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Blog for ETS of Summer - FALL, 2012
Deep Tremor News: Aug, 2012 - Oct. 2012
Again a BLOG is being thrown together to document and provide information on a Northern Washington ETS that should start this late summer or fall, 2012. There are no special experiments in the field for this ETS since the Array of Array (AofA) experiment was completed last year. We will be tracking and reporting on this ETS using data only from the real-time regional networks (UW, CN, TA, PB, CN) and/or information reported to us by others.
ETS dates: Aug 30 - Oct 11 (618 hours detected tremor)
Information will be posted on this page (latest at the top) and references to figures from time to time.
NEWS (latest at the top)
Nov 6, 2012: Post Script - Indeed the main ETS was over on Oct 12 as previously stated. Additional short bursts of tremor took place in the weeks following in several different parts of Cascadia. However, on Oct 18 a persistent patch of tremor started in northern California and has continued every day since then. Starting just to the west of Lake Shasta it first migrated slowly northward and then expanded to the south where it seems to be dying out just southwest of Red Bluff. Typically, northern California ETS events are smaller and shorter but more frequent than those to the north. The current one seems to fit right into that pattern.
Oct 12, 2012 - Officially this ETS is over! We have gone more than a day with no tremor so that defines the end of this episode. It is virtually tied with last year's main ETS for duration, 42 days from start to finish but is clearly the hands down winner for total hours of tremor, 618 hours out of about 1,000. So tremor was detected by the wech-o-meter clearly over half the time. From previous work, using more sensitive arrays we know that the wech-o-meter can detect and locate stronger tremor and that weak tremor is missed. Thus it is likely that at least some tremor was active pretty much all the time.
While one of the biggest ETS events yet monitored (as determined by duration, hours of tremor and extent of area with tremor) it remains to be seen if the geodetic slip makes it the largest ETS. Even a seismologist can see that slip has taken place on the PANGA processed GPS instruments, particularly for the longitudinal components of the BC Vancouver Island, the Puget Lowlands and the Olympic Peninsula groupings of stations. If all lumped together this seismologist would guess that the moment release for this event might exceed an equivalent magnitude 6.5 earthquake. However, we really must wait for the geodesists to do their black magic of fitting modeled slip to the measured date before a definitive result.
Some might argue that this has really been two different ETS events, one from central Vancouver Island to the north and the other from Southern Vancouver Island to the south. Both of these regions have gone as individual, separate ETS events in the past. This time they really were intertwined with tremor jumping back and forth and overlapping through both regions blurring the distinctions. Some have suggested that a normal central Vancouver Island ETS triggered a normal southern Vancouver Island-northern Puget Sound ETS since it was near time for the latter to go in any case (due roughly by the end of October) and they just merged together as one larger event. While loath to credit the Canadians with starting the main ETS that we normally start in the southern Puget Sound, this blogger feels this is as good a model as any.
Unless something interesting takes place in the next few days, this blog will terminate here and a new one will be started when a new event presents itself.
Oct 11, 2012 - Maybe it's over..... but not officially. The last day has seen very little tremor; just a few hours right at the south end of the zone near Centrailia. To declare it officially over there must be at least one full day with no located tremor. Tomorrow may be it. When it is officially over this blog will sumarize the whole thing and try and put it into context with other ETS events in this region.
Oct 8, 2012 - In the past two days the tremor front has jumped from about Olympia south to Centralia. This is about as far south as most of previous recorded standard Puget Sound ETS episodes have extended. See the comparative density maps for estimates of previous ETS extents. Thus, in all likelyhood the current episode should be ending soon.
Oct 5, 2012 - The ETS continues. After what looked to be a few days of this episode winding down it has picked back up the last day with locatable bursts half the time. Oct 2-3 had fairly weak tremor and was spread out a bit in the east-west direction just north of Olympia; but, yesterday and today it has picked back up and seems to be again moving slowly south with the leading edge just south of Olympia. It may be in its final phase. The tremor in central Oregon has died back to a low level in the past couple of days and doesn't seem to be moving north at all. Thus, this whole period may be near the end with the chance of a connection between the two area becoming much less likely.
Oct 1, 2012 - This tremor episode may not yet be the longest lasting (33 days) but Ken Creager reports that it is now the most active of any tremor episode detected and located by the wech-o-meter. In a note today he says:
Sep 29, 2012 - It continues as predicted. The leading edge is now just south of Seattle, pretty much directly under HDW. However, it seems to be less constant today than the past three days. There are even short periods where little to no tremor is taking place. Just after mentioning that the north Vancouver Island area was active again, it quit. Maybe it is over at the far north, but then if I say that it will start up again. Perverse tremor! All of the summary plots have been updated to include today.
Sep 21, 2012 - The tremor has clearly left Canada late today, though there were two very brief and small tremor bursts yesterday at the very far north end of the central VI zone. A report from Evelyn Roeloffs (studies the borehole strain data for the USGS) yesterday says: As of end-of-the-day UTC on Sept 18, the strain signal appears "over" at B012 (Ucluelet). It's "in progress" at the others in southern Vancouver Island and northern Olympics and not yet showing up south of Puget sound.
