New Earthquake Swarm Underway Near Pāhala, Hawaiʻi

A new earthquake swarm, designated S20260701.1, has been detected on the Island of Hawaiʻi. The swarm began on June 30, 2026, at 18:51 UTC, with its epicenter located approximately 2 kilometers southwest of the town of Pāhala in the Kaʻū district. Over the initial 41 hours, the seismic network registered 32 earthquakes associated with this sequence.

This activity is occurring in a region well-known for its persistent and deep seismic events. The Pāhala area is one of the most seismically active regions in Hawaiʻi and is closely monitored by scientists for insights into the deep magmatic processes that fuel the island's active volcanoes, primarily Kīlauea and Mauna Loa.

Geological Setting of the Pāhala Seismic Zone

The Island of Hawaiʻi sits atop a mantle plume, or "hotspot," a stationary source of immense heat rising from deep within the Earth. As the Pacific Plate moves northwestward over this hotspot, magma is generated, rises through the mantle and crust, and erupts on the seafloor, building the Hawaiian Islands over millions of years.

The earthquakes beneath Pāhala are distinct from the shallow seismic activity typically associated with volcanic inflation or magma movement directly beneath the calderas of Kīlauea and Mauna Loa. The Pāhala events occur at significant depths, generally between 20 and 50 kilometers (12 to 31 miles) below sea level. This places them deep within the upper mantle, near the boundary between the rigid oceanic lithosphere and the more ductile asthenosphere below.

The leading scientific hypothesis for this deep seismicity is that it is caused by the movement of magma through a complex network of conduits originating from the mantle hotspot. As magma forces its way upward, it stresses the surrounding rock, causing the small to moderate earthquakes that are frequently observed in swarms. This deep plumbing system is believed to be the primary pathway that transports magma to the shallower reservoirs beneath Kīlauea and Mauna Loa.

Characteristics of Swarm S20260701.1

The current swarm exhibits characteristics consistent with previous seismic activity in the region, though with some notable features. The 32 earthquakes recorded in the initial period ranged in magnitude from 1.4 to 3.3. The strongest event, a magnitude 3.3, occurred on July 1, 2026, at a depth of 31 kilometers. Due to their small magnitudes and significant depths, most of these earthquakes are not felt by residents.

The depth of the events in this swarm is particularly informative. The vast majority of the earthquakes, 29 of the 32, occurred at depths between 26 and 50 kilometers, firmly within the deep Pāhala seismic zone. However, the dataset also includes two unusually shallow events recorded at 0 kilometers depth. While deep seismicity is the hallmark of the region, such shallow events can indicate minor crustal adjustments possibly triggered by the deeper stresses.

A breakdown of the notable events includes:

• First event: Magnitude 1.7 at 28 km depth on June 30.
• Strongest event: Magnitude 3.3 at 31 km depth on July 1.
• Deepest event: Magnitude 2.3 at 50 km depth on July 1.
• Shallowest events: Two magnitude 1.8 events at 0 km depth on June 30 and July 2.

Historical Context and Increasing Frequency

Earthquake swarms are not a new phenomenon for Pāhala. SeismoSight records indicate that 36 distinct swarms have occurred in this area since January 2000. However, the frequency of these swarms has shown a marked increase in recent years.

Historical data reveals the following trend in the number of swarms per year:

• 2014: 1
• 2015: 1
• 2018: 4
• 2019: 2
• 2020: 7
• 2021: 4
• 2022: 2
• 2023: 8
• 2024: 6

This trend, particularly the elevated activity since 2019, has been noted by the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO). This sustained increase in deep seismicity suggests a potential increase in the supply of magma from the mantle hotspot into the volcanic plumbing system of the island.

While these swarms are a key indicator of the dynamic geological processes occurring deep beneath Hawaiʻi, they are not considered a direct precursor to an eruption. They represent the long-term, foundational process of magma transport that feeds the volcanoes. Scientists at HVO continuously analyze data from seismometers, GPS stations, and other monitoring tools to assess any changes that might signal a more immediate volcanic hazard. The current swarm is being closely watched as part of this ongoing effort to understand and forecast volcanic activity on the Island of Hawaiʻi.

References

• U.S. Geological Survey, Hawaiian Volcano Observatory (HVO). (2024). Website and Updates. Retrieved from https://www.usgs.gov/observatories/hawaiian-volcano-observatory
• Mattox, S. (2021, May 20). "Volcano Watch: Why do so many deep earthquakes happen beneath Pāhala?". U.S. Geological Survey. Retrieved from https://www.usgs.gov/observatories/hvo/news/volcano-watch-why-do-so-many-deep-earthquakes-happen-beneath-pahala
• Gonnermann, H. M., & Wright, T. J. (2022). "The Pāhala deep earthquake swarm: Aseismic slip on a décollement fault driven by mantle melt". Science Advances, 8(33), eabn1855.
• Neal, C. A., & Brantley, S. R. (2020). "The 2018 eruption of Kīlauea Volcano, Hawaiʻi". U.S. Geological Survey. Retrieved from https://www.usgs.gov/news/preliminary-summary-kilaueas-2018-lower-east-rift-zone-eruption-and-summit-collapse