Earthquake Swarm S20210205.1: Seismic Activity Near Pāhala, Hawaii

An earthquake swarm designated S20210205.1 occurred in the region 4 km south-southwest of Pāhala on Hawaii’s Big Island. The sequence began at 11:10 on 4 February 2021 and concluded at 08:07 on 2 March 2021, spanning 620 hours and 57 minutes. During this period, 475 earthquakes were recorded.

Analysis of the first 100 events reveals a tightly clustered distribution of magnitudes between 1.4 and 2.7, with the majority falling in the 1.7–2.4 range. Depths concentrated between 29 km and 37 km, although a few events reached 43–44 km. The initial event measured magnitude 1.7 at 34 km depth, followed within hours by several magnitude-2+ shocks at similar depths. Peak magnitudes of 2.7 occurred on 5 February at depths of 43–44 km, indicating brief downward migration of seismicity before the swarm stabilized at mid-crustal levels.

Pāhala lies on the southern flank of Kīlauea volcano within the active volcanic zone of the Hawaiian hotspot. The island chain forms as the Pacific plate moves over a mantle plume, producing successive shield volcanoes. Kīlauea and neighboring Mauna Loa remain the most active, with frequent magma intrusion and flank movement generating both volcanic tremor and discrete earthquake swarms. Seismic swarms in this district commonly reflect pressurization of the volcanic system or slow slip along décollement faults at the base of the volcanic edifice.

Since 1 January 2000, seventeen swarms have been documented in the immediate area. Activity increased markedly after 2017, with three swarms in 2018, two in 2019, and eight in 2020. The 2021 swarm fits this recent pattern of elevated seismicity south of the summit.

Such sequences typically last days to several weeks and rarely produce damaging ground shaking because magnitudes remain below 3.0. They nevertheless provide valuable data on subsurface fluid migration and stress changes within the volcanic edifice. Continued monitoring by the Hawaiian Volcano Observatory supports timely assessment of any escalation toward eruptive activity.

References

• SeismoSight internal swarm catalog (S20210205.1 parameters)
• U.S. Geological Survey Hawaiian Volcano Observatory seismic records
• Geological Society of America publications on Hawaiian hotspot volcanism and flank instability