Originally posted by TheRedneck
reply to post by JustMike
I know you seem to have had a 'sense' for these things in the past... do you still feel something is yet to come north of California?
Crumbs, that's quite a question!
I know you asked about what is basically the Pacific Northwest, especially OR up through WA and into BC, but first I'd like to address Nth CA, because
of its complexity and possible importance.
To begin with I'll mention something that you doubtless know but some of our readers might not. Namely, although Nth Cal is not as seismically
“busy” as the south, they get a fair number of quakes. Most of them are small – especially the on-land ones. The larger ones tend to occur in
the sub-sea plate regions. (More on that in a minute.)
What is interesting about Nth Cal is the various interactions there. The subducting Gorda plate can cause quakes on land, but if it does they'd tend
to be deeper than the sub-sea events and generally would also be at lower energy levels. But there is also the Juan de Fuca plate, which on a typical
map is shown with its deformation front some miles off land:
(Map from Geomaps via USGS at this link
However, while that is the main front and we'd expect the hypocenter/s of any major subduction event to be close to it, the subducted plate itself
extends some miles inland, going deeper down further away (to the E) from the deformation front. The interactive Tremor map from the Pacific Northwest
Seismic Network (here
) shows the subducted plate at various depths:
Yes, all those red dots are epicenters for tremors – just for yesterday! These quakes are so small that they are almost never felt, but each one
indicates some movement going on. (By the way this is not an unusual number to see in one day. It varies, though.)
Anyway, the yellow lines in the above image show the plate's assumed subducted depth at 20, 30 and 40 km. For example, the line that passes closest to
Portland indicates the subducted plate is about 40 km down; off the coast from there, the first yellow line shows a depth of 20 km.
So, we can see that things are rather complicated. There are four plates active in or close to Nth Cal – the Pacific, the Nth American and the Juan
de Fuca – and sandwiched in among all that, the Gorda. And two of them – the Juan de F. and the Gorda – are subducting beneath the Nth American
plate. Then when we add the San Andreas fault into the mix and also possible effects of localized volcanic activity, it's one heck of a puzzle!
Now onto those places nth of CA. I observe the PNW region more often and more closely than just about any other place on the globe, because it is
one (in the US and BC) that not only can cause devastation over an enormous area, but has the possibility of a major tsunami as well.
As I mentioned elsewhere I've been uneasy for several days and that mag 7.7 not far north of Vancouver Island did little to help. The only reassuring
fact I can point to is yesterday's quake was the biggest in that region since 1949, when they had a mag 8.1. And even after that, the CSZ did not let
go with its potential mag 9 quake.
Vancouver and Seattle and the coastal region there, to 600 miles or more down the coast is the worry. But Nth Cal could tie in as an indicator. If
there is a major increase in quake activity there, that might mean that things are on the move and the Juan de Fuca could be ready for a big
subduction event. That is one of the things I'm looking out for: an increase in activity at either end of the plate. Such activity on land, at depths
down to about 40 km, would be very concerning indeed. Odd as it seems, really shallow quakes would probably be less of a worry.
Significant quakes at sea near the deformation boundary are also a concern, especially if land-based ones at the “right” (subducted plate) depths
follow shortly after. That's what I'm watching for now and hoping not to see.
Not an ideal answer, I know, but the best I can do for now. In short, lots of smaller quakes along that inland subduction line, especially if not near
known, shallow faults, could be a strong indicator. Big quakes on the main boundary are always a concern, but the little ones can be very significant
EDIT: with a large quake within a subduction zone, there is the concern that it's a foreshock to a larger one. This was the case in Japan, when the
mag 7.3 on March 9 was followed by the mag 9.0 on March 11. And the big quake occurred only fractions of a degree of lat. and long. from the
foreshock. So that's why I'm nervous about the mag 7.7 on Oct 28 (UTC time). The 1949 event showed that a megaquake doesn't have to follow, but all
the same if I lived in the region I'd be wary. Not panicked, just wary.
edit on 29/10/12 by JustMike because: (no reason given)