NSF Org:	EAR Division Of Earth Sciences
Recipient:
Initial Amendment Date:	July 27, 2022
Latest Amendment Date:	December 18, 2024
Award Number:	2204587
Award Instrument:	Fellowship Award
Program Manager:	Aisha Morris armorris@nsf.gov  (703)292-7081 EAR  Division Of Earth Sciences GEO  Directorate for Geosciences
Start Date:	January 1, 2023
End Date:	November 30, 2024 (Estimated)
Total Intended Award Amount:	$180,000.00
Total Awarded Amount to Date:	$174,084.00
Funds Obligated to Date:	FY 2022 = $174,083.00
History of Investigator:	Evans Onyango (Principal Investigator)
Recipient Sponsored Research Office:	Onyango, Evans Awere Albuquerque NM  US  87131-0001
Sponsor Congressional District:	01
Primary Place of Performance:	University of Alaska Fairbanks Fairbanks AK  US  99775-7320
Primary Place of Performance Congressional District:	00
Unique Entity Identifier (UEI):
Parent UEI:
NSF Program(s):	Postdoctoral Fellowships, EPSCoR Co-Funding
Primary Program Source:	01002223DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s):	9150, 7137
Program Element Code(s):	713700, 915000
Award Agency Code:	4900
Fund Agency Code:	4900
Assistance Listing Number(s):	47.050, 47.083

ABSTRACT

Evans Onyango has been awarded an NSF EAR Postdoctoral Fellowship to conduct research and education activities related to developing a multiscale seismic velocity model for the Gulf of Alaska. This work will take place at the University of Alaska Fairbanks under the mentorship of Dr. Carl Tape. The surface of the Earth is divided into plates that are in constant motion and collide, slide past, or pull away from each other along the boundaries between plates. Plate interactions build mountains, cause volcanic activity, and produce large earthquakes. Alaska hosts one of the world?s most seismically active plate boundaries, which produced the 1964 magnitude 9.2 earthquake, the largest ever recorded in the United States. Scientists rely on three-dimensional models of Earth?s subsurface material to understand plate tectonic processes and to assess geologic hazards. In order to better understand large earthquakes, scientists must understand how seismic waves interact with Earth?s subsurface materials. A primary limitation to modeling the seismic wavefield in southern Alaska is the lack of a three-dimensional offshore model. Permanent seismic stations are on land, far from offshore earthquakes, which makes it difficult to model ground motion at coastal cities. This project seeks to build and test a three-dimensional subsurface model for the Gulf of Alaska to better understand the complex earthquake ground motion in coastal regions. The goal is to create a comprehensive three-dimensional model by combining existing two-dimensional models from previous studies. Computer simulations of seismic wave propagation within this model will be performed to generate predicted seismograms that will be compared with recorded seismograms from moderate earthquakes in the region. The final model will be publicly accessible via the IRIS Earth Model Collaboration, a public repository for Earth models. The project will involve outreach to local coastal communities prone to seismic and tsunami hazards.

Offshore regions at tectonically active margins, such as southern Alaska or southern California, often exhibit extreme structural complexity and produce a wide range of earthquakes in terms of magnitudes and mechanisms. In southern Alaska, the active collision and accretion of the Yakutat microplate beneath the North America plate and the structure of the accretionary wedge at this active subduction zone are examples of factors contributing to the complex structure of the region. Small-scale and large-scale deployments of seismic instruments on land such as the EarthScope Transportable Array have yielded numerous tomographic models of the mainland Alaska, most of which include inherent smoothing that cannot capture the sharp interfaces of the offshore setting. By comparison, the offshore region of the Gulf of Alaska has only been studied with a sparse assortment of high-resolution 2D velocity models derived from marine studies between the 1980s and 2000s. This project proposes leveraging these existing images from the Gulf of Alaska to create a comprehensive 3D velocity model that will have many uses such as modeling the seismic wavefield, understanding active tectonics, and modeling geodynamics of the subduction system. Synthetic waveforms calculated from the resultant 3D model will be compared with seismograms recorded from moderate and large earthquakes in the study area to validate the model. Seismogram misfit values will also be compared with the performance of other available tomographic models. This project will develop a 3D velocity model for the Gulf of Alaska that will be publicly accessible via the IRIS Earth Model Collaboration, a public repository for Earth models.

This project is jointly funded by the Earth Sciences Postdoctoral Fellowship program and the Established Program to Stimulate Competitive Research (EPSCoR).

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.

PROJECT OUTCOMES REPORT

Disclaimer

This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

 The plate tectonics boundary in southern Alaska is characterized by the subduction of the Pacific Plate beneath the North America Plate and the ongoing collision and accretion of the Yakutat microplate. It is also associated with geological features including the accretionary wedge, the Aleutian and Wrangell volcanic arcs, and major fault systems such as the Denali and the Fairweather-Queen Charlotte fault systems. This dynamic tectonic setting not only exhibits extreme structural complexity but also generates a wide range of earthquakes, both in magnitude and mechanism. However, because permanent seismic stations are located on land, far from offshore earthquakes, modeling ground motion in coastal cities remains a significant challenge. A primary limitation has been the absence of a 3D offshore seismic velocity model for the region. The main objective of this project was therefore to use advanced seismic imaging techniques and high-performance computing to construct and validate a 3D seismic velocity model for the Gulf of Alaska.

To build the seismic velocity model, we leveraged sparse but high-resolution 2D velocity models derived from marine seismic surveys conducted since the 1980s. We developed a suite of Jupyter notebooks to read velocity models from previous 2D studies and adapted them into 3D models in netCDF format. I also created a brief tutorial explaining the use of various flags in SPECFEM3D_GLOBE's mesher, with the goal of lowering the barrier to entry for new users. Although this project focused on a regional area, we used the global mesher to take into account Earth's curvature and ellipticity, factors that can influence wave propagation over large distances. The project contributed to functionality in SPECFEM3D_GLOBE, the open-source software used to simulate seismic wave propagation, allowing it to read external velocity models in netCDF format. This enhancement was incorporated into the latest version (v8.1.0).

Using the tools developed in this project, I conducted 3D wavefield simulations of earthquakes and virtual sources on the 3D velocity model, producing synthetic seismograms and correlograms. I validated the model by comparing these synthetics with observed waveforms from well-recorded earthquakes and ambient noise cross-correlations from prior studies. Project results were presented in public science talks and poster presentations, including Science for Alaska (2023) and the Seismological Society of America meetings (2023, 2024). This project has laid the foundation for a better understanding of the plate boundary between the Pacific and North American plates in the Gulf of Alaska by helping to quantify subsurface structures and processes that impact nearby coastal communities, including Yakutat, Kodiak, Cordova, and Sitka.

Last Modified: 03/25/2025
Modified by: Evans Awere Onyango

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