Abstract
We use shear wave splitting (SWS) analysis and double-difference relocation
to examine temporal variations in seismic properties prior to and
accompanying magmatic activity associated with the 2008 eruption
of Okmok volcano, Alaska. Using bispectrum cross-correlation, a multiplet
of 25 earthquakes is identified spanning five years leading up to
the eruption, each event having first motions compatible with a normal
fault striking NE-SW. Cross-correlation differential times are used
to relocate earthquakes occurring between January 2003 and February
2009. The bulk of the seismicity prior to the onset of the eruption
on 12 July 2008 occurred southwest of the caldera beneath a geothermal
field. Earthquakes associated with the onset of the eruption occurred
beneath the northern portion of the caldera and started as deep as
13 km. Subsequent earthquakes occurred predominantly at 3 km depth,
coinciding with the depth at which the magma body has been modeled
using geodetic data. Automated SWS analysis of the Okmok catalog
reveals radial polarization outside the caldera and a northwest-southeast
polarization within. We interpret these polarizations in terms of
a magma reservoir near the center of the caldera, which we model
with a Mogi point source. SWS analysis using the same input processing
parameters for each event in the multiplet reveals no temporal changes
in anisotropy over the duration of the multiplet, suggesting either
a short-term or small increase in stress just before the eruption
that was not detected by GPS, or eruption triggering by a mechanism
other than a change of stress in the system.
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