Southern End of Cascadia Fault More Unstable

And thus more likely to have a quake soon:

Northwest Earthquake Risk in U.S. Looms Large: 40% Chance of Major Earthquake Within 50 Years

ScienceDaily (Aug. 1, 2012) — A comprehensive analysis of the Cascadia Subduction Zone off the Pacific Northwest coast confirms that the region has had numerous earthquakes over the past 10,000 years, and suggests that the southern Oregon coast may be most vulnerable based on recurrence frequency.

[...]

"The southern margin of Cascadia has a much higher recurrence level for major earthquakes than the northern end and, frankly, it is overdue for a rupture," said Chris Goldfinger, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences and lead author of the study. "That doesn't mean that an earthquake couldn't strike first along the northern half, from Newport, Ore., to Vancouver Island.

"But major earthquakes tend to strike more frequently along the southern end -- every 240 years or so -- and it has been longer than that since it last happened," Goldfinger added. "The probability for an earthquake on the southern part of the fault is more than double that of the northern end."

[...]

The Goldfinger-led study took four years to complete and is based on 13 years of research. At 184 pages, it is the most comprehensive overview ever written of the Cascadia Subduction Zone, a region off the Northwest coast where the Juan de Fuca tectonic plate is being subducted beneath the continent. Once thought to be a continuous fault line, Cascadia is now known to be at least partially segmented.

This segmentation is reflected in the region's earthquake history, Goldfinger noted.

"Over the past 10,000 years, there have been 19 earthquakes that extended along most of the margin, stretching from southern Vancouver Island to the Oregon-California border," Goldfinger noted. "These would typically be of a magnitude from about 8.7 to 9.2 -- really huge earthquakes.

"We've also determined that there have been 22 additional earthquakes that involved just the southern end of the fault," he added. "We are assuming that these are slightly smaller -- more like 8.0 -- but not necessarily. They were still very large earthquakes that if they happened today could have a devastating impact." [...]

This fits in with a previous post I did, "Oregon Tsunami Inundation Maps and Reports", which referenced the Gorda plate, a seismic zone that extends from Southern Oregon to Northern California. More plates, more faults, thus more activity.