Seismic Swarm S20041223.1: Analysis of the December 2004 Event Near Volcano, Hawaii

Seismic swarm S20041223.1 was recorded south of Volcano, Hawaii, beginning at 17:07 on 22 December 2004 and concluding at 21:47 on 24 December 2004. Over 52 hours and 39 minutes, the sequence produced 32 earthquakes. The events clustered within a compact area approximately 12 km south of Volcano, consistent with shallow crustal processes beneath the southern flank of Kilauea volcano.

The swarm initiated with a magnitude 4.0 earthquake at 3 km depth, followed by a rapid succession of smaller events. Magnitudes ranged from 1.2 to 4.0, with the majority between 1.3 and 2.3. Depths remained predominantly shallow (0–8 km), although two outliers reached 42 km and 7 km. The temporal distribution showed highest activity in the first 12 hours, tapering over the subsequent two days. This pattern aligns with fluid-driven or magma-related triggering common in volcanic settings.

Hawaii’s geology is defined by hotspot volcanism. The Hawaiian-Emperor seamount chain formed as the Pacific plate moved over a mantle plume, producing the Big Island’s five major volcanoes. Kilauea, located on the southeastern flank, remains one of the world’s most active volcanoes. Its south flank experiences frequent seismicity due to magma intrusion, gravitational spreading, and faulting along the Hilina fault system. Earthquake swarms in this region often reflect dike propagation or pressurization of the shallow magmatic system rather than tectonic plate boundary motion.

Historical records maintained by SeismoSight indicate 15 swarms have occurred in the area since 1 January 2000. Yearly counts include three swarms in 2000, one in 2001, eight in 2003, and three in 2004. The December 2004 swarm represents the final sequence recorded that year. Such recurrent swarms underscore the persistent volcanic-tectonic interaction beneath Kilauea’s southern sector.

Analysis of swarm S20041223.1 reveals several notable characteristics. The initial magnitude 4.0 event likely served as a trigger, releasing accumulated strain and facilitating subsequent smaller ruptures. Most events clustered at depths of 1–2 km, suggesting activity within the volcanic edifice rather than deeper mantle sources. The brief duration and modest total count differentiate this swarm from longer, higher-rate sequences sometimes associated with major eruptive episodes.

The geological setting explains the observed parameters. Kilauea’s east rift zone and south flank accommodate extension through repeated intrusion and fault slip. Swarms of this scale typically indicate minor magma movement or hydrothermal fluid migration without leading to surface eruption. No significant deformation or eruptive activity was linked to this specific sequence.

Continued monitoring of similar swarms remains essential for understanding Kilauea’s behavior. The December 2004 event contributes to a growing catalog that helps refine models of volcanic seismicity and improves hazard assessment for nearby communities.

References

USGS Hawaiian Volcano Observatory reports on Kilauea seismicity.
Hawaii Volcanoes National Park geological summaries.
SeismoSight internal swarm classification database.