Volcanologists identify magma reservoirs by monitoring earthquakes. Seismic waves plunge through Earth’s innards before being detected by surface seismometers. They move more slowly through hot and partially molten rock, and scientists use their travel times to interpret how molten parts of the subsurface are.
But this traditional seismic imaging technique is imperfect. Seismic waves sometimes bend around molten pockets. This method also assumes that seismic waves travel in a simplified manner, from the quake directly to the seismometer; in reality, seismic waves emanate in all directions, and critical information about Earth’s underbelly is lost.
For the new study, the authors turned to a 20-year-long recording of Yellowstone’s background seismic noise — generated by distant ocean waves, the wind and human activity — to zero in on the volcano’s overlooked melt. They dispelled with traditional seismic simplifications and used supercomputers to represent the voyages of seismic waves more accurately.
The team found that seismic waves slowed to a crawl when traveling 2 miles to 5 miles down — corresponding to the upper segment of where the Yellowstone volcano’s shallower magma reservoir is thought to be. This suggests that up to 20 percent of this entire reservoir is molten.
Fortunately, this is nothing to lose sleep over. A rule of thumb is that reservoirs cannot produce eruptions without being 35 percent to 50 percent molten, when things are “kind of like a crystal soup,” said Ross Maguire, a seismologist at the University of Illinois Urbana-Champaign and an author of the study. That value, which volcanologists frequently debate, is likely to vary between volcanoes. Regardless, Yellowstone’s 20 percent is “still well below that critical threshold,” Dr. Maguire said.
For those hoping to unearth the secrets of other volcanoes, this study confirms that this relatively new technique “is a really good way to go,” said Diana Roman, a geophysicist at the Carnegie Institution for Science in Washington, D.C., who wasn’t involved with the study.
“It’s a bit like getting a new lens on an old camera,” Dr. Poland said. “It’s the same camera, but you’ve got finer resolution now. You see with more clarity.”