Seismic Swarm S20201225.1 Near Pāhala, Hawaii: Geological Context and Event Analysis

The seismic swarm designated S20201225.1 occurred approximately 10 km east-northeast of Pāhala on the Island of Hawaii. It initiated at 13:22 on 24 December 2020 and concluded at 13:06 on 2 February 2021, spanning 959 hours and 43 minutes. During this interval, 662 earthquakes were recorded. This activity aligns with the persistent seismic patterns driven by Hawaii's volcanic systems.

Hawaii lies above a mantle hotspot that has generated the Hawaiian-Emperor seamount chain over millions of years. The Big Island's volcanoes, including Kilauea and Mauna Loa, exhibit frequent seismic swarms linked to magma intrusion and movement within the crust and upper mantle. Pāhala sits on the southern flank of Kilauea, where tectonic stresses from volcanic loading and gravitational spreading contribute to elevated seismicity. Earthquake depths in this swarm, primarily 30–37 km, reflect processes occurring near the base of the volcanic edifice and within the underlying lithosphere.

Seismic swarms have characterized the region since at least 2000, with 49 documented episodes through 2020. Annual counts vary: 2000 recorded 2 swarms, 2003 and 2004 each had 5, while 2018 reached 7 and 2020 totaled 10. These events often precede or accompany volcanic unrest, providing indicators of subsurface magma dynamics.

Analysis of the first 100 events reveals a concentration of low-to-moderate magnitudes between 1.5 and 3.5. Depths remained consistent around 30–37 km, with the majority clustered near 32–34 km. The sequence began with events of magnitude 1.9 and 1.8 on 24 December, followed by a notable pair of 3.3 and 3.5 events early on 25 December. Subsequent activity showed steady repetition of magnitudes near 2.0–2.3, interspersed with occasional larger shocks up to 2.8. This pattern suggests episodic stress release without immediate escalation to higher magnitudes.

The swarm's duration and event count underscore the value of continuous monitoring in Hawaii's dynamic volcanic environment. Such sequences contribute to refined models of magma transport and flank stability along the island's southern coast.

References
USGS Hawaiian Volcano Observatory reports on regional seismicity.
Hawaii Earthquake Information from the USGS Earthquake Hazards Program.
SeismoSight internal swarm classification data.