Seismic Swarm VS20210329.1: Analysis of Activity Near Honaunau-Napoopoo, Hawaii

Seismic swarm VS20210329.1 occurred in a tectonically active region on the Island of Hawaiʻi, centered 22 km east of Honaunau-Napoopoo on the southwestern flank. This area lies within the volcanic province influenced by the Hawaiian hotspot, where the Pacific Plate moves over a mantle plume, fueling ongoing volcanism at Kīlauea and Mauna Loa. The swarm's shallow focus aligns with typical patterns of magma-induced seismicity in the island's rift zones and volcanic flanks.

The event sequence began at 11:26 on 29 March 2021 and concluded at 21:45 on 6 April 2021, spanning 202 hours and 18 minutes. During this period, 279 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from 0.7 to 2.4. The majority clustered between 0.8 and 1.5, indicating microseismic character consistent with fluid migration or minor fault adjustments rather than large tectonic release. Depths remained shallow, mostly between 2 and 5 km, with occasional outliers at 7 km or negative values likely reflecting measurement variability in near-surface settings.

Temporal distribution showed peak frequency in the initial hours after onset, with events spaced closely in the afternoon and evening of 29 March. Magnitudes above 2.0 appeared sporadically, including events at 2.3, 2.4, and 2.2, but did not escalate into a sustained energetic phase. This pattern suggests a contained swarm driven by localized volcanic processes rather than regional stress accumulation.

Hawaiʻi has experienced repeated seismic swarms linked to its dynamic volcanic system. Since 2000, 22 such swarms have been documented in comparable settings. Earlier episodes occurred in 2004 (five swarms), 2006 (one), 2012 (one), 2014 (one), 2015 (four), 2016 (one), 2017 (three), 2018 (one), 2020 (three), and 2021 (two). These events commonly accompany magma intrusion along rift zones or flank movement, reflecting the island's continuous volcanic evolution over hundreds of thousands of years.

The geological context underscores the role of basaltic volcanism in generating such swarms. Lava flows and shield-building processes have shaped the terrain, while subsurface magma pathways produce the observed shallow seismicity. Updated monitoring continues to track these phenomena to refine understanding of eruption precursors in this high-activity environment.

References SeismoSight internal swarm classification VS20210329.1 USGS Hawaiian Volcano Observatory geological summaries