Seismic Swarm S20211011.1 Near Naalehu, Hawaii: Analysis of October 2021 Activity

An earthquake swarm designated S20211011.1 occurred 26 km south-southeast of Naalehu on Hawaii's Big Island. The sequence began at 21:48 on 10 October 2021 and concluded at 05:40 on 13 October 2021, spanning 55 hours and 51 minutes. During this interval, 35 earthquakes were recorded, with magnitudes ranging from 1.0 to 6.2 and focal depths clustered between 31 and 39 km.

The swarm initiated with a magnitude 6.2 event at 35 km depth, followed within minutes by a magnitude 4.2 shock at identical depth. Subsequent events included multiple magnitude 2.5–3.2 quakes, with notable repetitions at 35 km depth. Activity persisted through 11 and 12 October before tapering, ending with a magnitude 2.3 event. Depths remained consistent, indicating a stable source volume within the oceanic lithosphere beneath the island.

This region lies on the southern flank of Mauna Loa volcano, where hotspot-driven magmatism interacts with regional stresses. Hawaii's geology stems from the Pacific plate's passage over a mantle plume, producing shield volcanoes and associated fault systems such as the Hilina and Ka'oiki faults. Earthquakes here often result from volcanic inflation, dike intrusion, or gravitational sliding of the volcanic edifice, rather than plate-boundary tectonics.

Historical records since 2000 show only three prior swarms in the immediate area: one in 2005, another in 2017, and this 2021 sequence. These infrequent clusters contrast with more continuous background seismicity tied to ongoing volcanic processes at nearby Kilauea and Mauna Loa.

The 2021 swarm's depth distribution aligns with known mantle-level events beneath Hawaii, where brittle failure occurs in the cool lithosphere above the hotspot. No surface deformation or eruptive activity was linked to this sequence, suggesting it represented stress release without direct magmatic involvement.

References:
USGS Earthquake Catalog
Hawaii Volcano Observatory reports on regional seismicity
Geological Society of America publications on Hawaiian hotspot dynamics