NOTE the Update below on May 20, 2014.

The two avalanches on the afternoon and evening of May 14, 2014 at Mount St. Helens had some of the same characteristics of a large avalanche in the spring of 2011.  These seem to have originated high on the east flank of Mount St. Helens and descended onto the Plains of Abraham where they converted into a very wet and slow slide/flow that continued down drainages for several kilometers.  The first event at 2:44 pm PDT (21:44 GMT) showed up as a very strong seismic signal at station EDM indicating that it may have passed quite close to this station on East Dome. It only lasted about two minutes indicating that it may have been smaller volume than the later slide.  The second starting at  5:47 pm PDT (01:47 GMT) did not produce as strong shaking at EDM but lasted for about eight minutes.  Both events had what looked to be a "stopping phase" near the ends of the signals.  We think this is when wet snow slides become highly chanalized and while flowing as a mass (slug flow) they loose enough fluidity to start stopping.  At some point the  whole thing refreezes and stops as a unit putting a large force into the ground that we see as a seismic stopping phase.

A friend and colleague flew over Mount St. Helens on May 15 and reported seeing two avalanches on the east side.  Additional details and photos will be posted here on May 19.

 

Here are the seismograms for the avalanche starting at 2:44 pm PDT as seen on eight seismic station on and around Mount St. Helens (see map below for stations locations).  Note that on station, EDM the signal is clipped (saturating the recording system) for almost two minutes.  Also, note that there is a small extra burst of energy just after 21:46 GMT that shows on most stations.  This we think is the "stopping phase" generated by a large mass of snow coming to a stop all together.

 

Here are the seismograms from the same stations for the second and much longer lasting avalanche that started just after 5:47 pm PDT. Note that the shaking is strong on EDM for about eight minutes but is never clipped.  The brief, sharp signal on station SEP at about 01:51:40 GMT is probably a very tiny, local event on the dome and unrelated to the avalanche. Note the very strong "stopping phase" in this case showing well on all stations just before 01:55 GMT. Estimate of the seismic wave arrival times on the different stations allows us to get an approximate location of this event which is about 8 km ESE of the center of the crater.

 

Here is a map of Mount St. Helens showing the location of the seismic stations recording these avalanches. Approximate locations as observed and photographed on May 15 were added to this map on May 20 after getting photos and descriptions (see below).

 

 Today I received some photos taken by C Dan Miller, a retired volcanologist with CVO who flew over Mount St. Helens on May 15.  His photos and descriptions and annotations provided by Rick Lahusen, another CVO volcanologist (currently active) point out that my previous interpretation was off a bit.  It seems these avalanches were not as big as I previously thought and certainly did not travel nearly as far as I estimated before.  Below is one of several photos provided by Dan.  I have added arrows showing where the two avalanches start (upper arrows) and terminate (lower arrows).  The second and larger avalanche is on the left with right pointing arrows and the first and smaller one is on the right with left pointing arrows.  The approximate locations of seismic station, EDM is shown as a circle.  Clicking on the photo will provide you with the original higher resolution version.

Thus the photos confirm the two avalanches and their relative sizes but my previous interpretation of the distance traveled by the second avalanche was greatly over estimated.

 

Now, in response to the comments posted by John McBride, as far as I know there is not a comprehensive catalog of all seismically observed avalanches.  Of course, such events would be only a small set of the actual avalanches taking place.  I think that the Northwest Avalanche Center maintains a catalog of reported avalanche observations.  This also would be a small subset of all that take place since many avalanches are not observed and many that are observed are not reported.  We do have some other avalanche observations in our PNSN notes and catalogs and I may try to recover those in some useful way for future reference.  Thanks for your suggestion. I do think the recent ones at Mount St. Helens will have no effect on the trail system since they probably terminated above the trail.

I don't know how to respond to the comment of Beej Baily since our work is actually based on observations and data analysis and her comments don't seemed to be based on anything that I know of.  There currently is no evidence that there is anything unusual going on in the Cascades or anywhere else based on actual observations.

A large explosion was reported in the early morning hours of April 25 in North Bend, WA. I reviewing the seismic records we find signals consistent with this report. For a preliminary report.....
PNSN instruments picked up the ground vibrations generated by the deadly Oso landslide.

Legacy web site content returns

March 17, 2014

by Steve Malone

Two years ago the PNSN web site changed format in a big way. New features and capabilities were added and the look and feel was greatly improved. But, many of the old popular pages were left behind. We have now converted many of these pages to generic documents that can be linked from the new pages but are still in the old format. For a summary of what we have now....

Ice avalanches on Cascade volcanoes

February 28, 2014

by Steve Malone

With the recent heavy snows in the mountains after a long, cold dry spell the Cascades could be primed for big snow avalanches. However, just in the past couple of days we have seen two big seismic sources that we interpret to be, at least initiated as ice avalanches at Mount Rainier and Glacier Peak. For some details and photos...... (and an update)

A New View On What's Shaking on the Cascade Volcanoes

February 26, 2014

by Jon Connolly

We have added a new interactive graphic to the PNSN home and volcano page that provides a quick summary of the latest Cascade volcanic seismicity. This graphic replaces a table view of the same data. We have strived to make the PNSN landing page a quick summary view of immediate information that allows a user to drill down for more info if desired. The table view for recent volcanic seismicity was a bit clumsy and fell short of this goal.

Seismic Spectrograms - A new way to look at wiggles

February 13, 2014

by Steve Malone

Many people are familiar with seismograms - charts showing vibrations from a seismograph over time - but far fewer know or understand spectrograms. Still, these plots showing the strength of seismic vibrations over time at different frequencies are very useful for seismic analysts once they have some experience with them. At the PNSN we have been using them for several years, particularly for volcano stations. Now we are providing them for anyone to look at. For an introduction........

