Seismo Blog

Continued Landslide Monitoring

February 11, 2018

by Steve Malone

Seismic monitoring of a slow landslide near Union Gap in eastern Washington continues with several new analysis techniques being developed and applied.  For details on the PNSN mobilization and initial monitoring procedures see the extensive Observations and updates for the January period.  This current post is a continuation of reports on our observations and analyses started on February 11, 2018.

Update on February 15, 2018 (A mystery and forensic seismology)
Update on February 27, 2018 (A rockfall and other observations)
Update on March 16, 2018 (No change in the slide but some new monitoring efforts)

Update: February 11, 2018 (A review of monitoring and a new monitoring parameter)

Here is a list of products available on the web that, along with other techniques, we routinely use to review the seismic activity related to the landslide:

  1. Raw Webicorders (12 hour-long, 10 minute-trace seismograms)
  2. Spectrograms (time versus frequency)
  3. RSAM  (Realtime Seismic Amplitude Monitor)
  4. REDPy (cluster identification over time)
  5. UGAP QuickShake (realtime seismograms)
  6. Tuned event counts (very local, tiny earthquake counts over time. Described below)
  7. Two real-time cameras (several images per minute.  Provided by U of Oregon)

We also have access to daily reports from an automatic motion detection system, and we participate in weekly discussions with other organizations monitoring the landslide. Should we notice unusual seismic activity associated with the slide we will immediately notify the monitoring group and participate in discussions regarding interpretations of the situation.

Number 6 above is a new monitoring tool.  To help us quickly recognize changes in the number of tiny earthquakes we record we are now using an automatic "tuned event counter".  Alex Hutko of our staff, who is a specialist in studying seismic noise and characterizing the quality of seismic data, has developed the system to try to automatically minimize the detections of cultural signals and maximize the detection of landslide related, tiny earthquakes. Using a process of filters and amplitude thresholds this counter runs on all of our seismic stations.  The top part of the display shows counts of only events that record on more than one station.  Here is an annotated snapshot of part of the display for today. Of course, as time goes on the hourly updated display will change.

There are several things to note on this plot.  The numbers along the bottom are day-of-the-year (Julian day).  When there are gaps in the data for one station the combination plot at the top will show no data even if the other station has data.  Because of a problem in our waveserver there is frequently a short gap at the end of many days giving a drop in the counts (seen on the 10 day plot, not on this example above). Note, in the bottom plot the "power" at a station (not the number of events but the overall signal level in the seismogram) has a very diurnal pattern, due to the the diurnal nature of traffic on the interstate highway.

One thing we are not sure how to interpret is the apparent notable increase in the average number of counts on station UGAP3 and perhaps UGAP6 (though the record is shorter) over the past month.  Unfortunately there was a period of time when UGAP3 had electrical problems that looked a little like tiny events to the automatic counting software (day 24 through day 27).  However, if one excludes that time period there apears to be an increase in the raw counts over the whole period from about 25 events per hour to around 50 events per hour.  This level has stayed steady for the past week or two.  Perhaps this averge count level is representative if the speed of the landslide which was slowly increasing up until mid January when it stabilized at about 7.5 cm/day.

Another thing to note is that the count levels on UGAP5 are quite high, UGAP6 a bit less and UGAP3 and UGAP2 less so.  The station UGAP5 is located just a few meters from the largest part of the landscape scarp and, based on approximate event locations covered in an earlier post, one might expect that near this station is where most events are taking place.  The count numbers are consistent with this.

 Update of February 15, 2018 - A Mystery and Forensic Seismology

Whenever seismologists see seismic signals unrelated to earthquakes our interest is piqued.  The seismic stations monitoring the Rattlesnake Ridge landslide near Union Gap, WA are filled with non-earthquake signals. Most we know the source for and consider to be "noise" (ie windy periods, trains, trucks, cars, helicopters and other aircraft). Recently we noticed a strange signal that showed up on all of our stations in the area and lasted all night (Feb 12 -13).  Here is one view of when it occurred.

