Earthquake swarm S20121101.1 was recorded from 23:02 UTC on 31 October 2012 until 22:12 UTC on 2 November 2012, centered 104 km east of McCarthy, Alaska. In 47 hours and 10 minutes, the sequence produced 32 events with magnitudes between 1.0 and 2.9 and focal depths from 0 to 14 km. No single mainshock dominated; instead, activity occurred in a diffuse cluster, consistent with swarm behavior driven by fluid migration or minor stress adjustments along local faults.
The largest event reached magnitude 2.9 at 09:00 UTC on 1 November at 10 km depth. Subsequent events remained below magnitude 2.0, with the final recorded shock of magnitude 1.5 occurring at 22:12 UTC on 2 November at 10 km depth. Depths were predominantly shallow, averaging 8 km, indicating activity within the upper crust where brittle failure is common.
McCarthy lies in the eastern Chugach-St. Elias Mountains, a region shaped by ongoing convergence between the Yakutat microplate and the North American plate. This collision, active since the Miocene, produces rapid uplift rates exceeding 10 mm per year and accommodates strain through both thrust and strike-slip faulting. The swarm location aligns with the northern margin of the St. Elias fold-and-thrust belt, where sedimentary and volcanic rocks overlie accreted terranes. Regional seismicity is further influenced by the nearby Denali Fault system and the Wrangell volcanic field, though the 2012 events showed no direct volcanic association.
Since 1 January 2000, only one swarm has been identified in this sector, making S20121101.1 the sole documented instance. The low-magnitude, high-frequency nature of the sequence suggests it released localized strain without triggering larger rupture on major structures.
USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Alaska Earthquake Center, University of Alaska Fairbanks
USGS National Earthquake Information Center tectonic summaries for the Yakutat collision zone