Seismic Swarm VS20160704.1: Analysis of Activity Near Honaunau-Napoopoo, Hawaii

Seismic swarm VS20160704.1 occurred in a volcanically active region of Hawaii's Big Island, approximately 23 km east of Honaunau-Napoopoo. The event sequence began at 21:52 on 3 July 2016 and concluded at 14:38 on 10 July 2016, spanning 160 hours and 45 minutes. During this period, 84 earthquakes were recorded, providing valuable data on subsurface dynamics in an area shaped by the Hawaiian hotspot.

The Hawaiian Islands formed through prolonged volcanic activity as the Pacific Plate moved over a mantle plume. This process has built shield volcanoes such as Mauna Loa and Kilauea, with the southwest coast near Honaunau-Napoopoo lying on the flanks of these systems. Shallow seismicity here often reflects adjustments along volcanic rift zones, fault movement, or minor magma migration. Depths in the swarm ranged primarily from 0 to 5 km, consistent with crustal processes above the deeper magmatic sources.

Magnitudes remained low throughout, with the largest event reaching 3.2 on 10 July 2016 at 0 km depth. Most events clustered between 0.6 and 2.6, indicating microseismic to minor activity. Notable clusters appeared on 4 July, featuring multiple events above magnitude 2.0, and on 9 July with several readings near 2.0–2.3. Negative depth values in the dataset likely represent surface or very shallow sources typical in volcanic monitoring.

This swarm aligns with the region's history of episodic seismic clusters. Since 2000, eight swarms have been documented in the area, occurring in 2004 (four events), 2006 (one event), and 2015 (three events). Such patterns suggest recurring stress release linked to the island's ongoing volcanic and tectonic evolution.

Insights from the sequence reveal a gradual buildup followed by intermittent peaks, with activity tapering after 9 July. The predominance of events at 2–4 km depth points to brittle failure in the upper crust, potentially influenced by regional extension or volcanic loading. No escalation to higher magnitudes occurred, supporting the interpretation of a contained swarm rather than a foreshock sequence to a larger event.

Overall, VS20160704.1 underscores the persistent low-level seismicity characteristic of Hawaii's dynamic geology, where hotspot-driven volcanism continues to shape the landscape.

References

USGS Earthquake Catalog Hawaii Volcano Observatory reports SeismoSight internal swarm classification VS20160704.1