  Mechanical spectroscopy, i.e. cyclic deformations at varying frequencies, is
used theoretically and numerically to measure dissipation in model glasses.
From a normal mode analysis, we show that in the high-frequency THz regime
where dissipation is harmonic, the quality factor (or loss angle) can be
expressed analytically. This expression is validated through non-equilibrium
molecular dynamics simulations applied to a model of amorphous silica
(SiO$_2$). Dissipation is shown to arise from non-affine relaxations triggered
by the applied strain through the excitation of vibrational eigenmodes that act
as damped harmonic oscillators. We discuss an asymmetry vector field, which
encodes the information about the structural origin of dissipation measured by
mechanical spectroscopy. In the particular case of silica, we find that the
motion of oxygen atoms, which induce a deformation of Si-O-Si bonds is the main
contributor to harmonic energy dissipation.
