Seismic swarm S20130714.1 was recorded 99 km east of McCarthy, Alaska, beginning at 05:47 on 14 July 2013 and concluding at 19:20 on 20 July 2013. Over 157 hours and 32 minutes, the sequence produced 300 earthquakes. The first 100 events exhibited a clear temporal clustering, with the majority occurring within the initial 48 hours. Magnitudes ranged from 0.5 to 4.4, and focal depths remained shallow, predominantly between 0 and 20 km. The sequence opened with a magnitude 3.3 event at 05:47 on 14 July, followed rapidly by a magnitude 1.2 shock seven minutes later. Subsequent activity included a magnitude 3.4 at 08:10 and a magnitude 4.4 at 08:15, marking the largest event in the initial hundred. Depths for these early larger shocks averaged around 9–11 km. Smaller events (magnitudes 0.5–2.0) dominated thereafter, with many registering at depths of 4–16 km. Later notable shocks within the first 100 included additional magnitude 4.4 and 3.3 events on 15 July at depths of 11 km. The pattern showed episodic bursts separated by brief lulls, consistent with swarm behavior rather than a classic mainshock-aftershock sequence. This swarm occurred in a region shaped by long-term plate-boundary processes. Eastern Alaska near McCarthy lies within the St. Elias orogen, where ongoing collision and accretion of the Yakutat terrane against the North American margin drive crustal deformation. The Pacific Plate subducts northwestward beneath Alaska along the Aleutian megathrust to the south, while the Yakutat block’s flat-slab subduction and collision produce transpressional stresses that extend inland. These forces have uplifted the Wrangell and St. Elias Mountains and created a network of active faults capable of generating both volcanic and tectonic seismicity. Wrangellia terrane rocks dominate the local geology, comprising Paleozoic to Mesozoic volcanic and sedimentary assemblages accreted to North America during the Mesozoic. Subsequent Cenozoic deformation, including right-lateral strike-slip motion along regional faults and thrust faulting associated with Yakutat indentation, continues to release strain through frequent small earthquakes. Historical records indicate that the area has experienced multiple seismic swarms; since 1 January 2000 only two such swarms have been documented, with the first occurring in 2012. Swarm S20130714.1 provides further evidence of the persistent low-to-moderate level of seismicity that characterizes this portion of the Yakutat–North America collision zone. Depths consistently less than 20 km align with the brittle upper crust where strain accumulates and releases along mapped and unmapped faults. Continued monitoring remains essential for understanding how these transient clusters relate to longer-term tectonic loading in one of Alaska’s most rapidly deforming regions.
Alaska Earthquake Center, University of Alaska Fairbanks – regional seismicity summaries.
United States Geological Survey Earthquake Hazards Program – tectonic framework of southern Alaska.
Geological Society of America – Wrangellia and Yakutat terrane accretion history.