Seismicity and seismic structure at Okmok Volcano, Alaska

Okmok volcano is an active volcanic caldera located on the northeastern portion of Umnak Island in the Aleutian arc, with recent eruptions in 1997 and 2008. The Okmok area had ~900 locatable earthquakes between 2003 and June 2008, and an additional ~600 earthquakes from the beginning of the 2008 eruption to mid 2009, providing an adequate dataset for seismic tomography. To image the seismic velocity structure of Okmok, we apply waveform cross-correlation using bispectrum verification and double-difference tomography to a subset of these earthquakes. We also perform P-wave attenuation tomography using a spectral decay technique. We examine the spatio-temporal characteristics of seismicity in the opening sequence of the 2008 eruption to investigate the path of magma migration during the establishment of a new eruptive vent. We also incorporate the new earthquake relocations and three-dimensional (3D) velocity model with first-motion polarities to compute focal mechanisms for selected events in the 2008 pre-eruptive and eruptive periods.

Through these techniques we obtain precise relocations, a well-constrained 3D P-wave velocity model, and a marginally resolved S-wave velocity model. We image a main low Vp and Vs anomaly directly under the caldera consisting of a shallow zone at 0–2 km depth connected to a larger deeper zone that extends to about 6 km depth. We find that areas of low Qp are concentrated in the central to southwestern portion of the caldera and correspond fairly well with areas of low Vp. We interpret the deeper part of the low velocity anomaly (4–6 km depth) beneath the caldera as a magma body. This is consistent with results from ambient noise tomography and suggests that previous estimates of depth to Okmok's magma chamber based only on geodetic data may be too shallow. The distribution of events preceding the 2008 eruption suggest that a combination of overpressure in the zone surrounding the magma chamber and the introduction of new material from below were jointly responsible for the explosive eruption. Magma escaping from the top of the main magma chamber likely reacted with both a smaller shallow pod of magma and groundwater on its way up below the Cone D area. The earthquakes in the 2008 pre-eruptive and eruptive periods are found to have a mixture of strike-slip, oblique normal, and oblique thrust mechanisms, with a dominant P-axis orientation that is nearly perpendicular to the regional tectonic stress. This may indicate that the stresses related to magmatic activity locally dominated regional tectonic forces during this time period.