Collaborative Research: Incoming plate and forearc structure of the Semidi and SW Kodiak Segments offshore Alaska Peninsula from 3-D active-source and local earthquake tomography award

The largest earthquakes and associated tsunamis as well as volcanic eruptions are found at subduction zones where one tectonic plate dives beneath another. A prominent example in North America is the subduction zone offshore of the Alaska Peninsula, where some of the largest earthquakes ever recorded in modern times have occurred, like the great earthquake of 1938 and the devastating 1964 M9.2 Prince William earthquake. In this region the history of large and small earthquakes and how strongly the two plates are locked together vary substantially and rapidly along the margin. Many questions, however, remain unanswered about the ability of the subduction zone to generate large earthquakes. This project will analyze and interpret recordings of earthquakes that were collected as part of a larger NSF-GeoPRISMS funded Alaska Amphibious Community Seismic Experiment (AACSE) in 2018-2019. The seismic data, recorded by a dense array of offshore and onshore seismometers, will allow the development of a three-dimensional image of the geological structure and physical properties of this subduction zone down to a depth of 100 km. The quality of the images will be high enough to examine structures that have been previously proposed to explain changes in the seismic behavior of the plate boundary in this area and at other subduction zones worldwide. The project supports the training of two graduate students.

Three-dimensional seismic tomography studies will be conducted of P- and S-wave structure based on controlled-source and local earthquake data recorded by the AACSE amphibious array. The project will address the following key question: To what extent is the segmentation of the Alaska Peninsula margin in seismogenic properties, seismicity during the interseismic period, and plate coupling controlled by along-strike and downdip variations in upper continental plate structure, composition, and/or downgoing oceanic plate physical properties and structures such as seamounts, bending faults and fracture zones? The 3-D seismic velocity models will be used to investigate along-strike variations in water input into the subduction zone, as well as depth extent of the presumed hydrated oceanic mantle and whether or not water is delivered to seismogenic depths where it could influence megathrust seismic properties. By mapping seismic velocity anomalies along the plate interface, the plate interface relief and the presence (or lack thereof) of subducted structural anomalies such as seamounts chains, swell and fracture zones, and their presumed role in contributing to rupture segmentation and variations in the geodetically inferred plate interface locking, will be examined. An improved understanding of subduction zone processes associated with large earthquakes is important for assessing earthquakes and tsunami hazards at the Alaska Peninsula subduction zone. Although this project focuses on the Alaska Peninsula subduction zone, the scientific question applies across many subduction zones worldwide.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.