Volcano Watch: Understanding magma storage and migration in the active East Rift Zone of the Big Island’s Kīlauea

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. Today’s article is written by HVO geophysicist Ninfa Bennington and colleagues from ETH Zürich Federica Lanza and Alicia Rohnacher.

As signals created by earthquakes move through the ground, they are influenced by the structure of a volcano, including the presence of magma and/or fault zones.

These structures can cause the seismic waves to travel faster or slower, which is recorded on seismometers. Seismologists can use that data to create images of where magma is located and track its underground path.

The Hawaiian Volcano Observatory has about 80 permanent seismometers on the Island of Hawaiʻi.

Using only data from these permanent seismometers provides a fuzzy picture of underlying magma storage structures.

However, if the number of seismometers at the surface is increased, more of the seismic waves traveling through regions of magma storage will be recorded, yielding a crisper picture of the subsurface.

In late June, seismologists from ETH (Eidgenössische Technische Hochschule) Zürich and the Hawaiian Volcano Observaory deployed 115 seismic nodes — tiny, portable seismometers — across Kīlauea’s East Rift Zone, fortuitously before significant unrest began.

Data recorded on these nodes will be used to image the location and volume of magma within the East Rift Zone at a level of detail not previously possible, and the resulting model will help us better understand the volcanic hazards in this region.

Given the timeline of their deployment, they recorded earthquakes associated with intrusions of magma into the East Rift in July and August, as well as the September 15-20 middle East Rift Zone eruption.

In fact, 4 of the nodes had to be rescued from the field to prevent them from being impacted by the recent eruption.

These densely spaced seismic instruments will continue to record through October.

ETH Zürich and Hawaiian Volcano Observatory seismologists are working together now to analyze a subset of data from the nodes already collected from late June through late August.

Specifically, they are using an analysis tool called ambient noise interferometry, which takes advantage of continuous seismic signals created through the interaction between ocean swells and the ocean crust, to identify what was happening below the surface leading to the September eruption.

Magma moving through a volcano opens and closes fracture systems causing changes in the speed at which ocean noise signals travel through the ground. Scientists can monitor these ocean noise signals for signs that magma is accumulating beneath the surface.

Ocean noise traveling through the ground below the upper East Rift Zone of Kīlauea volcano between early July and late August showed changes as magma began to enter this area.

The most dramatic change we observed is a rapid velocity decrease that began July 21, indicating the opening of cracks and fractures because of magmatic intrusions in this region.

At the same time, swarms of earthquakes were occurring because of stresses created from intrusion of magma into the subsurface.

This example shows how ambient noise interferometry, along with other volcano monitoring datasets, can be used to understand the changes occurring beneath the surface of a volcano.

While this example focuses on changes in velocity at a single pair of nodes, future analysis will be carried out for the entire 115 instrument array.

This complete analysis will contribute to our understanding of where magma migrated across the East Rift Zone in the time leading to the September 2024 eruption.

Volcano Activity Updates

Kīlauea is not erupting. Its Volcano Alert level is at Advisory.

The middle East Rift Zone eruption that began Sept. 15 near Nāpau Crater, within Hawaiʻi Volcanoes National Park, ended Sept. 20.

In total, more than 156 acres were covered by lava that erupted from a fissure system more than 1 mile long.

Since the eruption ended, rates of ground deformation and earthquakes in the summit and East Rift Zone eruption area have decreased, with about 40 earthquakes located in Kīlauea summit region and 50 in the upper-to-middle East Rift Zone.

Middle East Rift Zone sulfur dioxide (SO2) emission rates have decreased since the eruption ended, with no SO2 detected from the eruption site Sept. 23.

The most recent measurement of SO2 emission rate at the summit was 60 tonnes per day on Sept. 17.

Mauna Loa is not erupting. Its Volcano Alert Level is at Normal.

Three earthquakes were reported felt around Hawaiʻi Island during the past week:

• Magnitude-3.2 earthquake 19 miles south-southwest of Pa‘auilo at at a depth of 11 miles Sept. 25 at 1:01 p.m.
• Magnitude-4.0 earthquake 2 miles south of Pāhala at a depth of 20 miles Sept. 22 at 6:21 a.m.
• Magnitude-3.1 earthquake 14 miles south of Volcano at a depth of 19 miles Sept. 19 at 9:31 p.m.

The Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the observatory’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.