2022 Pacific volcano eruption made a deep dive into Alaska

Atmospheric waves from an enormous 2022 South Pacific volcanic eruption created seismic waves that penetrated Earth to at the least 5 kilometers in Alaska, creating a possibility to make use of an uncommon technique of peering into the state’s deep subsurface.

Ken Macpherson, a scientist on the University of Alaska Fairbanks Geophysical Institute, and different researchers analyzed the coupling of atmospheric stress waves with the bottom to find out the pace at which seismic waves journey by means of Alaska’s higher crust. 

Subsurface materials properties equivalent to hardness, which controls seismic velocity, might be inferred by means of examination of the relative energy of incoming stress waves and the ensuing seismic waves.

Think of it like blowing onto the floor of a bowl of Jell-O versus blowing on a pan of brownie batter with equal pressure: You may see the Jell-O shake, however the brownies wouldn’t transfer a lot as a result of the fabric is stiffer.

“Hunga-Tonga’s pressure waves have provided us much more information about how seismic waves propagate in Alaska,” Macpherson mentioned.

Macpherson’s analysis of seismic velocity to depths of 30 meters, 2 kilometers and 5 kilometers was revealed July 8 in Seismica.

The eruption

The explosive eruption of Hunga Tonga–Hunga Ha’apai volcano, within the Kingdom of Tonga and about 6,000 miles from Alaska, occurred on Jan. 15, 2022. The eruption’s atmospheric waves have been the most important identified from a volcano because the 1883 Krakatau eruption.

“Hunga Tonga was an unprecedented explosion in the instrument age,” mentioned Macpherson, who’s with the institute’s Wilson Alaska Technical Center. “Those pressure waves shook Alaska, 6,000 miles away, which I just think is so remarkable. And many of those were long-period waves and consequently shook Earth to a great depth.”

A community of 150 co-located barometers, infrasound sensors and seismometers in Alaska recorded the eruption’s information utilized in Macpherson’s analysis.

Gaining data through air-to-ground coupling at a depth of 5 kilometers is unusual. That’s as a result of seismic waves produced by means of coupling often have a shorter wavelength — except the facility supply is one thing actually, actually huge.

“Because the Hunga explosion was so enormous, the pressure waves that traveled the long distance to Alaska were still powerful enough to shake the Earth, and were therefore ideal for a coupling study,” Macpherson mentioned.

Air-to-ground coupling

How does an atmospheric stress wave shake the bottom? And how can it rattle the bottom 5 kilometers under the floor?

Powerful stress waves from a volcanic eruption or explosion create speedy adjustments in air stress as they journey by means of the environment. When these stress waves contact the bottom, they push and pull on the floor in a course of known as air-to-ground coupling that transfers power into Earth’s inside. 

The power transfers by means of a course of described as Newton’s Second Law, which states that an utilized pressure causes particles to maneuver by overpowering their inertia. That particle movement creates seismic waves containing mechanical power in two varieties — kinetic power from the transferring particles and elastic power from the crust’s momentary deformation because the wave passes by means of.

Waves of data

The velocity analysis might be a further instrument for seismic hazard evaluation as a result of wave pace impacts the extent of floor movement. 

“If a propagating wave is in deep material and going fast but suddenly hits a softer material, the conservation of energy says, ‘Well, I’m going slower, but I still have the same energy,’” Macpherson mentioned. “That means amplitudes get bigger, causing stronger shaking.”

“Just knowing those upper crustal velocities is good for seismic hazard analysis,” he mentioned. “It’s [also] good for network operators like the Alaska Earthquake Center because they can accurately apply the crustal velocity beneath a particular seismic station to potentially increase earthquake location accuracy.”

Macpherson’s work additionally might be notably useful in tomography, a way seismologists use to create three-dimensional photographs of Earth’s inside by analyzing how seismic waves journey by means of totally different supplies. Tomography reveals variations in properties equivalent to density or velocity, serving to scientists map the deep subsurface.

“To do tomography properly you have to do what’s called a crustal correction because velocities in the upper crust are so much different from those deeper velocities that you’re trying to get at,” he mentioned. “If you know something about the crust, you can apply a correction that improves tomography for tens to hundreds of kilometers.”

The analysis paper’s co-authors embrace analysis professor David Fee, postdoctoral researcher Stefan Awender, assistant professor Bryant Chow and seismo-acoustic researchers Juliann Colwell and Sam Delamere, all with the UAF Geophysical Institute’s Wilson Alaska Technical Center. Matthew Haney of the U.S. Geological Survey can be a co-author.

The Nuclear Arms Control Technology Program on the U.S. Defense Threat Reduction Agency financially supported the analysis.