  It has been found that photospheric magnetic fields can change in accordance
with the three-dimensional magnetic field restructuring following solar
eruptions. Previous studies mainly use vector magnetic field data taken for
events near the disk center. In this paper, we analyze the magnetic field
evolution associated with the 2012 October 23 X1.8 flare in NOAA AR 11598 that
is close to the solar limb, using both the 45 s cadence line-of-sight and 12
minute cadence vector magnetograms from the Helioseismic and Magnetic Imager on
board Solar Dynamic Observatory. This flare is classified as a circular-ribbon
flare with spine-fan type magnetic topology containing a null point. In the
line-of-sight magnetograms, there are two apparent polarity inversion lines
(PIL). The PIL closer to the limb is affected more by the projection effect.
Between these two PILs there lie positive polarity magnetic fields, which are
surrounded by negative polarity fields outside the PILs. We find that after the
flare, both the apparent limb-ward and disk-ward negative fluxes decrease,
while the positive flux in-between increases. We also find that the horizontal
magnetic fields have a significant increase along the disk-ward PIL, while in
surrounding area, they decrease. Synthesizing the observed field changes, we
conclude that the magnetic fields collapse toward the surface above the
disk-ward PIL as depicted in the coronal implosion scenario, while the
peripheral field turns to a more vertical configuration after the flare. We
also suggest that this event is an asymmetric circular-ribbon flare: a flux
rope is likely present above the disk-ward PIL. Its eruption causes the
instability of the entire fan-spine structure and the implosion near that PIL.
