  We carry out a comprehensive investigation comparing the three-dimensional
magnetic field restructuring, flare energy release, and the helioseismic
response, of two homologous flares, the 2011 September 6 X2.1 (FL1) and
September 7 X1.8 (FL2) flares in NOAA AR 11283. In our analysis, (1) a twisted
flux rope (FR) collapses onto the surface at a speed of 1.5 km/s after a
partial eruption in FL1. The FR then gradually grows to reach a higher altitude
and collapses again at 3 km/s after a fuller eruption in FL2. Also, FL2 shows a
larger decrease of the flux-weighted centroid separation of opposite magnetic
polarities and a greater change of the horizontal field on the surface. These
imply a more violent coronal implosion with corresponding more intense surface
signatures in FL2. (2) The FR is inclined northward, and together with the
ambient fields, it undergoes a southward turning after both events. This agrees
with the asymmetric decay of the penumbra observed in the peripheral regions.
(3) The amounts of free magnetic energy and nonthermal electron energy released
during FL1 are comparable to those of FL2 within the uncertainties of the
measurements. (4) No sunquake was detected in FL1; in contrast, FL2 produced
two seismic emission sources S1 and S2 both lying in the penumbral regions.
Interestingly, S1 and S2 are connected by magnetic loops, and the stronger
source S2 has weaker vertical magnetic field. We discuss these results in
relation to the implosion process in the low corona and the sunquake
generation.
