  This paper studies the magnetic topology of successively erupting active
regions (ARs) 11429 and 12371. Employing vector magnetic field observations
from Helioseismic and Magnetic Imager, the pre-eruptive magnetic structure is
reconstructed by a model of non-linear force-free field (NLFFF). For all the
five CMEs from these ARs, the pre-eruptive magnetic structure identifies an
inverse-S sigmoid consistent with the coronal plasma tracers in EUV
observations. In all the eruption cases, the quasi-separatrix layers (QSLs) of
Large Q values are continuously enclosing core field bipolar regions in which
inverse-S shaped flare ribbons are observed. These QSLs essentially represent
the large connectivity gradients between the domains of twisted core flux
within the inner bipolar region and the surrounding potential like arcade. It
is consistent with the observed field structure largely with the sheared
arcade. The QSL maps in the chromosphere are compared with the flare-ribbons
observed at the peak time of the flares. The flare ribbons are largely
inverse-S shape morphology with their continuity of visibility is missing in
the observations. For the CMEs in the AR 12371, the QSLs outline the flare
ribbons as a combination of two inverse J-shape sections with their straight
parts being separated. These QSLs are typical with the weakly twisted flux
rope. Similarly, for the CMEs in the AR 11429, the QSLs are co-spatial with the
flare ribbons both in the middle of the PIL and in the hook sections. In the
frame work of standard model of eruptions, the observed flare ribbons are the
characteristic of the pre-eruptive magnetic structure being sigmoid which is
reproduced by {\bf the} NLFFF model with a weakly twisted flux rope at the
core.
