In this paper, one-dimensional (1D) nonlinear wave equations
u(tt) - u(xx) + V(x)u = f(u),
with periodic boundary conditions are considered; V is a periodic smooth or
analytic function and the nonlinearity f is an analytic function vanishing
together with its derivative at u = 0. It is proved that for "most" potent
ials V(x), the above equation admits small-amplitude periodic or quasi-peri
odic solutions corresponding to finite dimensional invariant tori for an as
sociated infinite dimensional dynamical system. The proof is based on an in
finite dimensional KAM theorem which allows for multiple normal frequencies
.