Seismo Blog

Exotic Events (not erotic events)

April 1, 2016

by Steve Malone

The main purpose of the Pacific Northwest Seismic Network (PNSN) is to record and study earthquakes to better understand them for the purposes of hazard mitigation.  But, there are many things that can shake the ground that are not earthquakes.  We call these exotic (erotic in the title was just to get your interest up) events.  They are exotic because they are different from the bread and butter earthquakes in which we are mostly interested.  Some have waveforms similar to earthquakes while others look totally different.  Here we describe general characteristcs of some of these and introduce a new web page listing some of these Exotic Events studied in the past.

Anything that shakes the ground, even by the minutest amount can be recorded by our very sensitive instruments.  A person walking within 20 meters of a seismic station will show up as small blips on the seismograms.  A car driving within a few hundred meters will show up as a rumbling lasting for seconds or tens of seconds.  A train passing within a couple of kilometers of a station may show as a strong rumble lasting for minutes.  These sorts of wiggles we call "cultural noise"; "cultural" because they are generated by human activity and "noise" because they are not what we are interested in and often can cover up or hide the real signals (from earthquakes) in which we are interested.  Non cultural sources of "noise" include the shaking generated by water moving in nearby streams or rivers or the pounding of surf on the coast.  A ubiquitous source of noise is the shaking of the ground due to wind, which can be particularly strong when there are bushes or trees near a seismic station.  The wind-like noise of tremor associated with slow slip can go on for many minutes during ETS events.  Of course animals such as deer walking near a seismic station will generate blips not unlike a human walking nearby.  All of these sources of noise, while not earthquakes, are usually not considered to be "exotic events".  They are too common and are of little or no interest.

There are lots of different sorts of seismic signals generated by volcanoes when they are erupting or even just becoming restless.  Low-frequency volcanic earthquakes, Long-Period earthquakes and volcanic tremor are the names we give some of these signals.  Such events, while certainly "exotic" in the day-to-day world of the PNSN are not reported here but show up in descriptions of volcanoes, particularly Mount St. Helens in separate web pages.

Below is a list of some of the types of exotic events we have recorded with an example from a single station's seismogram.  In most cases an analyst will need to see the seismograms from several stations to reliably identify exotic events. Experience, thoughtful reasoning and, best of all, direct confirmation from an eye witness observer may be needed to nail down the source details.

  • We start with a typical seismogram from a real earthquake (NOT exotic) for contrast. (Note: In most of these figures the small tics along the top are 1-second intervals.)  A real earthquake usually has a sharp beginning (P-wave) that may quickly die down and then a larger arrival (S-wave) that may transition into a longer set of waves (surface-waves or, more generally, coda-waves):

  • Often, looking very like a real earthquake, a quarry blast or mining type of explosion will have strong P-waves and weaker S-waves and because they are at or near the ground surface may exite relatively strong S-waves.  If the quarry blast was at the surface and produced an audible "bang" in the air we may record that on a nearby seismograph as a short high-frequency phase many seconds later. However, there are many cases where, without direct confirmation from the blasting operation a seismic analyst cannot really be sure and thus may classify such events as "probable explosion". These are not really all that exotic because they are fairly common. Sometime we record more explosions per day than we do real earthquakes.

  • Speaking of explosions; what about when an explosion takes place above ground?  In this case we may only see the air-wave because not enough energy shakes the ground to generate recordable seismic signals.  Two recent accidental explosions from gas and propane leaks were reported in a recent blog.  A strange set of signals have been seen on seismic stations in eastern Washington now and then.  When we go to extra efforts to locate them, they seem to be coming from the Yakima Firing Range.  Because this is a closed military area we are not sure what they are doing but assume it most be artillery practice or some similar thing going "bang" in the air.

  • Another type of "bang" in the air is the explosion of a bolide or meteor as it enters the atmosphere.  In the few cases where we have been able to reliably identify such things there were news reports of people seeing a flash and maybe hearing a distant "boom".  Here are records from two stations for the same event.  The major tics in this plot are minutes apart rather than seconds.

  • Back to things on the ground. There are all sorts of geological avalanches or slides that shake the earth.  The signals from these can be quite complicated.  Because the source is usually stretched out in time the signals can likewise last a long time.  Here is an example from a rock/ice-fall high on the west side of Mount Adams recorded at a seismic station near Mount Rainier.  Apparently a very large section of rock and ice fell steeply for several thousand feet and then slid for a longer period of time on a shallower slope below.

