  Flares are known to restructure the magnetic field in the corona and to
accelerate the gas between the field lines, but their effect on the photosphere
is less well studied. New data of the Solar Optical Telescope (SOT) onboard
Hinode provide unprecedented opportunity to uncover the photospheric effect of
a solar flare, which associates with an active region NOAA AR 10930 on 2006
December 13. We find a clear lateral displacement of sunspot penumbral regions
scanned by two flare ribbons. In the impulsive phase of the flare, the flare
ribbons scan the sunspot at a speed of around 18 km/s, derived from Ca II and
G-band images. We find instantaneous horizontal shear of penumbral fibrils,
with initial velocities of about 1.6 km/s, produced when a flare ribbon passes
over them. This velocity decreases rapidly at first, then gradually decays, so
that about one hour later, the fibrils return to a new equilibrium. During the
one hour interval, the total displacement of these fibrils is around 2.0 Mm,
with an average shear velocity of 0.55 km/s. This lateral motion of the
penumbral fibrils indicates that the magnetic footpoints of these field lines
being rearranged in the corona also move.
