Seismic Gaps

When the irresistible force of the North Pacific tectonic plate meets the immovable object Alaska, the result is an interlinked assemblage of volcanoes and earthquakes. Moving steadily to the northwest at about 6 cm (2.4 inches) per year, the North Pacific plate rubs against British Columbia and southeast Alaska and then slides down under the western Gulf of Alaska and the Aleutian Islands.

Rocks at the boundaries between the moving plate and the fixed continent try to absorb the differential motion by bending. The rocks in the Gulf of Alaska region seem to have enough strength to accept 3 to 5 meters (10 to 16 feet) of bending before they break to initiate the event we call an earthquake. The seismic waves created in the impulsive snapping of the rocks are stronger the greater the slippage is along the fault. In a large earthquake the slippage might be roughly 5 meters along a fault that may extend horizontally 100 hm to perhaps more than 1000 km.

The time taken to build up enough strain (bending) in the rocks to create a 5-meter break is 5 meters divided by the rate of slippage, 6 cm per year, 1.e., about 80 years. On this basis, seismologists think that the entire boundary between the North Pacific plate and the continent should undergo breakage every 60 to 100 years.

Three regions of the North Pacific boundary have not experienced large earthquakes in 80 or more years. These seismic gaps are in the vicinity of Yakataga, near the Shumagin Islands in the Aleutians and near the Commander Islands west of Attu. According to the seismic gap hypothesis, these three regions are more likely to experience large earthquakes than are the regions along the plate boundary to either side.