Seismic Swarm S20231207.1 Near Pāhala, Hawaii
A notable seismic swarm, designated S20231207.1, occurred 13 km northeast of Pāhala on Hawaii's Big Island. The sequence began at 18:42 on 6 December 2023 and concluded at 05:35 on 17 December 2023, spanning 250 hours and 52 minutes. During this period, 136 earthquakes were recorded. This activity aligns with the region's established pattern of episodic seismicity driven by volcanic and tectonic processes.
Pāhala lies on the southern flank of Kīlauea volcano within the Kaʻū district. The area's geology is shaped by active shield volcanism, where magma movement, dike intrusions, and gravitational spreading of the volcanic edifice generate frequent earthquakes. Depths in the swarm ranged from near-surface events to approximately 43 km, reflecting both shallow crustal faulting and deeper magmatic interactions. Magnitudes remained modest, predominantly between 1.0 and 2.7, with one event reaching 3.6.
Analysis of the first 100 recorded events reveals two primary depth clusters. A significant portion occurred at 28–36 km, consistent with deeper magmatic or mantle-derived processes beneath the volcanic flank. Shallower events, concentrated between 0–8 km, likely relate to brittle failure in the upper crust. The largest event (magnitude 3.6 at 32 km depth) occurred early in the sequence on 7 December at 17:13, followed by numerous aftershocks of lower magnitude. Temporal distribution showed peak activity on 7–8 December and again around 14 December, with events spaced irregularly over the multi-day period.
Hawaii’s south flank experiences recurrent earthquake swarms due to the interplay between Kīlauea’s magma system and regional fault zones such as the Hilina and Koaʻe fault systems. These swarms often precede or accompany eruptive phases but can also occur independently. Historical records since 2000 document 94 swarms in the vicinity, with notable concentrations in 2018 (9 swarms), 2020 (11), and 2023 (16). This distribution underscores the persistent volcanic-tectonic stress regime.
The December 2023 swarm fits within this long-term pattern without exceeding typical intensity thresholds for the area. No surface rupture or major structural damage was associated with the events. Continued monitoring by the U.S. Geological Survey Hawaiian Volcano Observatory remains essential for distinguishing background volcanic seismicity from potential precursors to renewed eruptive activity.
References
- SeismoSight internal swarm classification data
- U.S. Geological Survey Hawaiian Volcano Observatory earthquake catalogs (2000–2023)