Seismic Swarm S20170608.2 Near Fern Forest, Hawaii: Geological Context and Event Analysis

Seismic swarm S20170608.2 occurred in a volcanically active region 15 km south of Fern Forest on Hawaii's Big Island. The sequence began at 03:31 on 8 June 2017 and concluded at 13:40 on 11 June 2017, spanning 82 hours and 9 minutes. During this period, 47 earthquakes were recorded, providing insight into the dynamic processes beneath the island's southeastern flank.

The swarm featured predominantly low-magnitude events, with the majority registering below 2.0. Depths ranged from surface levels to 37 km, though most clustered between 0 and 10 km. A notable outlier was a magnitude 5.2 earthquake on 8 June at 17:01:19, occurring at 6 km depth. This event was accompanied by several immediate aftershocks, including magnitudes 2.1, 1.0, 2.2, and 1.5 within minutes. Subsequent activity on 9 and 10 June included clusters of events around magnitudes 2.4–2.8, often at shallow depths of 0–8 km. The final recorded event on 11 June reached magnitude 1.7 at 21 km depth. Overall, the sequence displayed typical swarm characteristics: a rapid onset, variable magnitudes without a single dominant mainshock-aftershock pattern, and a gradual decline in frequency.

This area lies within the East Rift Zone of Kīlauea volcano, where magma intrusion and flank movement frequently trigger seismic swarms. Hawaii's geology is shaped by its position over a mantle hotspot, resulting in shield volcanoes built from successive basaltic lava flows. The south flank of Kīlauea experiences persistent deformation due to gravitational spreading and magmatic pressure, leading to recurrent earthquake swarms. Depths in this swarm align with known zones of magma transport and crustal adjustment in the region.

Historical records indicate 43 swarms in the area since 1 January 2000. Yearly distribution shows elevated activity in 2003 (8 swarms) and 2004 (7 swarms), with fewer occurrences in intervening years such as 2005, 2008, and 2013 (1 swarm each). The 2017 swarm represents the sole event recorded that year up to the data cutoff. These patterns reflect ongoing volcanic and tectonic stresses characteristic of the Hawaiian hotspot environment.

Such swarms offer valuable indicators of subsurface magma movement and stress accumulation. Monitoring continues to support hazard assessment in this high-activity volcanic setting.

References

USGS Earthquake Catalog (Hawaii region data)
Hawaii Volcano Observatory geological summaries
SeismoSight internal swarm classification records