A document by Sarah Jaye Oliva. Click on the document to view its contents.

Oceanic plates experience extensive normal faulting as they bend and subduct, enabling fracturing of the crust and upper mantle. Debate remains about the relative importance of pre-existing faults, plate curvature and other factors in controlling the extent and style of bending-related faulting. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on bending-related faulting as the orientation of abyssal-hill fabric with respect to the trench and plate curvature vary along the margin. Here we characterize bending faulting between longitudes 161°W and 155ºW using newly collected multibeam bathymetry data. We also use a compilation of seismic reflection data to constrain patterns of sediment thickness on the incoming plate. Although sediment thickness increases by over 1 km from 156°W to 160°W, most sediments were deposited prior to the onset of bending faulting and thus have limited impact on the expression of bend-related fault strikes and throws in bathymetry data. Where magnetic anomalies trend subparallel to the trench (<30°) west of ~156ºW, bending faulting parallels magnetic anomalies, implying bending faulting reactivates pre-existing structures. Where magnetic anomalies are highly oblique (>30°) to the trench east of 156ºW, no bending faulting is observed. Summed fault throws increase to the west, including where pre-existing structure orientations do not vary between 157-161ºW, suggesting that the increase in slab curvature directly influences fault throws. However, the westward increase in summed fault throws is more abrupt than expected for changes in slab bending alone, suggesting potential feedbacks between pre-existing structures, slab dip, and faulting.

We present seismic reflection images of the plate interface up to depth of ~65 km across a ~130,000 km 2 area offshore the Alaska Peninsula. These images capture systematic along-strike and downdip variations in the thickness of the plate interface reflection package, which we interpret in terms of megathrust properties and slip nature. A relatively thin reflection package (<2 km) characterizes the megathrust at depths <24-34 km, marking an area of localized brittle deformation that is likeliest to host the largest seismic asperities. The plate boundary reflection band thickness transitions from 2-5 km over depths of ~24-41 km, suggesting a thickening zone of cumulative deformation and greater heterogeneity corresponding to conditionally stable megathrust behavior. At depths >35-41 km, the reflection package maintains a thickness of 5-6 km and marks the predominantly aseismically slipping megathrust areas of which only the shallower part can host earthquake slip. Historic and recent megathrust earthquakes in the SW Kodiak Asperity and Semidi Segment generally occur within the thin and/or transitional reflection bands. In contrast, the 2020 Simeonof and earlier M7.x earthquakes in the Shumagin Gap are estimated to also rupture the shallowest section of megathrust characterized by the thick reflection band (~35-41 km depth). Although the shallow part of the plate boundary ( in the Semidi and Shumagin segments did not rupture in recent 2020 M7.8 Simeonof and 2021 M8.2 Chignik earthquakes, the seismic properties of the megathrust suggest it may be capable of earthquake slip in the future, as may have occurred in 1788.

The Lower St. Lawrence Seaway (LSLS), in eastern Canada, is an important habitat for several species of endangered baleen whale. As we seek to reduce the hazards that these endangered species face from human activity, there is increasing demand for detailed knowledge of their habitat use. Only a sparse network of hydrophones exists in the LSLS to remotely observe whales. However, there is also a network of onshore seismometers, designed to monitor earthquakes, that have sufficiently high sample rates to record fin and blue whale calls. We present a simple method for detecting band-limited, regularly repeating calls, such as the 20 Hz calls of fin and blue whales, and apply the method to build a catalog of fin and blue whale detections at 14 onshore seismometers across the LSLS, over approximately a four-year period. The resulting catalog contains >600000 fin whale calls and >60000 blue whale calls. Individual calls are rarely detected at more than one seismometer. Fin whale calls recorded onshore appear to travel mainly through solid earth, rather than only entering the earth at the shoreline, and they often have a complex ~2 s sequence of P-like and S-like phases. Onshore seismometers provide a valuable, previously unused source of data for monitoring baleen whales. However, in the LSLS, the current seismometer network cannot provide high-precision whale tracking alone, so a denser deployment of onshore and/or offshore seismometers is required.