Seismic swarm S20130823.1 occurred approximately 98 km east of McCarthy, Alaska, within the tectonically dynamic Wrangell Mountains region. The sequence began at 00:06 on 23 August 2013 and concluded at 06:17 on 25 August 2013, spanning 54 hours and 10 minutes. During this period, 50 earthquakes were recorded, providing a clear example of clustered seismic activity in a remote, geologically complex area.
The swarm unfolded with an initial series of low-magnitude events on 23 August, followed by a peak in activity later that day. A magnitude 4.4 earthquake at 21:28:57 marked the strongest event, occurring at a depth of 13 km. Subsequent shocks included multiple events between magnitudes 1.1 and 2.1, concentrated at shallow depths ranging from 0 to 19 km. Activity continued at a reduced pace through 24 August, with events such as a magnitude 2.6 shock at 20:19:40 (14 km depth), before tapering off on 25 August. Depths remained predominantly crustal, consistent with regional faulting patterns.
Geologically, the location sits near the convergence of the Pacific and North American plates, where subduction drives ongoing deformation. The Wrangell Mountains feature volcanic edifices and associated fault systems that accommodate strain release through both isolated quakes and swarm sequences. Historical records indicate limited swarm occurrences in the vicinity since 2000, with only three documented episodes: two in 2012 and one prior instance in 2013. This scarcity underscores the episodic nature of such clusters amid the broader background of tectonic seismicity.
Analysis of the event timings reveals a classic swarm progression, with the magnitude 4.4 shock triggering an immediate increase in smaller aftershocks that decayed over the following days. Magnitudes clustered below 2.0 for the majority of events, reflecting efficient energy dissipation along local structures without escalation to a larger mainshock-aftershock sequence. Depths averaging under 10 km suggest involvement of near-surface fault networks influenced by the region's volcanic and sedimentary geology.
Such swarms contribute to understanding strain accumulation in Alaska's interior ranges, where monitoring supports hazard assessment for nearby communities and infrastructure. Continued observation of similar patterns aids in distinguishing tectonic signals from other geophysical phenomena in this high-latitude setting.