Seismic swarm S20180127.1 occurred approximately 16 km east-southeast of Chickaloon in south-central Alaska. The sequence began at 21:10 UTC on 26 January 2018 and concluded at 10:27 UTC on 30 January 2018, spanning 85 hours and 16 minutes. During this period, 58 earthquakes were recorded, with magnitudes ranging from 0.6 to 4.6 and focal depths between 2 km and 14 km. The largest event reached magnitude 4.6 at a depth of 13 km on 27 January, while an initial magnitude 4.3 shock occurred at 3 km depth shortly after the swarm onset.
The temporal distribution showed the highest activity on 27 January, including multiple events above magnitude 2.0. Depths remained predominantly shallow, clustering between 4 km and 13 km, consistent with upper-crustal faulting. Lower-magnitude events dominated later phases, tapering toward the swarm's termination on 30 January.
South-central Alaska lies within a tectonically active margin where the Pacific Plate subducts beneath the North American Plate along the Aleutian megathrust. This convergence drives regional seismicity, including both megathrust events and crustal earthquakes associated with the Castle Mountain Fault system and related structures near the Matanuska Valley. Chickaloon sits near the northeastern extent of these fault zones, where strain accumulation produces frequent small-to-moderate earthquakes.
Historical records indicate that south-central Alaska has experienced significant seismic events, most notably the 1964 Great Alaska Earthquake of magnitude 9.2, whose epicenter lay roughly 200 km to the southwest. Post-1964 aftershock sequences and background seismicity have continued to shape the region. Since 2000, only one prior swarm was documented in the immediate area, occurring in 2016, underscoring the relative rarity of clustered activity compared with isolated events.
The 2018 swarm's characteristics align with known patterns of induced or triggered seismicity in the region, potentially influenced by fluid migration or stress transfer along pre-existing faults. No damage or felt reports of consequence were associated with these events, reflecting their modest magnitudes and depths.