UI researchers study megathrust earthquakes

The day after Christmas in 2004, an undersea earthquake occurred near the Indonesian island of Sumatra, triggering 30-foot waves that roiled the Indian Ocean as far as East Africa and killed at least 225,000 people in a dozen countries.

In 2011, an immense underwater earthquake shook Japan, triggering a tsunami that destroyed the Fukushima nuclear reactor and decimated a large swath of the country’s eastern coast.

While earthquakes are common, some, like the megathrust quakes off Indonesia and Japan, are particularly devastating.

“On a global scale, they produce the Earth’s largest earthquakes because they occur along the Earth’s largest faults,” says Bill Barnhart, an earthquake specialist at the University of Iowa.

Barnhart is studying megathrust earthquakes by examining the largest one this century that took place entirely on land, the megathrust quake that struck Nepal in 2015. With funding from the U.S. National Science Foundation, Barnhart and his research team, along with collaborators from the University of Colorado Boulder, will use satellite imagery, ground seismometers, and GPS stations to map the Gorkha megathrust quake and gain greater insights into why it occurred.

Researchers seek to answer many questions.

A central issue is to understand subduction zones that trigger megathrust quakes. Subduction occurs where one tectonic plate slides beneath another, causing slippage and scraping that ripples through the Earth’s interior. With the Gorkha earthquake, the Indian and Eurasian continental plates collided, spawning two major temblors that leveled much of Nepal’s capital city, Kathmandu. Those quakes registered 7.8 and 7.3 on the Earthquake Moment Magnitude scale.

The researchers will map the width, depth, velocity, and physical features of the Gorkha earthquake region, a 1,500-mile-long fault line called the Main Himalayan Thrust. The computer models and simulations are expected to yield the location and structure of the fault zone that triggered the quakes.

“You start to look at the stress areas and how that may transfer along the earthquake region,” says Barnhart, assistant professor in the Department of Earth and Environmental Sciences. “You can then model how the stress evolves.”

The Gorkha quake is somewhat unique in that it did not create any openings on the surface.

“So, that opens the question: Why do you have deep quakes, and others that go all the way to the surface?” Barnhart asks.

Shaoyang Li, a postdoctoral researcher in Barnhart’s lab, will attempt to answer that question by finding out what controls the size and timing of megathrust earthquakes. Since the Gorkha earthquake was located deep underground, Li says a large section of the shallow fault zone, about 10 miles below the surface, remains intact. Eventually, it too will give way.

“An earthquake affecting this shallow portion is expected to happen, and it will be more dangerous than the 2015 event not only because of its potentially larger magnitude, but also because it will be much closer to the Earth’s surface, causing more above-ground destruction such as collapsing buildings and landslides,” says Li, who began working on the project in January 2018.

The researchers hope their monitoring and modeling of the Main Himalayan Thrust will yield new approaches to studying subduction zones embedded beneath the oceans, where nearly all megathrust earthquakes—including the ones affecting Indonesia and Japan—have occurred.

“There’s a fundamental science question of what these oceanic plate boundaries look like at depth,” Barnhart says. “A fault may look very different at depth than what appears at the Earth’s surface.”

Even with better information, earthquakes likely will continue to defy complete forecasting.

“There’s a randomness to earthquakes,” Barnhart says, “that suggests we may never truly define them.”