  Volatility of intra-day stock market indices computed at various time
horizons exhibits a scaling behaviour that differs from what would be expected
from fractional Brownian motion (fBm). We investigate this anomalous scaling by
using empirical mode decomposition (EMD), a method which separates time series
into a set of cyclical components at different time-scales. By applying the EMD
to fBm, we retrieve a scaling law that relates the variance of the components
to a power law of the oscillating period. In contrast, when analysing 22
different stock market indices, we observe deviations from the fBm and Brownian
motion scaling behaviour. We discuss and quantify these deviations, associating
them to the characteristics of financial markets, with larger deviations
corresponding to less developed markets.
