  We present a multifractal analysis of Mount St. Helens seismic activity
during 1980-2002. The seismic time distribution is studied in relation to the
eruptive activity, mainly marked by the 1980 major explosive eruptions and by
the 1980-1986 dome building eruptions. The spectrum of the generalized fractal
dimensions, i.e. Dq vs q, extracted from the data, allows us to identify two
main earthquake time-distribution patterns. The first one exhibits a
multifractal clustering correlated to the intense seismic swarms of the dome
building activity. The second one is characterized by an almost constant value
of Dq 1, as for a random uniform distribution. The time evolution of Dq (for
q=0,2) calculated on a fixed number of events window and at different depths,
shows that the brittle mechanical response of the shallow layers to rapid magma
intrusions, during the eruptive periods, is revealed by sharp changes, acting
at a short time scale (order of days), and by the lowest values of Dq (0.3).
Conversely, for deeper earthquakes, characterized by intense seismic swarms, Dq
do not show obvious changes during the whole analyzed period, suggesting that
the earthquakes, related to the deep magma supply system, are characterized by
a minor degree of clustering, which is independent of the eruptive activity.
