2 Geologic Background and Previous Geophysical Studies of the 1964 Rupture Area
Kodiak Island (Qikertaq in Alutiiq ) is part of an archipelago that represents an exposed section of the Mesozoic-Tertiary Alaska-Aleutian accretionary complex uplifted either via duplex accretion and underplating (Sample & Fisher, 1986), out-of-sequence splay faulting (e.g., Rowe et al., 2009), or a combination of these processes. The surface exposures consist of Jurassic to Eocene formations bounded by NW-dipping and NE-striking thrusts (Wilson et al., 2015; Fig. 1b). The thrust-bounded units get progressively younger towards the southeast, approaching the current subduction trench offshore (Fig. 1b). Potentially active Quaternary fault systems include the Albatross Bank, Kodiak Shelf and Narrow Cape fault zones (Figs. 1b, 1c and 1d)). Paleocene granitic intrusions (~58-50 Ma) from ridge subduction (Ayuso et al., 2009; Farris et al., 2006; Fig. 1b) form the mountainous spine of the island interior. In the duplex accretion and underplating scenario for Kodiak Island formation and deformation, a stacked section of marine sediments builds up near the subduction decollement, forming a series of flat-ramp-flat geometries of imbricated material at depth within the overriding plate (Sample & Fisher, 1986). The build-up of the underthrust material causes the accretionary prism to grow vertically, with minimal fault penetration or deformation within the overlying sediments. In the splay fault model, the island was uplifted due to deformation on one or several seaward-vergent thrusts possibly rooted at the megathrust.
Prior to our study, the 2007-2008 Multidisciplinary Observations of Onshore Subduction (MOOS; J. Li et al., 2013; Fig. 1a) measured structure and seismicity beneath the Kenai Peninsula in the northern 1964 rupture zone. The MOOS experiment included 34 broadband seismometers deployed at 10-15 km station spacing. Major results include RF imaging showing a 3-5 km-thick low velocity zone (LVZ) sandwiched between the overriding North American plate and the subducting Yakutat microplate (Y. Kim et al., 2014). This low-velocity zone suggests the presence of subducting sediments and/or the presence of fluids within or below the plate interface. Imaging via autocorrelation of P-wave coda from local earthquakes replicates these results and further suggests that S-wave velocity within this zone decreases with depth (D. Kim et al., 2019).
A more recent study of the subducting crust beneath southcentral Alaska suggests that the LVZ extends far beyond the location of the MOOS array. In their scattered-wave imaging of the subduction zone beneath southcentral Alaska, Mann et al. (2022) analyzed seismic data recorded by 218 broadband seismometers across southcentral Alaska. Using data from the Wrangell Volcanism and Lithospheric Fate (WVLF; Fig. 1a) array, the Broadband Experiment Across the Alaska Range (BEAAR; Fig 1a) array, the Transportable Array (TA) and the MOOS array, they found that the LVZ covers > 450 km of the subducting Yakutat terrane (Mann et al., 2022). Our study tests whether these features extend southward, controlling structure beneath northeast Kodiak Island.