Earthquake Swarm S20151016.1 on the Island of Hawaii

The Island of Hawaii, commonly known as the Big Island, lies above the Hawaiian hotspot, a mantle plume responsible for the Hawaiian-Emperor seamount chain. This geological setting produces persistent volcanic and seismic activity as the Pacific Plate moves northwestward over the hotspot at approximately 7–10 cm per year. The island hosts five major shield volcanoes, with Kilauea and Mauna Loa remaining the most active. Magma intrusion, flank movement along rift zones, and occasional tectonic adjustments generate frequent earthquake swarms, which are clusters of events without a distinct mainshock-aftershock sequence.

Swarm S20151016.1 was recorded on the Island of Hawaii between 19:34 UTC on 15 October 2015 and 23:42 UTC on 18 October 2015. Over this 76-hour period, 45 earthquakes were detected. Event magnitudes ranged from 0.4 to 3.9, with the majority occurring at shallow depths between 0 and 10 km; one outlier reached 29 km. The strongest event, magnitude 3.9, occurred at 06:41 UTC on 16 October at 6 km depth. Subsequent notable shocks included several events of magnitude 2.5–2.9 distributed across the following days, concentrated primarily in the first 48 hours.

Analysis of the temporal distribution shows peak activity on 16 October, with 22 events recorded that day. Depths remained consistently shallow for most shocks, consistent with processes occurring within the volcanic edifice or upper crustal layers. Such patterns commonly reflect magma migration or pressurization beneath the island’s active volcanoes rather than purely tectonic faulting.

Historical records indicate 37 swarms have occurred on the island since 1 January 2000. Yearly counts include three in 2000, one in 2001, eight in 2003, five in 2004, one each in 2005 and 2006, four in 2007, one in 2008, three in 2011, four in 2012, one in 2013, two in 2014, and two in 2015. This frequency underscores the recurrent nature of swarm activity driven by the island’s dynamic volcanic environment.

The 2015 swarm fits within established patterns of Hawaiian seismicity, where shallow events often precede or accompany changes in volcanic unrest. Continued monitoring by regional networks remains essential for distinguishing between benign swarms and those signaling potential eruptive activity.

References

• SeismoSight internal swarm classification database
• USGS Hawaiian Volcano Observatory geological summaries
• NOAA Pacific Tsunami Warning Center regional tectonic reports