  We consider longwave mode of the interface instability in the system
comprising of two immiscible fluid layers. The fluids fill out plane horizontal
cavity which is subjected to horizontal harmonic vibration. The analysis is
performed within the framework of "high frequency of the vibration"
approximation and the averaging procedure. The nonlinear equation (having the
form of Newton's second law) for the amplitude of interface deformation is
obtained by means of multiple scales method. It is shown that (in addition to
previously detected quasistationary periodic solutions) the equation has a
class of quasistationary solitary solutions.
