Seismic Swarm S20200723.2 Near Pāhala, Hawaii: Geological Context and Event Analysis
Seismic swarm S20200723.2 occurred in the region 4 km south-southeast of Pāhala on the Island of Hawaiʻi. The sequence began at 05:04 on 22 July 2020 and concluded at 10:50 on 9 August 2020, spanning 437 hours and 45 minutes. During this interval, 194 earthquakes were recorded.
Pāhala lies on the southern flank of Kīlauea volcano within the Kaʻū district. The area experiences persistent seismic activity driven by the interaction between the Pacific Plate and the underlying Hawaiian hotspot. Magma movement, volcanic inflation, and gravitational spreading of the volcanic edifice generate frequent earthquake swarms. Depths commonly range from 30 to 45 km, corresponding to the lower crust and upper mantle where magma reservoirs and transport pathways reside.
Analysis of the first 100 events reveals a characteristic swarm pattern. Magnitudes clustered between 1.5 and 2.7, with the largest events reaching 2.7. The majority of hypocenters were located between 30 and 42 km depth, although a few shallower events occurred near 6 km and 14 km. Temporal distribution showed elevated rates on 22–23 July and 26–30 July, consistent with episodic fluid or magma migration. No single mainshock dominated; instead, the energy release remained distributed across numerous events of similar size.
Historical records indicate that earthquake swarms are recurrent in this sector. Since 1 January 2000, 47 swarms have been documented, with notable years including 2004 (6 swarms), 2018 (7 swarms), and 2015 (5 swarms). These sequences reflect the ongoing volcanic and tectonic evolution of the island, where south-flank motion and occasional dike intrusions produce clustered seismicity.
The 2020 swarm aligns with established patterns of deep seismicity beneath the Pāhala region. Such activity often precedes or accompanies subtle changes in volcanic deformation, although surface eruptions did not occur during this specific sequence. Continued monitoring remains essential given the proximity to populated areas and critical infrastructure on the southern coast.
References
USGS Hawaiian Volcano Observatory earthquake catalog (2000–2020)
Hawaiian Volcano Observatory geologic summaries of Kīlauea south flank
SeismoSight internal swarm classification database