Scientific American - November 2018

84 Scientific American, November 2018

Bomb size

1.1–2.1 kilotons

4.6–10.4 kt

8–27 kt

8–19 kt

20–87 kt

104–150 kt

Test date

1

October 9, 2006

2

May 25, 2009

3

February 12, 2013

4

January 6, 2016

5

September 9, 2016

6

September 3, 2017

SOURCES:“THE COUPLED LOCATION/DEPTH/YIELD PROBLEM FOR NORTH KOREA’S DECLARED NUCLEAR TESTS,”

BY MICHAEL E. PASYANOS AND STEPHEN C. MYERS, IN SEISMOLOGICAL RESEARCH LETTERS. PUBLISHED ONLINE

AUGUST 8, 2018 (bomb sizes and blast depths); “ABSOLUTE LOCATIONS OF THE NORTH KOREAN NUCLEAR TESTS

BASED ON DIFFERENTIAL SEISMIC ARRIVAL TIMES AND INSAR,” BY STEPHEN C. MYERS ET AL., IN SEISMOLOGICAL

RESEARCH LETTERS. PUBLISHED ONLINE AUGUST 15, 2018 (test locations); WWW.NORSAR.NO (seismograms);

“THE PUNGGYE-RI NUCLEAR TEST SITE: A TEST TUNNEL TUTORIAL,” 38 NORTH, MAY 23, 2018 http://www.38north.org/2018/05/

punggyetunnel052318 (tunnel locations); U.S. GEOLOGICAL SURVEY (earthquake magnitudes)

GRAPHIC SCIENCE

Text and Graphic by Katie Peek

Watching North Korea

Illicit nuclear detonations are anything but secret

In September 2017 North Korea tested its largest nuclear bomb yet. It was 10 times the blast

strength of any of the five previous underground detonations (map). How do we know? A global net-

work of more than 300  earthquake-monitoring stations stands sentry. After an explosion, seismom-

eters pick up two types of shock waves within minutes and alert intelligence ocers. Scientists learn

even more afterward. Since the last blast, they have combined the seismic signals with satellite imag-

es and other data to pinpoint more details, recently published, such as the precise location and bomb

size (chart). Should North Korea—or any other nation—try another bomb, the world will know.

1 km

1111

33333

2222

4 6 55

How to Tell a Blast

from an Earthquake

How to Tell a Blast

from a Cave-in

Six Nuclear Bomb Tests Compared

Test region

Mt. Mantap summit North Korea

Test loc ations

(ovals represent

uncertainties)

Tunnel entrances

Blast depth

340–480 meters

340–500 m

270–450 m

480–670 m

440–630 m

530–670 m

Seismogram

Magnitude 4.3

4.7

5.1

5.1

5. 3


6. 3

Test loc ations

North Korea’s six explosions were

conducted at the Punggye-ri test

site, located underneath Mount

Mantap. In each case, seismome-

ters around the world picked up

the shock waves. Researchers

compared the arrival times of the

waves at multiple stations to deter-

mine where the blast originated.

Bomb sizes

The 2017 test created the equiva-

lent of a magnitude 6.3 earth-

quake. Seismologists initially used

a standard relation, based on old

French and Russian detonations,

to estimate the bomb’s strength,

or yield, as equivalent to 100 to

200 kilotons of TNT. The most

recent estimate, based on a more

detailed analysis, puts the blast at

125 kilotons. The 1945 blast at

Hiroshima was 13 to 18 kilotons.

Blast depths

Scientists determine whether an

explosion was shallow or deep by

modeling the test site design and

simulating a range of strengths

and depths.

The signature of a mine collapse or

other cave-in looks like a blast,

except that the initial rock motion

is inward; a blast’s is outward.

Seismometers record compression

waves and shear waves in the

earth. An explosion compresses

rock more than shears it, so those

signatures are stronger.

Earthquake:

Shear waves are stronger

Compression Shear

Explosion:

Compression waves are stronger

Waves:

Collapse:

Initial rock motion is inward

Explosion:

Initial rock motion is outward