Physics
Scientific paper
Dec 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987pepi...49..283m&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 49, Issue 3-4, p. 283-293.
Physics
18
Scientific paper
The two classes of earthquakes which preceded the explosive eruption of Mount St. Helens, Washington, on 18 May 1980, can be related to two different forms of energy release. This is manifest both in the predominant periods of the two classes of seismic energy release at 1.0 and 0.55 s, and in the ratio of their source dimensions, in the range 2 : 1 to 6 : 1, inferred from characteristic magnitudes of the two possible classes of earthquakes apparent in the discrete frequency-magnitude distribution.
The observed increase in the ratio of long to short period amplitudes of surface waves recorded on MSO, a local WWSSN station, can be numerically related to the growth of a magma chamber or conduit within which are generated the transient oscillations thought to be responsible for volcanic tremor. These calculations predict an increase in the chamber's diameter at a rate of 2-7 m day-1 at depth in the 2 months preceding the eruption, and this is consistent with an observed growth rate of 2 m day-1 of the precursory bulge which appeared at the surface on the mountain's north flank at the time.
The frequency-magnitude distribution is analysed in two parts corresponding to a power law at low magnitudes and a Gaussian distribution of typical fault lengths at high magnitudes-an extension of the characteristic earthquake model first developed for individual fault zones. The power law can in general have a non-integer exponent, which can be equated to a fractal self-similarity dimension D. Temporal changes in D, which is proportional to the seismic b-value, are proposed as an alternative and complementary description of the way seismic energy is released to the more commonly used viewpoints of changes in stress and heterogeneity.
Despite a reassessment of the completeness of the magnitude catalogue, no clear seismic precursor to the explosive eruption is observed in the b-value, although an earlier phase of phreatic eruptions was strongly associated with a b-value anomaly.
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