This is a two day helicorder-type seismogram (one line is 30 minutes) for the time around the night of Feb 13.  Remember the times along the left edge are in Universal Standard Time (GMT or Z) which is 8 hours ahead of Pacific Standard Time (PST).  The approximate period of the strange signal is from 10pm PST on Feb 12 until about 8am PST on Feb. 13.  In this very compacted view one can just see that there is something different (larger signals) than at other times.  We can easily see that there is this elevated signal level on all stations in the vicinity via the RSAM Plot.  What about the character of that signal?  If we zoom in to a part of the signal we can look at it in both the time (seismogram) and the frequency (spectrogram and spectra) domains.  Here is an example of a couple of minutes at about 3:30 am PST on Feb 13.

Note in the spectrogram the many redish lines in the frequency range of 60 - 80 Hz.  These can be seen as the bumps in the spectral plot at the bottom as well.  Also, note the significant peak at about 18 Hz.  This is particularly bothersome to our event detection and counting software that filters the data from 15 - 40 HZ.  This 18 Hz signal, while it varies a little in strength, is almost always there during this period and records on both our UGAP monitoring stations and stations 1.1 km (UGP4) and 4.2 km (YA2) away.  Below are the same sorts of plots for a two minute period almost exactly a day later; early morning of Feb 14.

The scale is the same as before.  It's obvious that the overall signal levels are much lower, and the many peaks, particularly the one at 18 Hz, are gone.  One can also see near the beginning of the seismogram a small event (probably from the landslide) that would have been totally obscured the night before.

So, what is this mystery signal? We have not seen it on other nights.  None of the other monitoring techniques have reported anything unusual going on with the landslide, and from years of looking at seismograms we don't think it could have anything related to the landslide. We were very suspicious that it must have an anthropogenic origin.  It must be either a very strong source, such as a boring machine or rock crusher, or a bunch of distributed sources.  We know of nothing in the area that is big enough or would be operating in the middle of the night for the former.  A possibility we thought of was that the people at the Yakima Firing Range were doing some sort of night time manuvers and driving dozens of tanks all over the place all night.  Calling them turned up no activity.  After sending out queries to some people who live in the area we got a response from Ray Wolverton who lives only a few miles southwest of the landslide.  Here is part of the e-mail he sent to us:

I think I might have the answer to the constant mysterious seismic signal in that area.  The farmers were running their frost fans in that area-----some of them starting around 8:00 or 9:00 p.m. and continuing until around 8:00 a.m. the next morning.  I remember looking out my living room window that morning and seeing at least 30 of those machines operating within a mile or two of the landslide area. Typical size of these machines is 100+ horsepower engine driving an 18-20 foot prop. These machines create the noise and turbulence of a helicopter. When running, these can be heard/felt for considerable distances.

Indeed, we think Ray is correct. Several of these fans going at about the same speed could easily induce vibrations we would pick up several kilometers away.  There was still a bit of a question as to why we had not seen this before.  Looking at the temperature data from the Yakima airport for the past month or so helped to convince us even further that Ray was right.  Here is the plot for February.

Note that the only time the temperature was significantly below freezing for an extended period was the night of the 13-14th. While January had many cold days, the temperature never went much below 30 degrees F and then only for short periods.  In talking with Ray he even predicted that we might see these strange signals again.  The forecast is for very cold nights early next week, and so the frost prevention fans are likely to be turned on again.  Thanks for your help, Ray.

Update on February 27, 2018 - (A Rockfall and Other Observations)

The landslide as reported by the monitoring group continues to still be moving in about the same way as it has for the past month to six weeks, steady at 1.7 feet/week.  Yesterday a rockfall into the quarry was reported by the Yakima Valley Office of Emergency Management. It was visible from the cameras watching the incident and from the public driving by the incident site. In reviewing time-lapse camera images it seemed to have taken place at about 9:47 am PDT.  Here is an image from one of the cameras taken shortly after the rockfall started.  Note the dust cloud rising from the edge of the pit, and note the time-stamp of this photo in the bottom right corner.

