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Repeating coupled earthquakes at Shishaldin Volcano, Alaska

Journal of Volcanology and Geothermal Research

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DOI: 10.1016/j.jvolgeores.2005.01.011

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Abstract

Since it last erupted in 1999, Shishaldin Volcano, Aleutian Islands, Alaska, has produced hundreds to thousands of long-period (1-2 Hz; LP) earthquakes every day with no other sign of volcanic unrest. In 2002, the earthquakes also exhibited a short-period (4-7 Hz; SP) signal occurring between 3 and 15 s before the LP phase. Although the SP phase contains higher frequencies than the LP phase, its spectral content is still well below that expected of brittle failure events. The SP phase was never observed without the LP phase, although LP events continued to occur in the absence of the precursory signal. The two-phased events are termed "coupled events", reflecting a triggered relationship between two discrete event types. Both phases are highly repetitive in time series, suggestive of stable, non-destructive sources. Waveform cross-correlation and spectral coherence are used to extract waveforms from the continuous record and determine precise P-wave arrivals for the SP phase. Although depths are poorly constrained, the SP phase is believed to lie at shallow (<4 km) depths just west of Shishaldin's summit. The variable timing between the SP and LP arrivals indicates that the trigger mechanism between the phases itself moves at variable speeds. A model is proposed in which the SP phase results from fluid moving within the conduit, possibly around an obstruction and the LP phase results from the coalescence of a shallow gas bubble. The variable timing is attributed to changes in gas content within the conduit. The destruction of the conduit obstacle on November 21, 2002 resulted in the abrupt disappearance of the SP phase.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Repeating coupled earthquakes at Shishaldin Volcano, Alaska
Series title:
Journal of Volcanology and Geothermal Research
DOI:
10.1016/j.jvolgeores.2005.01.011
Volume
145
Issue:
1-2
Year Published:
2005
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
151
Last page:
172
Number of Pages:
22