  We study the magnetic field evolution in the active region (AR) 12673 that
produced the largest solar flare in the past decade on 2017 September 6. Fast
flux emergence is one of the most prominent features of this AR. We calculate
the magnetic helicity from photospheric tangential flows that shear and braid
field lines (shear-helicity), and from normal flows that advect twisted
magnetic flux into the corona (emergence-helicity), respectively. Our results
show that the emergence-helicity accumulated in the corona is
$-1.6\times10^{43}~Mx^2$ before the major eruption, while the shear-helicity
accumulated in the corona is $-6\times10^{43}~Mx^2$, which contributes about
79\% of the total helicity. The shear-helicity flux is dominant throughout the
overall investigated emergence phase. Our results imply that the emerged fields
initially contain relatively low helicity. Much more helicity is built up by
shearing and converging flows acting on preexisted and emerging flux. Shearing
motions are getting stronger with the flux emergence, and especially on both
sides of the polarity inversion line of the core field region. The evolution of
the vertical currents shows that most of the intense currents do not appear
initially with the emergence of the flux, which implies that most of the
emerging flux is probably not strongly current-carrying. The helical magnetic
fields (flux rope) in the core field region are probably formed by long-term
photospheric motions. The shearing and converging motions are continuously
generated driven by the flux emergence. AR 12673 is a representative as
photospheric motions contribute most of the nonpotentiality in the AR with
vigorous flux emergence.
