So-called "tremor-and-slip" events have occurred about every 15 months since they were first detected in 2002. The latest was found early Sunday north of Olympia and west of Tacoma, and is expected to travel north under the peninsula toward Vancouver Island. UW scientists say it can't be felt at the surface, but over the course of several weeks can release as much energy as a magnitude 6 earthquake.
I blame the seismic instruments! If you didn't have those, you'd never know this was going on! And you'd be in the same boat, because we don't know what this means. But I'm sure it must add to the account for the future big earthquake.