“In the preliminary analysis conducted within hours of the event, NORSAR reported an event magnitude of 5.8. This estimate was obtained using analysis procedures developed from investigations of the previous five North Korean test explosions, all of which were around the magnitude range 4 – 5.
However, the 3 September 2017 test was an order of magnitude larger than any of the previous explosions and resulted in seismic signals dominated by radiated energy at lower frequencies. Our analysis procedures have been revised accordingly to include the lowermost part of the signal spectrum. The revised assessment estimates the magnitude to 6.1 and makes this explosion clearly the strongest so far.”
September 15 2017 — NORSAR and 38NORTH have revised their initial yield estimate for the magnitude of the event on 3 September 2017. This new estimate — mb = 6.1 and a yield of about 250 kilotons TNT — is significantly higher than the yield previously suggested. In comparison, the explosive yield of the nuclear bomb dropped on Hiroshima on 6 August 1945 was estimated at approximately 15 kilotons TNT, while the bomb dropped on Nagasaki three days later was estimated at approximately 20 kilotons TNT. However, depending on the burial depth of the explosion, the yield could be significantly higher than currently reported. A yield as high as 500 kt cannot be ruled out. Follow us on Twitter: @INTEL_TODAY
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UPDATE (September 15 2018) — The information contained in this post is now widely accepted as true and accurate. For instance, in a report published in January 2018 by the Bulletin of Atomic Scientists — North Korean nuclear capabilities, 2018 — the authors conclude that:
“on September 3¸ 2017, North Korea demonstrated clearly that it could potentially produce nuclear devices with yields in the range of thermonuclear warheads. A nuclear explosion with a yield of several hundred kilotons showed that North Korea had managed to design a thermonuclear device, or one that used a mixed-fuel (composite) design.”
I have added satellite pictures of the test site before and after the H-bomb test of September 2017.
END of UPDATE
The apparent yield of a nuclear test is estimated from the magnitude which, in turn, is estimated from the amplitude of the waveform arriving at a seismic station.
Time and Location
Government agencies and independent institutions agree that North Korea’s 3 September 2017 nuclear test occurred at UTC 2017/09/03 03:30, (2017/09/03 12:00 local time), at the location of 41°17’53.52”N — 129°4’27.12”E.
Measurements of the seismic magnitude
Theory — There are several magnitude scales. Richter Magnitude was the first, there is also body wave magnitude (mb) and surface wave magnitude (Ms). They are all estimated in a similar fashion and as such the values obtained are very similar.
“For a specific event the magnitude is calculated for each seismogram arriving at each station. An average is then obtained using estimates from each station in the network. The magnitude estimates from different networks often vary slightly as different groupings of stations were used in each calculation.”
At this point, there seems to be a consensus that the magnitude (mb) of the event is at least 6.0 and possibly as high as 6.4. [Only South Korea is reporting a lower magnitude.]
South Korean Government’s detected a 5.7 magnitude earthquake. That is by far the lowest value currently reported.
NORSAR Seismology Center initially reported a magnitude 5.8 tremor.
However, on September 12 2017, this value was revised upward.
“However, the 3 September 2017 test was an order of magnitude larger than any of the previous explosions and resulted in seismic signals dominated by radiated energy at lower frequencies.
Our analysis procedures have been revised accordingly to include the lowermost part of the signal spectrum. The revised assessment estimates the magnitude to 6.1 and makes this explosion clearly the strongest so far.”
The German Federal Institute for Geosciences and Natural Resources says it detected a 6.1 tremor.
The Japan Meteorological Agency also detected a 6.1 magnitude tremor.
“North Korea tests most powerful nuclear bomb yet”. ABC. CNN, Scripps National Desk. September 3, 2017.
USGS: Initial reporting from the US Geological Survey (USGS) first claimed the magnitude to be 5.2, but quickly upgraded the event to magnitude 6.3.
The China Earthquake Administration also detected a 6.3 magnitude earthquake.
“China Earthquake Administration detects ‘suspected explosion’ in North Korea” Wen, Philip (September 2, 2017) — Reuters
The Geophysical Service of the Russian Academy of Sciences registered a 6.4 magnitude earthquake.
“North Korea likely to have conducted another nuclear test — Japanese TV”. TASS. Government of Russia. September 3, 2017.
CTBTO — So far, no final result has been released. The following message was posted:
“The event seems to have been larger than the one our system recorded in September last year and the location is very similar to that event. Our initial location estimate shows that the event took place in the area of the DPRK’s nuclear test site. ( 03-SEP-2017 03:30:06 LAT=41.3 LON=129.1 )”
However, the Exec Secretary of the Comprehensive Test Ban Treaty Organization (CTBTO) tweeted a few preliminary data. The first analysis gives magnitude 5.8.
Currently, the CTBTO webpage displays the following information:
“CTBTO’s monitoring stations picked up an unusual seismic event in the Democratic People’s Republic of Korea (DPRK) on 3 September 2017 at 03:30 (UTC). Over 100 of our stations contributed to the analysis. The magnitude of the event was 6.0. Review of the data is still ongoing. The event seems to have been larger than the one our system recorded in September last year and the location is very similar to that event. Our initial location estimate shows that the event took place in the area of the DPRK’s nuclear test site.”
