Tectonic Setting of the Gulf of Alaska
Seismicity of the Gulf of Alaska region is characterized by very shallow earthquake sources confined to the Pacific Plate crust. Typically, these events activate strike-slip faulting on preexisting fracture zones formed during the ocean ridge spreading processes hundreds of millions of years ago or break along the new fault lines. The M7.8 and 7.7 events in 1987 and 1988 were the largest earthquakes previously recorded within the Pacific plate in the Gulf of Alaska region. These were right-lateral strike-slip faulting events that occurred along the NS-oriented faults. The previously recorded earthquakes indicate that the outer rise region of the Alaska-Aleutian subduction zone experiences normal faulting events only as far north-east as the Shumagin Islands region. Farther north-east towards Kodiak Island and Kenai Peninsula, the outer rize events are either characterized by strike-slip or less commonly reverse faulting. On January 23, 2018 a magnitude 7.9 occurred in the outer rise region of the Aleutian megathrust, about 200 miles SE of Kodiak island. This was once again a strike-slip earthquake, possibly activating complex faulting on conjugate faults. The 2012 M8.6 and 8.2 earthquakes that occurred offshore Northern Sumatra within the Indian plate demonstrated that a complex system of faults in the outer rise could be activated during a single earthquake.