What's causing the near-daily collapses and 'earthquakes' at Halemaumau?

KILAUEA VOLCANO, BIG ISLAND (HawaiiNewsNow) - Simply put, it's the movement of magma. That's what's causing the face of Halemaumau Crater to change almost every day, according to USGS scientists.

For over three months, Hawaii Island residents near Kilauea's summit have dealt with ongoing ground shaking, civil defense alerts and light ashfall.

The rumbles often release energy equivalent to a 5.2- to 5.4-magnitude earthquake. Since May, scientists have recorded almost 60 of those magnitude-5 or higher tremblors.

By now, it's known these "earthquakes" don't generate a tsunami threat and the shaking is short-lived.

But according to U.S. Geological Survey scientists, they're not traditional earthquakes (in which plates underground move). And they're no longer caused by steam-driven ash explosions as they were in previous months.

USGS scientist Janet Babb says what triggers the temblors are "collapse events" at Halemaumau.

Here's the science behind them, according to Babb:

Magma moves from the summit reservoir, an area that acts as the crater's magma storage system. This is the magma that's moving down the east rift zone and erupts in lower Puna, likely at fissure no. 8.
As magma moves out of the reservoir, stress builds on the layers of rock above. This stress results in a handful of smaller earthquakes, around magnitude 3.0 to 4.0. These smaller quakes happen about 25 times a day.
"Eventually, enough magma is removed that the overlying rock collapses downward, causing a pressure pulse that releases energy equivalent to a magnitude-5+ earthquake and causes shaking felt by nearby residents," Babb said.
After each collapse, seismic activity is reduced and the process starts over again. This cycle happens every 30 to 40 hours.

The photo below shows the abundant lava lake at Halemaumau Crater on April 6, before the frequent explosions and eruptions began.

The lava in the above photo has since drained, and the drastic changes are noted in the aerial photo below, taken on July 13.

Over a period of just more than three months, the surrounding walls now appear fragmented and weak, as if ready to fall in at any moment.

The surrounding area is dusted in a thick layer of ash from previous plumes that sent volcanic material thousands of feet in the air.

Babb added that because these collapses and rumbles are not caused by ash explosions, plumes of ash are no longer seen and pose little to no widespread threat to residents.

"The remaining ash concern is the existing ash deposits and rock dust that are stirred up by slumping and rockfalls within Halemaumau and along the summit caldera walls. It is possible that trace (tiny) amounts of this ash/dust can be blown by wind, perhaps reaching downwind areas," Babb said, in an email.

For now, this is the cycle Halemaumau is trapped in: A 30-40 hour process of magma movement, collapse and shaking.

There's no telling when Kilauea will fall silent again. But for now, occurrences like this will continue almost daily.