This page will have frequent updates from observations and studies of
Cascadia deep tremor during the Winter and Spring of 2008
associated with the
CAFE experiment including imaging the structure of the
Cascadia subduction zone and a study of deep tremor associated with Episodic
Tremor and Slip (ETS).
News will be posted on this page (latest at the top) and references to figures from time to time.
Map of CAFE stations in operation during most of the CAFE experiment.
NEWS (latest at the top)
- June 12, 2008 - This is the wrap-up of this ETS since nothing new has
been seen since the tremor on Vancouver Island seems to have died out on June 2.
This may have been one of the longer lasting ETSes recorded based on the
tremor record. While there were short bursts of tremor in April the more
continuous record didn't start until May 4 and was located under the southeast
corner of the Olympics (just north of the southern end of Hood Canal. After
slowly moving north northwest for about a week it was under the Straights of
Juan de Fuca about May 11-12. On May 15 it clearly divided into a patch
just moving onto southern Vancouver Island and a second patch in souther Puget
Sound (near Olympia). This southern patch only lasted for three days while the
northern patch seemed to stick around souther Vancouver Island before moving
on northward around May 18-20. While the GPS signals from this ETS were reported
to be weaker than some previous ones they can be clearly seen on some stations
in northwest Washington as well as southern Vancouver Island (in particular
P418 & P436 starting in early to mid May, SC02 & ALBH in mid May, BAMF &
CLRS in late May).
The automatic tremor location plots will
continue to be made periodically since this is a good tremor detection
method to compliment the hand review of envelope plots.
- June 4, 2008 - Garry Rogers of the PGC reports that there
has only been very weak sporatic tremor over the last 24 hours and
thus this ETS may be over. Unless we see or the PGC sees more tremor
in the next few days, this will be the last report for this ETS.
- May 31, 2008 - Garry Rogers of the PGC reported yesterday that "tremor
continues to move slowly northwest on southern Vancouver Island. It has
been observed in 46 of the past 48 hours and contains a number of strong sequences."
So, while for us in Washington it seems to be pretty much over, the ETS is still
ripping along in Canada. Aaron's automatic tremor locations sometimes give
bogus results and are now producing far fewer good locations than it would if
our stations were closer.
- May 27, 2008 - In reviewing spectrograms for the current ETS there are some
interesting observations. We have set up a
set of tremor summary pages
for key stations for each recent ETS. These are manually generated from the automatic
daily multi-station spectrograms
- May 26, 2008 - The ETS related tremor has moved totally out of Washington
and is now under Vancouver Island. Our capability for locating tremor in
this area is not as good as it is in Washington. The PGC
seismic stations see the tremor much better when it
is north of the Straights of Juan de Fuca. Garry Rogers of PGC reports
"The character has changed to be more similar to previous ETS tremor sequences in
strength and length of the tremor sequences. The tremor source area is now
north of Victoria and over the weekend the eastern part of the tremor region
along the east coast of Vancouver Island has become active."
- May 19, 2008 - After three days of having tremor both on southern Vancouver Island
and in the southern Puget Sound the south Sound batch seems to have died out a
day ago. Interestingly during the three days of renewed south Sound activity it slowly
moved further south than it has been during any of this ETS. The herd of Texans at
Big Skidder will be rounded up and brought back to the corral this Wednesday since no
tremor seems to be in this central area any more.
- May 16, 2008 - Gary Rogers from PGC reports that tremor is now under southern
Vancouver Island though not as continuous and strong as for some ETSes. Herb
Draggert from PGC notes that the GPS displacements seem to have started later and
may be smaller than for other ETSes. Maybe this one will not go the full distance.
Interestingly, based on Aaron Wech's "realtime" tremor location plots,
yesterday a batch of tremor appeared back in the south Sound area along
with the tremor in southern Vancouver Island; ie two distinct and widely separated
batches of tremor. The Big Skidder Texan array is scheduled to be serviced tomorrow
leaving everything running for one more four-day period.
