  Spatiotemporal chaos (STC) exhibited by the Kuramoto-Sivashinsky (KS)
equation is investigated analytically and numerically. An effective stochastic
equation belonging to the KPZ universality class is constructed by
incorporating the chaotic dynamics of the small KS system in a coarse-graining
procedure. The bare parameters of the effective theory are computed
approximately. Stability of the system is shown to be mediated by space-time
defects that are accompanied by stochasticity. The method of analysis and the
mechanism of stability may be relevant to a class of STC problems.
