  Different mechanisms for the creation of strange nonchaotic attractors (SNAs)
are studied in a two-frequency parametrically driven Duffing oscillator. We
focus on intermittency transitions in particular, and show that SNAs in this
system are created through quasiperiodic saddle-node bifurcations (Type-I
intermittency) as well as through a quasiperiodic subharmonic bifurcation
(Type-III intermittency). The intermittent attractors are characterized via a
number of Lyapunov measures including the behavior of the largest nontrivial
Lyapunov exponent and its variance as well as through distributions of
finite-time Lyapunov exponents. These attractors are ubiquitous in
quasiperiodically driven systems; the regions of occurrence of various SNAs are
identified in a phase diagram of the Duffing system.
