  The interstellar turbulence is magnetized and thus anisotropic. The
anisotropy of turbulent magnetic fields and velocities is imprinted in the
related observables, rotation measures (RMs), and velocity centroids (VCs).
This anisotropy provides valuable information on both the direction and
strength of the magnetic field. However, its measurement is difficult
especially in highly supersonic turbulence in cold interstellar phases due to
the distortions by isotropic density fluctuations. By using 3D simulations of
supersonic and sub-Alfv\'enic magnetohydrodynamic(MHD) turbulence, we find that
the problem can be alleviated when we selectively sample the volume-filling
low-density regions in supersonic MHD turbulence. Our results show that in
these low-density regions, the anisotropy of RM and VC fluctuations depends on
the Alfv\'enic Mach number as $\rm M_A^{-4/3}$. This anisotropy-$\rm M_A$
relation is theoretically expected for sub-Alfv 'enic MHD turbulence and
confirmed by our synthetic observations of $^{12}$CO emission. It provides a
new method for measuring the plane-of-the-sky magnetic fields in cold
interstellar phases.
