Type of Culminating Activity
Master of Science in Geophysics
Lee M. Liberty
I present marine seismic reflection results from Prince William Sound, Alaska that document the location of active faults related to the subduction zone processes.
Subduction zones along convergent margins experience large earthquakes, magnitude >8, with recurrence intervals on the order of centuries. Smaller magnitude earthquakes with shorter recurrence intervals are probable along the same subduction zone fault zones. Convergent margin earthquakes also are associated with high uplift rates and tsunami generation, yet the location and uplift history of most tsunamigenic faults are unknown. In this thesis, I present the processed results of high resolution marine seismic reflection data of Prince William Sound (PWS), Alaska. I then use these results to create a new tectonic model for the region that constrains fault histories, earthquake hazards, and distribution of active faults within a megathrust subduction zone system.
The epicenter of the M9.2 Great Alaska Earthquake of 1964 was in northern PWS. This earthquake caused tsunamis that impacted shoreline communities as far as California. Earthquake damage and local tsunamis affected towns and infrastructure throughout the PWS region. Future earthquakes may occur independently on these faults or they may rupture only during a large earthquake.
Data were collected in four locations within PWS: Gravina Bay, Orca Bay, Hinchinbrook Entrance, and Montague Strait. Seismic results show high angle faults that offset three primary stratigraphic layers (Holocene, older Quaternary, and Tertiary). Regionally extensive bathymetric lineations connect mapped faults on seismic reflection profiles to define fault lengths that are a proxy for maximum earthquake magnitude. Bathymetric lineations where bedrock surfaces on the sea floor help identify additional active or relic faults. Megathrust splay faults that control motion along the subduction plate boundary surface on Montague Island and recorded the greatest offset during the M9.2 Great Alaska Earthquake of 1964. I interpret Montague Island as the outerarc high of the subduction zone system and suggest splay faults may extend across PWS. Lineations in Hinchinbrook Entrance represent the surface expression of splay faults identified in unpublished crustal seismic data. I interpret normal faults in Orca Bay and Montague Strait, within the forearc of the subduction zone, to form in the hanging wall of the main PWS splay fault that defines the outer arc high. I calculate slip rates along hanging wall normal faults of 1.8 m/kyr in Orca Bay and 6.25 m/kyr in Montague Strait. These faults may extend 40 km, potentially causing an independent M7 earthquake that could damage local infrastructure. Furthermore, landslides identified in Orca Bay and Montague Strait demonstrated a potential hazard from local tsunamis within these locations.
Finn, Shaun Patrick, "Megathrust Splay Fault Geometry in Prince William Sound, Alaska" (2012). Boise State University Theses and Dissertations. 720.