Seismic Swarm S20231213.2: Analysis of Activity Southwest of Volcano, Hawaii

Seismic swarm S20231213.2 occurred from 22:12 on 12 December 2023 to 02:41 on 14 December 2023, approximately 11 km southwest of Volcano, Hawaii. Over 28 hours and 28 minutes, the event registered 38 earthquakes, all at shallow depths between 0 and 2 km. Magnitudes ranged primarily from 1.1 to 2.9, with the strongest events reaching 2.9 on three occasions.

This swarm unfolded in the Southwest Rift Zone of Kilauea volcano, a region shaped by Hawaii's position over a mantle hotspot. The Hawaiian Islands formed through prolonged volcanic activity as the Pacific Plate moves northwest over a fixed hotspot, producing a chain of shield volcanoes. Kilauea, one of the world's most active, features frequent magma-driven seismicity due to intrusions along rift zones. Shallow depths in this swarm align with typical patterns of magma migration or pressurization within the volcanic edifice.

Key phases included an initial cluster on 12 December with events up to magnitude 2.9, followed by sustained low-magnitude activity through 13 December. Notable peaks occurred around midday and evening on 13 December, including magnitudes of 2.7 and 2.9. Activity tapered with isolated events early on 14 December before concluding. Such sequences reflect episodic stress release common in rift environments where magma movement induces fracturing in the brittle upper crust.

Historical records since 2000 show 103 swarms in the region, with notable increases in frequency during 2018 (9 swarms), 2020 (13 swarms), and 2023 (20 swarms). This trend correlates with ongoing volcanic unrest at Kilauea following the 2018 caldera collapse and subsequent summit activity. Earlier decades recorded fewer events, such as 8 in 2003 and 5 in 2004, underscoring variability tied to eruptive cycles.

Swarm events like S20231213.2 provide insight into subsurface dynamics. The predominance of magnitudes below 3.0 and consistent shallow foci indicate localized adjustments rather than large-scale tectonic rupture. Monitoring such patterns aids in distinguishing magmatic from tectonic signals in this high-volcanic-hazard setting.

References SeismoSight internal swarm classification S20231213.2 USGS Hawaiian Volcano Observatory geological summaries on Kilauea rift zones