2014 Earthquake Swarm in Northwest Brooks Range, Alaska

Friday, 19 December 2014
Natalia A Ruppert and Stephen G Holtkamp, University of Alaska Fairbanks, Fairbanks, AK, United States
An unusual sequence of earthquakes in NW Brooks Range region of Alaska began with two magnitude 5.7 earthquakes within minutes of each other on April 18, 2014. These events were followed by a vigorous aftershock sequence with many aftershocks reaching magnitude 4 and higher. Later, three more magnitude 5.7 earthquakes occurred in the same source region on May 3, June 7 and June 16. Earthquake source mechanisms indicate normal faulting on SE-NW striking fault planes. The source region is located ~20 km NE of the Noatak village and ~40 km S of the Red Dog Mine. A magnitude 5.5 occurred in this area in 1981. The 1981 sequence also exhibited a swarm-like behavior over the course of 6 months.

Detection and reporting of these earthquakes is complicated by sparseness of seismic network in NW Alaska. At the time of April 18 earthquake the nearest seismic site was located at the Red Dog Mine, with the next nearest station 350 km away. Following the May 3 event, the Alaska Earthquake Center installed two additional temporary stations, one in Noatak and another in Kotzebue, 85 km S of the source area. Overall, 450 events were reported in this sequence through end of July. The catalog magnitude of completeness with the additional stations was about ~2.2.

We applied waveform template matching algorithm to detect additional events in this sequence that could not be detected with the standard network processing. The template matching resulted in ~600 additional event detections. The waveform cross-correlation indicates that most of the events are not repeating sources. From the catalogued events, only 6% of event pairs have correlation coefficients of 0.75 or higher. We were able to identify only a few families of repeating events. Only one family seemed to be present throughout the entire sequence, while other event families were mostly short-lived. We find preliminary evidence that the earthquakes migrated to shallower depths throughout the sequence, consistent with the observation that most event families are short-lived.

We speculate that these earthquakes are concentrated in a zone of weak crust that allows intrusion of upper mantle materials, such as fluid and volatiles. Western Alaska is characterized by extensional tectonics and accommodates a distributed plate boundary between Bering micrpolate and North American plate.