  • The signal from a landslide can be quite strong and long even though the total vertical distance traveled may be no more than a few hundred feet.  Here is an example from the Oso landslide showing that it occurred in two separate chunks separated in time by over three minutes.

  • Snow avalanches usually don't shake the ground very much.  Even though they can be devastating to anything caught in them their mass is usually not enough to cause much ground shaking.  However if the avalanche is big enough and very dense (wet snow) they too can generate diagnostic seismic signals.  Here is an example of a fairly large wet snow avalanche off the east side of Mount St. Helens.  Note that this signal lasts for several minutes indicating that the avalanche must have been traveling fairly slowly (wet flow) or its source kept growing over quite a long time.


There are lots of other exotic signals that we record. For many we never really figure out the source.  But it is fun detective work to try and do so when something shows up that we don't understand. Check out our Exotic Event page for details of ones we have studied.

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.

West Coast Earthquake Early Warning System on the Horizon

February 9, 2016

by Shelley Chestler

Last week the White House hosted the first ever Earthquake Resilience Summit. One of the main goals of the meeting was to discuss the potential for fully funding a west coast earthquake early warning system.

Back-to-back ETS events, maybe

February 6, 2016

by Steve Malone

Following the recent "standard" northern Washington ETS another has apparently started heading south toward Oregon.

Slow Earthquake Trembles beneath Vancouver Island

January 7, 2016

by Shelley Chestler

Did you know that there is a type of earthquake that happens so slowly that we can’t feel it? One of these slow earthquakes is happening under Vancouver Island and northern Washington right now!

A Perspective on Tremor Activity

January 7, 2016

by Aaron Wech

With tremor activity occurring in the Pacific Northwest, it's important to provide perspective.

Don't Sweat the Little Ones

December 14, 2015

by Shelley Chestler

The recent earthquakes around the Puget Sound are probably not indicative of the “big one.”

A new look for pnsn.org

December 4, 2015

by Jon Connolly

The new pnsn.org website aims to provide a better user experience for all devices, prioritize features, and provide robust availability during a seismic event.
The July 2015 New Yorker article “The Really Big One,” by Kathryn Schulz, shook up the Pacific Northwest (PNW) more than any earthquake has since the Magnitude-6.8 Nisqually earthquake in 2001. In the article’s most dooming statement, the head of the Cascadia FEMA division was quoted saying, “everything west of I-5 will be toast.” This assertion scared the living daylights out of PNW residents, creating a sense of terror and hopelessness that was the antithesis of what the article meant to do: to spur the region into preparing for this potentially devastating event.

Unusual earthquake swarm south of Bend, OR

October 23, 2015

by Steve Malone

A somewhat unusual earthquake swarm started early on Oct 22, 2015 in an area about 65 km (40 miles) southwest of Bend, OR (25 km WSW of La Pine, OR). 36 events have been detected and located by the PNSN as of noon on Oct. 23, the largest only Magnitude 2.5. This ongoing swarm is in the same area that had similar swarms in 2001 and 2012 and is likely just the same sort of thing taking place again. For more details and updates....

How big was that earthquake?

September 24, 2015

by Steve Malone

Determining an earthquake's size seems to often result in different and inconsistent estimates. The "Magnitude" of an earthquake can be determined by several different methods, all of which should have some relationship to one another and, at least be consistent one earthquake to another. Unfortunately that's often not the case. PNSN seismologists spend lots of time estimating (measuring and calculating) earthquake magnitudes and end up discussing (arguing over) different techniques and complaining (whining) about inconsistencies and criticizing (belittling) certain results. Recently some effort is being made to try and refine (improve) our standard, routine ways of determining magnitude. We are starting to upgrade our published catalog with these "improved" magnitude estimates so you may see these estimates change from what was in the catalog before. Don't worry. The earthquakes have not changed, just our estimate of how big they are. For the gory details of how this is being done......

Summer rockfall time, yet again

August 21, 2015

by Steve Malone

With hot dry weather it is not surprising that the seismic records for volcano stations show lots of signals consistent with rockfall/avalanches and other exotic seismic events. In fact it is a bit of a surprise that we have not seen more and bigger such events this summer........ so far. Recent activity at Mount Rainier has included a debris flow (probably related to a jökulhlaup) and a moderate sized rockfall from high on the southwest side of the volcano. For some of the seismic details.....