- May 13, 2008 - Finally it is an official ETS. Herb Draggert from PGC reports
significant displacements are taking place at north-western Washington sites.
The Mount Olympus site is moving nearly 5mm westward which is significant.
The "footprint"
of the displacement also seems to be expanding. However, he still doesn't see
much movement north of the Juan de Fuca Straights. Usually by this time there
would be hints of motion up north. Time will tell.
Tim Melbourne of CWU agrees that motions have started in Washington and that
his long baseline tilt
meter records show tilts that are significant compared to the past three months.
- May 12, 2008 - The tremor has moved to along the north coast of the
Olympic Peninsular as determined by the automatic locations. Amplitudes
are consistently lower on southern Sound stations and higher in the north.
A short animated film clip shows
the time progression of tremor locations over the past couple of weeks.
Note that there is a early burst of tremor in the northern Sound
on Apr 23-25. This is not seen on a
signal amplitude plot starting on Apr. 28
that shows the earlyest tremor on HDW starting about Apr. 30 but really
picking up on May 2 while again decreasing starting on May 9.
- May 10, 2008 - The tremor is definitely moving north. Yesterday Garry Rogers
of the PGC reported that "the tremor from beneath the Olympic Mts in Washington
is starting to show up on the southernmost Vancouver Island seismographs.
Since yesterday afternoon (0200 Z May 9) at least to 2200Z ther have been 11 hours
containing tremor activity, all coming from south of Vancouver Island". Herb
Dragert of PGC reports that there are no GPS signals yet indicating the
detection of slip.
The Big Skidder Texan array was serviced yesterday while the tremor locations seem
to be slightly to the northwest. From the autolocations the tremor is
definitely spread out over a larger area than it was a few days ago.
- May 8, 2008 - The tremor is really ripping over the last few days with most
hours having moderate to strong tremor that is showing on many stations.
According to Aaron's automatic tremor loation plots tremor seem to have started
moving to the northwest and is now directly under the herd of Texans at
the Big Skidder site. Justin has scheduled a service run for these 80 stations on Friday.
- May 5, 2008 - News Flash! Tremor has really picked up. Jenny Hanna
reports 15 or more hours of tremor per day during each of the last three
days from analyzing the detailed
May, 2008 envelope plots. Is this the start of the 2008 ETS event?????
Wech's automatic tremor locator
has recently gone through a thorough tuning process and is tracking tremor
day-by-day showing strong tremor the last few days under Hood canal. The UW
crew lead by Justin Sweet deployed about 80 "Texans" today in a small-aperture
array on the Olympic Peninsula to record all this activity with high fidelity.
- May 2, 2008 - Low-level tremor continues off and on since Apr 30
in the south Sound area; easily seen on HDW and weakly seen on a few
other stations.
- Apr 30, 2008 - After no tremor since Apr 23 we have had low level tremor
again back in the south Sound area from 02:00Z off and on until at least 20:00
today.
- Apr 24, 2008 - Suddenly, after weeks of almost no tremor in Washington,
there were several hours of moderate tremor yesterday, the strongest occurring
around 16:30Z. Thanks for the early heads up by Wendy McCausland. The tremor
seems to be located in the north Puget Sound between the Olympics and San Juan
Islands.
By 20:00Z it had died out and up until 22:00Z today there doesn't
seem to be much more if any. Is this just a teaser for things to come or
a typical "burble" that happenes from time to time unrelated to a real ETS?
- Apr 4, 2008 - It has been very quite around here. No tremor in Washington
for several weeks. However the PGC dudes (Honn Kao, Garry Rogers, Herb Dragert)
report fairly strong tremor in the north Vancouver Island area lasting for
four days and then quiting yesterday and without any detectable GPS motions.
Maybe that section is mimicing the southern Puget Sound section and "faking"
and ETS event starting.
