Seismic Swarm VS20210408.1: Analysis of Activity on Hawaiʻi Island

Earthquake swarms represent clusters of seismic events occurring in a localized area over a short period, often linked to volcanic or magmatic processes rather than a single mainshock-aftershock sequence. Swarm VS20210408.1 was recorded on the Island of Hawaiʻi, beginning at 13:35 HST on 7 April 2021 and concluding at 14:03 HST on 19 April 2021. Over this 288-hour, 28-minute interval, a total of 300 earthquakes were detected.

The Island of Hawaiʻi lies above a mantle hotspot, where the Pacific Plate moves northwestward over a fixed plume, fueling the growth of shield volcanoes including Kīlauea and Mauna Loa. Seismic activity here frequently arises from magma intrusion, dike propagation, and associated faulting within the volcanic edifice and surrounding flanks. Shallow depths predominate because brittle failure occurs in the upper crust, typically within the first few kilometers.

Analysis of the first 100 events reveals predominantly low-magnitude earthquakes, with values ranging from 0.7 to 2.4. The majority clustered between 0.8 and 1.3, indicating microseismicity consistent with fluid migration or minor fracturing. Depths were shallow, concentrated between 0 and 6 km, with several events registering negative values that may reflect near-surface or atmospheric coupling in volcanic settings. Notable peaks included a 2.4 event on 9 April at 17:58 and repeated 2.1–2.2 readings earlier in the sequence. Temporal distribution showed elevated rates during the initial 48 hours, followed by a gradual decline, typical of swarm decay without a dominant mainshock.

Historical records maintained by SeismoSight indicate 15 swarms on the island since 1 January 2000. Documented episodes occurred in 2004 (four swarms), 2006 (one), 2015 (two), 2016 (one), 2017 (two), 2020 (two), and 2021 (three). These recurrent swarms align with ongoing volcanic unrest at Kīlauea’s summit and rift zones, where similar patterns have accompanied eruptions in 2018 and subsequent years.

Such activity underscores the dynamic nature of Hawaiʻi’s geology, where continuous monitoring by the Hawaiian Volcano Observatory supports hazard assessment. Swarm VS20210408.1 fits established patterns of short-duration, low-magnitude clusters driven by subsurface magmatic movement.

References

USGS Hawaiian Volcano Observatory reports on regional seismicity and volcanic monitoring.
SeismoSight internal swarm classification database.