  We give predictions for the up-down asymmetry in the Nadir angle dependence
of the ratio Nmu/Ne of the rates of the mu-like and e-like multi-GeV events
measured in water-Cerenkov detectors (Super-Kamiokande, etc.) in the case of
3-neutrino oscillations of the atmospheric nu_e (antinu_e) and nu_mu
(antinu_mu), driven by one neutrino mass squared difference, |Delta m^2_{atm}|
= |Delta m^2_{31}| ~ (2.0 - 3.0) 10^{-3} eV^2 >> Delta m^2_{21} = Delta
m^2_{sol}. This ratio is particularly sensitive to the Earth matter effects in
the atmospheric neutrino oscillations, and thus to the values of sin^2(theta13)
and sin^2(theta23), theta13 and theta23 being the neutrino mixing angle limited
by the CHOOZ and Palo Verde experiments and that responsible for the dominant
atmospheric nu_mu -> nu_tau (antinu_mu -> antinu_tau) oscillations,
respectively. It is also sensitive to the type of neutrino mass spectrum which
can be with normal (Delta m^2_{atm} > 0) or with inverted (Delta m^2_{atm} < 0)
hierarchy.