Reviewing the seismograms we find a distictive rockfall signal starting at 9:43 am PST (17:43 Z) with a possible small precursory event about 15 seconds earlier and nothing at 17:47 Z.  Checking with the camera operators they confirm that the time-stamping clock for the cameras was not synced to realtime, and the several minutes' difference was due to this discrepancy.  Here is a two minute segments of the seismograms from the four monitoring stations on or very near the slide block.  Note that the classical cigar-shaped envelope for the rockfall lasts about 35 seconds with two parts.  This event appears to be very similar, perhaps a little bit larger, to the rockfall of January 19, 2018 from about the same place.

Yesterday's event did last longer than the January one and had several very small follow-on falls over the next hour or so. If we look at the same records in the frequency domain via spectrograms we again see the characteristics of a rockfall.

 Note that it is strongest on UGAP6, the station actually out on the slide block.  It is only slightly closer to the source than 5 or 2, so why does it seem so much stronger?  We feel this may be because the seismic waves traveling directly through the slide block to this station are not as attenuated as those that must transition through the edges where the rock probably is not as competent.  Again UGAP1 is contaminated by so much traffic noise (and is farther away) so this rockfall can not be discriminated.  Recognizing rockfalls on 12 hour webicorders for this period is not easy, particularlly since after this event geologists were driving around the hill slope near our stations generating even more cultural noise.  The following is a more detailed seismogram, easier to review, that we sometimes use in the lab where we can change scales and duration for a better view.  Even then it is not imediately obvious what is a rockfall versus the many other types of signals.

Before leaving the topic of rockfalls we are a bit surprised that we have not seen more of these.  We may be missing small ones, particuarly those that take place during seismically noisy periods (wind or lots of traffic), but we don't think that rockfalls of the size of the one yesterday are being missed.  Given that the slide has been moving about 1.7 feet per week for the past six or more weeks (and moving some for the previous two months) it seems that the total movement could be as much as 20 feet. One might expect that this movement would have dumped up to 20 feet off the edge of the slide into the quarry.  This is a bit of a puzzle to this seismologist.

Enough of rockfalls.  What about the more interesting (to a seismologist) tiny earthquakes that seem to continue at about the same rate.  Our several monitoring techniques (listed at the beginning of this post) show little change over the period we have been watching; some ups and downs dependent on background noise levels but nothing that look to be significant changes in landslide events.  However, over the past several days there does appear to be an increased number of cluster events as determined by REDPy.  Below is a snapshot of part of the REDPy display showing events in each cluster as marks on a time line.  Note that many near the top of the plot were all occurring in the past several days.  These new clusters seem to be mostly in the same areas as previous clusters (along the east edge of the slide block) but may represent new points of failure along this edge.

This appears to be a subtle change if it is a change at all.  Other parameters we look at don't seem to indicate a change in total number of events, though a few recent events have been slightly larger than what we have seen before. 

We plan to continue the monitoring in some form into the future.  However, to save effort we may remove some of the seismic stations that are not providing useful information.


 Update on March 16, 2018 (No change in the slide but some new monitoring efforts)
The report from the enginners geodetically monitoring the landslide is that it is continuing to move at about the same rate as it has for the past two months; ~1.7 ft/week.

January 2018 Oregon Tremor Event Update

January 31, 2018

by Nancy Sackman

Oregon tremor over!
In the previous blog post about The M9 Project, we talked about how the Cascadia Subduction Zone can generate an M9.0 earthquake. However, our understanding of what an earthquake of this scale would actually look like is less advanced. While we have evidence of past earthquakes (e.g., native oral histories, tsunami records), we have no quantitative observations of how strong the shaking would be during a megathrust earthquake in the Pacific Northwest.

Seismic monitoring of a slow landslide

December 30, 2017

by Steve Malone

Active landslide at Union Gap. Updated Jan 31, 2018.

December 2017 Oregon Tremor Event - Update

December 27, 2017

by Nancy Sackman

Update to Central Oregon Tremor - moving toward Portland and Medford

December 2017 Oregon Tremor Event

December 15, 2017

by Nancy Sackman

Over the past 9-10 days, it appears that tremor in central Oregon has picked up.
What is “The Big One” going to look like? How soon will we know it’s coming? How are our cities and communities going to fare?

