A new earthquake swarm, designated S20260721.1, has commenced in the seismically active region beneath the southern part of the Island of Hawaiʻi. The swarm began at 14:04 HST on July 20, 2026, with its epicenter located approximately 4 kilometers (2.5 miles) south-southwest of the town of Pāhala. In its first 22 hours, the Hawaiian Volcano Observatory (HVO) network registered 24 small-magnitude earthquakes associated with this event.
The earthquakes in this swarm have been relatively small, with magnitudes ranging from 1.7 to a peak of 2.9. The largest event, a magnitude 2.9, occurred at 03:06 HST on July 21. A defining characteristic of this seismic sequence is its depth. With the exception of a single shallow event (magnitude 2.1 at 0 km depth), the earthquakes are clustered deep within the Earth's lithosphere, at depths between 27 and 33 kilometers (17 to 20.5 miles). This deep origin means the earthquakes are too small and too far beneath the surface to be felt by residents.
The Island of Hawaiʻi owes its existence to a mantle plume—a stationary upwelling of hot rock from deep within the Earth's mantle. As the Pacific Plate moves northwestward over this hotspot, magma penetrates the oceanic crust, building the chain of volcanic islands. The Pāhala region sits directly above a critical part of this deep magmatic plumbing system.
The persistent and often intense seismicity beneath Pāhala is not caused by the shallow faulting associated with volcanic eruptions at the surface of Kīlauea or Mauna Loa. Instead, scientists believe these deep earthquakes are the result of stresses generated by the movement of magma from the mantle hotspot upwards into the deep roots of the volcanoes. As magma forces its way through the rigid oceanic lithosphere, it creates stress changes that are released as small earthquakes. The location of these swarms, at the base of the lithosphere, provides a unique geophysical window into the processes that supply magma to Hawaiʻi's active volcanoes.
The depths of the current swarm, primarily between 30 and 33 km, are consistent with this interpretation and align with the location of a well-documented zone of deep, long-period earthquakes that has been active for decades.
This latest swarm is not an isolated event but part of a significant, well-documented increase in seismic activity in the Pāhala region over the last several years. Since the beginning of 2000, a total of 38 distinct earthquake swarms have been identified in this area. The frequency of these swarms has shown a marked acceleration since 2019.
Historical data reveals the trend:
The current event is the second swarm recorded in 2026, continuing this pattern of elevated activity. Scientists at the HVO interpret this sustained increase in deep seismicity as a potential indicator of changes in the magma supply from the mantle. This could involve a greater volume of magma, a change in the pathways it travels, or variations in pressure within the deep magmatic system that feeds both Kīlauea and Mauna Loa.
While this activity does not signal an imminent eruption, it underscores the dynamic and ever-changing nature of the volcanic processes deep beneath the Hawaiian Islands. The continuous monitoring of the Pāhala seismic zone is crucial for understanding the long-term behavior of Hawaiʻi's volcanoes and for assessing future volcanic hazards. The S20260721.1 swarm serves as another valuable dataset, offering further insight into the fundamental geological forces that continue to shape the island.