Cd. Stephens et Ba. Chouet, Evolution of the December 14, 1989 precursory long-period event swarm at Redoubt Volcano, Alaska, J VOLCANOL, 109(1-3), 2001, pp. 133-148
The intermittency pattern and evolution in waveforms of long-period (LP) se
ismic events during the intense, 23-h swarm that preceded the December 14,
1989 eruption of Redoubt volcano are investigated. Utilizing cross correlat
ion to exploit the high degree of similarity among waveforms, a substantial
ly more complete event catalog is generated than was available from near re
altime detection based on short-term/long-term. amplitude ratios, which was
saturated by the high rate of activity. The temporal magnitude distributio
n of the predominant LP events is found to have an unusual banded structure
in which the average magnitude of each band slowly increases and then decr
eases through time. A bifurcation that appears in the uppermost band shortl
y after the peak in magnitudes is characterized by a quasi-periodicity in i
ntermittency and magnitude that is reminiscent of one of the classic routes
to chaotic behavior in some non-linear systems. The waveforms of the predo
minant events evolve slowly but unsteadily through time. These gradual chan
ges appear to result from variations in the relative amplitudes of spectral
peaks that remain stable in frequency, which suggests that they are due to
differential excitation of a single, resonant source. Two other previously
unrecognized, repetitive waveforms are also identified, but the signals fr
om these secondary events are not clearly recorded at distances beyond the
closest station. Similarities among the spectra of the predominant and seco
ndary events suggest that the signals from these events also could represen
t different modes of exciting the same source. Significant changes in the r
ates and the sizes of the largest of these secondary events appear to coinc
ide with the peak in the size distribution of the predominant LPs. At least
some of the non-repetitive LP waveforms in the swarm appear to be the resu
lt of the superposition of signals from the rapid repetition of predominant
LP source, thus placing a constraint on the repeat time of the triggering
mechanism for this source. A lone hybrid event, which has a waveform charac
ter intermediate between the predominant LP events and high-frequency volca
no-tectonic events, was also identified in the swarm; the occurrence of thi
s event provides important evidence that the low-frequency character of the
LP events is a source rather than a path or site effect. (C) 2001 Elsevier
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