(BIVN) – Kīluaea volcano is not erupting, and scientists recently toggled the alert level from ADVISORY to WATCH and back to ADVISORY, after a swarm of earthquakes beneath the south part of the volcano caldera started on August 23 and then diminished two days later.

The earthquake activity in the area has since increased again, although the alert level – as of Friday morning – remains at YELLOW / ADVISORY.

In this week’s Volcano Watch article, U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates wrote about the recent activity:

Late Monday afternoon, earthquake activity picked up at Kīlauea’s summit. At about 1:30 a.m. HST on Tuesday, that activity intensified, and it became clear that seismicity and increasing deformation were indicating a new intrusion of magma. The seismicity extended southward from Hālemaʻumaʻu crater, to an area south of Kīlauea caldera.

An “intrusion” occurs when magma is injected into rocks underground, and new intrusions can be the first indication of a potential eruption. Because seismicity indicated the intrusion was within 1–2 km (0.6–1.2 miles) of the surface, the USGS Hawaiian Volcano Observatory (HVO) increased Kīlauea’s Volcano Alert Level and Aviation Color Code from Advisory/Yellow to Watch/Orange at 5:34 a.m. on Tuesday.

Heightened rates of seismicity occurred throughout the day on Tuesday, and summit tiltmeters showed ground motions consistent with continued growth of the intrusion. Seismicity and deformation activity peaked on Tuesday evening; then, both decreased considerably, and the intrusion appeared to have halted.

Top panel shows tiltmeter data from the Sandhill (SDH) instrument, located within Kīlauea’s south caldera region. The line indicates that the instrument tilted towards an azimuth of 300° (northwest) as the ground in the area inflated. Occasional spikes are due to sloshing of instrument’s bubble level during earthquakes. Bottom panel is an hourly histogram of earthquakes in Kīlauea’s summit region. Both show the increase in activity late Monday, August 23, and subsequent slowdown the following Wednesday. USGS plots.

In seeking to understand current events, volcanologists often look to the past. We look for “analog” events that are similar to what is happening and that will give us a sense for what the future might hold. A very close analog event for this week’s activity was an intrusion in 2015.

The 2015 sequence of events started with inflation of the shallow Halemaʻumaʻu magma reservoir, which triggered the overflow of Kīlauea’s summit lava lake onto the floor of the crater. Seismicity then migrated south to the same area that has been active during the past few days. The episode last for about 4 days and also ended with an intrusion in an area south of the modern caldera outline.

These two south caldera intrusions (2015 and 2021) have strikingly similar patterns of seismicity and deformation, even though the 2015 event started quite differently. The patterns of seismicity and deformation suggest that the location and orientation of the intrusions are also quite similar.

Map showing the seismic activity beneath the south part of Kīlauea caldera, within Hawaiʻi Volcanoes National Park, from August 23, 2021, at noon through the same time on August 25, 2021. The earthquake swarm began in that region at around 4:30 p.m. HST on August 23 and continued until the morning of August 25, 2021. Almost 500 earthquakes were detected during this swarm. The map shows earthquake locations, with earthquakes colored by the time that they occurred and their magnitude (see key at bottom of figure). The plots above and below the map show earthquake depth below sea level (0) by longitude (bottom) and depth below sea level (0) by latitude (top), with longitude and latitude lines corresponding to the map. USGS map and plots.

Regional uplift patterns observed by satellite confirm that the center of the intrusive activity in both cases is well to the south of the caldera boundary faults. A third similar event was also recorded in 2006, prior to the formation of the lava lake within Halemaʻumaʻu.

This area is well known for being the beginning of the path that magma follows to the lower regions of the Southwest Rift Zone of Kīlauea. Earthquakes have been used to track the progress of magma intrusions through this pathway and HVO scientists call this “the seismic Southwest Rift Zone.”

Magmatic intrusions followed this pathway in the 1960s, 1970s, and early 1980s, but only one of these events led to an eruption. In December 1974, an intrusion began following this path southward, but erupted as a series of short fissure segments with a total length of 5 km (3 miles) as it turned southwest.

The caldera collapse events in 2018 radically changed the surface landscape of Kīlauea’s summit. However, it was unclear how much of the sub-surface magma storage and plumbing might have been changed. The fact that magma is being intruded and stored close to the caldera shows that some of the magmatic connections remain unchanged by the 2018 collapse.

The similarities of the 2015 and 2021 intrusions suggest that at least some of the summit connections are still in place. This is yet another line of evidence that while our view of Kīlauea’s surface is entirely new, the volcano is not as much changed underground.

The magma storage system beneath Kīlauea’s summit has clearly been refilling and inflating as it recovers from the 2018 eruption. Events like the recent summit eruption and this intrusion are the expected signs that the system is repressurizing due to continued input of magma. We expect to see continued intrusive activity and possible eruptive activity near the summit as magma reconnects with old pathways and storage regions.

On Thursday morning, the Volcano Alert Level and Aviation Color Code for Kīlauea was changed again from Watch/Orange, back down to Advisory/Yellow as seismic activity and deformation had returned to pre-intrusive levels within the region. However, at the time of this writing on the evening of August 26, seismic activity and ground deformation had resumed in Kīlauea’s south caldera region. Perhaps, this is only the first chapter in a much longer story!