The 2015 Mw 7.1 earthquake on the Charlie-Gibbs transform fault: Repeating earthquakes and multimodal slip on a slow oceanic transform

The 2015 M-w 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.