  We present a new technique in order to quantify the dynamics of spatially
extended systems. Using a test on the existence of unstable periodic orbits, we
identify intermediate spatial scales, wherein the dynamics is characterized by
maximum nontrivial determinism. This method is applied to earthquake catalogues
containing time, coordinates and magnitude. As a result we extract a set of
areas with significant deterministic and low-dimensional dynamics from the
data. Finally, a simple model is used to show that these scales can be
interpreted as local spatial coupling strengths.
