If the ultrahigh-energy ( UHE ) neutrino fluxes produced from a distant astrophysical source can be measured at a km ^ { 3 } -size neutrino telescope , they will provide a promising way to help determine the flavor mixing pattern of three active neutrinos .
Considering the conventional UHE neutrino source with the flavor ratio \phi _ { e } : \phi _ { \mu } : \phi _ { \tau } = 1 : 2 : 0 , I show that \phi ^ { D } _ { e } : \phi ^ { D } _ { \mu } : \phi ^ { D } _ { \tau } = ( 1 - 2 \Delta ) : ( 1 + \Delta ) % : ( 1 + \Delta ) holds at the detector of a neutrino telescope , where \Delta characterizes the effect of \mu - \tau symmetry breaking ( i.e. , \theta _ { 13 } \neq 0 and \theta _ { 23 } \neq \pi / 4 ) .
Current experimental data yield -0.1 \leq \Delta \leq + 0.1 .
It is also possible to probe \Delta by detecting the \overline { \nu } _ { e } flux of E _ { \overline { \nu } _ { e } } \approx 6.3 ~ { } { PeV } via the Glashow resonance channel \overline { \nu } _ { e } e \rightarrow W ^ { - } \rightarrow~ { } { anything } .
Finally , I give some brief comments on the possibility to constrain the mixing between active and sterile neutrinos by using the UHE neutrino telescopes .