Seismic Swarm S20170629.1 Near Naalehu, Hawaii: Event Analysis and Regional Context

Seismic swarm S20170629.1 occurred approximately 36 km east-southeast of Naalehu on the Island of Hawaii. The sequence began at 04:47 on 29 June 2017 and concluded at 14:11 on 1 July 2017, spanning 57 hours and 23 minutes. During this period, 51 earthquakes were recorded, with magnitudes ranging from 1.0 to 3.2 and focal depths primarily between 4 km and 14 km.

The temporal distribution showed peak activity on 29 June, particularly between 13:00 and 16:00, when multiple events of magnitude 2.0 or greater clustered within a short interval. Notable shocks included a magnitude 3.2 event at 16:18:15 on 29 June at 11 km depth and a magnitude 3.0 event at 13:19:57 the same day. Subsequent days exhibited lower rates, with isolated events on 30 June and 1 July, the final registering magnitude 1.7 at 8 km depth.

Depth profiles indicate a concentration around 11–12 km, consistent with activity along crustal structures influenced by Hawaii’s volcanic edifice. Shallower events (0–7 km) occurred sporadically, while deeper shocks remained limited. The swarm’s compact duration and moderate magnitudes suggest a localized stress release rather than widespread tectonic rupture.

Hawaii’s geology is dominated by hotspot volcanism, with the Island of Hawaii formed by overlapping shields including Kilauea, Mauna Loa, and Mauna Kea. The Naalehu region lies on the southern flank, where seismic activity often links to magma migration, flank instability, or rift-zone dynamics. Historical records show seismic swarms remain infrequent in this precise locale; since 2000, only two prior swarms have been documented—in 2001 and 2005—each comprising a single reported sequence.

This low recurrence rate underscores the swarm’s significance within the area’s seismic catalog. The 2017 event aligns with typical patterns of Hawaiian seismicity, where swarms frequently accompany volcanic unrest yet can also arise from purely tectonic adjustments along pre-existing faults.

Further monitoring of similar sequences aids in refining hazard assessments for southern Hawaii communities. Continued data collection supports improved understanding of how such swarms interact with the island’s dynamic volcanic and structural framework.

References

SeismoSight internal swarm classification records
USGS Hawaiian Volcano Observatory geological summaries
Hawaii State Earthquake Catalog (2000–2017)