Volcano Watch: Kīlauea Summit experiencing seismic swarms in October

The most recent eruption at Kīlauea’s summit began on Sept. 10, 2023. Following weeks of heightened unrest, fissures opened on the Halema’uma’u crater floor and to the east on the “down-dropped block,” a remnant of the older caldera floor that collapsed in 2018.

The eruption was short-lived, ending just a week later, on Sept. 16. Lava stopped flowing. Gas emissions diminished. Seismicity and ground deformation returned to “background” levels. Things got quiet. But not for long.

A new series of earthquake swarms began suddenly at Kīlauea’s summit on Oct. 4, 2023. Activity escalated quickly and more than 250 earthquakes were recorded in the south caldera region on Oct. 5. Seismicity at this scale has been observed prior to recent Kīlauea summit eruptions, but as we know a new eruption did not begin in early October.

Instead, intermittent seismic swarms have continued, varying from less than 20 events per day to more than 150 events on Oct. 22. Most earthquakes have been smaller than magnitude-2 and have occurred at depths of around 0.6 to 2 miles below the surface. Seismic signals indicating magma movement — such as low-frequency tremor — have also been observed, most recently on Oct. 23.

Ground deformation rates increased in early October as well. The Sand Hill tiltmeter, located southwest of the caldera, has recorded approximately 120 microradians of change over the past three weeks.

GPS and satellite radar data confirm that more than 4 inches of uplift has occurred in the south caldera region since late September. Northeast of the caldera, the Uēkahuna tiltmeter has recorded approximately 20 microradians of change over the past three weeks.

Deformation has also been intermittent, with days-long periods of increased rates alternating with similar-duration periods of little or no displacement. Increased numbers of earthquakes have generally coincided with increased rates of ground motion at Sand Hill, as seen on October 4–6, 16–18 and 21–23.

These observations indicate that magma is accumulating in the geologically complex south caldera region. Examining history, this is not surprising.

Numerous intrusions have been recorded here in the past, most recently in 2021 and 2015. In August 2021, an intrusion here occurred over about a week and was followed by an eruption within Halema‘uma‘u about a month later (Sept. 29, 2021). In May 2015, an intrusion here lasted less than a week and occurred during ongoing eruptions within Halema‘uma‘u and at Pu‘u‘ō‘ō.

Intrusions also occurred here in the 1960s, 1970s, early 1980s and in 2006. But only one, in December 1974, led to an eruption. It migrated southward from the caldera and then reached the surface, erupting as a series of short fissure segments with a total length of 3 miles, as it turned southwest.

Of note is the fact that before the 1974 eruption earthquakes had migrated farther southwest than Hawaiian Volcano Observatory has observed during recent unrest.

Although it is not possible to forecast an exact outcome from current unrest, here are three possible scenarios that could play out in the coming days to weeks:

• Magma could continue to accumulate but eventually stop, with no eruption.

• Magma could continue to accumulate, followed by a summit eruption inside the caldera—similar to recent eruptions at Halema‘uma‘u. If this happens, we expect to see accelerating deformation and seismicity beneath the caldera 1 to 2 hours before an eruption.

• Magma could continue to accumulate, with an eventual summit eruption outside the caldera, to the south or southwest. If this happens, we expect to see earthquakes migrating southward as they did in December 1974, followed by accelerating deformation and seismicity 1 to 2 hours before an eruption.

The timeframe for any scenario is uncertain. With permission from Hawai‘i Volcanoes National Park, the observatory has installed two new webcams to better monitor the south caldera region: MITDcam – Kīlauea’s upper Southwest Rift Zone, looking north and S2cam – Kīlauea’s upper Southwest Rift Zone, looking southwest. Considering potential volcanic hazards, the National Park has closed trails near areas of unrest.

Unrest continues at Kīlauea summit. Hawaiian Volcano Observatory is watching closely. Time will tell what happens next.

Editorʻs Note: Volcano Watch is a weekly article and activity update written by US Geological Survey Hawaiian Volcano Observatory scientists and affiliates.