Characteristic Spatial Scales in Earthquake Data

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.