  High-frequency (HF) seismic radiation of large earthquakes is approximately
represented by P wave trains recorded at teleseismic distances. Observed
envelopes of such signals look random and intermittent, suggesting non-trivial
stochastic structure. Variogram and spectral analyses were applied to instant
power calculated from band-filtered observed P-wave signals from eight large
(Mw=7.6-9.2) earthquakes, with 8-30 records per event and eight
non-overvlapping frequency bands analyzed (total frequency range 0.6-6.2 Hz,
bandwidth 0.7 Hz). Estimates for both variograms and power spectra look linear
in log-log scale, suggesting in most cases self-similar correlation structure
of the signal. The range for the individual-event values of the Hurst exponent
H is 0.71-0.80 (averaged over bands and stations) when estimated from
variograms, and 0.78-0.83 when estimated from spectra. No systematic dependence
on station or frequency band was noticed. The values of H around 0.8 may be
characteristic for large earthquakes in general. The result suggests that the
space-time organization of earthquake rupture process has significant fractal
features. Also, a useful constraint is established for application-oriented
earthquake strong motion modeling.
