Why earthquakes disappear

May 31, 2015

by Renate Hartog

Just after midnight last Thursday I woke up to the jingle of multiple text messages followed by several pages. DUBIOUS ALERT M3.0, DUBIOUS ALERT M4.4, DUBIOUS ALERT M4.4... Multiple earthquakes in very close succession may happen in movies but are uncommon in real-life, so I immediately suspected that our network was set off by the seismic waves from a large earthquake far away (a so-called teleseism). This is in fact fairly common.  The DUBIOUS qualifier in front of the pager messages meant that the earthquake information had not been released yet and that I needed to review the waveforms to see if these were real, local earthquakes.

I immediately went to the U.S.G.S. website earthquake.usgs.gov and saw that there was a bright red, large, dot in Alaska indicating that a very recent, large earthquake had just happened there. There was the culprit!  I also saw one small red dot in Washington State and a larger red dot in California.  My first thoughts were, "Shoot! We released a smaller bogus event" and "I wonder if that event in California is real?". I called into the conference line in case others also had woken up and wanted to know what was going on. There were three of us on the line, John Vidale, Erin Wirth, and I, and we sat there waiting for waveforms to show up in the desktop analysis program so we could take a closer look at the smaller event that was distributed to the U.S.G.S. and the world.  I tagged the bigger false events as teleseisms via our Duty Review Page. Meanwhile, it was clear from seismograms on our PNSN webpage that the small event had been real (see the Figure below, recorded near Mt. Shuksan in WA). When the system was finally done harvesting all the waveforms (took extra long this time because it saved almost every channel in our network multiple times because multiple solutions had been generated) we quickly re-analyzed the one real earthquake and confirmed that indeed it was small.  We were lucky, none of the bogus solutions had left our building. The California event (initially estimated as a M5.1 earthquake), however, also turned out to be a bogus solution for the large Alaska earthquake and was therefore cancelled. 

This is why earthquakes disappear.

Seismogram of small local event followed by waves from large earthquake in Alaska

Very, very early this morning the pager started going crazy again. A DUBIOUS M5.2, DUBIOUS M5.0, DUBIOUS M4.1, DUBIOUS M4.2, and a DUBIOUS M5.3. Our system had been tricked again! It immediately started saving waveforms for each of these big events which bogged down the system. This time my fellow seismologist on the conference line to commiserate with was Steve Malone.  The U.S.G.S. page showed a big red dot south of Japan (Bonin region), which was labeled M8.5 at the time (it was later revised to M7.8) and there was a red dot in California, again. I knew to be skeptical about that one! We had gotten lucky again and our DUBIOUS filter had trapped our poor earthquake solutions in our building and never let them escape. The California event later was cancelled because it too had been a wrong solution for the Bonin earthquake.

Our Demo Earthquake Early Warning System did send 5 false warnings, it claimed that five magnitude 5 to 6 earthquakes occurred in a region spanning from northern Vancouver Island, Washington, Oregon, into northern California. Hopefully all the beta-users slept through the alerts or had their computers off.  If we could have, we would have cancelled all those events because NONE of them were real!

This is why earthquakes disappear.

            

Earthquakes disappear because automatic systems are computer algorithms that cannot think for themselves.  When waves from distant earthquakes or anthropogenic noise sources create waveforms that look similar to  those from local earthquakes, the system may get it wrong. Look at the waveforms from the Japanese earthquake recorded in Washington (top) and northern California (bottom) in the Figure on the left and you'll understand why the computer algorithms thought they were from local earthquakes!!! To a trained seismologist it is completely clear that these are waves from a distant earthquake (although it is impossible to tell from the lower three, clipped, records in the top panel).

Because we aim to notify the public as soon as possible, we typically send our automatic solutions out without review by a person.  At the PNSN we do require that the automatic solutions have relatively small uncertainties before we send them out. This is what protected us from sending out the poor solutions for these teleseisms. The California networks have their own way of preventing notifications of bogus earthquakes caused by far away events and it usually works.  However, just like our system is not completely fool-proof neither is theirs.  This is the downside of open science; if you want to see everything our systems detect, you will also see its mistakes. Hopefully this post helps clear up some confusion! If people are interested in how often teleseisms trigger our network and how often our bad solutions make it out into the world, let me know in the comments and I'll write a different post about that.

 

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......

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.

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