One should also check out the 30 Day Raw Plots for Station B004, in particular the CH0 component. Evelyn has compared this to previous major ETS signals and says: This 2012 event looks like it started at B004 about Sept 4, so it's only half as long as the 2009 one so far and is already 30-50% larger on the CH0 component.
Sep 19, 2012 - Here it comes, into Washington. As of today the tremor is centered in the Straights of Juan de Fuca overlapping into southern Vancouver Island and the coast of the Olympic Peninsula. Herb Dragert says he is "amazed that Homeland Security didn't prevent the encroachment of an obviously Canada-initiated ETS episode into the US...." Though Honn Kao, the developer and operatior of TAMS/SSA, says: "For the past three days, the main cluster appeared to be right around the border. Perhaps the Department of Homeland Security IS doing something to stop it from entering the US". While I guess one is reluctant to say anything is normal about ETS, at least the past 7 major ETS events have started in Washington and migrated north, sometimes with a slightly bilateral migration with a late phase back to the south. This ETS seems significantly different, clearly starting in mid-Vancouver Island and after some jumping around (bilatteral migration?) centering on southern Vancouver Island and now propagating south into Washington. Garry Rogers, who looked back through older Canadian records says: " We have not seen that behaviour before since we have had continuous digital data to analyse; i.e. April 1997 event onwards."
Shown in the figure to the left is the number of hours of tremor detected by the wech-o-meter plotted by region (NV-northern Vancouver Island, VI- southern Vancouver Island, NW- northern Washington; these regions are overlapping and detections can extend a bit outside a region if the tremor is strong). Note that both NV and VI have similar amounts of tremor detected early in the sequence and the NV numbers drop quickly after Sep 14. NW gradually picks up starting around Sep 13 and is as high as VI by Sep 18.
I haven't determined if it is unique yet or not but there have also been periods when this tremor seems VERY strong compared to previous ETS events. A closer comparisons of the amplitudes and the exact locations for this event with previous is yet to be made..... when I have more time.
Sep 14, 2012 - Well, it looks like we are all wrong. This tremor episodes seems to have escaped Canada and is heading into Washington. Up until Sep 14 it was active at both the south and north ends, even the north batch moving slightly more northerly. Then on Sep 15 the northern batch stopped and the southern batch extended south into the Straights. Today the southern edge extends all the way to the coast of the Olympics. It looks like there is a good chance this will now keep going and fill in the normal 14-month ETS area that goes from southern Vancouver Island to southern Puget Sound. If that is the case it could take another two to three weeks to finish making this one of the longer and largest ETS sequence we have observed. The next couple of days should provide the data to make this prediction more sure (and then it too will probably end up being wrong).
Sep 11, 2012 - This tremor episode in southern Vancouver Island continues but may be about to quit..... then again it may escape its normal pattern. As of yesterday around noon local time there was the last strong tremor burst in over 24 hours; however, weaker tremor bursts are still being located by the realtime monitor. For the past few days tremor has been concentrated near both the north and south end of the zone and little migration is seen. The map to the left shows the progression of tremor from Aug 30 (dark blue) to Sep 10 (dark red). One can see that both early and late tremor was concentrated near the ends of the zone. The yellow to green locations are for the Aug 2-7 time period mostly. Some here at UW feel that this sequence has extended over a larger region than is typical of the "mid-Vancouver Island" ETS. But, we don't have as much experience looking at this area as do the Canadians. Lets hope they know what is going on.
Sep 7, 2012 - We have independent predictions that this tremor episode will stop within a day or two and NOT emigrate to the US from Canada. Both Garry Rogers and Ken Creager agree that this is a "mid Vancouver Island" ETS that is shorter and covers less area than the main one we usually are interested in. Ken's comments:
This event is now up to 118 hours duration, so if it follows the pattern of the previous two events (133 and 148 hours) that occurred in the same area it will die out pretty soon. Interestingly the time intervals between these three events is 15 months. We (by which I mean wech-o-meter) only have good recordings in this area to see these three events. Honn has a much longer recording history and perhaps knows more about this pattern? If history repeats itself it will not cross the border.
And Garry's comments:
Last predominantly "Mid Island" ETS event was from about May 20 to June 5, 2011 (~15 months ago). It migrated from north to south. It ended where the north migrating last Puget Sound / South Island event stopped September 3, 2011. The area that is lighting up with tremor now seems to be the same area of the May 20 to June 5, 2011 event. I would say so far, the world is unfolding as it should.
OK, you heard it here... seismic predictions. It is still continuing into Sep 8 as can be seen on the tremor spectrograms (just updated with the more diagnostic stations, B926 and MGB).
Sep 6,2012 - Oh dear. Are we seeing something a bit different. Since the end of August there has been a persistent patch (sometimes two patches) of tremor on southern Vancouver Island. Locations from the wech-o-meter show a patch NW of Port Alberni on Aug 30 and Sep 2. On Aug 31 a larger, stronger patch starts just south of Port Alberni and seems to spread out a bit over the following couple of days and seems to be moving south. By the end of today it is about the latitude of Sidney and is partly overlapping the area covered by the end of the last (northward moving) ETS in Aug-Sep, 2011. The ETS before that (2010) extend farther north into the area with the current tremor. An interesting aspect of the current tremor is that some of it seems to be a bit more up-dip of previous tremor; HOWEVER, the wech-o-meter has only had access to a good number of the Canadian seismic stations since early this year. Thus location biases when using different sets of stations may account for this apparent difference.
