On 23 January 2018 a Mw 7.9 earthquake occurred in the Gulf of Alaska, 175 miles southeast of Kodak Island (56.06°N, 148.38° W). It was the largest Alaskan earthquake since the 1964 Mw 9.2 earthquake, which is the largest on record worldwide. The earthquake generated a small tsunami (<30 cm/1 ft) that was observed on local coastlines as well as across the Pacific, including the French Polynesian island Niku Hiva, 7250 km (4500 mi) distant, where the wave measured 24 cm.

The region is marked by the seismically active Alaska-Aleutian Trench, where the Pacific Plate is moving to the north-northwest at approximately 59 mm/yr (about 2 inches/yr) toward the continental North American Plate. The trench marks the boundary of the subduction zone where the Pacific Plate sinks beneath the North American Plate. Most often, earthquakes near subduction zones occur on thrust faults beneath the overriding plate where tension builds up between the oceanic and continental plates, eventually leading to a fault rupture. In the case of the January 23rd earthquake, the location and mechanism show it originated approximately 120 km (75 mi) offshore of the trench, at a depth of 15 km (9.3 mi) on a strike-slip fault system within the Pacific Plate, rather than at the actual plate boundary. Subsequent analysis revealed a complex network of multiple faults rupturing in two different directions (e.g., Krabbenhoef et al. 2018, http://doi.org/10.1038/ s41598-018-32071-4; Guo et al. 2020 https:// doi.org/10.1029/2019JB019267).

Top: Regional tectonics and seismicity Mw 6 or greater, by depth. Epicenter of the 23 January Mw 7.9 earthquake is shown by the yellow star. Bottom: Geographic setting.