Using Seismic b -Values to Interpret Seismicity Rates and Physical Processes During the Preeruptive Earthquake Swarm at Augustine Volcano 2005–2006

Document Type

Article

Publication Date

2010

Abstract

We use seismic b-values to explore physical processes during the Augustine Volcano 2005?6 preeruptive earthquake swarm. The preeruptive earthquake swarm was divided into two parts: the ?long swarm,? which extended from April 30, 2005, to January 10, 2006; and the ?short swarm,? which started 13 hours before the onset of explosive activity on January 11, 2006. Calculations of b-value for each of these swarms and for a background period were performed. The short swarm, directly preceding the eruption, had the lowest calculated b-value. In addition to the low value, the shape of the b-value plot for the short swarm appears to have two separate slopes, a shallower slope for magnitudes as great as 1.2 and a steeper slope for magnitudes greater than 1.2. Calculations of b were also run for three precursory deformation stages suggested by a separate investigation of deformation at Augustine Volcano. The highest b-value, found in stage 2, may indicate an increase in pore pressure and in thermal gradient, which matches the geodetic interpretation of a proposed dike intrusion. Finer resolution changes of b are explored through calculations of b-value versus time. An initial drop in b-value in late 2004 preceded the onset of increased seismicity. The temporal nature of this change and its timing are corroborated by atmospheric temperature data recorded on the summit of the volcano, which increased at approximately the same time. Stress at Augustine Volcano was also studied using 79 earthquakes that returned acceptable focal mechanisms between January 1, 2002, and January 10, 2006. These mechanisms and an attempted stress-tensor inversion imply that stresses within the Augustine edifice are highly variable and do not display a dominant faulting style. A population of high-frequency volcano-tectonic earthquakes during the short swarm is found to have accompanying very-long-period (20 seconds and greater) energy. Statistical analysis indicates that these earthquakes are a separate population of events. We interpret this population of earthquakes to represent a separate and distinct physical process that was not seen before the 13 hours preceding the eruption. The b-value time series also indicates that when changes in stress, pore pressure, and thermal gradient occur simultaneously, that stress effects dominate the observed b-value.

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Citation / Publisher Attribution

U.S. Geological Survey Professional Paper, art. 1769, p. 59-83

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