Seismic Swarm S20211216.1: Analysis of Earthquake Activity Near Naalehu, Hawaii

A seismic swarm designated S20211216.1 was recorded 43 km east-southeast of Naalehu on Hawaii’s Big Island. The sequence began at 09:23 on 15 December 2021 and concluded at 17:04 on 17 December 2021, spanning 55 hours and 40 minutes. During this interval, 38 earthquakes were detected, with magnitudes ranging from 0.8 to 3.1 and focal depths predominantly between 3 km and 11 km. A small number of events occurred at greater depths, including one at 31 km and another at 38 km.

The swarm exhibited a typical pattern of clustered, low-to-moderate magnitude events without a single dominant mainshock. The largest shocks reached magnitude 3.1 at depths of 7 km and 9 km. Most activity remained shallow, consistent with processes occurring within the volcanic edifice or upper crustal faults. Temporal distribution showed peak rates during the afternoon and evening of 15 December, followed by a secondary cluster on the morning of 16 December.

Hawaii’s geology is shaped by hotspot volcanism, where the Pacific plate moves northwestward over a mantle plume, generating the Hawaiian-Emperor seamount chain. The Big Island hosts active volcanoes including Kīlauea and Mauna Loa, whose flanks and rift zones experience frequent seismicity linked to magma intrusion, flank slip, and volcanic-tectonic interactions. The swarm location lies south of the island’s main volcanic centers, in a region influenced by both volcanic loading and regional fault systems.

Historical records since 2000 indicate six comparable swarms in the broader area, occurring in 2001, 2005, 2017, 2020, and twice in 2021. These episodes underscore the recurrent nature of clustered seismicity in this tectonically and volcanically active setting.

The December 2021 swarm fits within established patterns of Hawaiian seismicity, where short-lived clusters often reflect transient stress changes without leading to larger tectonic events or eruptive activity. Depths and magnitudes remained well below thresholds associated with significant surface impact.

References

SeismoSight internal classification data for swarm S20211216.1
USGS Earthquake Hazards Program – Hawaiian region overview (updated 2023)
Hawaiian Volcano Observatory geologic summaries (2022–2024)