Post- and inter-seismic behavior in the central Queen Charlotte fault: implications for earthquake cycle

The ~900 km long Queen Charlotte Fault (QCF), which separates the Pacific and North American plates, is the fastest-slipping oceanic–continental transform fault on Earth. Since the early 20th century, six major earthquakes with moment magnitudes greater than 7.0 have struck along the QCF, posing significant hazard threats to western America and Canada. The most recent event, the 2013 Mw 7.5 Craig earthquake, ruptured the central segment of the QCF and has been proposed as the first reported oceanic interplate earthquake exhibiting supershear rupture (Yue et al., 2013, JGR, 10.1002/2013JB010594), attracting widespread attention within the seismological community. Nevertheless, owing to the lack of long-term near-field monitoring, the seismic behavior of this region remains poorly understood.
In this study, we analyzed data from a dense ocean-bottom seismometer (OBS) network (network code YI) deployed along the central QCF from late August 2021 to early September 2022. The network consists of 25 OBS stations with an average spacing of ~15 km, providing an exceptional opportunity to characterize microseismicity along the central QCF. Using PickBlue, a machine-learning–based phase picker trained on OBS data, we constructed a high-resolution seismicity catalog comprising 502 well-located earthquakes with moment magnitudes ranging from 1.0 to 3.3. Our catalog delineates a steeply dipping (75°–80°) subvertical fault plane and reveals distributed seismicity within the Pacific plate, suggesting that transpressive convergence along the central QCF is largely accommodated by slip on the dipping fault plane and by offshore deformation of the Pacific plate.
Furthermore, along the central QCF, a highly segmented seismic behavior was revealed. Two primary earthquake clusters were detected in the southern section near the epicenter of the 2013 Mw 7.5 Craig earthquake, whereas the northern section remains nearly aseismic. The most active cluster was located at the margin of the main coseismic rupture area and coincides with a slightly curved fault segment, which may have decelerated northward rupture propagation during the 2013 Craig earthquake while accommodating most deformation. Further south, in the largest coseismic slip region, an additional cluster is observed within the area of maximum coseismic slip, suggesting progressive stress reloading on the previously ruptured fault plane. To better understand the stress evolution of the 2013 Craig earthquake, we also relocated a 21-day local aftershock catalog recorded ~4 months after the mainshock (Walton et al., 2019, EPSL, 10.1016/j.epsl.2018.11.021). Notably, the spatial distributions of aftershocks and interseismic events display a pronounced complementary pattern in the largest coseismic slip region, with interseismic events distributed at the center of the rupture zone and aftershocks beautifully surrounding it. Together, these observations illuminate the stress evolution of the 2013 Craig earthquake from the postseismic to the interseismic period and provide new insights into understanding the seismic cycle.