A High Resolution Aftershock Catalog of the Magnitude 7.5 Craig, Alaska, Earthquake on 5 January 2013

The M-w 7.5 Craig, Alaska, earthquake on 5 January 2013 was the second largest earthquake on the Queen Charlotte fault (QCF) in 40 yrs, representing the transform plate boundary between the Pacific and North American plates. Earthquake hazard along the QCF was known from historical earthquakes, but the absence of major earthquakes in the digitally instrumented era has limited studies of earthquakes. Here, we present an analysis of aftershocks of the Craig earthquake. We first use waveform cross correlation and double-difference relative relocation to relocate the aftershock catalog of the Alaska Earthquake Center (AEC). Our relocated catalog reveals a complex fault structure activated in response to the mainshock and provides insight into the faulting mechanics of this graduated transpressive plate boundary. We then use network-matched filtering (NMF) to detect additional aftershocks. Using 351 aftershocks located by the AEC in 2013 as template events, we identify 1785 aftershocks through May 2013. This is a greater than fivefold increase in aftershock detection rate compared with the standard single-event network processing. Utilizing regional seismic stations and waveform modeling, we are able to characterize the faulting parameters for 12 aftershocks. Six aftershocks show focal mechanisms consistent with the mainshock faulting parameters, and six aftershocks show either thrust or strike-slip mechanisms with nodal planes rotated similar to 45 degrees from the mainshock mechanism. The six aftershocks that are inconsistent with the QCF have P axes that are perpendicular to the plate boundary, suggesting that transpression remains at this location. We show that five of these large aftershock families were activated within a day of the mainshock, whereas seven families were delayed by multiple days before activating. We show evidence for migrating patterns of seismic quiescence and elevated seismicity, relative to the predicted Omori-Utsu decay, which is related to the largest aftershock in the sequence.