5.1 Absence of Oceanic Crust Arrival
In subduction zone environments, RFs are commonly used to investigate plate interface structure since the method exploits the conversion of incident P waves from a teleseismic event to S waves at significant seismic-velocity discontinuities. RFs have identified LVZs along the plate interfaces in subduction zones globally as negative amplitude pulses atop positive amplitude pulses at slab depth (Bostock, 2013; Audet & Kim, 2016). This dipole character has been observed in the Japan (Kawakatsu & Watada, 2007; Akuhara et al., 2017), Cascadia (Janiszewski & Abers, 2015; Ward et al., 2018), Costa Rica (Audet & Schwartz, 2013), Mariana (Tibi et al., 2008), Alaska (Ferris et al., 2003), and the central Mexico (Pérez-Campos et al., 2008; Y. Kim et al., 2012) subduction zones. Depending on how far down dip the study area is located, the negative pulse is typically interpreted as hydrated oceanic crust or mantle hydrated by fluid expelled from the subducting slab due to the low S-wave velocities observed, while the positive amplitude pulse is generally the slab Moho. In Cascadia, Janiszewski and Abers (2015) interpreted the LVZ as metamorphosed sediments, while Bangs et al. (2009) interpreted the LVZ in Nankai as high porosity underthrust sediment. In the northern 1964 segment, Y. Kim et al. (2014) also observed this typical negative-to-positive character, attributing the negative arrivals to an LVZ of subducted marine sediments along the plate interface.
Neither our observed nor the preferred synthetic RFs (Figs 2 and 3) feature the negative-positive dipole character observed within the northern 1964 asperity, highlighting a major difference in RF character. The lack of major arrivals before the positive slab Moho suggests that beneath Kodiak, the seismic properties at the base of the upper plate and top of the subducting slab may be similar. late interface material is commonly inferred from trench sediment input to the subduction zone (Morgan, 2004; Underwood, 2007) At the trench near Kodiak, both pelagic sediments and sediment from the Surveyor Fan (von Huene et al., 2012; Reece et al., 2011; Fig. 1a) comprise the subduction input. We suggest that the subduction zone environment has altered the properties of any subducted sediment at the interface, thus suppressing contrast between the sediment and the surrounding rock. 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 padding the plate interface. 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. The metasedimentary rocks on Kodiak Island are close enough in velocity (Miller et al., 2021) and density to the Pacific crust that there is no significant discontinuity at the interface to resolve with Ps RFs.
While the Ps RFs presented use relatively high frequencies for teleseismic imaging (1.2 – 2.5 Hz), there may be coherent structural layers that are too thin to be resolved. For example, using controlled source seismic reflection data, J. Li et al. (2015) estimated a thin 600-900 m low-velocity channel at ~8-10 km depth along the plate interface south of Kodiak Island inside the 1938 Mw 8.2 Semidi rupture zone. A synthetic test of 2.5 Hz Ps RFs showed that a 750 m thick LVZ would not be well resolved (Fig. S4(a)). We also tested using higher frequency observations, 4.8 Hz, but the signal-to-noise ratio of teleseismic sources decreases and the prominent velocity increase interpreted as the slab Moho is only resolved sporadically across the array (Fig. S4(b)). In areas where potential slab Moho arrivals are observed in the 4.8 Hz RF image, we still do not find evidence for an overlying LVZ (Fig. S4(b)). Thus, we cannot rule out a thin LVZ like that imaged at shallower depths in the Semidi rupture zone (J. Li et al., 2015), but we can be confident that a thicker LVZ (~3-5 km) like that imaged by Kim et al. (2014) in the Kenai asperity would be resolvable if it existed beneath our study area. Mann et al. (2022) used scattered P and S coda of teleseismic P waves to successfully image a continuous ~7-km thick low-velocity layer lining the top of the subducted Yakutat crust. While we see reverberations in sections of our profile (e.g., Fig. S5), their quality is too low to allow for interpretation. The short deployment window (~25 days) and the limited backazimuth distribution of the events used in this study limits the usefulness of later arrivals.