Seismic Swarm S20191114.1 Near Pāhala, Hawaii: Geological Context and Event Analysis

The seismic swarm designated S20191114.1 occurred approximately 4 km east of Pāhala on the Island of Hawaii. This region lies within the Kaʻū district, south of Kīlauea volcano, where the Hawaiian hotspot drives persistent volcanic and tectonic activity. The swarm initiated at 15:05 on 13 November 2019 and concluded at 13:57 on 11 December 2019, spanning 670 hours and 51 minutes during which 335 earthquakes were recorded.

Hawaii’s geology stems from the Pacific Plate’s movement over a mantle plume, producing the Hawaiian-Emperor seamount chain. Pāhala sits on the flank of the active volcanic system, where magma intrusions and crustal adjustments frequently generate earthquake swarms. Depths of 30–41 km place these events in the lower crust and upper mantle, consistent with processes beneath the volcanic edifice rather than shallow surface faulting.

Analysis of the first 100 events reveals a tightly clustered sequence. Magnitudes ranged from 1.6 to 2.7, with the majority falling between 1.7 and 2.0. Depths remained stable between 30 and 41 km, showing little temporal migration. Early activity included events of magnitude 2.1, 2.3, and later peaks at 2.7, all occurring within the initial 48 hours. Subsequent events maintained similar characteristics, indicating sustained but non-escalating energy release typical of volcanic swarms rather than tectonic mainshock-aftershock sequences.

Historical data underscore the recurrence of such swarms in this locale. Since 1 January 2000, 51 swarms have been documented near Pāhala. Annual counts include three in 2000, one in 2001, seven in 2003, five in 2004, and one each in 2005 and 2006. Further activity comprised two in 2007, one in 2008, two in 2011, four in 2012, one in 2013, two in 2014, four in 2015, two each in 2016 and 2017, eight in 2018, and three in 2019. This pattern reflects ongoing magmatic and hydrothermal influences beneath the south flank of Kīlauea.

The 2019 swarm fits within this established framework, exhibiting comparable magnitude distributions and depths to prior episodes. No surface deformation or eruptive activity was linked to the sequence, reinforcing its classification as a deep volcanic swarm. Monitoring continues to track potential precursors to larger seismic or volcanic events in the region.

References

SeismoSight internal swarm classification records.
USGS Hawaiian Volcano Observatory geological summaries.
Hawaiian hotspot and Kīlauea flank studies (updated through 2023).