Seismic Swarm S20111127.1 Near Yakutat, Alaska: Geological Context and Event Analysis

A seismic swarm designated S20111127.1 occurred approximately 58 km north-northwest of Yakutat, Alaska, between 22:44 on 26 November 2011 and 04:55 on 28 November 2011. Over this 30-hour period, 33 earthquakes were recorded, with the majority exhibiting magnitudes between 1.3 and 2.2 and focal depths ranging from 1 to 16 km. The largest event reached magnitude 3.7 at 02:21 on 27 November at a depth of 11 km, followed by a magnitude 2.9 event later that day. Activity remained clustered in time and space, characteristic of swarm behavior rather than a typical mainshock-aftershock sequence.

Yakutat lies within the tectonically complex transition zone between the strike-slip Queen Charlotte-Fairweather fault system and the convergent margin associated with the Yakutat microplate. This microplate, an oceanic plateau fragment, has been colliding with and subducting beneath the North American plate since the Miocene, driving rapid uplift of the St. Elias Mountains and elevated seismicity across southeastern Alaska. The region experiences both shallow crustal earthquakes and deeper events linked to the underthrusting slab, with historical records documenting magnitude 7–8 events along the Fairweather fault, including the 1958 earthquake that produced significant coastal deformation.

The November 2011 swarm aligns with the shallow crustal regime typical of this collision zone. Most events occurred at depths of 8–15 km, consistent with brittle failure in the upper crust where strain accumulates from ongoing convergence at rates exceeding 40 mm per year. The tight temporal clustering and absence of a dominant mainshock suggest fluid migration or aseismic slip may have contributed to triggering, processes observed in other Alaskan swarms near convergent boundaries.

Subsequent monitoring by regional networks has confirmed that such swarms are recurrent features in the Yakutat area, often occurring without producing surface rupture or significant damage due to their moderate magnitudes and offshore or remote locations. Updated seismic hazard assessments for the region continue to emphasize the potential for larger events driven by the same tectonic forces responsible for the 2011 activity.

References:
USGS Earthquake Catalog
Alaska Earthquake Center Annual Reports
Geological Society of America Special Paper on Yakutat Terrane Tectonics