Seismic Swarm S20210318.1: Analysis of Activity Near Volcano, Hawaii

Seismic swarm S20210318.1 occurred approximately 20 km west-northwest of Volcano, Hawaii, on the Big Island. The sequence began at 11:10 on 18 March 2021 and concluded at 03:02 on 21 March 2021, spanning 63 hours and 51 minutes. During this interval, 113 earthquakes were recorded.

Analysis of the initial events reveals predominantly low-magnitude seismicity. Magnitudes ranged from 0.7 to 3.5, with the majority below 2.0. Notable peaks included a 3.5 event at 18:23 on 18 March and a 3.1 event shortly afterward at 18:27. Depths were primarily shallow, clustering between -2 km and 3 km, consistent with activity in the upper volcanic crust. A smaller number of events reached depths of 4–8 km, indicating limited deeper propagation.

The swarm exhibited typical characteristics of clustered, low-energy releases rather than a single large rupture. Temporal distribution showed higher frequency in the first 24 hours, with events occurring at intervals of minutes during peak periods. Magnitudes remained modest overall, suggesting fluid or magma-related processes rather than tectonic stress accumulation.

Hawaii’s geology is shaped by its position over a mantle hotspot, driving the formation of the Hawaiian-Emperor seamount chain. The Island of Hawaii hosts five major volcanoes, with Kilauea among the most active. Its east rift zone and summit caldera frequently generate seismic swarms due to magma intrusion, dike propagation, and associated fracturing. Such swarms have been documented for decades, often preceding or accompanying eruptive episodes.

Historical records indicate 73 swarms in the region since 1 January 2000. Annual counts vary significantly: 2000 recorded three, 2003 seven, 2004 eight, 2018 eight, and 2020 fourteen. Earlier years such as 2001, 2005, 2008, and 2013 each recorded one. This pattern underscores persistent volcanic unrest linked to ongoing magmatic supply beneath Kilauea.

The March 2021 swarm aligns with this established behavior. Its shallow focus and rapid succession of small events mirror prior episodes associated with rift-zone inflation or summit reservoir adjustments. No surface rupture or major eruptive activity was directly tied to this specific sequence based on the provided classification.

Seismic monitoring in Hawaii relies on dense networks operated by the U.S. Geological Survey Hawaiian Volcano Observatory. These systems enable precise location and magnitude determination, supporting real-time assessment of volcanic hazards. Continued observation remains essential given the island’s dynamic subsurface environment.

References

SeismoSight internal swarm classification records.
U.S. Geological Survey Hawaiian Volcano Observatory geological summaries.
Hawaii Volcanoes National Park volcanic history documentation.