Experiments on thin layers of granular material subjected to vertical vibra
tion reveal a series of subharmonic bifurcations from flat surface to stand
ing wave patterns. From pressure and surface layer dilation measurements we
show that a solid-liquid type transition precedes surface wave bifurcation
, indicating that these waves are associated with the fluid-like behavior o
f the layer. Other structures observed, like the flat with kinks state, are
linked to an inverse liquid-solid type transition.