Analysis of the 2004 Earthquake Swarm Near Honaunau-Napoopoo, Hawaii

The earthquake swarm designated VS20040829.1 occurred 31 km east of Honaunau-Napoopoo on the island of Hawaii. It began at 09:13 on 28 August 2004 and concluded at 21:58 on 21 September 2004, spanning 588 hours and 44 minutes. During this interval, 397 earthquakes were recorded, reflecting a prolonged period of seismic unrest typical of Hawaii's volcanic environment. Examination of the first 100 events reveals a consistent pattern of low-magnitude activity. Magnitudes ranged between 1.0 and 2.6, with the majority falling between 1.5 and 2.5. Depths clustered primarily between 35 km and 55 km, although occasional shallower events near 8 km and deeper ones up to 65 km were noted. The sequence began with events at approximately 51 km depth and maintained a dominant mid-crustal signature throughout the initial days, indicating a stable source region rather than progressive migration. Hawaii Island lies above the Hawaiian hotspot, where magma ascent drives both eruptive and intrusive processes. The Kona coast region, including areas east of Honaunau-Napoopoo, experiences frequent earthquake swarms linked to magma movement within the volcanic edifice and along its flanks. These swarms differ from tectonic events elsewhere because they arise from fluid pressure changes and dike propagation rather than plate-boundary slip. Historical records show similar swarms preceding or accompanying activity at Kilauea and Mauna Loa throughout the twentieth century. The 2004 swarm aligns with documented patterns of deep seismicity beneath the island's southwest sector. Depths in the 40–50 km range correspond to the lower crust and uppermost mantle where magma accumulates before upward migration. The modest magnitudes and lack of a dominant mainshock are characteristic of swarm behavior driven by volcanic rather than purely tectonic forces. No surface rupture or significant ground deformation was associated with this particular episode, consistent with many non-eruptive swarms recorded on the island. Continued monitoring by the Hawaiian Volcano Observatory has refined understanding of such events. Updated models emphasize the role of pressurized magma reservoirs and hydrothermal systems in generating prolonged seismic sequences. The 2004 activity contributed to broader datasets used to map subsurface plumbing beneath the Kona region and to improve forecasts of future unrest. References
SeismoSight internal swarm classification VS20040829.1
USGS Hawaiian Volcano Observatory historical seismicity reports
USGS Professional Paper series on Hawaiian volcanism and earthquake mechanisms