Interseismic Slip Deficit at the Edge of a Locked Patch: Shumagin Islands, Alaska

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Alaska is a premier location for studying what controls variations in seismic activity along a margin where tectonic plates converge. This is because the width of the seismic zone is known to differ between segments along the Alaskan subduction zone. The Alaska Peninsula segment includes the transition from a wide, locked region on the plate interface to a dominantly creeping section. The fact that a chain of islandsruns across this segment provides an ideal setting for measuring deformation, and these data will be used to determine the distribution of recent slip (or lack thereof) along the plate boundary fault. This is the first time that a detailed view of how the seismogenic zone varies from a locked to a creeping section will be obtained. The findings will inform assessment of earthquake and tsunami hazards, both in relation to the Alaska Peninsula and along the US west coast due to trans-Pacific tsunamis. Investigators will conduct public lectures and work with a teacher in the school district of the local community of Sand Point, Alaska, in the Shumagin Islands. Lesson materials will be developed on the topics of earthquakes and tsunamis in Alaska, subduction and its impact on their local environment.GPS measurements will be made at sites that have not been surveyed since the 1990s, providing new highly precise plate velocities. Most of the sites have been measured once, and the new measurements will come 20 years after the first, resulting in velocity uncertainties that should be 0.5 mm/yr. These data will be used to estimate the distribution of locked and creeping parts of the plate interface fault, and spatial changes in the seismogenic zone will be compared with spatial changes in seismicity and properties of thedowngoing plate. Results will address how abrupt the along-strike transition is, from the wide locked region of the Semidi segment to the much narrower and/or creep-dominated locked region of the Shumagin segment. The structure within the partially locked region of the Shumagin segment will be addressed through modeling. The study will also investigate potential correlation between observed slip variations and features on the overriding or downgoing plates, the reflection character of the plate interface itself, seismicity patterns, or other geologic observations.