That makes 5 mag 4+ earthquakes in the PNW in 13 days. Not normal.

That's exactly where the Cascadia Fault subduction zone is.

http://standeyo.com/NEWS/08_Earth_Changes/080101.OR.tsunami.fleet.html

More
http://www.google.com/search?q=cascadia+fault&hl=en&rlz=1C1GGGE_enUS464US464&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=X&ei=g8s7T8z6KKikiQLP7ciSDA&ved=0CFoQsAQ&biw=1680&bih=987

If the big one is on the way then it makes sense that some warning shakers would give us a little heads up. Remind me to go east rather than west for my next weekend away.

Lots of little ones are way better than one big one. Let that pressure out gradually!

Hear hear, Smarana. What's absolutely amazing about all of Cascadia is the relative paucity of really destructive earthquakes, and that's mostly due (based on the last models I've read about) to the huge layer of sediment atop the subduction zone. That sediment apparently acts as an insulation layer, allowing the oceanic plate to bend from mantle heat instead of breaking. True, the eventual melting of the plate material produces Cascadia's volcanoes as the water vapor and carbon dioxide is released during that melting, but it beats all hell out of potentially catastrophic earthquakes on a regular basis in populated zones.

Pre-shocks are certainly more rare than after shocks, but not unheard of. Small, non-damaging earthquakes can mean that stress is being released in small doses rather than building up to result in a larger earthquake. Then again, small earthquakes can also transfer more strain to other segments of the fault.

Must have been started by all the bricks being thrown last night by the Blazers!