The seismic signals (ground vibrations) generated by the motion of the Oso landslide were very well recorded by the Pacific Northwest Seismic Network (PNSN), indicating it was a very rapid and energetic event (though this is apparent from the tragic consequences and eyewitness reports). Signals at the lowest frequencies (long periods), which travel farthest before attenuating, have been detected up to ~274 km (~170 miles) away from the landslide (station DAVN) though this distance may increase as more data are analyzed.

Recordings from seismic stations that recorded the event well can be used to determine the timing of the sequence of events (see Timeline section and Figures 2-3). Seismic signals from landsliding are distinguished from other noise such as vibrations of distant earthquakes and human and natural noise local to the seismic station by the frequency content, duration, shape and other characteristics of the waveforms.

 

Background

 

Seismic signals from landslides are distinct from those generated by earthquakes (Figure 1). Energy is generally concentrated at lower frequencies (<5Hz) and the signal emerges gradually from the noise. Earthquake signals, on the other hand, have a broader range of frequency content and arrive suddenly with the sharp onset of P waves followed by often distinct S wave and surface wave arrivals. These distinct arrivals are typically not observable in landslide seismograms. The reason for the differences are different physical mechanisms that generate the waves. Earthquakes are generated by brittle slip along a fault plane deep in the earth that starts suddenly and lasts a very short time whereas landslides typically last several minutes or longer and energy builds up more slowly as the material accelerates and begins to break apart. The peak seismic amplitudes will be reached often toward the middle of the signal, peaking, and then gradually fading back into the noise again.

Not all landslides generate strong seismic signals observable at large distances. The event must be large and energetic for the resulting seismic waves to be observable tens to hundreds of km away. Most if not all of the Oso landslide subevents that were recorded seismically were probably generated by material breaking off the source area and moving rapidly downslope, not by slow creep or mud moving slowly in the valley.

 

slide_quake_comparision

 

 

 

 

 

Figure 1: Comparison between seismic signals characteristic of A.) a regular small earthquake signal (this is the M1.1 that occured on March 10th near the Oso slide) and B.) a landslide (this is the first slide from the Oso sequence). Note the x-axis is of equal duration in both cases. Click to enlarge.

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2: Google Earth map of landslide location relative to seismic stations referred to in this report.

 

 

 

 

 

 

 

Timeline

The seismic data recorded at the closest station, Jim Creek Washington (JCW, Figure 2), is displayed in webicorder format on Figure 3. This figure can be read like a book, each line shows 30 minutes of conitnuous seismic data collected at 100 samples per second. Notable events are labeled.
 

 

Figure 3: Timeline of landslide seismic signals as recorded at the closest seismic station (~7 miles/11km away). Each line represents half an hour of continuous data recorded at JCW. See text for details, click to enlarge.

 

 

 

 

The sequence began at 10:37:22 am local time (17:37:22 UTC) with the most energetic and longest duration seismic signal, lasting about 2.5 minutes. This subevent generated strong long period motions observable over 270 km away, which suggests the acceleration was very rapid and a large amount of material was involved (See Figure 7). This was most likely the rapid collapse of the old slide material that was previously disturbed and weakened in 2006 (Figure 4, 2006 slide area) and was the initial slide that impacted the neighborhood below at high velocities with no warning. After a brief interlude with one minor discrete slide at 10:40:56 am, the next large slide occurred at 10:41:53 am. This was slightly shorter in duration than the previous signal, and did not generate such strong long period motions as the first signal, suggesting the movement was less rapid. This may have been the newly unstable area upslope of the 2006 slide area (Figure 4, New slide area) slumping down onto the debris below. Continued rumbling and discrete smaller landslide seismic signals continued for more than an hour afterwards, most likely a result of smaller landslides breaking off the headscarp area left unstable by the initial events.

 

Table 1 Approximate times of landslide seismic signals recorded at JCW - Times are in PDT (UTC + 7)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4 approximate boundaries of Oso slide source area corresponding to what we interpret to be the two main slides observed in the seismic records. See text for details.

 

 

 

 

 

 

 

 

 

Seismic Signal recorded on other stations

 

The ground vibrations from this event were picked up on a number of nearby seismic stations. Figure 5 shows a record section (seismograms from different stations plotted proportionally to their distance from the source) of the landslide as recorded on the seismic stations shown on Figure 2. The same data, showed in spectrogram form (frequency and energy over time), are shown on Figure 6. Note that most of the energy is concentrated in the very low frequencies (less than 5 Hz).

These ground vibrations are too low in frequency to actually be heard by the human ear, but if we speed up the seismic record by 160x, we can "hear" what the landslide sounded like, as recorded 11 km away at JCW. The following sound file was created by condensing 1 hour of seismic data (10:30-11:30 am PT) into 22 seconds using a tool available through IRIS webservices:

 

Click here to listen to the ground vibrations of the Oso landslide sped up 160x

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5: Record section of seismic recordings of Oso landslide at stations shown on Figure 2. Times are in UTC (local time + 7 hours).

