Association of the November 30, 2018 M 7.1 Earthquake near Anchorage, Alaska to the Tectonics of Southcentral Alaska

Abstract

The November 30, 2018 M 7.1 earthquake near Anchorage, Alaska was caused by a deep normal fault. It was limited to a Yakutat plate as based on a new proposed tectonic model which reflected the movement of a block of this westsouthwest subducting plate into the Cook Inlet subduction zone. This movement was strongly influenced by a deeper and faster north-northwest subducting Pacific plate. It is evident that the Cook Inlet subduction zone has two distinct subducting plates in contrast to the Aleutian subduction zone to the immediate southwest which has just one. Similar normal fault events also exist for the region, but ones comparable to the November 30 size are unknown. However other deep to shallow M 7± earthquakes associated with large strike-slip faults in the Yakutat plate have been common and are called slice faults. In addition, varying types of megathrust earthquakes occur. The great 1964 Alaska earthquake reflected a sudden megathrust slip principally between the Pacific and the Yakutat plates for the offshore region of southcentral Alaska. Based on paleoseismic data a similar oceanic megathrust is expected in about 800 years. Paleoseismic evidence and the mechanics of the November 30 event indicate more continental megathrust earthquakes also occur along the bottom and top of the Yakutat plate with the Pacific and the North American plates, respectively. In fact, based on this paleoseismic data and on present crustal folding for the Cook Inlet region, a more continental megathrust earthquake is expected in about 230± years which would be due to west-southwest thrusting between the Yakutat and the North American plates. The November 30, 2018 earthquake would be small compared with such a predicted megathrust, but it is a very important precursor to it. The November 30 event also helps to confirm the existence of a Redoubt slice fault within the Yakutat plate by defining its boundaries and nature. Collectively these elements verify the existence of a west-southwest subducting Yakutat plate with its regional strike-slip slice faults which helps to explain the seismicity and some of the complex geology of southcentral Alaska.