Seismic Swarm S20200821.1: Analysis of Activity East of Pāhala, Hawaii

Seismic swarm S20200821.1 was recorded 5 km east of Pāhala on Hawaii's Big Island. The sequence began at 18:09 on 20 August 2020 and concluded at 02:21 on 23 September 2020, spanning 800 hours and 11 minutes. During this period, 392 earthquakes were detected, providing insight into ongoing crustal dynamics in a volcanically active region.

The Pāhala area lies on the southern flank of Kilauea volcano within the Kaʻū district. This zone experiences frequent seismicity driven by the interaction of magmatic processes, gravitational spreading of the volcanic edifice, and movement along the basal decollement fault. Depths of recorded events clustered primarily between 29 km and 37 km, consistent with activity near the interface between the volcanic pile and the underlying oceanic crust. Shallower events, such as one at 0 km, occurred infrequently and may reflect localized fracturing or measurement variation.

Analysis of the first 100 events reveals a predominance of low-magnitude earthquakes. Magnitudes ranged from 1.5 to 3.4, with the majority falling between 1.7 and 2.3. Notable larger events included a magnitude 3.4 earthquake on 25 August at 35 km depth and a magnitude 3.0 event on 23 August at 34 km depth. Depths remained stable around 30–35 km for most events, indicating a consistent source region. Timing showed initial clustering in the first 48 hours, followed by sustained but declining activity through late August.

Hawaii's geology is shaped by hotspot volcanism. The Big Island hosts active shields including Kilauea and Mauna Loa, where magma transport and flank instability generate swarms. Historical records since 2000 document 55 such swarms in the region, with elevated counts in 2003 (6), 2004 (5), 2018 (8), and 2020 (4). These episodes often correlate with periods of volcanic unrest or aseismic slip along the south flank.

The 2020 swarm fits established patterns of deep seismicity beneath the Pāhala area, where stresses accumulate from both volcanic loading and tectonic forces. Event rates and magnitude distributions align with typical swarm behavior rather than mainshock-aftershock sequences. No surface rupture or significant ground deformation was associated with this particular episode based on available monitoring.

Continued seismic monitoring remains essential for understanding magma movement and flank stability in this dynamic setting. Data from networks operated by the U.S. Geological Survey Hawaiian Volcano Observatory support long-term assessment of hazard potential.

References

U.S. Geological Survey Hawaiian Volcano Observatory earthquake catalogs SeismoSight internal swarm classification records