Seismic Swarm S20231021.1: Analysis of Activity Southwest of Volcano, Hawaii

Seismic swarm S20231021.1 was recorded 8 km southwest of Volcano, Hawaii, beginning at 18:57 on 20 October 2023 and concluding at 08:16 on 1 November 2023. Over 277 hours and 19 minutes, the sequence included 165 earthquakes. This event occurred in a region known for recurrent seismic activity linked to volcanic processes on the Big Island.

The first 100 events displayed magnitudes predominantly between 0.2 and 2.5, with most falling below 2.0. Depths were primarily shallow, concentrated between 0 and 5 km, although several events reached 27–32 km. Notable examples include a magnitude 2.5 event at 3 km depth on 21 October and multiple magnitude 1.7–2.1 events clustered at shallow levels throughout the initial days. Later entries in the sequence showed a mix of shallow and deeper activity, such as magnitude 2.2 events near 30 km on 25 and 26 October. These patterns suggest a combination of near-surface fracturing and possible deeper magmatic influences.

Hawaii’s geology is dominated by hotspot volcanism, where the Pacific Plate moves over a mantle plume, fueling the Hawaiian-Emperor seamount chain. On the Big Island, Kilauea volcano remains one of Earth’s most active systems, with frequent magma intrusions that generate earthquake swarms. The swarm location lies on Kilauea’s southwest rift zone flank, an area subject to repeated dike propagation and stress changes from ongoing volcanic unrest. Historical records indicate that such swarms commonly precede or accompany eruptive episodes, reflecting the dynamic interplay between tectonic stress and magmatic movement.

Since 1 January 2000, 94 swarms have been documented in the region. Annual counts show variability, with earlier decades recording fewer events (for example, three in 2000 and six in 2003) and recent years exhibiting higher frequencies, including 14 in 2020, 11 in 2021, and 12 in 2023. This trend aligns with sustained monitoring improvements and persistent volcanic activity at Kilauea, which has produced multiple eruptions and intrusive events in the 21st century.

The 2023 swarm fits within this established context of elevated seismicity. While individual events remained modest in magnitude, their clustered timing and spatial distribution provide valuable data on subsurface stress migration. Continued surveillance by networks such as the Hawaiian Volcano Observatory supports refined understanding of precursory signals in this high-hazard volcanic setting.

References

USGS Hawaiian Volcano Observatory reports on Kilauea seismicity (2000–2023).
SeismoSight internal swarm classification database.