Seismic Swarm S20240721.1: Analysis of Earthquake Activity Near Volcano, Hawaii

Seismic swarm S20240721.1 was recorded beginning at 20:33 on 20 July 2024 and concluding at 05:07 on 23 August 2024. The events were centered 6 km SSW of Volcano, Hawaii. Over approximately 800 hours and 34 minutes, the swarm produced 1,535 earthquakes.

The location lies on the Island of Hawaii within the active volcanic zone associated with Kīlauea. This shield volcano sits above the Hawaiian hotspot, where magma generated by a mantle plume rises through the Pacific Plate. The crust in this area is thin and fractured, allowing frequent magma intrusions that trigger shallow seismicity. Depths recorded during the swarm were predominantly between 0 and 2 km, consistent with processes occurring in the upper volcanic edifice and shallow rift zones.

Kīlauea has a long history of persistent activity. Since the early 19th century, the volcano has exhibited nearly continuous summit and flank eruptions. Major episodes include the prolonged Puʻuʻōʻō eruption from 1983 to 2018 and the dramatic 2018 lower East Rift Zone event that destroyed hundreds of structures. These eruptions are typically preceded or accompanied by seismic swarms as magma migrates through dikes and sills.

Analysis of the first 100 events in swarm S20240721.1 reveals a pattern of low-to-moderate magnitude earthquakes. Magnitudes ranged from 0.5 to 3.5, with the majority falling between 1.5 and 2.5. Depths remained shallow for nearly all events, and a notable cluster of activity occurred within the first 24 hours, including several events of magnitude 2.5–3.5. Later events in the initial sequence showed slightly greater depth variation, including isolated occurrences near 19 km and 6 km, though most stayed within the uppermost 4 km. This distribution indicates repeated small-scale fracturing likely driven by fluid or magma movement rather than large tectonic stress release.

Historical records document 52 seismic swarms in the region since 1 January 2000. Annual counts have varied, with notable increases in 2018 (6 swarms), 2020 (7), 2021 (7), and 2023 (11). The 2024 count stands at 4 swarms through the date of this event. Such swarms are a recognized feature of Kīlauea’s volcanic system and frequently correlate with periods of elevated magma supply or changes in eruptive style.

The combination of high event count, shallow focal depths, and temporal clustering observed in S20240721.1 aligns with established characteristics of volcanic seismicity at this location. Continued monitoring remains essential for interpreting any potential links to future eruptive activity.

References

USGS Hawaiian Volcano Observatory reports on Kīlauea seismicity and eruption history.
Hawaiian Volcano Observatory geological summaries of the Hawaiian hotspot and shield volcano structure.
Internal SeismoSight classification data for swarm S20240721.1.