McCarthy M1.1 | Alaska Earthquake Center

Tectonic position of the Yakutat Block

The Yakutat microplate is the most recent addition to the assemblage of accreted terranes that comprise southern Alaska. The microplate was transported northward along margin-parallel transform faults, including the Queen Charlotte and Fairweather faults. Eventually, the microplate encountered the southern Alaska continental margin, where ongoing collision creates complex seismotectonic interactions resulting in a very active seismic belt. Two large earthquakes in 1899 (magnitude 8.1 and 8.2) altered landforms in the Yakutat Bay area, resulting in uplift of up to 40 feet and land subsidence of 6 feet. Another notable event in this region was the magnitude 7.7 Lituya Bay earthquake in 1958, which occurred from the head of Yakutat Bay along the entire length of the Fairweather fault. This earthquake caused a massive landslide that collapsed into Lituya Bay and generated a 1,720-foot-high tsunami wave. The last major earthquake in the region was the magnitude 7.4 Saint Elias earthquake in 1972. In the offshore zone, the most prominent structures are the Transitional Fault separating the Yakutat Microplate from the Pacific Plate and the Northern Fault Zone of the Gulf of Alaska. A series of strong earthquakes in the Gulf of Alaska, including a magnitude 7.7 earthquake in 1987 and a magnitude 7.8 earthquake in 1988, caused significant ground motions that were felt along the entire southeastern and southern Alaska coast. Aside from a magnitude 6.7 event in 1973 at the southeastern end of the fault, no significant earthquakes on the Transitional Fault are known. This region has high background seismicity. Hundreds of earthquakes are recorded each year, most of which are shallow and within the 50-mile-wide coastal zone.