Seismic Swarm S20200104.1: Analysis of Activity Near Pāhala, Hawaii

Seismic swarm S20200104.1 was recorded 2 km east-southeast of Pāhala on Hawaii’s Big Island. The sequence began at 05:43 on 3 January 2020 and concluded at 04:03 on 19 April 2020, spanning 2,566 hours and 19 minutes. During this interval, 1,689 earthquakes were detected.

Pāhala lies on the southern flank of Kīlauea volcano within the active volcanic zone shaped by the Hawaiian hotspot. The region experiences persistent seismicity driven by magma movement, volcanic loading, and crustal adjustments along the south flank. Earthquake depths in the swarm predominantly ranged between 29 km and 38 km, consistent with the deeper seismic layer beneath the volcanic edifice where magma transport and pressurization commonly occur.

Inspection of the first 100 events reveals a narrow magnitude distribution, with events between 1.7 and 3.5. The majority clustered between 1.8 and 2.3, while four events reached or exceeded 3.0. Depths remained stable near 32–36 km for most events, although one outlier registered at only 1 km depth. This pattern suggests sustained deep crustal processes punctuated by occasional shallower fracturing.

Historical records maintained by SeismoSight document 52 swarms in the same area since 1 January 2000. Annual counts include three swarms in 2000, one in 2001, seven in 2003, five in 2004, and single swarms in 2005 and 2006. Further activity occurred in 2007 (three), 2008 (one), 2011 (two), 2012 (four), and 2013 (one). Additional swarms were noted in 2014 (two), 2015 (four), 2016 (two), 2017 (two), 2018 (eight), and 2019 (four). The 2020 swarm fits within this established recurrence, underscoring the persistent seismic character of the Pāhala region.

The prolonged duration and high event count of S20200104.1 distinguish it from many prior swarms, yet the depth and magnitude characteristics align closely with earlier episodes. Such swarms contribute to ongoing monitoring of volcanic and tectonic stress accumulation along Hawaii’s south flank.

References

SeismoSight internal swarm catalogue, 2000–2020.
USGS Hawaiian Volcano Observatory seismic bulletins, 2020.
Hawaiian Volcano Observatory geologic summaries of Kīlauea south flank.