Seismic Swarm S20230324.2: Analysis of Earthquake Activity on Hawaiʻi Island

Seismic swarm S20230324.2 was recorded on the Island of Hawaiʻi from 20:30 on 23 March 2023 to 17:47 on 23 May 2023. Over 1461 hours and 16 minutes, the swarm comprised 740 earthquakes. This event occurred within a region shaped by the Hawaiian hotspot, where the Pacific Plate moves over a mantle plume, fueling ongoing volcanism at Kīlauea and Mauna Loa volcanoes.

Hawaiʻi Island lies at the southeastern end of the Hawaiian-Emperor seamount chain. Magma ascent, crustal deformation, and fault slip along rift zones and flank structures generate frequent seismicity. Depths of 30–36 km typically reflect events within the volcanic edifice or deeper magmatic system, while shallower activity may indicate surface fracturing or hydrothermal processes. The 2018 lower East Rift Zone eruption and summit collapse at Kīlauea demonstrated how magma movement can trigger intense seismic sequences, reshaping the landscape and providing modern analogs for swarm behavior.

Historical records since 2000 document 64 swarms on the island. Annual counts show variability: 2000 (2), 2003 (5), 2004 (4), 2005 (1), 2006 (1), 2007 (3), 2008 (1), 2011 (1), 2012 (2), 2013 (1), 2014 (1), 2015 (4), 2016 (2), 2017 (2), 2018 (8), 2019 (3), 2020 (11), 2021 (9), and 2022 (3). Elevated activity in 2018, 2020, and 2021 aligns with documented eruptive and intrusive episodes.

Examination of the first 100 events reveals a dominant pattern of low-magnitude earthquakes. Magnitudes ranged primarily from 1.3 to 2.7, with most events between 1.6 and 2.3. Depths clustered between 29 and 36 km during the initial days, consistent with mid-crustal magmatic or tectonic sources. On 24 March, several events occurred at depths of 0–7 km, including magnitudes 2.3 at the surface and 2.5 at 5 km, suggesting possible shallow fracturing. Similar shallow signals appeared sporadically through 29 March. Later events in the sequence maintained the deeper focus, with occasional repetitions at identical times and depths indicating closely spaced sources or aftershock-like sequences.

This distribution points to an initial phase of deeper unrest followed by intermittent shallow adjustments. Such patterns are characteristic of fluid or magma migration beneath the island’s volcanic centers. No large-magnitude events dominated the early swarm phase, aligning with the overall low-energy release typical of Hawaiian swarms driven by volcanic rather than tectonic forces.

Continued monitoring by the U.S. Geological Survey Hawaiian Volcano Observatory remains essential for interpreting whether future swarms precede renewed eruptive activity. The 2023 sequence adds to the long-term record of seismic unrest that defines Hawaiʻi’s dynamic geology.

References

• U.S. Geological Survey, Hawaiian Volcano Observatory: Hawaiian seismicity and volcanic history reports.
• U.S. Geological Survey Earthquake Catalog: Regional earthquake data for the Island of Hawaiʻi.