Recent Earthquake and Tsunami History

Southern Alaska and the adjoining Aleutian Island chain together constitute one of the most seismically active zones of the world. Approximately 7% of the annual release of seismic energy occurs within this region. Between 1899 and 1965, seven earthquakes were recorded with a Richter magnitude of 8 or larger and over 60 earthquakes were recorded with a Richter magnitude of 7 or larger. Significant changes in ground elevation (uplift and subsidence) as well as horizontal ground movements were associated with many of these earthquake events. Enormous vertical movements of the sea floor associated with earthquakes within the shelf area of the Gulf of Alaska have generated large tsunamis which have propagated across the Pacific Ocean (e.g. the 1946 and 1964 teletsunamis). In addition to the major teletsunamis that are generated in this area, many damaging local tsunamis have originated within the narrow steep sided fjords that characterise much of the coastline of southern Alaska. Many of these local tsunamis are generated along the Fairweather Fault system. The Fairweather is a major tectonic feature of the southern Alaska region and is much like the San Andreas system in California. Whilst the type of movement along this fault system (horizontal rather than vertical) means that tsunamis are not really generated as a direct result of earthquake activity; earthquakes on this system do cause large underwater sediment slumps at the heads of river deltas entering the sea. These sediment slumps in turn trigger tsunamis. Tsunamis have also been generated by a variety of processes associated with erupting volcanoes in the region.

Alaska has a complex tsunami history and it is highly probable that the tsunami history of the area is incomplete because of factors such as low population density and a short written record. The shores of Alaska have been impacted by both locally generated tsunamis and by those that have propagated to the State's shores from outside the region. Landslide generated tsunamis triggered by earthquakes are common in the Prince William Sound and southeastern Alaskan areas. Such tsunamis are considered unique in the world because of their frequency and magnitude. For the period 1737 to 1996, 96 tsunami are reported to have affected the shores of Alaska (see Table 1).