Seismic Insights: The December 2005 Earthquake Swarm Near Naalehu, Hawaii

The December 2005 earthquake swarm, classified internally as S20051207.1, occurred approximately 39 km east-southeast of Naalehu on Hawaii's Big Island. This sequence began at 05:33 on 7 December 2005 and concluded at 17:19 on 10 December 2005, spanning 83 hours and 45 minutes. During this period, 117 earthquakes were recorded, providing a detailed snapshot of seismic activity in a volcanically active region.

Analysis of the first 100 events reveals predominantly shallow focal depths between 7 and 23 km, with the majority clustered around 10-16 km. Magnitudes ranged from 1.3 to 4.5, featuring a notable peak event of magnitude 4.5 at a depth of 21 km on 7 December at 11:42. Other significant shocks included magnitudes of 3.5, 3.3, and multiple events between 3.0 and 3.2. The temporal distribution showed intense activity in the initial hours, with frequent events every few minutes, gradually tapering over subsequent days. This pattern is characteristic of volcanic swarms, where fluid migration or magma movement induces repeated fracturing rather than a single large rupture.

The Naalehu area lies within the southeastern sector of the Island of Hawaii, influenced by the Hawaiian hotspot. This geological setting features active shield volcanoes, including nearby Kilauea and Mauna Loa, whose flanks experience persistent seismicity due to gravitational spreading, magma intrusion, and faulting along the south flank. Depths in the 10-20 km range align with the brittle-ductile transition zone in the oceanic crust beneath the volcanic edifice. Historical context indicates limited swarm occurrences in recent decades, with the prior event noted in 2001, underscoring the episodic nature of such clusters amid background tectonic and volcanic stresses.

Hawaii's seismic history reflects ongoing volcanic construction, with the archipelago formed over millions of years by hotspot volcanism. The 2005 swarm fits within this framework, where low-to-moderate magnitude events often precede or accompany eruptive phases without widespread surface damage. Updated monitoring by regional networks continues to track similar activity, emphasizing the importance of real-time data for hazard assessment in this dynamic environment.

References

USGS Earthquake Hazards Program reports on Hawaiian seismicity.
Hawaii Volcano Observatory geological summaries of Kilauea and Mauna Loa activity.