Seismic Swarm S20070814.1: Analysis of Activity Near Fern Forest, Hawaii

An earthquake swarm designated S20070814.1 was recorded 15 km south-southeast of Fern Forest on Hawaii's Big Island. The sequence began at 14:44 on 13 August 2007 and concluded at 19:43 on 20 August 2007, spanning 172 hours and 58 minutes. During this interval, 87 earthquakes were detected, with magnitudes ranging from 0.8 to 5.4 and focal depths primarily between 0 and 10 km, except for isolated deeper events reaching 47 km.

The swarm featured a notable mainshock of magnitude 5.4 at 05:38 on 14 August 2007 at a depth of 9 km. This event was followed by numerous aftershocks clustered in the first 48 hours, including multiple events of magnitude 2.0–3.0. Subsequent activity declined steadily, with smaller events (mostly magnitude 1.0–2.2) persisting through 20 August. Depths remained shallow for the majority of shocks, consistent with volcanic or shallow crustal processes.

Hawaii lies above a mantle hotspot that has generated the Hawaiian-Emperor seamount chain over millions of years. The Big Island comprises five coalesced shield volcanoes, with Kilauea and Mauna Loa remaining the most active. Magma movement through rift zones frequently triggers earthquake swarms as pressure changes fracture surrounding rock. Fern Forest sits on the southeast flank of Kilauea, an area influenced by both volcanic inflation and gravitational spreading of the volcanic edifice.

Seismic swarms in this region often precede or accompany eruptive episodes, though not every swarm leads to surface activity. Depths under 10 km typically reflect brittle failure within the volcanic pile or along décollement surfaces at the base of the volcano. The 2007 swarm's shallow focus and rapid onset align with documented patterns of magma intrusion along Kilauea's east rift zone.

Since 1 January 2000, 21 swarms have occurred in the same general area. Yearly counts show notable clusters in 2003 (8 swarms) and 2004 (4 swarms), with additional activity recorded in 2000, 2001, 2005, 2006, and three swarms in 2007. These recurrent episodes underscore the persistent volcanic-tectonic stress regime beneath the southeast flank of Kilauea.

The S20070814.1 sequence illustrates typical swarm behavior: an energetic initial phase dominated by the largest event, followed by a prolonged tail of smaller shocks. Such patterns aid in distinguishing swarm activity from classic mainshock-aftershock sequences and support real-time volcanic monitoring efforts.

References
USGS Earthquake Catalog (historical Hawaiian seismicity)
Hawaiian Volcano Observatory, Volcano Watch summaries (2007 regional context)
SeismoSight internal swarm classification S20070814.1