Entiat area earthquakes and other seismicity

August 9, 2017

by Steve Malone

A few questions have popped up about earthquakes near Entiat, WA. I might as well address these and a few other questions.

Earthquake swarm NE of Bremerton

May 11, 2017

by Renate Hartog

Earthquake swarm near Bremerton; what is going on?

Volcano Preparedness May 2017

May 1, 2017

by Nancy Sackman

May is Volcano Preparedness Month for Washington State
On Monday morning (April 10) the Pacific Northwest Seismic Network (PNSN) was buzzing with activity, but not seismic activity. The Network hosted a press conference to announce the rollout of a new version of the earthquake early warning (EEW) system, ShakeAlert, which is now fully integrated across the entire West Coast of the United States.

Next ETS Expected any time now

January 24, 2017

by Steve Malone

Already over and then going again. Back-to-back ETS and finally over as of Apr 6.

Another Seahawks game experiment - Jan 7

January 6, 2017

by Steve Malone

Seahawks fans shake up the PNSN instruments.... again.
This Thursday, 50 million people around the world will drop, cover, and hold on for the 8th Annual Great ShakeOut, the largest earthquake drill in the world. This year at the PNSN, our motto is “drop, cover, hold on, and do something else too". We are thinking about other ways that we can enhance our preparedness for a major earthquake. This week on the SeismoBlog, we are outlining a few other strategies to supplement this year’s drill.

Cascade Volcano Seismology - a Tutorial

October 5, 2016

by Steve Malone

A newly modified tab on each volcano page gives a nice overview of each's earthquake history. Here is an introduction and some hints for interpreting these plots.

iMUSH: Adventures in the Field

September 12, 2016

by Lauren Burch

Seismology graduate student Mika Thompson shares a thrilling tale of wasps, fallen trees, and other impediments to science.

The Long Trek to MH09

September 8, 2016

by Shelley Chestler

Sometimes uninstalling a seismic monitoring station doesn't go quite as planned. Here is tale of my team's first attempt to take out a particularly stubborn station for the iMUSH (Imaging Magma Beneath St. Helens) project:
Both the Cascade Mountains and the Olympic Mountains are products of subduction, but not all mountain ranges are created in the same way.

Another debris flow avalanche at Mount Baker

June 15, 2016

by Steve Malone

Seismic signals on May 25 are evidence for the size and timing of yet another on of these avalanches.

All the mountains, oceans, and islands on Earth exist because of plate tectonics. Different plate boundaries produce different geologic features: divergent boundaries spread apart to form mid-ocean ridges and rift valleys, transform boundaries slide past one another to form strike-slip faults like the San Andreas, and convergent boundaries collide to form tall mountains, deep trenches, and volcanoes. This type of plate boundary is responsible for the numerous volcanic arcs around the Pacific Rim (often called the “Ring of Fire”), and formed our iconic Cascade Volcanoes. Here in the Pacific Northwest, the Juan de Fuca plate is subducting beneath the North American plate along a convergent plate boundary called the Cascadia Subduction Zone (CSZ). Subduction zones like this are the only fault systems capable of producing very large megathrust earthquakes, but they only do so occasionally - over the last 100 years, there have been 84 earthquakes of magnitude 8.0 or greater worldwide, and only 4 of them were greater than an M9.


The simplest answer to the question “Will there be another large earthquake on the CSZ?” is yes. However, the question of “when” is much more difficult to answer. Seismologists don’t know exactly when the next large earthquake will occur on the CSZ, but we do have a good picture of when they have happened over the past 10,000 years. If we divide 10,000 years by the number of ~M9 earthquakes found in that time period, the average recurrence rate for M9 earthquakes along the CSZ is roughly 550 years. We are 316 years past the last great CSZ earthquake in 1700, and we estimate that there is about a 15 % chance that an M9 will occur on this fault within the next 50 years. However, research on submarine landslide deposits shaken loose by big earthquakes indicate that M8+ earthquakes occasionally strike off the coast of Oregon in between “full rip” M9 events. This research suggests that there is a greater probability of reoccurence of a great earthquake in Southern Oregon than off the Washington coast, but there is not a consensus within the geophysical community as to specifically how much greater the hazard is.