Of course the Canadians may understand all of this better than we do. Their tremor location system (TAMS) show a very similar if not identical parttern to that of the wech-o-meter and so I aksed them for their thoughts. A note from Garry Rogers yesterday (the original discoverer of deep tremor in Cascadia):
This current event (start Aug 30) is immediately adjacent to where the last Puget Sound/South Island event stopped on September 3, 2011. I am not sure when the last Mid Island event occurred or where its southern boundary was. This event is probably 1) filling in the hole between the last South Island and Mid Island event, or 2) if the last Mid Island event was some time ago, this event will probably continue north.Northern boundary of this event has definitely moved north in last couple of days and tremor has not occurred south of where last ETS event stopped on Sept. 3, 2011. I still haven't had time to dig out where last mid island event stopped, that is a key piece of missing information at the moment. My bet is that this one is not going south.
OK, so not going south. But will it keep growing somewhere? A note from Herb Dragert (retiring geodesist and ETS expert) says:
In particular, the current event now has 58 hours (accounting for overlapping times from multiple sub regions) and is located at about the same place as an event in 2010 and one in 2011. Those two events started on Feb 10, 2010 and May 19, 2011 and had durations of 148 and 133 hours respectively.
Sep.1, 2012 - Probably nothing, but a batch of tremor has been going on the past two days on southern Vancouver Island very close to the northern end of the normal every ~14 month ETS tremor patch. This bunch seems a bit up-dip (SW) from previous patches which is interesting but nothing more than that at this point.
Aug 16, 2012 - Indeed Ken was correct. There has only been very weak tremor barely detectable on HDW spectrograms since yesterday afternoon. Thus, this was NOT the beginning of an ETS. Yet again this blogger was on the verge of making a fool of himself by pushing the panic button.
Aug 15, 2012 - Hold on! After talking to Ken Creager about the recent strong tremor he did some comparisns of previous ETS starts and inter-ETS bursts of tremor and provides the following:
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.
However, this current event follows right on the heals of a 69-hour swarm under the Strait of Juan de Fuca. My automatic cluster code considers the late July tremor in the Strait of Juan de Fuca to be a different event than the current one under Hood Canal. Together they add up to a respectable 100 Hours. This pair is nearly identical a similar pair in October 2007 (so far...)
So maybe we hold off on pushing the ETS start button. Today there is again a strong, but relatively short burst of tremor in the same place......
Aug 14, 2012 - For the past four days there have been bursts of strong tremor in the central Puget Sound area. These bursts are much stronger than the bursts to the east reported in July and have a pattern common to the start of the May, 2009 and Aug, 2010 (but NOT the July, 2011 nor earlier) ETS. It is still too early to be sure if this really a start. Despite the fact that Herb Dragert will be laughing at this bloger I am temped to punch the start button.
July 31, 2012 - There seem to have been a number of small and short tremor periods over the month of July. They were mostly to the east (down-dip) of the common main ETS tremor areas. Malone reported to the tremor e-mail list on July 30 the following:
Starting on July 19 and going until yesterday with a couple of down days, there has been persistent tremor in the east end of the Straights of Juan de Fuca. While only the tremor yesterday could be considered even medium strong there have been a few moderate bursts but mostly weaker periods. Located tremor occurs over about 60 hours out of the 11 days (264 hours) of this period. An interesting observation is a slight tendency for the tremor to migrate up-dip. See attached. The earliest period starts over near Lopez and Whidbey Islands and yesterday's is much farther west, almost to Port Angeles area. Tremor in the days in between was first in the south and then more northerly near Victoria.
This seems too early to be the main ETS. If on schedule this year's ETS should not start until October in the South Puget Sound. Of course the last ETS came a couple of months early. Could we be on a new schdule? Herb is shaking his head at my naivety.
Of course as soon as Malone sent out this e-mail the tremor went dead quiet.
- Steve Malone (UW retired) writes, edits and maintains this blog
- Ken Creager and Aaron Wech of UW developed the cross-correlation location plots
- Aaron Wech developed and helps maintain the Interactive Tremor Web pages.
- Garry Rogers of PGC provides routine update information on tremor observed in Canada
- Herb Dragert of PGC provides analysis plots of GPS data
- Tim Melbourne of CWU provides analysis and plots of GPS and long baseline tilt-meters data.
- Honn Kao of PGC provides routine tremor analysis for Canadian stations.
- Evelyn Roeloffs and Wendy McCausland of USGS provides analsyis of borehole strain meter data
- John Langbein of USGS provides strain meter analysis plots.
- Realtime seismic data is primarily from the PNSN but also uses data from the Canadian National Seismograph Network, the Northern California Seismic Network, University of California at Berkeley and the Plate Boundary Observatory and USArray, both of EarthScope.