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.