Why Alaskan plate tectonics drive massive earthquakes

For more than 15 years, UB geologist Margarete Jadamec has studied the Alaskan subduction zone, where two huge pieces of the Earth’s rigid outer layer — the North American Plate and the Pacific Plate — are converging. In this region, the Pacific Plate is being forced under the North American Plate.

“If you imagine the Earth to be an orange, the tectonic plates are the outermost layer, like a peel that’s broken. These plates are floating on the mantle and moving in different directions. In the Alaskan subduction zone, you have two plates that are moving toward one another with enormous force,” says Jadamec, assistant professor of geology, a faculty member in the university’s Computational and Data-enabled Science and Engineering Program, and an expert on Alaskan plate tectonics.

She explains that as the tectonic plates press against each other, their complex, 3D geometries mean that the Earth is strained in different ways, with some areas getting compressed (a compressional quake) and others stretching out (an extensional quake).

“In the region of south central Alaska where the Nov. 30 earthquake occurred, the shape of the plate boundary varies in three dimensions,” Jadamec says. “This can lead to faults in multiple directions, allowing for both compressional events as well as what the initial data indicate is an extensional event for the Nov. 30 earthquake.”

She says initial data suggest the Nov. 30 earthquake occurred inside the down-going Pacific Plate.

“In this part of Alaska, the 44-kilometer depth of the Nov. 30 earthquake calculated by the U.S. Geological Survey suggests the event occurred inside the Pacific Plate, near where my subduction models define the plate interface. Here, the Pacific Plate is curved, like a very broad, flat-topped hill spanning several hundred kilometers across. The Nov. 30 event appears to have occurred just to the west side of that arc within the Pacific Plate,” Jadamec says.

Historically, the Alaska subduction zone is known for its large earthquakes, where the Pacific and North American plates scrape against each other violently as one slides suddenly over the other in the subduction zone. This scenario generates megathrust earthquakes, such as the Great Alaskan earthquake, which took place farther southeast within the subduction zone in 1964 and is listed by the U.S. Geological Survey as the second-largest earthquake recorded in the world.

“Unlike the 1964 megathrust event, the Nov. 30 event was likely caused by normal faulting within the Pacific Plate, which suggests that part of the Pacific Plate was stretching apart. How the dynamics of this event fit into the larger compressional tectonics of the subduction zone is now actively being examined," Jadamec says.

Jadamec’s research is at the forefront of understanding the three-dimensional dynamics of subduction zones. Alaska is a major focus of her work, and her research group has created 3D visualizations of the Alaskan subduction zone that show the shape of the Pacific Plate as it descends into the Earth’s mantle beneath the North American Plate:

• http://www.acsu.buffalo.edu/~mjadamec/alaska1.html
• https://ub.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=67a5fc45-3073-400f-b5b2-a8f400f8819c
• https://ub.hosted.panopto.com/Panopto/Pages/Viewer.aspx?id=33af04d0-9459-4b46-942f-a8f400f6d0d5

“3D visualization is a cutting-edge area of research. It’s really useful in understanding the deformation of the Earth, especially in areas like the Alaskan subduction zone where the situation is complicated in three dimensions,” Jadamec says. “There are a lot of tectonic processes happening there that don’t fit into a simple two-dimensional paradigm.”