Volcano Watch — Another intrusion southwest of Kīlauea’s summit

Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

Last week, USGS Hawaiian Volcano Observatory (HVO) scientists were closely monitoring earthquakes and ground deformation in the region southwest of Kīlauea’s summit. The increased unrest prompted HVO to raise the Alert Level/Aviation Color Code for Kīlauea to WATCH/ORANGE on January 31 as another intrusive event began beneath the surface.

Intrusions are when magma breaks rock to create new pathways within the ground. As magma moves beneath the surface into its new space, the ground above it deforms to accommodate the new material. HVO detects intrusions through earthquake locations (which occur where rocks break) and changes in the ground surface, known as deformation (which can be recorded using tiltmeters, GPS, and satellite-based methods).

Earthquake swarms south of Kīlauea caldera began over the weekend, on Jan. 27. This activity was not surprising, as intermittent earthquake swarms have been occurring in the south caldera since October 2023, and there is a known body of magma in this area.

On Jan. 31, however, earthquake activity significantly increased, with over 700 earthquakes detected throughout the day. The number of actual earthquakes was likely much greater, as many of the smallest earthquakes went undetected in the intense swarm of activity.

Coincident with the increased earthquake activity, tiltmeters at Uēkahuna Bluff (the summit of Kīlauea) and Sand Hill (southwest of the caldera) began to show changes to the ground surface. Starting just before 4:30 a.m. in the morning, the tiltmeters began to show highly variable directions and rates, indicative of crack growth that can precede either an eruption or intrusion. The increased earthquake activity and complex tilt patterns prompted HVO to raise the Alert Level/Aviation Color Code for Kīlauea to WATCH/ORANGE at 4:41 a.m.

HVO seismologists were analyzing earthquake locations in real-time. Earthquakes beneath Halema‘uma‘u might signal potential for an eruption there (as occurred in the hours prior to recent eruptions in Halema‘uma‘u). HVO geologists and volcanic gas specialists poised themselves on the rim of Kīlauea caldera, ready to collect data in the event of a new eruption.

However, earthquake locations instead migrated farther to the southwest, along the Koa‘e fault system. By the evening, the tiltmeters showed a more consistent deflationary signal. Together, the two monitoring datasets indicated that magma had opened a new pathway and was moving from the storage system below Kīlauea summit to the southwest.

These observations were confirmed later with satellite-based InSAR data, which showed that between 6 p.m. on Jan. 31 through 6 p.m. on Feb. 1, 2024, Kīlauea’s summit area deflated as the region southwest uplifted (up 20 inches) and spread apart. In the center of the uplifted area, a narrow band of subsidence marks where the dike (a vertical sheet of magma) intruded, with a small section of ground dropping to accommodate lateral spreading.

On Feb. 2, the number of earthquakes began to decrease, along with ground deformation rates. On the morning of Feb. 3, HVO lowered the Alert Level/Aviation Color Code for Kīlauea back down to ADVISORY/WATCH.

While there have been several intrusions in the south caldera region of Kīlauea recently—in December and October 2023, 2021, and 2015—the recent intrusion extended farther southwest, towards Pu‘ukoa‘e and the Kamakai‘a Hills on the Southwest Rift Zone of Kīlauea. As a result of the intrusion, surface cracks, extending tens of meters in length and centimeters (inches) in width, formed in loose Keanakāko‘i tephra along the Maunaiki trail near Twin Pit Craters in the Kaʻū Desert of Hawai‘i Volcanoes National Park.

The south caldera area and Koa‘e fault system are informal names for specific geographic areas that we see on the surface of Kīlauea, but they extend into the subsurface. There is a well-known magma body beneath the south caldera region (the largest magma storage region of Kīlauea) and the Koa‘e faults are structural features related to Kīlauea’s south flank. How these features are connected beneath the surface, to each other, and to the Southwest Rift Zone of Kīlauea is not well understood.

Intrusions are a common process that volcanoes around the world experience, akin to a pressure-release valve for the magma reservoir. Although there hasn’t been an eruption at Kīlauea since September 2023, the recent intrusion shows us that the volcano has been very busy beneath the surface.

Volcano Activity Updates

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

Seismicity at Kīlauea’s summit and along the Koa‘e fault system southwest of the caldera continues to decrease following an intrusion of magma into the area that occurred Jan. 31-Feb. 1, 2024. Tiltmeters near Sand Hill and Uēkahuna bluff have recorded little change over the past week, both show mild deflationary trends. Periods of increased earthquake activity and rates of ground deformation can be expected to continue in this region. No unusual activity has been noted along the rift zones.

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

Webcams show no signs of activity on Mauna Loa. Summit seismicity has remained at low levels over the past month. Ground deformation indicates continuing slow inflation as magma replenishes the reservoir system following the 2022 eruption. SO2 emission rates are at background levels.

One earthquake was reported felt in the Hawaiian Islands during the past week: a M3.6 earthquake 6 miles east of Pāhala at 1 mile depth on Feb. 2 at 8:33 p.m.

HVO continues to closely monitor Kīlauea and Mauna Loa.

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 [email protected].