Seismic Swarm S20151025.1 Near Pāhala, Hawaii: Analysis and Context

A seismic swarm designated S20151025.1 was recorded in the region 5 km east-northeast of Pāhala on Hawaii’s Big Island. The sequence began at 20:27 on 24 October 2015 and concluded at 10:41 on 28 October 2015, spanning 86 hours and 14 minutes. During this interval, 70 earthquakes were registered. The events clustered at depths predominantly between 31 km and 37 km, with a smaller number occurring at shallow depths near 0 km or 8 km. Magnitudes ranged from 0.9 to 2.6, with the largest events reaching 2.6 on 25 October at 06:48 and again on 26 October at 22:23. Most events fell between magnitude 1.7 and 2.4, consistent with typical swarm behavior in which numerous small earthquakes occur over a short period without a single dominant mainshock. Pāhala lies on the south flank of Kīlauea volcano within the active volcanic zone of the Island of Hawaiʻi. The island’s seismicity arises primarily from hotspot volcanism, in which magma movement and associated stress changes generate earthquakes at both shallow and intermediate depths. Deeper events in the 30–40 km range often reflect adjustments within the volcanic edifice or the underlying oceanic crust as magma ascends from the mantle hotspot. The south flank of Kīlauea is also subject to seaward motion along a décollement surface, which can produce additional seismic activity. Historical records indicate that eight earthquake swarms have occurred in the same area since 2000. These took place in 2000 (one swarm), 2004 (two swarms), 2006 (one swarm), 2014 (one swarm), and 2015 (three swarms, including S20151025.1). Such recurrent swarms underscore the persistent volcanic and tectonic stresses that characterize this portion of the island. The 2015 swarm did not culminate in a larger mainshock or reported surface deformation beyond background levels. Sequences of this type are common in Hawaii and are routinely monitored by the Hawaiian Volcano Observatory to assess potential links to magmatic or flank movement processes.

References

United States Geological Survey, Hawaiian Volcano Observatory – Earthquake Catalog and Regional Tectonic Summaries.
Hawaiian Volcano Observatory, “Volcano Watch” archives and seismic monitoring reports (2000–2015).