Velocity Structure and 2008 Eruptive Seismicity at Okmok Volcano, Alaska
Abstract
Okmok Volcano is an active volcanic caldera located on the northeastern portion of Umnak Island in the Aleutian arc, and most recently erupted in 2008. We present updated results from Okmok seismicity between January 2003 and May 2009. The volcanic caldera has been well instrumented with a network of as many as nine short-period and four broadband seismometers, providing good azimuthal coverage. Since the 2008 eruption, the volcano underwent a period of increased seismicity (addition of over 700 earthquakes) making earthquake tomography more feasible. Using a combination of waveform cross-correlation and double-difference seismic tomography, we derive refined Vp and Vs models, relocate hypocenters in them, and carry out resolution tests on the models. We image two low Vp and Vs anomalies: one directly beneath the caldera extending to 4-5 km below sea level and another to the southwest below a well known geothermal area. In addition, we re-calculate focal mechanisms in the updated model for relocated events prior to and during the 2008 eruption. We observe high focal mechanism diversity, including many reverse-faulting events. Finally, we present the 2008 relocation results in the context of satellite ash observations and seismic tremor for a detailed assessment of Okmok's seismic activity during vent formation and stabilization. The earthquakes accompanying the onset of the eruption occurred in a vertical lineation at about 4 km below sea level to the northwest of Cone D, directly below the site of the new vent, and spread slightly towards the east and west over the next few hours. Four hours after the first earthquakes, a sequence of events began near sea level. As the eruption progressed, events expanded both to greater depths and outward throughout the caldera volume, mainly westward in the direction of Cone A. The shallower and deeper groups of events seen in the early eruption remained separated by a nearly aseismic region.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V33C2404O
- Keywords:
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- 7270 SEISMOLOGY / Tomography;
- 7280 SEISMOLOGY / Volcano seismology;
- 8428 VOLCANOLOGY / Explosive volcanism;
- 8440 VOLCANOLOGY / Calderas