Seismic Swarm S20180703.3 Near Pāhala, Hawaii

Seismic swarm S20180703.3 was recorded 7 km east of Pāhala on Hawaii’s Big Island. The sequence began at 15:43 on 2 July 2018 and concluded at 17:35 on 4 July 2018, spanning 49 hours and 51 minutes. During this interval, 30 earthquakes were detected, with magnitudes ranging from 1.2 to 4.1 and focal depths varying from 37 km to shallow or near-surface levels.

The swarm opened with two closely spaced events at 15:43 on 2 July, registering magnitudes 1.2 and 2.2 at depths of 37 km and 35 km. Activity intensified later that afternoon with a magnitude 4.0 earthquake at 6 km depth, accompanied by a magnitude 3.1 event at 5 km. Subsequent events clustered at shallow depths, including several between 0 km and 2 km, while deeper events persisted around 25–35 km. Peak magnitudes occurred on 4 July, including a magnitude 4.1 event at 05:31 and a magnitude 3.2 shock at 16:55.

Pāhala lies on the southern flank of Kīlauea volcano within the Hawaiian hotspot chain. The island’s geology is dominated by shield volcanoes built by the Pacific plate’s passage over a mantle plume. Frequent seismicity arises from magma intrusion, volcanic flank movement, and crustal adjustment. Depths recorded in this swarm align with both deep magmatic processes and shallow volcanic or tectonic sources typical of the region.

Since 1 January 2000, 43 seismic swarms have occurred in the area. Annual counts show elevated activity in 2004 (7 swarms), 2015 (5 swarms), and 2018 (7 swarms), indicating recurring episodic unrest linked to Kīlauea’s volcanic system.

The 2018 swarm reflects the characteristic pattern of clustered, moderate-magnitude events that accompany magma migration or south-flank slip. Shallow events dominated the later phase, consistent with surface deformation often observed during volcanic unrest on the Big Island.

References

SeismoSight internal swarm classification S20180703.3
USGS Hawaiian Volcano Observatory geological summaries
Hawaii Seismic Network historical records