The final football game analysis

January 19, 2014

by Steve Malone

The data and notes have been collected for our seismic recording of the NFC championship game between the Seattle Seahawks and San Francisco 49ers and some analysis has been done. While too early yet for a definitive conclusion on all aspects of the data, we can report some interesting results and speculations. This blog will be added to as more analysis is completed. (By the way... The Seahawks won so on to the Super Bowl.) In the meantime for some interesting observations.......

The Football Game Experiment Continues

January 14, 2014

by Steve Malone

During the Seattle Seahawk's-New Orelans Saints Divisional game of Jan 11, 2014 we experimented with adding seismic stations at the stadium, providing live seismogram feeds, near realtime seismograms and some interpretation of recorded events. Since the Seahawks won and will play again in CenturyLink Field, why stop now. We learned some things, are puzzled about some things and changed somethings and doing it again. For all the details......

Seismic Game Analysis

January 11, 2014

by Steve Malone

The PNSN, along with with many fans, took extra interest in yesterday's playoff game. With two extra seismic stations installed at the stadium seismologists watched the seismograms at the same time watching the game on TV. We now have some analysis of the wiggles and other observations on this multipart experiment. For all the details....

PNSN Earth-shaking Seahawks Experiment

January 8, 2014

by Jon Connolly

Here is the content of a press release PNSN issued today about the deployment of two strong motion sensors in CenturyLink Field. We will monitor the vibrations of the structure and ground produced by an excited and energized crowd of Seahawks fans during the playoff game against the New Orleans Saints on Saturday, 11 Jan., 2014. The experiment provides challenges at all turns, but we hope to learn something about how seismic waves are generated within a structure, how to sense them and transmit them in a very challenging environment for data telemetry, and how to process and present them to users in real time. We also hope the Hawks win (although a close game might produce more ground motion!). Go Hawks!

Large Mount Baker debris Avalanche this fall

October 29, 2013

by Steve Malone

Every few years a buildup of ice and snow on the north and west side of Sherman Peak (Mount Baker) produces a large debris avalanche that can go several kilometers down the Boulder Glacier. Such an event occurred recently as determined by a pilot report (with photos). Searching the seismic records for Mount Baker seismographs turned up the seismic signal for this event on the afternoon of Oct 21, much later in the year than for previous such events. For more details.....

Speedy ETS in the works

September 16, 2013

by Steve Malone

It seems that the expected ETS of Oct-Nov, 2013 is already underway. Significant tremor started on Sep 7 in south Puget Sound and has already moved into southern Vancouver Island. This one seems both early and speedy with strange jumps. Update on Oct 11, 2013: It is over. This one went from Sep 7 - Oct 8, 2013. For all the details of this whole event......

Peppy seismic swarm 20 km NW of Mount St Helens

August 24, 2013

by John Vidale

A series of M3 earthquakes are shaking the area of Mount St Helens, in one of the more vigorous bursts of seismic activity in a few years.
Say "jokulhlaup" three times real fast and then run up-slope to get away from it. This icelandic word describes a sudden release of water trapped in a glacier. Such sudden floods can rapidly "bulk up" with sediment scavenged from river banks generating a lahar (mud flow) that can be very dangerous and destructive. Such an event occurred in the early morning hours of May 31 from the Deming Glacier down the Middle Fork of the Nooksack River and was well recorded by the MBW seismic station of the PNSN.
To address our users' desire for a simple user interface to view the latest earthquakes in the Pacific Northwest, we have just released three features: a new recent events list, mobile views, and a Twitter feed that will tweet all PNSN events magnitude 2 or greater.

M3.5 event west of Tacoma early Sunday morning

April 8, 2013

by John Vidale

Deep event is typical of seismicity near Seattle, has some aftershocks.

Oregon ETS is over, but....

April 4, 2013

by Steve Malone

The ETS in central Oregon starting on Feb 24 seems to have finished on Mar 31. But, bursts of tremor continue in other parts of Cascadia. In fact during the Oregon ETS much of Cascadia has seen periods of tremor lasting from one to several days.

Small swarm near Mount McLoughlin last night

March 24, 2013

by John Vidale

It has mostly been seismically quiet recently, although last night and this morning a swarm has been active in southern Oregon.

Earthquake early warning workshop quick report

March 17, 2013

by John Vidale

A workshop with 50 people met last month to chart the path to Earthquake Early Warning in the Pacific Northwest. Progress is encouraging.

thePNSN Facebook discussions

March 15, 2013

by John Vidale

The PNSN's in-depth blogs are here, and meanwhile our liveliest discussions on happening on Facebook.

Deep Tremor over much of Cascadia

March 8, 2013

by Steve Malone

Following three months of relatively little deep tremor in Cascadia the past month has seen bursts of activity up and down the region including what appears to be a full blown ETS starting in northern Oregon and spreading south.

Small earthquakes under Gold Bar

February 28, 2013

by Kate Allstadt

Though the residents of Gold Bar may not have noticed, a swarm of hundreds of tiny earthquakes has been rumbling along just a few kilometers east of town since October 2012.
The last great Cascadia Subduction Zone Earthquake occurred 313 years ago. We need to do more before the next one strikes.

A flash in the sky, a thump in the ground

January 11, 2013

by Steve Malone

Reports of a bright flash in the sky in eastern Washington this morning caused us to search seismograms for stations in the area which turned up what seems to be a "sonic" source that weakly was recorded on 9 seismographs. It is fairly common for bright flashes in the atmosphere, sometimes referred to as "fireballs", "meteors" or "bolides" to end up being recored seismically. Such recordings can allow us to pinpoint the time and location more accurately than can be done from eyewitness reports.

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