Comprehensive observations of surface wave anisotropy across Alaska and the Aleutian subduction zone would help to improve understanding of its tectonics, mantle dynamics, and earthquake risk. We produce such observations, using stations from the USArray Transportable Array, regional networks across Alaska, and the Alaska Amphibious Community Seismic Experiment in the Alaska‐Aleutian subduction zone both onshore and offshore. Our data include Rayleigh and Love wave phase dispersion from earthquakes (28–85 s) and ambient noise two‐ and three‐station interferometry (8–50 s). Compared with using two‐station interferometry alone, three‐station interferometry significantly improves the signal‐to‐noise ratio and approximately doubles the number of measurements retained. Average differences between both isotropic and anisotropic tomographic maps constructed from different methods lie within their uncertainties, which is justification for combining the measurements. The composite tomographic maps include Rayleigh wave isotropy and azimuthal anisotropy from 8 to 85 s both onshore and offshore, and onshore Love wave isotropy from 8 to 80 s. In the Alaska‐Aleutian subduction zone, Rayleigh wave fast directions vary from trench parallel to perpendicular and back to parallel with increasing periods, apparently reflecting the effect of the subducted Pacific Plate. The tomographic maps provide a basis for inferring the 3‐D anisotropic crustal and uppermost mantle structure across Alaska and the Aleutian subduction zone.
As new techniques exploiting the Earth's ambient seismic noise field are developed and applied, such as for the observation of temporal changes in seismic velocity structure, it is crucial to quantify the precision with which wave‐type measurements can be made. This work uses array data at the Homestake mine in Lead, South Dakota, and an array at Sweetwater, Texas, to consider two aspects that control this precision: the types of seismic wave contributing to the ambient noise field at microseism frequencies and the effect of array geometry. Both are quantified using measurements of wavefield coherence between stations in combination with Wiener filters. We find a strong seasonal change between body‐wave and surface‐wave content. Regarding the inclusion of underground stations, we quantify the lower limit to which the ambient noise field can be characterized and reproduced; the applications of the Wiener filters are about 4 times more successful in reproducing ambient noise waveforms when underground stations are included in the array, resulting in predictions of seismic time series with less than a 1% residual, and are ultimately limited by the geometry and aperture of the array, as well as by temporal variations in the seismic field. We discuss the implications of these results for the geophysics community performing ambient seismic noise studies, as well as for the cancellation of seismic Newtonian gravity noise in ground‐based, sub‐Hertz, gravitational‐wave detectors.
more » « less- NSF-PAR ID:
- 10453865
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 124
- Issue:
- 3
- ISSN:
- 2169-9313
- Page Range / eLocation ID:
- p. 2941-2956
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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