- Mar 12, 2008 - No tremor for the past three days. A
21 day seismic envelope plot of the period
Feb 28 to Mar 12, 2008 shows the 7+ days of tremor that looked to be the
beginning of an ETS. Note that it is quite strong on HDW but not nearly as
obvious on the nearby stations, CPW, SMW and GNW though it can be seen
above the noise there too. Other noises showing up on many stations are
obviously diurnal cultural noises that even are lower levels on weekends.
The Auto-Tremor Map for the past few days
shows almost no tremor locations.
- Mar 9, 2008 - After some very strong tremor on Mar 7 tremor seems to have
decreased during Mar 8.
Over the past 24 hours (Mar 9 GMT) there has only been very weak
tremor now and then. It seems that the ETS has either stalled or this is not
the main event. As previously scheduled the almost 90 Texan seismographs in
the Olympics were serviced, swapping existing ones for new ones.
- Mar 7, 2008 - Tremor continues at moderate to strong levels
during periods over the past two days but does not seem to have
moved from its position in southern Puget Sound.
Tim Melbourne from Central Washington University reports that no
anamolous tilt is being observed on a long baseline tilt meter
located at Shelton. Herb Dragert of Pacific Geoscience Centre reports
that the GPS network of stations in the region are NOT showing any
lateral motions consistent with an ETS. However, Wendy McCausland
and Evelyn Roeloffs of the Cascade Volcano Observatory report that their
analysis of the PBO borehole strain meter B018 (south of Olympia) does
show a subtle signal that may be the beginning of an anamolous strain event.
- Mar 5, 2008 - Tremor is continuing at moderate levels but variable
at different times. 90 high-frequency verticals (Texans) were installed today in
a tight array in the central Olympics with the hope that the tremor will
soon move north west right under the array. TA site, C04A has had an
autonomous broad-band instrument installed. The experimental automatic
tremor location routine by Aaron Wech is plotting contours of number of
locations per area and shows where tremor is currently concentrated.
- Mar 4, 2008 - Tremor continues today at similar levels.
A large crew is headed to the field tomorrow to deploy a heard of Texans.
A CAFE station is being moved to the abandoned TA site of C04A to run
autonomously for at least a month.
The automatic tremor locator process is being
updated with some improvements but is not yet plotting correctly.
With everyone in the field tomorrow it will not be fixed until Thursday.
A simple press release announcing the ETS start is being prepared.
- Mar 3, 2008 - Tremor has been detected in the South Puget Sound area
starting yesterday (Mar 2). It is moderately strong on station HDW from
time to time but quite weak on most other stations in the area. Unfortunately
a key TA station C04A was removed a couple of weeks ago and thus is no longer
available for detecting tremor. The CAFE autonomous
stations were all serviced over the past two weeks and so should be working
fine now. Plans are being made to do the installation of a many component
array of high-frequency, vertical only sensors (Texans) at the Big Skidder site
on Wednesday, Mar 5.
An experimental auto-locator for tremor is being
developed by Aaron Wech. While only in the early testing mode we are putting
up its results on this web page as of today.
- Feb 25, 2008 - No ETS starting yet. Unfortunately, the USArray has
been pulling stations out of western Washington to move to their next
deployment much farther east. This and some heavy snow conditions causing
other station failures has reduced the number of good stations for studying
the next ETS. However, the Texan array deployment project is just about ready
to go. Most instrument sites have been found and marked so when tremor
starts we should be able to deploy most instruments in a very short time.
- Feb 7, 2008 - No obvious tremor for the past week or so. The second
half of Feb 5 looks like it might be tremor on the
RMS-RSAM plots but while the over all envelope
rises slowly and falls together on several stations the short term (minutes
to tens of minutes) details don't match. This pattern is consistent
with general wind noise due to a storm. For much higher details we
are now providing
large PDF files of hourly envelopes
for February, 2008 and for
March, 2008.
April, 2008.
May, 2008.
- Feb 1, 2008 - The semi-realtime seismic amplitude (envelope) plots have been
started again and should automatically update every hour or so.
- Jan 31, 2008 - A whole herd of Texans (RT125) have arrived at the PNSN labs.
