Seismo Blog

Rattlesnake Ridge Landslide slowing down

July 9, 2018

by Steve Malone

Detailed reports of an active landslide on Rattlesnake Ridge, near Union Gap just south of Yakima, WA are available from  "Seismic Monitoring of a Slow Landslide" in January, 2018 and "Continued Landslide Monitoring" in March 27, 2018. 

At the end of March our seismic event counter was recording about 180 tiny events per hour on two of our stations with the highest activity (UGAP6, on the slide and UGAP5 ~20 meters away right next to the high scarp on the east side of the slide).  Currently this count is down to about 100 events per hour.  Similar to the event counts the geologists monitoring the slide with geodetic techniques report that the rate of motion has gone from 1.7 feet per week to about 1.2 ft/wk on the fastest markers and to less than 1.0 ft/wk on some of the slower markers.  There is some scatter to both sets of data but the decrease is clearly seen when looking at the whole period.

Below is an extracted plot from our Tiny Event Counter Display (events per hour) for the whole period we have been monitoring (first of the year, 2018).  Note that from the time that UGAP5 (green) and UGAP6 (black) were installed in late January they have had the highest counts of our four stations.  UGAP5 started right out with about 180 evts/hr and UGAP6, while starting out at about 100 evts/hr caught up to and was tracking UGAP5 closely by the end of March.  Since then they both have been gradually decreasing in little bumps and flat periods. UGAP3 (red), located near the top of the slide may be mimicking this same pattern but at much reduced levels (~80 evts/hr to ~60evts/hr).  UGAP2 (blue) is located about 100 meters from the slide and only records the larger events so there is not enough resolution to see any pattern in its data (and it is partially cut off in this sample plot).

One might wonder what the significance of this slow down is.  While there is still a possibility that it could speed up and fail catastrophically, this scenario seems less and less likely.  The more likely forecast (from me, a seismologist with little to no geology or landslide expertise) is that it will continue to slow ever so slightly until it reaches a threshold rate and then will stop.  Of course, this rate may be so slow that the exact time of stop is hard to tell.  There is always some noise in any data thus defining a count threshold meaning "stopped" may not be obvious.

In the meantime we continue monitoring but at a lower priority than early in the sequence.  One might notice that about two weeks ago the seismograms for station UGAP5 changed to random motions and that the counts for it had a big jump and then goes to zero. This change took place in the very early morning and there was no evidence on the seismograms of a vehicle driving up or people walking near the instrument. Just today the cable to the seismometer for this station was found by a colleague to have been cut in two places.  While the cuts look fairly clean (see photo below) there is evidence of chew marks.  We think some sort of rodent doesn't like landslide monitoring and/or likes to eat seismometer cables.  In any case we will try to replace the cable next time someone is in the area but no special trip will be made for a repair.  I.e. repairs are a lower than normal priority since other stations are working fine.  Lets hope the rodent got indigestion from this bite and doesn't try to find more.

Anyone driving along I-82 near Union Gap can obviously see the landslide from the road.  While it does seem to be slowing down it is still pretty obviously not a normal geologic feature.  Here is a recent photo from the monitoring camera just across the river from the slide run by the UofO and UofN.

MSH Anniversary Media Round-Up

May 18, 2018

by Elizabeth Urban

A collection of news stories on the 38th anniversary of the Mount St. Helens eruption
The final blog about The M9 Project is going to focus on you. What are you going to experience during a megathrust earthquake? How do we connect science and community? What should you do to be prepared?

Landslide Study with Nodal Seismographs

March 26, 2018

by Amanda Thomas

A special seismic array for studying the RattleSnake Landslide. March 26, 2018

Seismology in the air

March 22, 2018

by Steve Malone

A bolide explosion off the coast is recorded on many seismic stations allowing us to get an approximate location.

Continued Landslide Monitoring

February 11, 2018

by Steve Malone

Steady landslide motion still of seismic interest. Update on March 26, 2018

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.

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:

 

https://pnsn.org/tremor