One is a guess, and the other is an educated guess.
Earthquakes happen on faults, but where are the faults in Oregon and Washington? The new "Display Faults" tool on the PNSN Recent Events map can help you explore the locations of faults in the Pacific Northwest.

Exotic Events (not erotic events)

April 1, 2016

by Steve Malone

Seismically recorded non-earthquakes now have their own page at the PNSN.

Negative depth earthquakes?

March 30, 2016

by Steve Malone

Why do some earthquakes in our list have negative depths now?

Explosion "Earthquakes"

March 10, 2016

by Steve Malone

Two recent large explosions generated acoustic waves recorded on seismographs.

December 2017 Oregon Tremor Event

December 15, 2017

by Nancy Sackman

December 2017 Oregon Tremor Event


Over the past 9-10 days, it appears that tremor in central Oregon has picked up (Figure 1).  The last slow slip and tremor event was in February 2016, 22 months ago.  


Figure 1

Figure 1. Age progression of tremor in central Oregon for the past 9 days.  Earliest tremor locations start from 12/5/2017 and propagate roughly outward, clustering near Salem and Roseburg.  Last update was December 14, 2017.


Tremor is the release of seismic noise from slow slip along the interface of the Juan de Fuca and North American plates and lasts for several weeks to months.  This process is known as Episodic Tremor and Slip (ETS).  Slow slip happens down-dip of the locked zone (Figure 2).  The locked zone is where tectonic stress builds up until it releases in a great earthquake or megaquake.  The recurrence interval of slow slip and tremor varies at different regions along the Cascadia Subduction Zone.  


Figure 2

Figure 2. Cross section of the subducting Juan de Fuca Plate.  Figure from Vidale, J. and Houston H.  (2012) Slow slip:  A new kind of earthquake (Physics Today, 2012 pages 38-43).


The last ETS event in Cascadia started in February 2017 around the western edge of the Olympic Mountains.  The duration was approximately 35 days with a two-week quiescent period.  Prior ETS events in northern Washington/Vancouver Island area was approximately December 2015.  


The last ETS event in central Oregon was 2016 and lasted just over a week before it stopped on March 1, 2016.  


ETS events are still being studied to understand the processes about slow slip and megathrust earthquakes.  

More information about slow slip and tremor can be found here on the PNSN website.

December 2017 Oregon Tremor Event - Update

December 27, 2017

by Nancy Sackman

Tremor has continued in Oregon since the last post on December 15th.  Current tremor activity has been ongoing since about 12/5/2017 (figure 1).  


Figure 1

Figure 1. Age progression of tremor in central Oregon for the past two weeks.  Earliest tremor locations start from 12/5/2017 and propagate northerly and southerly.  Last update was December 26, 2017.

Since December 19th, tremor has now migrated northerly toward Portland and southerly toward Medford.


Figure 2

Figure 2. Tremor activity from 12/19 to 12/26 showing progression in a northern and southerly direction.

More FAQs on Slow Slip and Tremor


On our previous blog post, we briefly discussed what ETS (episodic tremor and slip) is.  Let’s go through a couple of more frequently asked questions.


1.What is tremor?


Tremor in the Cascadia Subduction Zone is the seismic noise of slow moving earthquake along the interface of the subducting Juan de Fuca Plate and the North American plates.  Compared to normal earthquakes, tremor has lower frequency energy and can last for minutes, hours or weeks.


2. What about volcanic tremor?


Tremor can also be volcanic.  But ETS is deep, non volcanic signatures that are a result of plate motion, not magmatic movement.


3. How deep are the tremors?


As it states on our website - “This is a topic of ongoing research.”  But research suggests that it occurs near the plate interface at approximately 30 - 40 km deep.   


4. What is the magnitude of tremor?


More than likely less than a 2.  According to our website, we don’t assign a magnitude during our automatic detection of tremor.  


Check out the map on our web page: