A significant Magnitude 5.1 earthquake struck directly in the Cascadia Subduction Zone off the US/CANADA west coast at about 10:49 AM eastern US time this morning. The quake is of significant size because the Cascadia Subduction Zone (CSZ) is considered to be a far more dangerous area than all of the San Andreas Fault!
The CSZ is where the Pacific Tectonic Plate goes beneath (Subducts) the North American Plate. The Pacific Plate pushes north/northwest while the North American Plate pushes south. Over millions of years, part of the North American Plate got "snagged" by the Pacific Plate, and this "snag" has the effect of pushing the North American plate - which then created the Cascadia Mountain Range and, to a lesser extent, the Rocky Mountains.
Seismologists say that when the North American Plate becomes "un-snagged" that will be "the big one" for the West Coast. All the stress built-up over millions of years, could be released in one fell swoop, thus flattening out the Cascadia mountain range, as the newly-freed North America Plate returns to its pre-snagged state, moving to the west by upwards of 100 miles!
Of course, there is no way to accurately predict earthquakes, and no one is claiming that "the big one" is anywhere close to taking place. But whenever a Magnitude 5.1 quake strikes in that area, it is of great concern. Smaller earthquakes can sometimes loosen-up the ground and permit much larger earthquakes.
For folks on the west coast, it is merely a case of wait and see.
UPDATE 12:30 PM EST --
Another earthquake, this one magnitude 4.6 has just struck the CSZ very near where the M5.1 struck an hour ago.
Link to US Geological Survey earthquake report HERE