Time for a tremor update. All of the map data is from the PNSN interactive tremor map
This map is the past 4 days since my last map post:
And this is the map showing this most recent episode over time:
As you can see, the burst of tremor just south of Portland, Oregon has moved south and west over time. The western (up-dip) movement is important
during tremor episodes; I'll come back to that.
It's also interesting that Vancouver Island and central Washington have become active again, considering those areas had their longest recorded
episode ever, back in the fall 2012.
The 3 yellow lines on the maps indicate the depth of the plate interface (20, 30, & 40 kms--moving west to east) as the Juan de Fuca oceanic plate
gets shoved under the North American plate.
The central Oregon burst began fairly deep, around 45 kms down-dip, and has moved up-dip to approximately 25 kms.
Here is an illustrated cross-section of the fault:
After observing repeated ETS events, the geoscientists have concluded that the lower limit of the transition zone is at about 25 kms depth. Their
modeling shows the plate interface to be 100% locked up-dip, near 10 km depth. Then the coupling (locked-tight-ness
) drops to only 50% at 25 kms
deep. From 25 to 40 kms (the depth of the majority of tremor and slow-slip), the coupling drops rapidly from 50% to only 15%; and below 70 kms the
slab is freely slipping into the mantle. (I've paraphrased this info from here
Now, the website
WillowWisp linked a few posts above, written by W.D. Stanley, of USGS
(formerly?) (who published on ETS in 1999), hypothesizes that the central Oregon block (Siletzia terrane) is the main sticking point for the entire
fault line. He reasons that because the area is primarily composed of denser, stronger mafic rock, as opposed to the northern and southern ends of
the CSZ, the asperities at the plate interface, in this area, are tougher to break.
When a critical percentage of these sticking points crumble, that is when a major earthquake will happen.
He also proposes the following:
It seems likely that there is a very predictable pattern to the tremor and slip patterns. As time goes on, this web
site will try to point out the latest patterns in tremor, earthquakes and slip (when available). If there is a radical departure from the pattern then
this might be one signal of a forthcoming megaquake.
Let me put this together:
1. During an ETS event, the stress applied to the "locked zone" is increased, because the interface down-dip is "unlocked" for a short time, allowing
the full weight of the down-going slab to "pull" on the locked zone.
2. The 25 kms depth on the interface seems to be the westward, up-dip limit of tremor and slip.
3. If there is strong deviation from "normal" ETS patterns, it could signal the megathrust EQ is near.
So, we should try to establish what the "normal" ETS patterns are, and be on the lookout for radical departures from said patterns. Easy
(If I have misunderstood any of this, or just have it flat out wrong--please set me straight!)
Same area, different topic. I find this small quake interesting. Mag 2.1
Check out the
Ack! I can never get a post right on the first try. Or second.
The current tremor west of Salem and Eugene, Oregon is ongoing. Realtime tremor
It is also still
moving up-dip, into the transition zone.
Also, I'm not sure why I've never "clicked" the chart option on the interactive tremor map before, but it is fun!
(well, I'm a geek and easily amused)
This is a screencap of one option.
You can set it to show latitude or longitude movement, hours of tremor, # of discrete epicenters, etc, and you can play it as a movie. Check it
edit on 3/12/2013 by Olivine because: adding
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on 3/12/2013 by Olivine because: more mistakes
edit on 3/12/2013 by Olivine because: do you ever just have "one of those