Multi-fault segments in the Pernambuco Lineament, Brazil: Implications for fault architecture in intraplate areas

摘要

Here we report the intraplate seismicity in the Pernambuco Lineaments (PeL), a Neoproterozoic continuous and vertical ductile shear zone, 700 km long and 5 km wide similar to 600 Ma in intraplate Brazil. We observed one fault segment in the PeL, here defined as the Sao Caetano segment, to assess (1) the seismogenic reactivation of the ductile Precambrian basement fabric and (2) the correlation between seismological and structural data derived from satellite imagery to understand fault reactivation, and (3) the identification of a complex multi-fault zone architecture revealed by our seismological observations. In the Sao Caetano Fault segment, the overall spatial distribution of the seismic clusters follows approximately an E-W direction with a 64 degrees dip towards the N. However, the observed multi-fault architecture revealed by the hypocentral locations exhibits different clusters that do not form a single fault structure. Hence, composite solutions are not necessarily the only representative focal mechanism of an entire fault zone. The inversion of the stress tensor using the focal mechanism from the present study is consistent with S-1 nearly vertical and S-3 in the E-W direction. This indicates that the interplay between the stress field, inherited structures, and seismicity can be more complex and vary at the local scale of 10s of kilometers. In this intraplate region, we show that the stress field is variable, with long-wavelength variations overprinted by more local and subtle changes, which has profound implications for intraplate seismic hazard mapping since models to estimate hazard rely on simplified assumptions of the fault geometry.