Complex Faulting and Triggered Rupture During the 2018 MW7.9 Offshore Kodiak, Alaska Earthquake

In the early morning hours of January 23, 2018, a magnitude 7.9 earthquake struck 300 km offshore Kodiak Island, Alaska in the outer rise region of the Alaska-Aleutian subduction zone. While the moment tensor for the earthquake suggests predominantly strike-slip faulting, the true complexity of the source has only become evident through analysis of multiple datasets. We combine aftershock relocations, source mechanisms, teleseismic P-wave back-projection and GPS data inversion to constrain complex faulting geometry of the M_W7.9 earthquake. Relocated aftershocks delineate several N-S trends includinga prominent 110-km-long segment, as well as broad NE-SW trends. The N-S faults correlate with plate bending faults imaged by seismic imaging and high-definition bathymetry (including the most recent data collected in July 2018 by the Alaska Amphibious Seismic Community Experiment cruise Leg 2 onboard R/V Sikuliaq). The NE-SW faulting trends do not coincide with locations of oceanic plate fractures of other features detectable on the ocean floor, but correlate with orientations of fault planes of previously recoded earthquakes in the area. GPS modeling and back-projection indicate that the NE-SW trending left-lateral strike-slip segments released most energy dominating far field crustal deformation and radiated wavefield. Back-projection infers fast E-to-W rupture propagations superimposed on a slower S-to-N migration. We propose a five-segment model of the rupture that was partially driven by dynamic triggering.