August 20, 2012
by Steve Malone
After watching the Wech-o-meter for a few days I thought it might have already started early like the one last year did. Several days with strong tremor such as the 8 hours on Aug 14 shown here got our interest up. This region is close to that in which previous major ETS have started.
In preparation of being able to follow the next ETS we set up the a bLOG page for the summer-fall, 2012 ETS to put our observations as things progressed. We were ready, but did not "push the tremor start" button.
In discussing this amoung ourselves here at the UW calmer heads prevailed. Ken Creager did a few comparisons with previous ETS and with tremor bursts that were short lived. His comments and a figure are as follows:
To shed light on whether this current tremor activity represents the third August ETS in a row I have taken a look at previous moderate tremor swarms. Since 2006, Wech's catalog has 5 Northern Washington ETS events and 6 other Northern Washington tremor swarms that had durations ranging from 40 - 80 hours. The current event is just over 30 hours (through yesterday, being careful not to double count tremors from SW and NW). One of those 6 events was the 80-hour false alarm we called in March, 2008 that led to installing the Texans (and finding the first Cascadia LFEs!). So based on that I would give the current activity less than a 50-50 chance of developing into an ETS event.
Here is a map of the centroids of the 5 ETS events since 2006 (red) and of the 7 events (including the current one) with 30-81 Hours of tremor (blue).
Note that the blue dots are all clearly east-northest of the red dots. Ken argues that these down-dip tremor periods are distinctly different than the tremor when a main ETS occurs. Ken's comments generated a response from none other than the orginator of the Wech-o-meter, Aaron Wech who is sniffing out tremor amoung other things on a PostDoc in New Zealand. Aaron observes:
An unsolicited thought regarding this question:
I'm wondering if it really matters though. Don't get me wrong, this isn't some jaded ex-Cascadia pessimism coming out. But I think the question of "is this an ETS?" is a bit of an antiquated one. A better question is "how big will this event be?". Slip is slip. These things happen all the time, all up and down Cascadia. Updip and downdip. Are we asking whether this one will be large enough for GPS to be able to resolve it? Or are we asking whether this one belongs to a special family of geodetically confirmed quasi-periodic events particular to the updip region of Northern Washington? If the latter, why would that matter? Different parts of the fault have different pediodicities, so I'm not sure thinking of it in a special "ETS" sense is that illuminating, unless one thinks that the difference between an 80 hour and 150 tremor swarm is more than just size. But asking how big this will be, where it will rupture, etc. is a different matter. In this sense, rupture history via past tremor/slip catalogs becomes important, as Ken is showing here.
Ken's resonse to this, thoughtful as ever:
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