Posted by J. Vidale, 10/28/2012
The M7.7 earthquake last night was widely felt along the British Columbia coast. It struck near the Queen Charlotte Fault, about 200 km north of the our big coastal fault, the Cascadia subduction zone.
Oddly, this stretch of fault is a site of both lateral compression and sideways motion. There was an M8.1 earthquake in nearly the same spot in 1949. The 1949 earthquake was largely sideways motion; in contrast, the event tonight was largely lateral compression. Aftershocks filled a zone at least 200 km long, probably delimiting the extent of the section of ground that broke. Further study of the mainshock and the aftershocks in the coming weeks should provide a more clear picture of what is moving how along the Queen Charlotte Fault.
This earthquake likely applied some additional stress to the faults we are concerned with in the Puget Sound and will have local aftershocks that are prominent for weeks. As usual, the chance of further seismic activity in the immediate vicinity of this earthquake will be higher than normal for a while, but does not rise to the level at which any actions are warranted.
More details, maps, and the latest aftershocks may be found at the USGS link here.