Seismic Analysis of Swarm S20220620.1 Near Pāhala, Hawaii

Earthquake swarm S20220620.1 was recorded 9 km ENE of Pāhala on Hawaii’s Big Island. The sequence began at 12:57 on 19 June 2022 and concluded at 10:58 on 19 February 2023, spanning 5,878 hours and registering 3,458 events. This prolonged activity is characteristic of the region’s volcanic setting, where magma movement and crustal stress release frequently produce clustered seismicity rather than isolated mainshock-aftershock sequences.

Pāhala lies on the southeastern flank of Kīlauea volcano within the active Hawaiian hotspot chain. The island chain forms as the Pacific plate moves over a mantle plume, generating persistent basaltic volcanism. Seismic events in this area commonly originate at depths of 30–35 km, corresponding to the interface between the oceanic crust and upper mantle where magma ascends and interacts with surrounding rock. Such depths distinguish these swarms from shallower tectonic earthquakes elsewhere in the Pacific.

Historical records document 84 swarms in the same locale since 1 January 2000. Annual counts vary significantly: 14 swarms occurred in 2020 and 10 in 2021, while earlier years such as 2003 and 2004 each recorded seven. Elevated swarm frequency in recent years aligns with renewed eruptive phases at Kīlauea and Mauna Loa, underscoring the link between magma supply and seismic clustering.

Examination of the first 100 events reveals a narrow magnitude range dominated by events between 1.5 and 2.5, with two peaks reaching 3.8 and 3.7 on 19 June. Depths remained tightly clustered between 30 and 35 km for the majority of events, although a few outliers appeared at 0 km and 4 km. The initial 24 hours contained the largest events and highest rate, after which activity transitioned to a steady background rate of smaller quakes. This temporal pattern is typical of fluid-driven swarms, in which initial pressure pulses trigger larger slips followed by sustained microseismicity.

Collectively, the swarm parameters and regional statistics illustrate the persistent volcanic-tectonic coupling that defines seismicity near Pāhala. Continued monitoring of depth distributions and event rates provides valuable indicators of magma transport beneath the Hawaiian volcanoes.

References

• United States Geological Survey, Hawaiian Volcano Observatory earthquake catalog and regional tectonics summaries.
• SeismoSight internal swarm classification database for event parameters of S20220620.1.