Seismic Analysis of Swarm S20201103.1 Near Pāhala, Hawaii

The earthquake swarm designated S20201103.1 occurred approximately 8 km east-northeast of Pāhala on the Island of Hawaii. It began at 02:24 on 2 November 2020 and concluded at 08:50 on 30 November 2020, spanning 678 hours and 26 minutes. During this interval, 288 earthquakes were recorded. This activity aligns with the broader tectonic and volcanic framework of the Hawaiian Islands, where seismicity frequently results from magma movement beneath active shield volcanoes.

Hawaii’s geology is shaped by hotspot volcanism, with the Big Island hosting Kīlauea and Mauna Loa. Pāhala lies in the Kaʻū district, south of these volcanoes, where the crust experiences both volcanic inflation and regional stresses. Earthquake swarms in this area often occur at depths of 25–35 km, reflecting interactions between the volcanic edifice and the underlying oceanic crust. Depths near 30 km commonly indicate magma transport or pressurization within the lower crust, while shallower events may signal fracturing closer to the surface.

Analysis of the first 100 events reveals consistent patterns. Magnitudes ranged primarily from 1.5 to 2.9, with the largest event reaching 3.4 on 2 November at 11:47. Most events clustered between 1.8 and 2.3. Depths averaged approximately 31 km during the initial phase, with the majority falling between 28 and 35 km. A gradual increase in event frequency occurred over the first 48 hours, followed by sustained activity. Later events within this subset showed occasional shallower depths, including several at 4–12 km on 7–10 November, suggesting possible migration of stress or fluid movement.

Temporal distribution indicates a typical swarm progression: an initial energetic pulse, a period of moderate repetition, and intermittent larger events. No single foreshock-mainshock-aftershock sequence dominated; instead, the data reflect diffuse, swarm-like behavior characteristic of volcanic regions.

Since 1 January 2000, 19 swarms have been documented in the vicinity. Their yearly distribution includes single swarms in 2000, 2007, and 2014; four in 2015; five in 2018; two in 2019; and five in 2020. This recurrence underscores ongoing magmatic and tectonic processes beneath southern Hawaii.

Seismic monitoring in the region supports hazard assessment and volcanic surveillance. Continued observation aids in distinguishing between background seismicity and potential precursors to eruptive activity.

References

SeismoSight internal swarm classification S20201103.1
USGS Earthquake Catalog
Hawaiian Volcano Observatory reports