Fig. 4From: On data reduction methods for volcanic tremor characterization: the 2012 eruption of Copahue volcano, Southern AndesComparison of the 52 long-duration events. Comparison of the 52 long-duration events. Permutation entropy versus energy (a) and centroid frequency (b), and dominant polarization degree versus energy (c) and centroid frequency (d). The blue dashed line in a and b depicts the background level of the permutation entropy. For detailed information of each event see Additional file 6Back to article page