For people living near volcanoes, danger goes well beyond lava flows and clouds of ash. Some explosive eruptions can lead to dramatic collapses of the sides of a volcano, like those at Mount St. Helens, Washington, and Anak Krakatau, Indonesia. The latter triggered tsunamis blamed for most deaths from its historic eruptions in 1883.
But the science and exact triggers behind such catastrophes remain largely unknown. To help scientists forecast collapse of volcano sides, also known as flanks or slopes, Christelle Wauthier, associate professor in Penn State's Department of Geosciences and computational sciences hub director in the Institute for Computational and Data Sciences, has led the development of new models that can gauge a volcano's stability.
The models, published in the Journal of Geophysical Research: Solid Earth, can help local authorities and communities by evaluating the potential for collapse long before ground may give way entirely and suddenly.
"The input of magma below the volcano puts the crust under a tremendous amount of pressure - much stronger than water pressure," Wauthier said. "It's exerting huge force on the rocks that can help destabilize the volcano and lead to collapse. But we don't really know the exact conditions that would favor instability, and evaluating the triggering factors is quite complex."
Drawing in part from real-world examples of slip of volcano slopes - including sites in Hawaii that experienced collapse - Wauthier and her research partners developed a way to predict how slopes would respond to rising magma under varying conditions. Magma is the molten rock that becomes lava once it emerges onto Earth's surface. They also evaluated where sliding of the surface would be more or less likely, in line with expected changes in stability.
Their new models build on prior knowledge of magma location. Magma rising under a volcano can force slippage on existing faults - fractured areas where two blocks of rock can move relative to each other. Slippage in these spots can lead eventually to collapse.
"If you have an idea of which area of the volcano is more susceptible to collapse, you could place ground-based sensors such as seismometers or GPS to monitor a risky flank on a minute-to-minute or hour-to-hour basis well before a collapse happens," said Wauthier, who is also a faculty affiliate of the Earth and Environmental Systems Institute.
To help make predictions, the research team focused on fault dips, or the angle of a fault or rock fracture relative to the horizontal surface. Researchers found ground more likely to give way on slopes with shallow fault dips under the surface - specifically if magma opens the crust under the volcano summit. Like side-by-side stacks of blocks on a playground slide, more vertical fault dips on steeper flanks also are prone to instability, the researchers said.
They noted that topography has a sizable impact on predictions of ground movement, a factor that is often neglected in other studies.
"This fundamental research can have useful applications to better assess specific collapse hazards and areas of the volcano that are more susceptible to instability," Wauthier said. "Over the long term, pushing this type of research could help volcano-adjacent communities by giving them time to prepare and evacuate ahead of a collapse if need be."
Historically, she said, collapses caused by volcanic activity have been especially menacing to human life. When Mount St. Helens erupted in May 1980, its collapse removed the cap of its magma reservoir, resulting in an even bigger, more violent explosive eruption. In all, 57 people died in the Mount St. Helens eruption; 27 bridges and nearly 200 homes were destroyed.
A century earlier, the August 1883 eruption of Anak Krakatau - another instance of volcanic collapse - led to more than 36,000 deaths and ruined dozens of villages. Waves from tsunamis were recorded at more than 100 feet high.
Following the volcano's collapse and eruption in December 2018, more than 400 people died amid a massive tsunami. Wauthier and colleagues also studied that event, finding the mountainside had been slipping for years.
"These collapses can be very, very dangerous," said Wauthier, whose research focuses on mitigating natural hazards from volcanoes, landslides and earthquakes, among others.
She said the most explosive volcanoes form along subduction arcs, where one tectonic plate is being subducted or buried beneath another. Many subduction-zone volcanoes are situated along coastlines, including in Indonesia and along the Aleutian Islands in Alaska. Volcanoes in Hawaii, too, can be unstable in places, although they're not as explosive as those in subduction areas, Wauthier said.
Follow-up research may focus on strengthening the model calculations and testing the models under other varying conditions, she said.
The other contributors to the study are Judit Gonzalez-Santana, a former graduate student and postdoctoral scholar within Wauthier's group in geosciences at Penn State; Jay Sui Tung, an assistant professor in geophysics at Texas Tech University; and Timothy Masterlark, a professor of geology and geological engineering at the South Dakota School of Mines and Technology.
Supporting the study were a Faculty Early Career Development (CAREER) grant awarded to Wauthier from the U.S. National Science Foundation; a NASA Earth and Space Science and Technology grant issued through the Science Mission Directorate's Earth Science Division to Gonzalez-Santana; and a Penn State Institute for Computation and Data Sciences seed grant awarded to Wauthier and Reuben Kraft, a professor of mechanical engineering.
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