Why earthquakes disappear

May 31, 2015

by Renate Hartog

Earthquakes have been appearing and disappearing from the U.S.G.S. webpages, this blog explains why.

PNW Earthquake Early Warning prototype goes live

February 18, 2015

by Steve Malone

Earthquake Early Warning (EEW) is now officially working for the Pacific Northwest (Washington and Oregon) in the same way it has in California for the past two years. It even got tested the first night in operations and worked...sort of. A workshop hosted by the PNSN at the University of Washington on Feb 17 introduced the topic and featured software to a group of about forty invited participants. For a few details on the workshop and the system's first live test......
The Seattle Seahawks' win over the Green Bay Packers in over time ended up so exciting that many of us serious scientists forgot to be serious and analyze the seismograms. Yikes! What a crazy ending. In fact, most of the game was seismically quiet (and disheartening for Seahawk fans), but the final half hour produced several seismic events that challenges the original "Beast Quake" for seismic supremacy. For our semi-scientific analysis......

Panther versus Seahawk Game Analysis

January 11, 2015

by Steve Malone

Both the PNSN experiment and the Seahawks were successful Saturday evening. Both got off to a slow start. The PNSN QuickShake display had several bad dropouts during the first half and at half-time the Seahawks were only ahead by four points. When working properly QuickShake provided us with "early Warning" of a successful play that would show up on TV a few seconds later. None of the signals compared to the size of those during the original "Beast Quake" of 2011 but some interesting patterns were seen. For more detailed analysis......
Last year the PNSN used the vibrations generated by enthusiastic Seahawk fans at CenturyLink Field to test instruments, data acquisition and web based displays. Some might say the seismic monitoring inspired fans to greater cheering resulting in the Seahawk's successful Super Bowl run. With new instruments recently acquired and improved data processing and display techniques developed we are again looking for somewhere to test them. With the Seahawks again in the playoffs with home field advantage why not watch/help them again? For the details.....

Canadian ETS morphing to Washington one?

November 18, 2014

by Steve Malone

Over two weeks of tremor in central Vancouver Island has been progressing southward. Though we don't know about the geodetic component we suspect that this represents a slip event that is propagating southward. Since a southern Vancouver Island-Northern Puget Sound ETS is due about now the question is will this current activity continue all the way to southern Puget Sound. For some details.....

Great ShakeOut, Great success!

October 16, 2014

by Angel Ling

Congratulations and thank you for your participation in the Great ShakeOut Earthquake Drill! Hopefully it gave you a great opportunity to practice "Drop, Cover and Hold on", and review and update emergency preparedness plans and kits/supplies. Thanks again to make the Pacific Northwest safer. Read more to see what the Great Washington ShakeOut looks like at the PNSN!

The Great ShakeOut 2014 is Tomorrow!

October 15, 2014

by Angel Ling

Tomorrow is the Great Washington Shake Out 2014! Please join us in the World's largest Earthquake Drill at 10:16 a.m. on October 16. Read more...

Three Cascadia ETS events in past month??

September 4, 2014

by Steve Malone

Since Aug 10 there have been three parts of the Cascadia subduction zone with extended periods of near continuous tectonic tremor. Typically if tremor continues in a zone for more than 10 days then the geodesists can easily see an accompanying slow-slip event coincident with it. While tremor in the three zones has not been exactly synchronous it is somewhat unusual for this much of Cascadia to "light up" this strongly all together. For some of the details....

Is Mount St. Helens seismicity increasing?

July 29, 2014

by Steve Malone

Looking at the "Quakes near volcanoes" plot today shows that 87 earthquakes have been recorded at Mount St. Helens over the past 30 days. This is way above the average for the past many years. Is this significant? Actually, no. Its called a sampling artifact that gives the impression of increased activity. For the details......
Several days of very warm weather has resulted in a couple of large snow avalanches at Mount St Helens but apparently no unusually large ones at other volcanoes. The seismic network at Mount St. Helens is particularly good at picking up the shaking due to large snow avalanches. Two such events on the afternoon of May 14 got our attention. For copies of seismograms and photos......
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.....