Seismic Swarm VS20041012.1: Geological Context and Event Analysis near Pāhala, Hawaii
Seismic swarm VS20041012.1 was recorded beginning at 11:09 on 11 October 2004 and concluding at 23:26 on 30 October 2004. The sequence was centered 28 km NNW of Pāhala in Hawaii’s Kaʻū district on the Island of Hawaiʻi. Over 468 hours and 17 minutes, the swarm produced 360 earthquakes, providing a focused window into subsurface activity within one of Earth’s most active volcanic provinces.
The Hawaiian Islands sit above a mantle hotspot that has generated the archipelago through successive shield volcanoes. Pāhala lies on the southern flank of Kīlauea, where the crust is built from layered basaltic flows and subject to both volcanic inflation and gravitational spreading. Earthquakes in this setting commonly arise from magma movement, fault slip along rift zones, or adjustment of the volcanic edifice under its own weight. Depths recorded during the swarm, predominantly 30–50 km, place many events near the base of the crust or within the uppermost mantle, consistent with the deeper expression of hotspot-related stresses.
The first 100 events displayed magnitudes between 1.0 and 2.7, with the majority falling in the 1.5–2.4 range. Depths clustered tightly around 35–48 km, although a few shallower outliers appeared near 7–9 km. Timing showed an initial energetic pulse on 11–12 October followed by sustained, lower-rate activity. This pattern—rapid onset, moderate magnitudes, and limited shallow events—suggests a deep-seated trigger rather than surficial volcanic unrest.
Historical records maintained since 1 January 2000 indicate only two comparable swarms in the immediate region: one in 2003 and the present 2004 sequence. The scarcity of similar episodes underscores the episodic nature of deep seismic release beneath the Kaʻū coastline.
Taken together, the 2004 swarm supplies a useful reference for understanding how deep crustal or mantle stresses are accommodated beneath the Hawaiian hotspot. Continued monitoring of comparable depth ranges remains essential for distinguishing background tectonic signals from any future magmatic incursions.
References
SeismoSight internal swarm classification VS20041012.1
USGS Hawaiian Volcano Observatory regional geological summaries