Carl Tape

Alaska is located at the northernmost point of the interface between the Pacific plate and the North American continent. The subduction of the Pacific plate generates arc volcanoes along the Aleutian trench, which stops to the east at the Denali Volcanic Gap. This volcanic gap has been linked to the underthrusting of the Yakutat terrane, which might alter the thermal state of the mantle wedge and prevent melt formation. This implies that the limits of the volcanic activity should mirror the extent of the Yakutat subduction. However, the transition from the Pacific slab to the Yakutat terrane at depth is not fully understood. To investigate this issue, we processed a new composite seismic data set from six arrays deployed in the region from 2000 to 2018. We apply a multi‐mode 3D Kirchhoff migration to obtain high‐resolution 3D scattering images of the region. Our results highlight a sharp lateral boundary in the slab structure, with a 10 km Moho step, just offshore Anchorage, and a more gradual slab transition beneath the southern part of the Kenai peninsula. Our images from the Yakutat slab plunge down to 150 km depth are consistent with previous estimates of the Yakutat slab extent below the Alaska Range. Although the steeply dipping boundaries of the subducting Pacific lithospheres are not fully recovered, deep coherent signals from the Pacific slab are observed down to 150 km depth. These observations suggest that the crust is still partially uneclogitized at these depths in both slabs.

Nenana basin in central Alaska is a long (90 km), narrow (12 km), and deep (7 km) sedimentary basin aligned with an active fault zone producing Mw≥6 earthquakes. From 2015 to 2019, 13 broadband seismic stations were deployed in the region as part of the Fault Locations and Alaska Tectonics from Seismicity project. These stations recorded a wide range of earthquakes, including Mw 3–4 directly below the basin as well as several regional earthquakes Mw>6. These 43 local and regional earthquakes, in addition to five teleseismic events and continuously recorded ambient noise, provide a data set that we use to quantify the response of Nenana basin to the seismic wavefield. We calculate spectral ratios between each station and a bedrock reference station for 48 earthquakes. We find amplification of 11–14 dB (amplification ratio 3.5–5.0) for low frequencies (0.1–0.5 Hz), and 8–15 dB (amplification ratio 2.5–5.6) for high frequencies (0.5–4.0 Hz) on the vertical component. At low frequencies, amplification of the earthquake wavefield agrees well with amplification of seismic noise, with both data sets exhibiting stronger amplification on the horizontal components, in comparison with the vertical component. Furthermore, stations overlying the deeper part of the basin exhibit stronger amplification, whereas stations at the margin of the basin exhibit minimal low-frequency amplification. At higher frequencies, amplification occurs at both deeper basin stations and also marginal basin stations. Our study establishes a catalog of diverse events for future theoretical and numerical studies that can use Nenana basin to better understand the complex influence of sedimentary basins on the seismic wavefield.