  So far, only transient Gravitational waves (GWs) produced by catastrophic
events of extra-galactic origin have been detected. However, it is generally
believed that there should be also continuous sources of GWs within our galaxy,
such as accreting neutron stars (NSs). In fact, in accreting NSs, centrifugal
forces can be so strong to break the neutron star crust (causing a starquake),
thus producing a quadrupole moment responsible for the continuous emission of
GWs. At equilibrium, the angular momentum gained by accretion and lost via GWs
emission should balance each other, stopping the stellar spin-up.
  We hereinafter investigate the above physical picture within the framework of
a Newtonian model describing compressible, non-magnetized, and self-gravitating
NSs. In particular, we calculate the rotational frequency need to break the
stellar crust of an accreting pulsar and we estimate the upper limit for the
ellipticity due to this event. Depending on the equation of state (EoS) and on
the mass of the star, we calculated that the starquake-induced ellipticity
ranges from $10^{-9}$ to $10^{-5}$. The corresponding equilibrium frequency
that we find is in good agreement with observations and, for all the scenarios,
it is below the observational limit frequency of $716.36$ Hz. Finally, we also
discuss possible observational constraints on the ellipticity upper limit of
accreting pulsars.
