Induced Velocity Changes and Decorrelation at Okmok Volcano Following the 2015 M6.9 Fox Islands Earthquake Mapped with Tomographic Coda-Wave Interferometry

Abstract:

The 2015 M6.9 Fox Islands Earthquake was a thrust-fault rupture along the Aleutian Arc, 150 km SW of Okmok Volcano, Alaska. Peak dynamic stresses between 0.1 and 0.4 MPa were calculated across a temporary array of seismometers deployed on the northeastern lobe of Umnak Island, on which Okmok resides. We performed ambient noise coda-wave interferometry between July 2015 and August 2016 from 13 broadband seismometers in the 0.1-2 Hz frequency band and measured induced relative velocity changes of between -0.05 and -0.5% across the array, as well as induced noise decorrelation. These changes were superimposed on a strong seasonal signal in the 7 intra-caldera stations that is likely related to snow pack formation. We observed the largest velocity changes on the intra-caldera stations, at early lag times, and on the horizontal components. Decorrelation was also concentrated on the intra-caldera stations.

To capture the heterogeneity of velocity changes and decorrelation in space and lag time, we performed a tomographic inversion of these measurements using isotropic acoustic sensitivity kernels for scattered waves under the 2-D radiative transfer approximation. Initial results show that the largest induced changes (relative velocity and decorrelation) are mostly confined within the caldera, mirroring the location and approximate extent of the shallow magma reservoir as observed by geodesy and seismic tomography. Because of the consistent calculated peak dynamic stresses, this also implies that velocity susceptibility (the ratio of relative velocity change to dynamic stress) is largest over the known magma reservoir location. While it has been theorized that magma plays a role in increased velocity susceptibility at other volcanic settings, we find that the shallowest layers produce a similar response to deeper layers (by proxy of the frequency bands measured). This may suggest that other fluids contribute to the caldera-focused velocity susceptibility and decorrelation at Okmok.