A major earthquake off the coast of Alaska sparked a tsunami warning alert for parts of the Alaskan and Canadian coast late Friday and early Saturday. The tremor, which had an initial magnitude of 7.7 (M7.7), but was later downgraded to M7.5, struck just before midnight local time on Friday, January 4, 2013, approximately 60 miles from the town of Craig.
The NOAA’s Pacific Tsunami Warning Center issued an alert for the coast of Alaska and British Columbia in which they warned that “a tsunami with significant widespread inundation is expected or is already occurring” and advised residents of low-lying coastal areas to move to higher ground. The alert was later cancelled and at the time of writing no damage or injuries had been reported.
January 2013 Earthquake: Tectonic Setting of Alaska
Like all of the Pacific coast of North America, Alaska forms part of the Pacific Ring of Fire, a seismically active belt of earthquake and volcanic activity that girds the Pacific Ocean, marking the boundaries of the Pacific and other tectonic plates.
The location of the January 5th Alaskan earthquake was on a part of the fault where the Pacific plate is sliding roughly northwards past the largely static North American plate at a rate of approximately 80 mm per year. This type of lateral movement is capable of producing significant earthquakes when the build-up of stress is released. This section of the plate margin is the northwards extension of the Queen Charlotte Fault off Canada, and earthquakes are common.
Although the earthquake was significant in scale and occurred offshore, conditions suited to the generation of tsunamis, it is not entirely surprising that the initial warning was cancelled. Tsunamis are generated by the displacement of large quantities of seawater – something which is more likely to happen where vertical movement of the earth’s crust occurs along the faults. Such movements are more typical of earthquakes where one plate is subducted (forced below another).
Earthquakes & Tsunamis in Alaska
Alaska is known as a highly seismic area. The nature of the plate boundary changes north of the Queen Charlotte Fault and subduction is the dominant process. This area has seen one of the world’s largest earthquakes. The M9.2 in Prince William Sound in 1964 was the second largest earthquake recorded since the birth of modern seismology at the turn of the nineteenth century and triggered a damaging tsunami that claimed more than 100 lives.
Although Alaska can never be said to be seismically calm, there were no major earthquakes (>M7.0) in the state or its waters in 2012: the largest recorded by the USGS was an M6.4 in the Andreanof Islands in September. However, further south along this section of the fault in Canadian waters, an M7.7 was recorded in the Queen Charlotte Islands in late October.
Though earthquakes in Alaska are common, the majority are relatively small. Of the larger events, those associated with transform sections of the fault do not typically displace large volumes of water and, therefore, are unlikely to generate a major tsunami. The main risk in the state’s waters is posed by the subduction zone to the north.
United States Geological Survey. M7.5 – 106km WSW of Craig, Alaska. (2013). Accessed 5 January 2013
United States Geological Survey. Historic Earthquakes: Prince William Sound, Alaska. Accessed 5 January 2013
NOAA. Tsunami Warning and Cancellation. (2013). Accessed 5 January 2013