A nearly 50-year-old earthquake fundamentally changed the evolution of Kilauea, scientists at the University of Hawaii say.

According to a study by UH Manoa researchers, a 7.7-magnitude earthquake beneath Kalapana in 1975 changed the way the south flank of Hawaii island has shifted due to volcanic activity.

The quake took place Nov. 29, 1975, with an initial 5.7-magnitude quake rocking the south shores of the island about an hour before the main event. The quake caused significant ground displacement, with some of the south coast sliding horizontally as far as 26 feet. The quake also generated a 20-foot-high tsunami.

Two people died and 28 people were injured following the event.

The study, published in November in the Journal of Geophysical Research: Solid Earth, studied 120 years of data on the volcano to analyze decades-long deformation patterns before and after the earthquake. That data included observations of 338,396 earthquakes and more than 15,000 measurements of displacements to create a computational model of the area before, during and after the quake.

“It’s a really impressive feat of data collation,” said Hawaiian Volcano Observatory research geophysicist Ingrid Johanson. “That may not sound very exciting, but it’s very impressive for those of us who work with data.”

Johanson said the study found that the 1975 quake was a “dividing line,” where patterns of ground deformation were observably different before and after the event.

In short, Johanson explained, “(Kilauea’s) south flank was deforming in a different way before the earthquake than it is now,” although she added that the differences are complicated.

In broad terms, fault creep within the region — where land gradually slips along a fault; in this case, toward the ocean — appears to have greatly diminished since the quake, reducing from an average of 10 centimeters per year before the quake to 4 centimeters per year afterward.

Lead author Lauren Ward Yong, who conducted the study as part of her doctoral dissertation, explained in a statement the findings suggest the fault was likely locked in place by friction before the earthquake, accumulating stress over time until the quake.

“Deciphering Kilauea’s history deepens our understanding of volcanic and seismic hazards,” said Yong. “It offers critical insights into how stress evolves in volcanic systems, guiding our ability to anticipate and interpret future earthquakes and magmatic events.”

“It suggests that the pattern we’re used to with the volcano wasn’t there before,” Johanson said. “It challenges us to think about how the volcano’s behavior might change in the future, how we can’t count on the same background processes that we’re used to being the same forever.”

Yong said in the statement that the changes in ground slip along the fault suggest other changes that will influence the region’s seismic and magmatic activity over long periods.

“Hawaii’s communities live alongside active volcanoes and face significant seismic risks,” Yong said. “This research enhances hazard preparedness and reinforces UH’s commitment to advancing science for the safety and well-being of Hawaii’s residents and ecosystems by shedding light on past significant events.”