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.

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As signals created by earthquakes move through the ground, they are influenced by the structure of the volcano, including the presence of magma and/or fault zones. These structures can cause the seismic waves to travel faster or slower inside the Earth and are recorded on seismometers. Seismologists can take advantage of these changes, to create images of where magma is located and to track its path underground.

The USGS Hawaiian Volcano Observatory (HVO) 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 2024, seismologists from ETH Zürich and HVO deployed 115 seismic nodes (tiny, portably seismometers) across Kīlauea’s ERZ, fortuitously before significant unrest began in the ERZ. Data recorded on these nodes will be used to image the location and volume of magma within the ERZ at a level of detail not previously possible, and the resulting model will help us to better understand the volcanic hazards in this region. 

These densely spaced seismic instruments will continue to record seismic signals through October 2024. Given the timeline of their occupation, they recorded earthquakes associated with intrusions of magma into the ERZ in July and August, as well as the September 15–20 eruption in the middle ERZ. In fact, four of the nodes had to be rescued from the field to prevent them from being impacted by the recent eruption. 

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While the dense seismic array continues to record data, scientists have collected a subset of data from these instruments including late June through late August 2024. ETH and HVO seismologists are working together to analyze these newly collected data. 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 that 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 ERZ of Kīlauea volcano between early July and late August 2024 showed changes as magma began to enter this area. The most dramatic change we observe is a rapid velocity decrease that begins on July 21, indicating the opening of cracks and fractures due to magmatic intrusions in this region. At the same time, swarms of earthquakes were occurring due to 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, which spans Kīlauea’s ERZ. This complete analysis will contribute to our understanding of where magma migrated across the ERZ in the time leading to the September 2024 eruption. 

Volcano Activity Updates

Kīlauea is not erupting. Its USGS Volcano Alert level is ADVISORY.

The middle East Rift Zone eruption of Kīlauea that began September 15 around Nāpau Crater, within Hawaiʻi Volcanoes National Park, ended on September 20. In total, more than 156 acres were covered with new rock 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 on Monday, September 23. The most recent measurement of SO₂ emission rate at the summit was 60 tonnes per day on September 17, 2024

Mauna Loa is not erupting. Its USGS Volcano Alert Level is at NORMAL.

Three earthquakes were reported felt in the Hawaiian Islands during the past week: a M3.2 earthquake 31 km (19 mi) SSW of Pa‘auilo at 17 km (11 mi) depth on September 25 at 1:01 p.m. HST, a M4.0 earthquake 4 km (2 mi) S of Pāhala at 32 km (20 mi) depth on September 22 at 6:21 a.m. HST, and a M3.1 earthquake 24 km (14 mi) S of Volcano at 32 km (19 mi) depth on September 19 at 9:31 p.m. HST.

HVO continues to closely monitor Kīlauea and Mauna Loa.

Please visit HVO’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.