Earthquake Segment Boundaries and Tsunamigenic Faults of the Kodiak Segment, Alaska-Aleutian Subduction Zone

摘要

The most recent megathrust earthquake to impact the Alaska subduction zone was the M9.2 Great Alaska earthquake of 1964. This multi-segment rupture spanned over 700 km of the plate boundary and engendered both local and trans-Pacific tsunamis. The Kodiak Islands region served as the southwestern limit to rupture. The nature of past megathrust segmentation for the Alaska subduction zone has been largely hypothesized through paleoseismological methods and the Kodiak region in particular has not received a comprehensive geophysical characterization of its inferred segment boundaries.I analyze multiple geophysical datasets (e.g. seismic reflection, earthquake, potential fields) to understand the spatiotemporal relationships between subduction, accretion, lower and upper plate structure, and tsunamigenic fault hazard in the context of the known megathrust earthquake record and other interseismic observations for the Kodiak region.The northeast Kodiak segment boundary is defined by the subducting 58° fracture zone, which can be traced below the forearc using magnetic and gravitational fields. Subduction of this feature is expressed on post-1964 seismicity, is consistent with oblique shortening, and manifests itself within the upper plate as the Portlock Anticline.The southwest segment boundary marks the transition between the Kodiak and Semidi segments. It is shown to be a region that shifts from significant margin erosion to a region of imbricate thrusting and margin growth. These two zones are bound by fracture zone subduction. I furthermore independently constrain and compliment paleoseismological models of joint Kodiak and Semidi segment rupture by identifying and characterizing a through-going marine fault zone across this segment boundary.Finally, I revisit the source mechanisms for the local tsunami that inundated the Kodiak Islands as a result of the 1964 earthquake. I provide a new tsunamigenic source model that suggests discrete uplift of the Kodiak Islands shelf fault system and illuminate its along-strike rupture variability throughout the Holoecene epoch.My findings suggest segment boundaries across Kodiak have a clear geophysical expression and a multi-dataset approach is necessary to decipher tectonic controls on megathrust segmentation.