3D Seismic Velocity Models for Alaska from Joint Tomographic Inversion of Body-Wave and Surface-Wave Data

Abstract

We present two new seismic velocity models for Alaska from joint inversions of body-wave and ambient-noise-derived surface-wave data, using two different methods. Our work takes advantage of data from many recent temporary seismic networks, including the Incorporated Research Institutions for Seismology Alaska Transportable Array, Southern Alaska Lithosphere and Mantle Observation Network, and onshore stations of the Alaska Amphibious Community Seismic Experiment. The first model primarily covers south-central Alaska and uses body-wave arrival times with Rayleigh-wave group-velocity maps accounting for their period-dependent lateral sensitivity. The second model results from direct inversion of body-wave arrival times and surface-wave phase travel times, and covers the entire state of Alaska. The two models provide 3D compressional- (VP) and shear-wave velocity (VS) information at depths ∼0–100 km. There are many similarities as well as differences between the two models. The first model provides a clear image of the high-velocity subducting plate and the low-velocity mantle wedge, in terms of the seismic velocities and the VP/VS ratio. The statewide model provides clearer images of many features such as sedimentary basins, a high-velocity anomaly in the mantle wedge under the Denali volcanic gap, low VP in the lower crust under Brooks Range, and low velocities at the eastern edge of Yakutat terrane under the Wrangell volcanic field. From simultaneously relocated earthquakes, we also find that the depth to the subducting Pacific plate beneath southern Alaska appears to be deeper than previous models.

Teleseismic Attenuation, Temperature, and Melt of the Upper Mantle in the Alaska Subduction Zone

“…The Alaska‐Aleutian subduction region has a typical geometry of subduction, showing a clear WBZ to >150 km depth and a high‐velocity and high‐ Q subducting lithosphere (Eberhart‐Phillips et al., 2006; Martin‐Short et al., 2018; Nayak et al., 2020; Page et al., 1989; Ratchkovski & Hansen, 2002). The ΔT results confirm this pattern, showing some of the fastest raypaths (earliest arrivals) over the shallower part of the subducting plate and slow paths (delayed arrivals) in the backarc (Figures 3 and 6).…”

“…In the Cook Inlet Basin, a 5 km sequence with mean S-wave velocity (V S ) of 2.2 km/s surrounded by basement of 3.5 km/s, as estimated in recent high-resolution velocity models (Figure S8 of Nayak et al, 2020), would explain the observed 1.0 s difference in ΔT S between regions 3 and 2. The P-wave velocity (V P ) has been observed to vary from about 5.0 in the basin to 6.5 km outside, averaged over the upper 10 km (Nayak et al, 2020). That difference explains the 0.35-0.40 s difference in ΔT P observed between the two regions.…”

Subduction of an Oceanic Plateau Across Southcentral Alaska: Scattered‐Wave Imaging

“…Dense WBZ seismicity ends abruptly around 146°W, although a distinct sparse WBZ has been observed beneath the Wrangell segment (Stephens et al., 1984; Page et al., 1989; Daly et al., 2021). Some tomographic studies of the region show a faint high‐velocity anomaly at mantle depths in the Wrangell segment that has been interpreted as subducting material as far east as 140°W (e.g., Feng and Ritzwoller., 2019; Gou et al., 2019; Jiang et al., 2018), but other studies see no such anomaly (e.g., Eberhart‐Phillips et al., 2006; Martin‐Short et al., 2018, 2016; Nayak et al., 2020). Teleseismic attenuation indicates an attenuating arc/backarc and low‐attenuation forearc, consistent with subduction (Soto Castañeda et al., 2021).…”

Mapping the Lithosphere and Asthenosphere Beneath Alaska With Sp Converted Waves

“…To calculate the Sp receiver functions, we deconvolved the SV component of the direct S arrival from the P component using a time-domain deconvolution method (Ligorría & Ammon, 1999). The resulting impulse responses were convolved with a Gaussian whose half-width is 0.9 s. To migrate the receiver functions to depth, we used the 3D velocity model from Nayak et al (2020).…”