Abstract

We investigate the 2013 Mw 7.5 Craig, Alaska, earthquake and nearby seismicity to understand better how temperature and composition may control the depth of seismic rupture along a strike‐slip fault offsetting contrasting lithosphere types. The Queen Charlotte–Fairweather (QCF) fault lies between the oceanic lithosphere of the Pacific plate and the accreted Insular superterrane of the North American continent. We use point‐source and finite‐fault modeling of teleseismic body waves to characterize the focal mechanism and the depth extent of seismic rupture of five Mw 5.9–7.5 earthquakes. Four of the five earthquakes are consistent with rupture on the QCF fault. We find that these four earthquakes have centroid depths between 11 and 18 km (±3  km) and aftershocks with an order of magnitude less than typical continental earthquakes. Finite‐fault modeling of the 2013 Craig earthquake favors bilateral rupture along a 150 km fault with a depth range of slip between 5 and 25 km, with faster rupture (4–5  km/s) to the north than the south (1  km/s). These results suggest that the transition of brittle to ductile deformation along this section of the Pacific–North American plate boundary is thermally controlled by a more mafic rheology than average continental crust, exhibiting behavior consistent with that of an oceanic strike‐slip fault.

Online Material: Figures of best‐fitting point‐source focal mechanism and waveform fits, dip sensitivity, directivity tests, aftershock distributions, and finite‐fault results.

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