The calm before the quake?

Abstract

With the exception of Alaska, the Cascadia subduction zone, extending 1,200 Km from northern California to Vancouver Island, is the largest tectonically active fault system in North America. Although is widely accepted that the North American continental plate is converging at a rate of 2.5-4.5cm yr^(-1), modern seismic activity has been minimal along the Cascadia Subduction zone. Are the Pacific oceanic plates sliding effortlessly below the continental margin? Or are the plates welded together, gradually accumulating elastic strain, to be released in another cataclysmic earthquake sometime in the coming centuries? There is geological evidence, much debated at a recent meeting, that there have been six massive earthquakes (energy magnitude M_w = 8.5-9.5: see box overleaf) in this region in the past 3,600 years, most recently sometime in 1680-90.