Kīlauea’s collapse

Kīlauea’s caldera collapsed not in one smooth descent, but rather like a sticky piston. Roughly every day and a half, the collapse block dropped by nearly eight feet in a matter of seconds, then stopped. That’s because as magma in the chamber below the caldera surged out to fissures in Kīlauea’s lower eastern flank, it took away support for the overlying rock. “Eventually, the pressure becomes low enough that the floor falls in and it starts collapsing, like a sinkhole,” Segall said.

By the time the 2018 Kīlauea eruption ended, the volcano’s piston-like collapse events repeated 62 times – with each one triggering an earthquake and every move tracked down to the millimeter every five seconds by an array of 20 global positioning system (GPS) instruments. During the first few dozen collapse events, the geometry of the rock surfaces changed, but they held stable for the final 30 halting descents.

The new research shows that for this type of eruption, when the eruptive vent is at a lower elevation, it leads to a bigger drop in pressure below the caldera block – which then makes it more likely that a collapse event will start. Once collapse initiates, the weight of the massive caldera block maintains pressure on the magma, forcing it to the eruption site. “If not for the collapse, the eruption would have undoubtedly ended much sooner,” Segall said.

Evolving friction

Segall and Anderson’s analysis of the trove of data from Kīlauea’s caldera collapse confirms that, even at the vast scale of this volcano, the ways different rock surfaces slip and slide past one another or stick at different speeds and pressures over time are very similar to what scientists have found in small-scale laboratory experiments.

The summit lava lake of Kīlauea volcano in May 2018, roughly 220 meters below the crater rim. (Image credit: USGS)

Specifically, the new results provide an upper bound for an important factor in earthquake mechanics known as slip-weakening distance, which geophysicists use to calculate how faults become unstuck. This is the distance over which the frictional strength of a fault weakens before rupturing – something that’s central to accurate modeling of the stability and buildup of energy on earthquake faults. Laboratory experiments have suggested this distance could be as short as tens of microns – equivalent to the width of a hair spliced into a few dozen slivers – while estimates from real earthquakes indicate it could be as long as 20 centimeters.

The new modeling now shows this evolution occurs over no more than 10 millimeters, and possibly much less. “The uncertainties are bigger than they are in the lab, but the friction properties are completely consistent with what’s measured in the laboratory, and that’s very confirming,” Segall said. “It tells us that we’re okay taking those measurements from really small samples and applying them to big tectonic faults because they held true in the behavior we observed in Kīlauea’s collapse.”

The new work also adds realistic complexity to a mathematical piston model, proposed a decade ago by Japanese volcanologist Hiroyuki Kumagai and colleagues, to explain a large caldera collapse on Miyake Island, Japan. While the widely embraced Kumagai model assumed the volcano’s rock surfaces changed as if by flipping a switch from being stationary relative to each other to slipping past one another, the new modeling recognizes that the transition between “static” and “dynamic” friction is more complex and gradual. “Nothing in nature occurs instantaneously,” Segall said.