**Figure 4: ** Results of the application of the Casdagli test: (a) logarithm of the prediction error versus the logarithm of the length scale r of the data for a broadband event; (b) the same for one of its surrogates; (c) logarithm of the average number of neighbours N_{ave} found at length scales r versus log r for the same broadband event; (d) the same for its surrogate.