Seismic Swarm S20210831.1: Analysis of Activity Near Pāhala, Hawaii
Seismic swarm S20210831.1 occurred 7 km east-northeast of Pāhala on Hawaii Island. The sequence began at 08:52 on 30 August 2021 and concluded at 18:51 on 1 September 2021, spanning 57 hours and 59 minutes. During this period, 94 earthquakes were recorded, with magnitudes ranging from 1.3 to 3.1 and focal depths primarily between 29 km and 40 km.
The swarm exhibited a steady rate of low-to-moderate magnitude events. The largest earthquake reached magnitude 3.1 at 10:00 on 30 August at 36 km depth. Subsequent activity included multiple events of magnitude 2.0–2.8 clustered at depths of 31–37 km. Event frequency remained elevated through 31 August before gradually declining on 1 September, with the final recorded event of magnitude 1.5 at 35 km depth.
Geologically, Pāhala lies on the south flank of Kīlauea volcano within the Hawaiian hotspot chain. The island’s crust is built from successive shield-volcano flows overlying the Pacific plate. Seismicity in this region commonly originates from magma transport, volcanic inflation, or gravitational spreading of the volcanic edifice. Depths of 30–40 km correspond to the lower crust and uppermost mantle, where magma accumulation and pressurization frequently trigger swarms.
Hawaii Island experiences recurrent seismic swarms linked to volcanic processes. Since 2000, twenty such swarms have been documented in the area, with notable increases in frequency during 2018–2020. These episodes reflect persistent magmatic and tectonic stresses beneath the active volcanic centers.
The 2021 swarm shares characteristics with prior events, including similar depth ranges and magnitude distributions. Such patterns suggest episodic fluid or magma movement at depth rather than shallow tectonic faulting. No surface deformation or eruptive activity was associated with this particular sequence, consistent with many deep swarms that dissipate without leading to eruption.
Continued monitoring by the Hawaiian Volcano Observatory remains essential. Seismic swarms provide critical indicators of subsurface volcanic conditions and help refine hazard assessments for communities on the island’s southern flank.
References
United States Geological Survey, Hawaiian Volcano Observatory seismic catalogs.
Hawaiian Volcano Observatory, Volcano Watch summaries, 2021.
USGS Earthquake Hazards Program, Hawaii regional tectonics overview.