Mount Aso

Mount Aso in Japan

CHO KOK BIN/National Geographic Creative

This volcano stopped an earthquake in its tracks, scientists say

Before they break a volcano’s heart, earthquakes must stop in the name of lava. That’s the conclusion of a new study, which reports that Japan’s 2016 Kumamoto earthquake may have been stopped in its tracks by the magma chamber underneath the active volcano Mount Aso. The study offers insight into the interplay between earthquakes and volcanoes—and it may also help explain Mount Aso’s explosive eruption this month.

As the products of slowly moving tectonic plates, large earthquakes and active volcanoes are closely related phenomena that often occur near each other. Large earthquakes can trigger volcanic eruptions—even at considerable distances—and the presence of underground bodies of magma can affect the patterns and segmentation of geological faults in volcanic regions. Despite these links, opportunities to study earthquake-volcano interactions are comparatively rare, and a lack of geological observations means that little has been known about how volcanoes might affect an earthquake rupture.

This is what made the earthquakes that hit Japan’s Kyushu Island earlier this year rather special. The shocks, which peaked with a magnitude-7.1 event on 16 April that caused widespread damage, struck only 30 kilometers southwest of Mount Aso, one of the world’s largest active volcanoes. The mountain features a caldera—a cauldronlike depression caused by a volcano collapsing into itself—about 22 kilometers in diameter.

Seizing this rare opportunity, geoscientist Aiming Lin of Kyoto University in Japan and colleagues raced to the epicenter the day after the main shock to investigate the quake’s fault ruptures and the impact on the caldera. The researchers combined satellite images from Google Earth with on-the-ground observations of broken roads and river channels to find out how much the earth had shifted near fault lines, a measurement known as displacement. They also used seismic imaging to study the structure of the crust below the ground.

The team found that the earthquake created a 40-kilometer-long swath of surface ruptures along the pre-existing Hinagu-Futagawa fault zone, with horizontal displacements of as much as 2.45 meters and vertical shifts of up to 0.9 meters. They also found a new set of faults that cut across the Aso caldera from the southwest toward the northeast. Near the northeastern edge of the caldera, however, the surface rupturing changed. There, the ruptures were dominated by vertical movements before abruptly stopping.

Surface ruptures cut across the rim of Mount Aso’s caldera.

Surface ruptures cut across the rim of Mount Aso’s caldera.

Aiming Lin et al.

Seismic imaging of the ground beneath Mount Aso revealed that underground rupturing ceased below a depth of 6 kilometers, an unusual cutoff. But that also happens to be the upper boundary of the volcano’s underground magma chamber.

“We found that the coseismic rupturing stopped at the volcano, inside the caldera,” says Lin, explaining that the partially molten magma chamber could not be ruptured by the earthquake. If the magma chamber had not been present, he says, the extent of the earthquake’s rupturing would likely have been longer. Instead, the presence of the magma chamber acted to change the stress field of the rock around it, leading to pulling-apart stresses that resulted in the vertical fault movements.

But Lin warns that the new ruptures produced beneath the caldera could become fresh conduits for escaping magma, potentially leading to future eruptions like the 11-kilometer-high ash cloud that exploded from Mount Aso on 8 October. With their initial study complete, the researchers are hoping to gather more data on how the volcano might have been affected by the earthquake rupturing.

The factors that control earthquake rupturing are very much a matter of ongoing research, with variations in the mechanical properties of fault zones playing an important role, notes geophysicist Margarita Segou of the British Geological Survey in Nottingham, who was not involved in this study. Alongside the impact of magma chambers on the properties of the shallow crust, she notes, are other potentially relevant factors such as the presence of low seismic velocity zones controlled by fluids.

“In the future,” she adds, “time-dependent seismic hazard studies—including earthquake and volcano forecasts—are expected to include more realistic representations of active stress fields, acknowledging the existence of critical perturbations from multiple sources.”