along the coast. The Maranhao 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. In addition, this event can be used as a test for detectability of the global IMS system, as it was not reported in the NEIC-USGS catalogue.