The exact location and size won't be known for a few days, until more seismic data comes in. The depth estimates are 49 km and 59 km. This means that it is most likely along the interface between the North American and Juan de Fuca plates. The magnitude is in the 6.8-7.0 range, from preliminary estimates.
Frictional forces cause the two plates to be locked together.
At some point the stress gets too great, and the fault ruptures, with the Juan de Fuca plate slipping down into the mantle, and Oregon and Washington snapping back up. Seismologists have long known of the potential earthquake hazards in this region. This earthquake is located in the same general area as a magnitude 7.1 earthquake on April 13, 1949. That earthquake killed 8 people and caused damage and landslides in the Olympia-Tacoma area. This one, located about 10 miles northeast of Olympia, has already done some damage, though the full extent is not yet known.