Seismic Swarm S20180501.1: Insights into Early Activity Near Fern Forest, Hawaii

The seismic swarm designated S20180501.1 began at 16:52 on 30 April 2018 and concluded at 15:52 on 12 August 2018. Centered 9 km south-southeast of Fern Forest on Hawaii’s Big Island, the swarm produced 42,368 earthquakes over 2,494 hours and 59 minutes. This prolonged sequence reflects typical unrest in a volcanically active region where magma movement and associated fracturing generate repeated seismic events.

Fern Forest lies on the southeast flank of Kīlauea volcano, within the East Rift Zone. The area experiences frequent seismic swarms driven by magma intrusion, dike propagation, and gravitational spreading of the volcanic edifice. Historical records since 2000 document 46 such swarms in the broader region, with notable clusters in 2003 (8 swarms), 2004 and 2011–2012 (4 each), and additional activity in 2018. These episodes commonly feature shallow seismicity linked to rift-zone dynamics and have preceded or accompanied eruptive phases.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.6 to 3.0, with the majority between 2.0 and 2.7. Depths were shallow, concentrated between 0 and 3 km, though occasional events reached 7–12 km. The initial event on 30 April registered magnitude 2.1 at 7 km depth. Subsequent events on 1 May showed rapid escalation, including multiple magnitude 2.7 shocks at depths of 0–1 km within the first few hours. Clustering was evident, with several pairs of nearly simultaneous events sharing identical times and locations but differing magnitudes.

Throughout the initial period, event rates remained elevated, averaging several per hour during peak intervals on 1 May. Depths stayed consistently shallow, indicating brittle failure in the upper volcanic crust. A notable magnitude 3.0 event occurred at 04:40 on 1 May at 7 km depth, one of the stronger early signals. Later events in the sequence maintained similar magnitude and depth distributions, underscoring persistent shallow fracturing.

This pattern aligns with documented behavior of Hawaiian rift-zone swarms, where repeated small-magnitude events often accompany subsurface magma transport. The high event count and extended duration distinguish S20180501.1 from shorter swarms recorded in prior years. Shallow depths and moderate magnitudes suggest efficient stress release through numerous microfractures rather than large single ruptures.

Overall, the swarm provides a clear record of sustained seismic unrest in a dynamic volcanic setting. Continued monitoring of such sequences supports improved understanding of magma pathways beneath the East Rift Zone.

References

USGS Earthquake Catalog (historical swarm statistics)
Hawaiian Volcano Observatory annual summaries (regional geology)
SeismoSight internal swarm classification S20180501.1