Spatiotemporal distribution of events during the first week of the 2012 Haida Gwaii aftershock sequence

We systematically relocated the 2012 Haida Gwaii aftershock sequence by re-examining continuous seismic waveforms of the Canadian National Seismographic Network (CNSN). Because of the sparse station density in the source area and the offshore location of the majority of events, precise determination of hypocenters is extremely challenging. Constraints on the back azimuth and incident angle of ray paths were derived from the cross correlation of three-component waveforms and incorporated into the locating process to improve the results. We located 1228 aftershocks for the first week after the mainshock, effectively double the size of the CNSN routine earthquake catalog. The distribution of aftershocks tends to form two linear trends roughly parallel to the strike of the plate margin in the northwest-southeast direction. The trend located updip from the mainshock within the subducting Pacific plate appears to have three clusters spanning a lateral distance of approximately 80 km. The other trend coincides with the surface trace of the Queen Charlotte fault (QCF), spanning the northern two-thirds of the main rupture zone. The overall area of aftershock distribution is approximately 120 kmX approximately 40 km. Aftershocks overlap the southern part of the estimated rupture zone of the 1949 earthquake, with no activity in the seismic gap farther to the south. Therefore, it is likely that the 2012 earthquake only partially released the accumulated stress on the strike-slip transform fault system. In this scenario, the possibility of a major strike-slip earthquake along the southernmost part of the QCF should not be ignored.