Tiny tremors, smaller than earthquakes, are shaking the Cascadia subduction zone deep beneath the Pacific Northwest.
The Cascadia subduction zone is where two of Earth's tectonic plates meet in an epic collision and one haltingly slides below the other. The Cascadia Fault stretches for almost 700 miles (1,100 kilometers) from Northern California up to Canada. The force required to shove a piece of ocean crust into Earth's mantle can produce megaearthquakes along the zone, as in Japan and Sumatra.
For the rest of the story: http://www.livescience.com/25566-tremors-linked-earthquakes-cascadia.html