Upper-plate structure and tsunamigenic faults near the Kodiak Islands, Alaska, USA

Abstract

The Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modeling and geophysical data analysis. Using seismic and bathymetric data, we characterize a regionally extensive seafloor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km-long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but we suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper-plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short wave travel paths and little tsunami warning time for nearby communities.

Exhuming an Accretionary Prism: A Case Study of the Kodiak Accretionary Complex, Alaska, USA

“…This work provides new insight into vertical motions across the Kodiak accretionary complex using a combination of RSCM temperatures, deformation structures, and relative chronology. The deformation kinematics Fisher & Holmes, 1980;von Huene et al, 2012) and onshore observations (Carver et al, 2013;Ramos et al, 2022). of observed structures in all units correspond principally to plane strain in the NW-SE direction from the Early Cretaceous to the present.…”

“…Thus, the exhumation can result from horizontal shortening in combination with surficial erosion. However, in the Narrow Cape Formation, shortening is not as pervasive as in the Sitkinak Formation, and an additional process is potentially required to exhume deeply buried sediments, such as uplift along the thrust such as the currently‐active Kodiak Shelf Fault Zone (Carver et al., 2013; Ramos et al., 2022). Carver et al.…”

“…A good example is the Albatross Basin, located southeast of the Kodiak Island (M. A. Fisher & Holmes, 1980; von Huene et al., 2012). This basin is limited trenchward by the Albatross Bank fault system and landward by the Kodiak Shelf Fault System (Ramos et al., 2022). Such a thrust fault could produce subsidence and accommodate space either as a piggy‐back basin or by subsidence in the footwall of active thrust faults.…”

“…(d) Exhumation of the frontal part of the complex. The recent profile is created based on seismic profiles (M. A. Fisher & Holmes, 1980; von Huene et al., 2012) and onshore observations (Carver et al., 2013; Ramos et al., 2022).…”

“…Thus, the exhumation can result from horizontal shortening in combination with surficial erosion. However, in the Narrow Cape Formation, shortening is not as pervasive as in the Sitkinak Formation, and an additional process is potentially required to exhume deeply buried sediments, such as uplift along the thrust such as the currently-active Kodiak Shelf Fault Zone (Carver et al, 2013;Ramos et al, 2022). Carver et al (2013) pointed to a higher current uplift rate (∼0.75 mm/yr) in the Narrow Cape Formation than in the rest of the Kodiak Island, related to the active upper crustal faulting in the Kodiak shelf fault zone (Figure 15).…”