Summary of mb measurements
South Korea 5.7
Correspondence between the seismic magnitude and explosive yield
Theory — The correspondence between the seismic magnitude and explosive yield of an underground nuclear test is associated with a very large uncertainty.
Yield is calculated from magnitude using the following equation:
mb = A + B log Y
where mb is body wave magnitude, Y is yield in kilotons.
A and B are constants, dependent on the geology local to the test. [The constants used by the U.S. government are classified]
Seismologists have published various estimates for A and B associated with known test sites.
NORSAR — The group suggests to use a value of A = 4.3 and B = 0.75.
[ Ringdal (1992) recommended values of A and B equal to 4.45 and 0.75 for North America and Central Asia while Murphy (1981) suggested values of 3.92 and 0.81 for the Nevada Test Site (NTS) . In our initial estimate, INTEL TODAY used the values 4.26 and 0.97.
By applying these values, NORSAR obtains a yield estimate of 250 kilotons TNT for their measured magnitude 6.1 event on 3 September 2017.
USTC [Lianxing Wen’s Geogroup] — This group has estimated yield of 108.3±48.1 kt. Considering the value of the magnitude reported by China and the previous publications of this group, I find this estimate a bit puzzling as I explain below.
Japan — The magnitude reported by the JCS and KMA is significantly lower than the 6.3 published by the USGS. No exact yield was announced so far.
“I can’t predetermine, but it is believed to be 100 kilotons at the most,” said Rep. Kim Young-woo, who chairs the National Assembly National Defense Committee.
Again, this statement is far from being ‘obvious’. In fact, it is probably incorrect.
On 5 September, the Japanese government gave a yield estimate of about 160 kilotons, based on analysing Comprehensive Nuclear-Test-Ban Treaty Organization seismic data, replacing an early estimate of 70 kilotons.
North Korean nuke test put at 160 kilotons as Ishiba urges debate on deploying U.S. atomic bombs”. The Japan Times. 5 September 2017
38 NORTH — At the time of 38 North’s first report on the sixth nuclear test, the preliminary seismic magnitude estimates varied from 5.8, as published by both the CTBTO and NORSAR, to 6.3 by the United States Geological Survey (USGS).
“More recently, both the CTBTO and NORSAR have officially revised their estimates upward to 6.1. This revision is significant because, rather than providing an equivalent yield of about 120 kilotons derived from the lower magnitude estimates, the application of standard formula with appropriate constants shows that the yield can now be estimated to have been roughly 250 kilotons (one quarter megaton). This large explosive yield is also quite close to what 38 North had previously determined to be the maximum estimated containable yield for the Punggye-ri test site.” [ 38NORTH ]
US Intelligence Community — On 6 September, an early assessment by U.S. Intelligence that the yield was 140 kilotons, with an undisclosed margin of error, was reported.
Panda, Ankit (6 September 2017). “US Intelligence: North Korea’s Sixth Test Was a 140 Kiloton ‘Advanced Nuclear’ Device”. The Diplomat.
Yield estimate corrected for the “burial depth”
The following equation has been suggested to taken into account the ‘burial depth’ of the explosion:
log10 (Y [kt]) = mb + 0.79 * log10 (h [meter]) – 5.9
The graph below shows the Yield as function of the depth for mb = 5.7, 6.0 and 6.3 respectively.
Indeed, a yield of about 160 kT is estimated for a magnitude 6.0 and a depth of 450 meters.
However, considerably larger value would be deduced if the magnitude 6.3 is used and if the depth of the explosion is greater. Some researchers have suggested that h could be 900 meters. In that case, the yield could be as high as 500 kt.
Estimate of the burial depth
According to a recent analysis by 38NORTH dated March 10 2017:
“Commercial satellite imagery of the Punggye-ri Nuclear Test Site shows that substantial tunnel excavation is continuing at the “North” Portal (previously the “West” Portal), which provided support for the last four of the five declared underground nuclear tests conducted by the Democratic People’s Republic of Korea.
The North Portal tunnels provide direct access under Mt. Mantap, where up to 800 meters of overlying rock is available for test containment. This locale provides the maximum overlying rock possible within the entire test site and is where the most recent and largest detected test occurred on September 9, 2016.”
North Korean Nuclear Test Site After Destruction and Before H-Bomb Test
UPDATE ( September 15 2018) — These pics were taken on May 20 2018 and November 2 2015. Clearly, North Korea’s nuclear test site has at least partially collapsed. It is unclear whether the underground infrastructure beneath mountain Mantap can still be used for any future nuclear tests. I doubt it vey much.
North Korea claims new bomb has ‘great destructive power’
N. Korea claims successful H-bomb test for ICBM — Korea Times
High-precision location and yield of North Korea’s 2013 nuclear test —
GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 2941- 2946
Making Yield Estimates — Arms Control Wonk
The Seismic Signal Strength of Chemical Explosions — Bulletin of the Seismological Society of America, Vol. 88, No. 6, pp. 1511-1524, December 1998
North Korea’s 3 September 2017 Nuclear Test Location and Yield: Seismic Results from USTC — Lianxing Wen’s Geogroup
Significant Uncertainties in the Yield Estimate of North Korea H Bomb
One Year Ago — Significant Uncertainties in the Yield Estimate of North Korea H Bomb