Shear Wave Splitting and Mantle Flow Beneath Alaska

Abstract

Shear wave splitting is often assumed to be caused by mantle flow or preexisting lithospheric fabrics. We present 2,389 new SKS shear wave splitting observations from 384 broadband stations deployed in Alaska from January 2010 to August 2017. In Alaska, splitting appears to be controlled by the absolute plate motion (APM) of the North American and Pacific plates, the interaction between the two plates, and the geometry of the subducting Pacific-Yakutat plate. Outside of the subduction zone's influence, the fast directions in northern Alaska parallel the North American APM direction. Fast directions near the Queen Charlotte-Fairweather transform margin are parallel to the faults and are likely caused by the strike-slip deformation extending throughout the lithosphere. In the mantle wedge, fast directions are oriented along the strike of the slab with large splitting times and are caused by along-strike flow in the mantle wedge as the slab provides a barrier to flow. South of the Alaska Peninsula, the fast directions are parallel to the trench regardless of sea floor fabric, indicating along strike flow under the Pacific plate. Under the Kenai Peninsula, the complex flat slab geometry may cause subslab flow to be parallel to Pacific APM direction or to the North America-Pacific relative motion.

The development of seismic anisotropy below south-central Alaska: evidence from local earthquake shear wave splitting

“…As indicated earlier, our local earthquake splitting observations suggest a substantial sub-slab contribution to the SKS datasets of Venereau et al (2019) and McPherson et al (2020). McPherson et al (2020) already support this hypothesis for the Kenai Peninsula, where there is likely little-to-no mantle above the plate interface.…”

“…As indicated earlier, our local earthquake splitting observations suggest a substantial sub-slab contribution to the SKS datasets of Venereau et al (2019) and McPherson et al (2020). McPherson et al (2020) already support this hypothesis for the Kenai Peninsula, where there is likely little-to-no mantle above the plate interface. Our observations are also not easily explained by the mantle flow hypothesis of Jadamec and Billen (2010) who, consistent with SKS measurements more broadly in Alaska, predict strong mantle wedge flow due to the proximity of the slab-edge in Alaska and its influence in generating a toroidal mantle flow field.…”

Anisotropy Variations in the Alaska Subduction Zone Based on Shear‐Wave Splitting From Intraslab Earthquakes

“…There have been numerous SKS splitting studies in Alaska and its subduction zone over the past decade (Christensen & Abers, 2010; Hanna & Long, 2012; McPherson et al., 2020; Perttu et al., 2014; Venereau et al., 2019). These studies benefitted from temporary enhanced station coverage in the easternmost subduction segment, where a gap in volcanism is associated with subduction of the thick Yakutat terrane (Figure 1) (Eberhart‐Phillips et al., 2006; Ferris et al., 2003; Rondenay et al., 2010).…”

“…In this eastern segment, the SKS splitting pattern abruptly transitions from arc‐perpendicular southeast of the 70 km depth contour of the subduction interface to arc‐parallel northwest of the 70 km contour (Christensen & Abers, 2010). The arc‐parallel pattern is usually attributed to along‐arc flow in the mantle wedge, while the arc‐perpendicular pattern is attributed to a combination of entrained asthenospheric flow in the plate convergence direction beneath the subducting Pacific plate, as well as fossil anisotropy within the subducting plate (Christensen & Abers, 2010; Hanna & Long, 2012; McPherson et al., 2020; Perttu et al., 2014; Venereau et al., 2019).…”

“…Although there are exceptions (Hammond et al, 2010;Schlaphorst et al, 2017), most local splitting studies around the globe have suggested that the mantle wedge is the main anisotropic structure in the subduction system (Abt et al, 2009;León Soto & Valenzuela, 2013;Long & van der Hilst, 2006;Nakajima & Hasegawa, 2004). In the Alaska subduction zone specifically, anisotropy has been suggested to be present in the mantle wedge and in the subducting Pacific lithosphere and subslab asthenosphere (Christensen & Abers, 2010;Hanna & Long, 2012;Karlowska et al, 2021;McPherson et al, 2020;Perttu et al, 2014;Song & Kawakatsu, 2013;Venereau et al, 2019 (Argus et al, 2011). Red lines denote active faults (Koehler et al, 2012).…”