Earthquake Swarm S20210303.2: Seismic Activity Near Pāhala, Hawaii

Earthquake swarm S20210303.2 occurred 4 km east of Pāhala in the Kaʻū district on Hawaiʻi Island. The sequence began at 22:09 on 2 March 2021 and concluded at 19:44 on 20 May 2022, spanning 10,653 hours and 34 minutes. During this period, 9,245 earthquakes were recorded. Hawaiʻi Island lies above the Hawaiian hotspot, where the Pacific Plate moves northwestward over a mantle plume. This setting produces active shield volcanoes, including Kīlauea and Mauna Loa. Pāhala sits on the southern flank of Kīlauea, a region characterized by frequent seismic swarms linked to magma intrusion, volcanic deflation, and movement along the Hilina fault system and décollement faults at the base of the volcanic edifice. The first 100 events of the swarm displayed magnitudes between 1.5 and 2.6 and focal depths predominantly between 29 and 38 km, with one shallower event at 8 km. These depths align with the brittle-ductile transition zone beneath the volcanic pile, where stress accumulation from magma movement or gravitational spreading of the volcano commonly triggers clustered seismicity. Historical records indicate 36 swarms in the region since 1 January 2000. Annual counts include single swarms in 2000, 2006, 2007, 2008, 2011, 2013, 2014, and 2016; three in 2003; two in 2004; four in 2015; six in 2018; three in 2019; nine in 2020; and one in 2021. This pattern reflects the persistent volcanic and tectonic stresses in the Kīlauea south flank. Swarm S20210303.2 fits within this established framework of episodic seismic clusters. Its prolonged duration and high event count underscore the ongoing dynamic processes beneath Pāhala, where magma-related pressure changes and flank instability continue to generate microseismicity without associated large-magnitude mainshocks.

References

USGS Hawaiian Volcano Observatory earthquake catalog Hawaiian Volcano Observatory geologic summaries of Kīlauea south flank SeismoSight internal swarm classification records