Seismic Swarm S20181130.2: Analysis of the 2018-2019 Point MacKenzie Sequence

The seismic swarm designated S20181130.2 began at 17:29 UTC on 30 November 2018 and concluded at 18:44 UTC on 11 April 2019. Its epicentral area lies 4 km southeast of Point MacKenzie, Alaska, within the Cook Inlet basin. Over 3169 hours and 15 minutes, the sequence produced 9026 recorded earthquakes.

The initiating event was a magnitude 7.1 earthquake at 46 km depth, located 1 km southeast of Point MacKenzie. This mainshock was followed immediately by a dense series of aftershocks. Within the first 100 events, magnitudes ranged from 7.1 down to 2.1, with the majority falling between 2.5 and 4.0. Depths clustered between 30 km and 50 km, consistent with the regional seismogenic zone.

The first hour alone included multiple events above magnitude 4.0, such as the 5.8 at 17:35 and the 5.2 at 18:00. Subsequent activity showed a classic aftershock decay pattern, with event rates highest immediately after the mainshock and gradually declining. Depths remained stable, indicating slip within the subducting slab interface rather than shallow crustal faulting.

Geologically, the Cook Inlet region sits above the convergent margin where the Pacific plate subducts beneath the North American plate at rates of approximately 6 cm per year. This setting generates both intraslab and interface earthquakes. The November 2018 sequence occurred within the intraslab environment, similar to other moderate-to-large events recorded in south-central Alaska.

Historical records since 2000 show only three prior swarms in the immediate vicinity: one event in 2002, one in 2005, and one in 2009. These earlier episodes were far smaller in both number and maximum magnitude. The 2018–2019 swarm therefore represents the most energetic cluster documented in the area during the instrumental era.

Examination of the initial 100 events reveals that 78 percent had magnitudes below 3.5, underscoring the swarm’s productivity at lower magnitudes. Depths averaged 40 km, with no systematic migration observed in the early phase. This distribution supports a model of triggered slip on preexisting intraslab fractures rather than fluid-driven migration.

The sequence remained confined to a compact volume roughly 10 km in radius. No surface rupture was reported, consistent with the hypocentral depths. Regional seismic networks recorded continuous activity through early 2019, with the final events marking the return to background seismicity levels.

References

USGS Earthquake Catalog
Alaska Earthquake Center annual reports
SeismoSight internal swarm classification database