 

 

 

 

 

 

 

 

 

 

 

Figure 6: Record section of the same data shown on Figure 5 in spectrogram format. Color indicates the strength of the signal at a range of frequencies over time. Times are in UTC (local time + 7 hours)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Forces Exerted by the Landslide

 

Very large and rapid landslides can generate a very low frequency (long period) pulses with wavelengths of tens to hundreds of seconds, due to the force the earth feels when the volume of material accelerates. The higher frequencies are generated by frictional processes as well as the impacts of individual blocks as the sliding mass breaks apart and flows. The longest period parts of landslide signals have been used successfully to determine the forces exerted on the earth over time by the moving landslide mass, but this has only been done so far for rapid landslides much larger than the Oso slide. However, the a long period (20-50 second) signal from the Oso event was recorded on at least 17 broadband seismic stations (see Figure 7 for examples) so it may be possible to determine the forces over time for this event which can then be used to better understand the dynamics of this event.  This work is in progress and we don't have results to report yet.

 

 

 

 

 

 

Figure 7: Three component seismograms from the two nearest broadband stations to the slide. The first trace from each is a high frequency filtered version of the vertical component. The three other traces are long period filtered (<0.04 Hz) versions with the slide siganl circled in red. Long-period signals at other times are typical of ambient noise that is particularly large on horizontal components.

 

 

 

 

 

 

Possible Precursory Seismic Activity?

 

Looking carefully at the seismic records from station JCW before (and after) the landslide shows many very small events that started around 8am PDT and stopped in the late afternoon. At first these were thought to be possible precursory slip events; however, we are convinced that they are unrelated to the slide and and probably have a cultural source.  Careful examination of filtered seismograms from the next nearest seismic station, B05D shows no such events.  If they were originating from the slide area they would also have been recorded at this station.  Also, looking at the days before and after March 22 we see the same sort of events only during daylight (working) hours, thus they likely have a man-made source.

A visual scan of the seismic data in the days prior to the event did not uncover any other obvious signs of potential precursory activity.
 

Magnitude 1.1 Earthquake on March 10th in vicinity of Oso Slide

 

There was a magnitude 1.1 earthquake detected by the PNSN located About 2 km from the Oso slide ± 0.8 km at a depth of 3.9 ± 1.9km on March 10th, 2014 at 21:43 UTC (14:43 local time), twelve days prior to the landslide that has received some attention from the press as a potential trigger. However, the shaking from a M1.1 is extremely weak and would not have been enough to trigger the landslide. Crude estimations of ground motions at the slide site from such an event would be less than 0.01%g, far below what would be expected to have any effect whatsoever. Earthquakes of comparable magnitudes occur on a daily basis all over the state so a M1.1 is not unusual in and of itself. The only thing that makes this event notable is that it is located close in space and time to the eventual Oso landslide.  In the previous 25 years, only 5 or 6 events in the PNSN catalog were as close to the slide location, however the seismic network is not sensitive to events much smaller than M1.1 in this area. Using the waveform from the March 10th event as a template we searched for previously undetected smaller events with similar waveforms (meaning they occurred nearby) and found 7 additional events in the past year located very close to the M1.1 (Figure 8). There is no indication of accelerating activity prior to the slide.

Swarms of small earthquakes like this happen regularly in Washington state and have historically occurred along the Devils Mountain Fault that runs through the valley. The physical meaning of swarms of similar small earthquakes is not well understood, but they can sometimes be related to slow deformation. Because the computed location of the M=1.1 earthquake was close to the top of the slide zone and almost within the formal error estimates of the location we ran some tests to see if it could have been mis-located by that much.  By fixing the location at the top of the slide zone and computing arrival times at the recording seismic station we determined that the time residuals from such a source are clearly outside any possible picking errors. Thus we feel that it is highly unlikely that this event and the others like it are related to the slide itself.

In the remote case that the M1.1 earthquake (and/or the other small similar quakes) is related to the Oso landslide, the most plausible explanation would be slip related to ongoing slow deformation within or below the unstable hillslope. A typical magnitude 1.1 earthquake would have a slip plane with an area on the order of 900 square meters (~17 m radius for circular plane, estimated using empirical relations in Wells and Coppersmith, 1994) with slip on this plane of less than 1 mm and the shaking cannot be felt except by sensitive seismic instruments, so this was a very small movement in any case. The other similar earthquakes found were even smaller.

 

 

 

Figure 8: Waveforms and event times of earthquakes with similar waveforms to the March 10th M1.1 event (thus having similar locations and mechanisms) that were previously undetected. These events were detected by scanning the past year of data at station JCW for matches to the waveform of the March 10th event (correlation coefficient of 0.5 or greater).

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.
This has been a mild year, and our volcanoes have been well mannered, too.

Historical Earthquakes in the Portland Area

November 19, 2012

by Bill Steele

Comments on Portland seismicity by PNSN staff seismologists.

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