The PASSCAL expert, Steve Alzevedo is training a group of us to configure
and operate the Texans. A siting trip will be made within the next week or so.
- Jan 29, 2008 - Again yesterday there were several hours with moderate
tremor in the central Puget Sound. It seems that there will be several quiet
days and then a day with some activity, but not enough to indicate
a major ETS is underway. A simple plot of hours
per day with tremor since the beginning of 2008 shows a few such peaks.
An explanation of our current analysis process and a summary of 2007 activity
is available under a
Tremor Tracking Page.
- Jan 14, 2008 - During yesterday and today several hours of weak to
moderate tremor was observed, mostly in the north Puget Sound region.
- Jan 10, 2008 - Discussions within the PNSN scientists are centering around
doing a prototype experiment this winter using a bunch (~100) Reftek-Texan
recorders in a very dense array to look for P-wave coherence from deep tremor.
This experiment will be used to see if it is worth using these short-period,
vertical-component only in a much larger experiment during the following ETS.
Disccusions with PASSCAL indicate that we could get a set of Texans by the
end of January, in time to deploy just as we first detect the arrival of
the next ETS. These units will be run in continuous mode for one, two or
three four day periods during the strongest tremor. They will be installed
at or around the Big Skidder array site where there are 6 three-component
instruments currently running.
- Jan 8, 2008 - New Tremor page started for the expected ETS of winter, 2008.
According to Herb Dragert of PGC and based on his average GPS start-time
interval for the 14 month ETS events of 450 days the estimated start time
on southern Vancouver Island would be Apr 24, 2008 +/- 76 days. Since there
may be a tendency for a long interval (previous interval was 506 days) to
be followed by a shorter interval and the tremor often starts in Puget Sound
area before Vancouver Island he estimates it could start as
early as mid February.
BACKGROUND INFORMATION
The University of Washington seismic group began studying deep tremor
associated with Episodic Tremor and Slip (ETS) events in 2003 using both
the regional PNSN and small aperture arrays. With NSF funding we
installed
Cascadia Arrays for Earthscope (CAFE) for both structural and tremor
studies.
The CAFE experiment is still installed as of winter, 2008; however the
transportable array stations (TA) of earthscope are starting to be pulled
out of western Oregon and many will be pulled out of western Washington
in February ....... just before the next ETS. Too bad.
Some of the more critical stations are being left longer as well as
some stations that the PNSN is taking over by purchasing the equipment
from IRIS using funds supplied by the Murdock Chairtable Foundation.
Using stations from the PNSN and the TA we use a
semi-automated processing
system to track the general occurence of where and when deep tremor takes place.
As in the past we plan to collaborate closely with the seismologists
and geodesists of the
Pacific Geoscience Centre (Natural Resources Canada) and
Central Washington
University to follow the development of the next ETS.
Previous Studies of ETS by UW scientists
Other information about ETS
Acknowledgments
- Steve Malone of UW maintains this blog
- Tony Qamar of UW wrote the original tremor envelope plotting software
- Ken Creager and Aaron Wech of UW provide the cross-correlation location plots
- Bob Crosson of UW provide envelope functions from which cross-correlations are done.
- Aaron Wech has developed the automatic cross-correlation, tremor location
processing routines.
- 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.
- USArray and Plate Boundary Observatory of EarthScope provide all sorts of data
Financial Support
- Routine operations of the Pacific Northwest Seismic Network is supported
by a cooperative agreement from the US Geological Survey, a contract from the
Battelle Pacific Northwest Laboratory, and funding from the State of Washington
and the School of Arts and Sciences of the University of Washington.
- Special funding for deep tremor studies has been made available from
EAR-Geophysics of the National Science Foundation and from EAR-EarthScope
Science of the National Science Foundation.
- The CAFE experiment PIs are: Ken Creager, Geoff Abers, Steve Malone,
Stephane Rondenay,
Brad Hacker and Tim Melbourne and is supported by the National
Science Foundation.
...... PNSN ETS HISTORICAL RECORD
