The actively deforming Indo‐Australian intraplate region off the Sumatra‐Andaman trench hosted the largest strike‐slip earthquake recorded by modern instruments, the 2012 Mw 8.6 Wharton Basin earthquake, closely followed by a Mw 8.2 aftershock. These two large events ruptured either parallel north‐south trending faults or a series of north‐south and nearly perpendicular east‐west fault planes. No active east‐west faults had been identified in the region prior to these earthquakes, and the seismic rupture for these two earthquakes extended past the 800°C isotherm for lithosphere of this age, deep into the oceanic mantle and possibly beyond the inferred transition to ductile failure. To investigate the seismic behavior of this region, we calculate moment tensors with teleseismic body waves for 6.0 ≤ Mw ≤ 8.0 intraplate strike‐slip earthquakes. The centroid depths are located throughout the seismogenic mantle and could extend through the oceanic crust, but are generally well constrained by the 600°C isotherm and do not appear to rupture beyond the 800°C isotherm. We conclude that while many earthquakes are consistent with a thermal limit to depth, large magnitude earthquakes may be able to rupture typically aseismic zones. We also perform finite‐fault modeling for Mw ≥ 7.0 earthquakes and find a slight preference for rupture on east‐west oriented faults for the 2012 Mw 7.2 and 2005 Mw 7.2 earthquakes. This lends support for the presence of active east‐west faults in this region, consistent with the majority of previously published models of the 2012 M8+ earthquakes.

The 2015 Mw 7.1 earthquake on the Charlie‐Gibbs transform fault along the Mid‐Atlantic Ridge is the latest in a series of seven large earthquakes since 1923. We propose that these earthquakes form a pair of quasi‐repeating sequences with the largest magnitudes and longest repeat times for such sequences observed to date. We model teleseismic body waves and find that the 2015 earthquake ruptured a distinct segment of the transform from the previous 1998 earthquake. The two events display similarities to earthquakes in 1974 and 1967, respectively. We observe large oceanic transform earthquakes to exhibit characteristic slip behavior, initiating with small slip near the ridge, and propagating unilaterally to significant slip asperities nearer the center of the transform. These slip distributions combined with apparent segmentation support multimode slip behavior with fault slip accommodated both seismically during large earthquakes and aseismically in between.

We investigate the 2013 M w 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 M w 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.