SCIENCE

North Korea just announced a successful underground test of a nuclear device, and seismic evidence supports that claim. How do scientists know the earthquake in North Korea was triggered by an underground nuclear blast and was not a naturally occurring tectonic event? (BBC)

Why are North Korea’s nuclear tests such a big deal, anyway? Use our great study guide to find out.

Teachers, scroll down for a quick list of key resources in our Teaches Toolkit.

Discussion Ideas

• Why do North Korea and other nuclear powers (including the United States) often test their nuclear weapons underground? Take a look at our study guide on North Korea’s nuclear program for some help.
  • Underground testing is one of the safest ways to test nuclear weapons. When the device being tested is buried deep enough, the explosion may be contained, with no release of radioactive materials to the atmosphere.

• How does underground testing trigger quakes big enough to be detected on the other side of the world?
  • Well, first of all, seismographs are incredibly sensitive these days and it doesn’t take much to set one off.
  • According to the good folks at UC Berkeley Seismological Laboratory, underground nuclear explosions cause “very strong forces [to] rapidly act inside the Earth. This leads to intensive shaking of the rocks around the hypocenter, which in turn generates elastic waves. [More on those below.] They can travel thousands of miles and are detected by sensitive seismometers.”

• How is a quake triggered by an underground nuclear explosion different from a naturally occurring earthquake? Read through this short article from the UC Berkeley Seismological Laboratory for some help.
  • According to UC Berkeley and their sister scientists over at the Lawrence Livermore National Laboratory (the good folks who made the great graphic above), there are two major differences that are easily detected by seismographs.
  • elastic waves
    • P-waves dominate the seismographs of explosion-triggered earthquakes.
      • P-waves are formed from a series of compressions (which increase the medium’s density) and rarefactions (which decrease the medium’s density). These changes are caused and effected by pressure, and for this reason, the P in P-wave can stand for pressure.
      • P-waves also have the highest velocity of all seismic waves, and are the first to be recorded by seismographs. For this reason, the P in P-wave can also stand for “primary.”
    • S-waves dominate the seismographs of naturally occurring earthquakes.
      • S-waves move perpendicular to the direction of wave propagation. This movement is called “shear”, and for this reason, the S in S-wave can stand for shear.
      • S-waves travel much more slowly in rock than P-waves, so they are recorded after P-waves on seismographs. For this reason, the S in S-wave can stand for “secondary.”
    • side note: P-waves and S-waves interact in weird ways near the Earth’s core. Read about it here.
  • wave origin
    • By performing a computation known as a “moment tensor solution,” seismologists “trace the [P- and S-waves] back to their origin. That not only pinpoints the precise location of the focus, it also shows the mechanism of the forces initially shaking the rocks. During an earthquake, rock breaks in a shear fracture, which results in the rapid sideways movement of two flanks of a fault. In an explosion, however, the origin is indeed a point, from which elastic pressure waves travel concentrically outward.”

• Can anything besides a nuclear explosion trigger activity on a seismograph?
  • Yes!! Lawrence Livermore describes “Earth’s never-ending seismic activity” as including meteor impacts, volcanoes, and even ocean waves crashing on shore. What other natural phenomena might cause the earth to shake?
  • Other man-made earthquakes include mine collapses (detailed in the Lawrence Livermore graphic up-top) and fracking activity. What other man-made activities might cause the earth to shake?

TEACHERS TOOLKIT

BBC: North Korea claims success in fifth nuclear test (article)

USGS: M5.3 Explosion – 19km ENE of Sungjibaegam, North Korea (poster and maps)

Nat Geo: Did North Korea Just Detonate a Hydrogen Bomb? (study guide)

University of California at Berkeley Seismological Laboratory: Of Nuclear Bombs and Earthquakes (short article)

Lawrence Livermore National Laboratory: Sleuthing Seismic Signals (infographic and article)