P‐ and S‐Wave Velocities of Exhumed Metasediments From the Alaskan Subduction Zone: Implications for the In Situ Conditions Along the Megathrust

Abstract

Subduction plate boundaries host Earth's largest and most damaging earthquakes and tsunamis. Continuous geodetic and seismological monitoring has led to the relatively recent discovery of a spectrum of slip behaviors along tectonic faults globally, and particularly along subduction megathrusts (Beroza & Ide, 2011;

The High‐Frequency Signature of Slow and Fast Laboratory Earthquakes

“…This depletion in high‐frequency energy can also be seen in the raw time‐domain signals (Kao et al., 2005 ; Shelly et al., 2007 ). Because slow earthquakes typically occur in frictionally transitional regions that bound the traditional seismogenic zone and may feature high in‐situ fluid pressures (Behr & Burgmann, 2020 ) or structural anisotropy (Miller et al., 2021 ), it is possible that variations in path effects, such as a low‐velocity zone within the source region could have a significant effect on the spectra of slow earthquakes (Bostock et al., 2017 ; Gomberg et al., 2012 ). Alternatively, the lack of high‐frequency energy in slow earthquakes could be linked to their source properties (Bostock et al., 2015 ; Thomas et al., 2016 ).…”

Subduction Zone Interface Structure Within the Southern MW9.2 1964 Great Alaska Earthquake Asperity: Constraints From Receiver Functions Across a Spatially Dense Node Array

“…In their study of P ‐ and S ‐wave velocities of exhumed Kodiak metasediments, Miller et al. (2021) reported anisotropy of ∼8–28% in Vp and ∼6.5–8% in Vs, with lower wave speeds perpendicular to the rocks' dominant fabric. This suggests an absence of foliation or obliquely foliated rocks conducive for higher wavespeeds beneath our study area.…”

“…Therefore, the absence of a well-defined LVZ channel at the plate interface beneath our study area does not necessarily mean an absence of subducted sediment. In their study of P-and S-wave velocities of exhumed Kodiak metasediments, Miller et al (2021) reported anisotropy of ∼8-28% in Vp and ∼6.5-8% in Vs, with lower wave speeds perpendicular to the rocks' dominant fabric. This suggests an absence of foliation or obliquely foliated rocks conducive for higher wavespeeds beneath our study area.…”

“…For possibility (a), we suggest that the subduction zone environment may have altered the properties of most of the subducted sediment at the interface, thus suppressing the velocity and density contrast between the sediment and the surrounding rock across most of the interface. There is ample evidence from magnetotelluric (Heise et al, 2012), laboratory (Miller et al, 2021) and field studies of exhumed metasedimentary rocks from subduction zone forearcs (Rowe et al, 2009(Rowe et al, , 2013 pointing to instances of hundreds of meters of metamorphosed sediments lining the plate interface. It is likely that the metasedimentary rocks exhumed on Kodiak Island are close enough in seismic properties (e.g., Miller et al, 2021) to the Pacific crust that there is no significant discontinuity at the interface to resolve with Ps RFs.…”

“…There is ample evidence from magnetotelluric (Heise et al, 2012), laboratory (Miller et al, 2021) and field studies of exhumed metasedimentary rocks from subduction zone forearcs (Rowe et al, 2009(Rowe et al, , 2013 pointing to instances of hundreds of meters of metamorphosed sediments lining the plate interface. It is likely that the metasedimentary rocks exhumed on Kodiak Island are close enough in seismic properties (e.g., Miller et al, 2021) to the Pacific crust that there is no significant discontinuity at the interface to resolve with Ps RFs. Therefore, the absence of a well-defined LVZ channel at the plate interface beneath our study area does not necessarily mean an absence of subducted sediment.…”