EXPLORING FOREARC DEFORMATION OFFSHORE KODIAK ISLAND USING AMPHIBIOUS SEISMIC OBSERVATIONS (Invited Presentation)

From Prince William Sound to the Alaska Peninsula, the Alaska subduction zone exhibits significant variations in forearc deformation and earthquake behavior. Changes in plate boundary locking, subduction of sediment or unique seafloor fabric, and morphological expressions of upper plate deformation have been identified across segments of the margin, many of which have generated great earthquakes in the last 100 years. The Alaska forearc also hosts relatively high rates of interseismic earthquakes, potentially providing insight as to how stress is accumulating and being released. However, until recently, it has been difficult to characterize source properties of subduction-related earthquakes that are located offshore. Where the region has been studied using dense seismic deployments on land, events have been found to occur primarily within the downgoing plate. At the same time, geological reconnaissance following large earthquakes and high-resolution geophysical surveying of the forearc have also identified upper plate splay faulting, which may accommodate deformation during both the coseismic and interseismic periods.

Here, we take a closer look at the morphology and earthquake behavior of the offshore portion of the Kodiak segment of the Alaska forearc. Seismic observations collected as a part of the Alaska Amphibious Seismic Experiment (AACSE), as well as new seismic reflection data, and multibeam bathymetry can provide a more detailed picture of how the forearc is deforming. The forearc offshore Kodiak Island represents a region of uniquely high seismicity rates. Mapped faults along the continental shelf and slope, including the Albatross Bank fault and Kodiak Shelf fault zone, may accommodate ongoing deformation and lead to elevated seismicity, however, the downgoing plate is also capable of generating significant earthquakes, as demonstrated by the 2018 Gulf of Alaska, and 2019 Anchorage earthquakes. In this region of the forearc, it has been difficult to associated small events with a particular fault source, or distinguish whether they occur in the forearc, downgoing plate, or along the plate interface. By integrating earthquake source properties with new structural interpretations, we aim to refine models of deformation and seismic hazards along the margin.