Seismic Monitoring of North Korea Nuclear Tests

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

People have been asking why Weston Observatory didn’t record the recent “seismic event” in North Korea.

It was much too small (magnitude 5.1) for us to see it this far away at our New England Seismic Network (or BC-ESP) stations. For us to see a seismic event at that distance, it would probably have to be about a magnitude 6.0 or higher. However, Weston Observatory seismologists also track recordings at seismic stations operated by collaborating observatories that are closer to North Korea.

Here are the seismograms at Weston, MA where it wasn’t recorded and at the IRIS/USGS station at Mudanjiang, China (MDJ) where it was recorded very well. This figure shows the MDJ seismogram and also the “spectrogram” (multi-colored plot, calculated by Dr. Jay Pulli, Visiting Scholar at Weston Observatory).

(Click to enlarge.)

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In this next figure (from IRIS and USGS) you can see the 1/6/2016 event seismogram, superposed on three other seismograms of previous North Korea nuclear tests. The seismograms are so similar that it is hard to distinguish the 2016 event (shown in red). This was one of the first clues that the event was probably a North Korea nuclear test. The biggest difference is just the relative sizes of the nuclear tests.

(Click to enlarge.)

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Seismologists will be studying these, and other, seismograms of North Korea nuclear tests for forensic analysis of details of the nature of the 1/6/2016 event.

Spectrograms: Visualizing How the Frequency of Seismic Waves Varies With Time

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences, Boston College

These (very cool, and beautiful, I think) graphics are “spectrograms” of some of our recordings of the magnitude 4.7 earthquake that recently occurred in Oklahoma.

The seismograph in Texas is operated by Kristi Rasmusson Fink, and the seismographs in MA are operated by Weston Observatory. The data processing was done by Jay Pulli.

A spectrogram is a particular way of visualizing the vibrations present in a seismogram. It shows how the frequency of the motion varies over time, and how different frequencies of vibration appear at different times in the record. Yellow colors represent stronger signals, and blue colors represent weaker signals.

More (Fracking Related?) Oklahoma Quaking…

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences, Boston College

Just this past week alone, there have been 32 earthquakes detected in Oklahoma (magnitude 2.0 to 4.7). Our seismograms and “spectrograms” of today’s magnitude 4.7 quake are shown below.

(The seismograph in Texas is operated by Kristi Fink and the seismographs in MA are operated by Weston Observatory. The data processing was done by Jay Pulli.)

Students Make Their Own Earthquake!

Clear and Simple Illustration of Mechanism to Explain Earthquakes in the Central and Eastern United States

This is a clear and simple illustration (adapted from the South Carolina Earthquake Education and Preparedness Program) of the commonly accepted “ancient zones of weakness” model for the cause of earthquakes in the Central and Eastern United States (and other “intraplate” regions far from plate boundaries). According to this model, preexisting faults and/or other geological features formed during ancient geological episodes persist in the intraplate crust, and, by way of analogy with plate boundary seismicity, earthquakes occur when the present-day stress is released along these zones of weakness.

Also see:

Why Does the Earth Quake in New England? and South Carolina Earthquake Education and Preparedness Program

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Broken Garage Door Springs, Earthquake Prediction, and Earthquake Triggering

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

(Originally Posted: April 27, 2012)

A few days ago, one of the huge springs on our garage door opener broke, with quite a big KABOOM!

It seems to me that this is a perfect example of the problem of earthquake prediction. Here’s what I see as the connection:

In the morning, the garage door repair guy (who happened to be there fixing something else) looked up at the spring and said, “That deformed spot in the spring is ‘metal fatigue’ – it might break in 5 minutes, or it might break in 10 or 20 years.” Then, two hours later, it broke! (Perhaps he disturbed the internal stress field while he was fixing something else?)

This seems to be a perfect analog of the Elastic Rebound Theory of earthquakes, and the phenomenon of Earthquake Triggering!

It also explains why we knew that an earthquake was lurking in Haiti, but we couldn’t tell when it would occur: The tectonic stresses were ready to be released in an earthquake, but nobody could tell if the earthquake would occur in minutes, in years, in decades, or in centuries…

New Updated Seismicity Maps for Northeast U.S.

Click on the maps below to see the Weston Observatory seismicity maps of the Northeast United States, updated as of early February 2014.

The first map shows historical seismicity (from June 1638 to December 1974), and the second map shows network seismicity (from January 1975 to early February 2014).

The Detective Work of Seismologists: Earthquake or Blast?

Justin Starr
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Residents of the city of New Bedford, MA and the surrounding area felt something very strange at 10:52 am on January 9th, 2014. They heard a loud blast and felt a distinct rumble.

A seismic event had occurred.

Weston Observatory scientists quickly determined that the magnitude of this seismic disturbance was 1.9 and that it was located very close to New Bedford. Not very big, but shallow enough to be heard by many people in the area.

While speaking to the Massachusetts Emergency Management Agency (MEMA), Weston Observatory scientists were informed that there may have been blasting in the New Bedford harbor around the same time as the earthquake and very close to the possible epicenter. So was this a real earthquake? It was up to the scientists to find out.

Several phone calls went out and eventually, Weston Observatory scientists reached the New Bedford harbormaster who put them in touch with the Captain of… the Kraken! The Kraken is a drilling and blasting barge located in New Bedford and is tasked with widening the shipping lanes. As it turns out, the Kraken did not blast until 12:09 pm, over an hour after the earthquake occurred. In fact, the Captain received many phone calls asking if it was they who blasted and caused the shaking… but it was the earthquake!

The two seismograms shown below are from a USArray seismic station in Tiverton, RI. On those seismograms, you can see the Kraken Blast and the earthquake. Notice the big difference in magnitude (the earthquake is an order of magnitude larger).

And notice (below) the well-defined, high amplitude wave labelled “Rg” on the seismogram of the blast. The presence of these “Rg” waves on a seismogram are one of the ways that seismic “detectives” use to identify blasts, and distinguish them from earthquakes.

These “Rg” waves have been studied by seismologists, and are one of the ways that seismologists can distinguish between earthquakes and explosions. See, for example:

Kafka (1990): Rg as a Depth Discriminant for Earthquakes and Explosions

Yes, There Really is a “Sandwich Plate”

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

An interesting pair of earthquakes recently occurred in the Scotia Sea region, south of Chile and Argentina: Magnitude 7.7 (November 17, 2013) and Magnitude 7.0 (November 25, 2013).

Fortunately, because of their remote location, there was little or no damage associated with these earthquakes, and they didn’t make a lot of headline news. But they illustrate some interesting earthquake and plate tectonic processes of the Scotia Plate and the Sandwich Plate.

The Scotia Plate/Sandwich Plate region is a tectonic region between the South American Plate and Antarctic Plate, and stretches from the South Sandwich Islands to the southern tip of South America. The Scotia Plate moves eastward relative to the South American plate, but the motion is complicated by the presence of a divergent boundary in the eastern portion of the Scotia Plate, delineating the western edge of the Sandwich Plate.

The two recent earthquakes had motion that is consistent with the long-term (millions-of-years time scale) plate motion (shown by the blue and white arrows in the top figure).

The second earthquake might be a remotely-triggered earthquake, an earthquake that is too far away from the first quake to be an aftershock, but (maybe?) too close in time to be just a coincidence…