Seismo Blog

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?  Every 12-15 months these slow earthquakes occur in the Cascadia Subduction Zone beneath southern Vancouver Island and northern Washington. One of these events is happening right now and is possibly connected to the magnitude 4.8 earthquake NE of Victoria, Canada that occurred on December 29.

I like to call slow earthquakes the banana slugs of earthquakes.  This is because while slow earthquakes can release the same amount of energy as a magnitude 6.5-6.8 normal earthquake, they release this energy more slowly.  For example, many of you who have been living in the Pacific Northwest for a long time probably remember the magnitude 6.8 Nisqually earthquake in 2001.  The shaking during this earthquake lasted about 15 seconds.  A slow earthquake of the same magnitude lasts for weeks; the magnitude 6.8 slow earthquake in 2010 lasted from August 8 to September 8.

The current slow earthquake began on December 21 beneath Vancouver Island, Canada.  Activity continued beneath Vancouver Island for ~8 days and now the slow earthquake is migrating southward beneath the Straight of Juan de Fuca. 

If we can’t feel these earthquakes, how do we track them?  Like normal earthquakes, slow earthquakes shake the ground.  But, the ground motions are so small only sensitive instruments called seismometers can record them.  These ground motion recordings, or seismograms, look like a squiggly lines.  If the ground moves up, you get an upward squiggle and if the ground moves down you get a downward squiggle. 

The seismograms produced by slow earthquakes look different than those produced by normal earthquakes.  In the figure below, the top seismogram is from the Nisqually earthquake.  There are well-defined peaks that indicate the arrivals of the seismic waves that shake the ground.  The bottom recording shows the ground motion from a slow earthquake, which is often referred to as tremor.  Unlike the recording from a normal earthquake, tremor looks like a disorganized jumble of seismic wave arrivals without distinct peaks.  In addition, the seismic waves from a large, normal earthquake like the Nisqually earthquake are much larger than the seismic waves that make up tremor.  That is why you can feel a normal earthquake but not a slow earthquake.

We track slow earthquakes by looking at where and when tremor is occuring.  You can look at tremor using this Interactive Tremor Map.  The example below shows tremor during the current slow earthquake from December 21, 2015 to January 4, 2016.  Each dot on the map, color coded by time, represents 5 minutes of tremor activity.  Unlike normal earthquakes, where a dot represents a single earthquake, all the tremor dots are part of the ongoing slow earthquake.

How is the current slow earthquake related to the earthquake near Victoria?  While the Victoria earthquake occurred on the southeastern edge of where tremor, and hence the slow earthquake was occurring, the Victoria earthquake was deeper than the tremor.  The Victoria earthquake occurred within the subducting Juan de Fuca plate while the slow earthquake is occurring on the boundary between the Juan de Fuca and North American plates (see figure below).  Despite the difference in location between the Victoria earthquake and the slow earthquake, the two events could still be related.  Changes in the distribution of built up energy due to the slow earthquake could have triggered the Victoria earthquake.  

Slow earthquakes happen almost every year and are thought to be part of the normal cycle of energy release in the subduction zone.  This event is in no way alarming! If you want more information about slow earthquakes make sure to read the previous PNSN blog post by Aaron Wech or watch this video of a talk I gave at Town Hall Seattle as part of the 2014 UW Science Now lecture series.  

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

December 4, 2015

by Jon Connolly

The new 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.....
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......