Seismic Swarm S20110311.1: Analysis of Activity Southwest of Leilani Estates, Hawaii

Seismic swarm S20110311.1 was recorded beginning at 00:53 on 11 March 2011 and concluding at 02:43 on 12 March 2011. The events were located 12 km southwest of Leilani Estates in Hawaii’s Puna district, with a total of 26 earthquakes detected over 25 hours and 49 minutes. This swarm occurred within the lower East Rift Zone of Kīlauea Volcano, a region shaped by persistent magmatic processes.

The swarm’s events exhibited magnitudes ranging from 0.0 to 4.5 and focal depths between 4 km and 13 km. The largest earthquake, magnitude 4.5, occurred at 08:58 on 11 March at a depth of 8 km. A notable cluster of activity took place between 09:03 and 09:24, featuring multiple events of magnitudes 2.4 to 2.7 at depths of 7–13 km. Additional prominent shocks included a magnitude 3.3 event at 06:18 and a magnitude 3.1 event at 19:46 on the same day. Depths remained consistently shallow, consistent with volcanic rather than tectonic origins.

These characteristics point to fluid migration or minor dike propagation along rift fractures. Such swarms commonly reflect pressure changes in the volcanic system without leading to surface eruption. The distribution of magnitudes, dominated by events between 2.0 and 2.9, follows a typical Gutenberg-Richter pattern for rift-zone seismicity.

Kīlauea’s East Rift Zone has hosted repeated intrusive episodes since at least the 19th century. Historical records document fissure eruptions and ground deformation tied to magma movement along the same structural trends. Leilani Estates lies near the site of the 1955 and 1977 eruptions, underscoring the area’s long-term volcanic susceptibility.

Since 1 January 2000, fifteen swarms have been identified in the region according to internal SeismoSight classification. Yearly counts include one swarm each in 2000, 2001, 2005, 2006, and 2008; four in 2003; two in 2004; three in 2007; and one in 2011. This pattern indicates episodic intrusive activity recurring every one to three years on average.

The March 2011 swarm fits within this established sequence. Its timing and parameters align with prior episodes that produced no immediate surface breakout yet contributed to gradual rift extension. Monitoring of similar swarms supports improved forecasting of potential eruptive windows along the lower East Rift Zone.

References
SeismoSight internal swarm catalog (S20110311.1 parameters and historical counts).
USGS Hawaiian Volcano Observatory summaries on Kīlauea East Rift Zone structure and activity.