Tectonic Setting of the Yakutat Block
The Yakutat microplate is the latest addition to the assemblage of accreted terranes that make up
southern Alaska. The microplate was transported northward along margin-parallel transform faults,
including the Queen Charlotte and Fairweather faults. Eventually, the microplate encountered the
continental margin of southern Alaska, where the ongoing collision creates complex seismotectonic
interactions resulting in a very active seismic belt. Two great earthquakes in 1899 (M8.1 and 8.2)
re-arranged landforms in the Yakutat Bay area, resulting in up to 40 feet of uplift and 6 feet of land
subsidence. Another notable event in this region was the 1958 M7.7 Lituya Bay Earthquake, which
ruptured from the head of Yakutat Bay along the entire length of the fault. This earthquake caused a
massive landslide that crashed into Lituya Bay, creating a 1,720 feet high tsunami wave. The most
recent major earthquake in the region was the 1972 M7.4 Saint Elias Earthquake. In the offshore zone,
the most notable structures are the transition fault that separates the Yakutat microplate, Pacific
Plate, and Gulf of Alaska fault zones. A sequence of strong earthquakes in the Gulf of Alaska (M7.7
and 7.8) in 1987 and 1988 produced significant ground motions that were felt along the entire southeast
and southern Alaska coast. The transition fault is not known to have produced significant earthquakes
except for a 1973 M6.7 event along the southeastern end of the fault. This region has an extremely high
level of background seismicity, with thousands of earthquakes recorded each year, the majority of which
are shallow and located within the 50 mile wide coastal zone.
