  We study the properties of sub-Alfvenic magnetohydrodynamic (MHD) turbulence,
i.e., turbulence with Alfven Mach number $M_A=V_L/V_A<1$, where $V_L$ is the
velocity at the injection scale and $V_A$ is the Alfven velocity. We
demonstrate that weak turbulence can have different regimes depending on
whether it is driven by velocity or magnetic fluctuations. If the turbulence is
driven by isotropic bulk forces, i.e. velocity-driven, in an incompressible
conducting fluid, we predict that the kinetic energy is $M_A^{-2}$ times larger
than the energy of magnetic fluctuations. This effect arises from the long
parallel wavelength tail of the forcing, which excites modes with $k_\|/k_\perp
< M_A$. We also predict that as the turbulent cascade reaches the strong regime
the energy of slow modes exceeds the energy of Alfven modes by a factor
$M_A^{-1}$. These effects are absent if the turbulence is magnetically driven
at the injection scale. We confirm these predictions with numerical
simulations. As the assumption of magnetic and kinetic energy equipartition is
at the core of the Davis-Chandrasekhar-Fermi (DCF) approach to measuring
magnetic field strength in sub-Alfvenic turbulence, we conclude that the DCF
technique is not universally applicable. In particular, we suggest that the
dynamical excitation of long azimuthal wavelength modes in the galactic disk
may compromise the use of the DCF technique. We discuss alternative expressions
that can be used to obtain magnetic field strength from observations.
