Detect and Identify Nuclear Explosions

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					      Detect and Identify Nuclear
              Explosions

                     Lupei Zhu

FALL 2004   EASA-130 Seismology and Nuclear Explosions
                          Topics
   Nuclear explosions as seismic sources.
   Detection thresholds
   Difference between nuclear explosions and earthquakes
    and discriminants
     – Frequency content
     – Relative strength of P and S waves
     – Radiation pattern
     – Depths
   Evasion
     – Decoupling
     – Multiple explosions
     – Hide in earthquakes or mining explosions
   A case study: the May 1998 Indian nuclear tests.
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    Underground Nuclear Explosion
 90 to 95% of yield is used to make a cavity near the
  device (about 1 km in radius for a 1 Mt bomb).
 A few % is spent on cracking rocks around the cavity.
 Only 1-5% of energy is radiated out in seismic waves.
 Earthquake also spends about 90 to 98% of total
  energy to break rocks.




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 Explosion Yield and Magnitude
 The energy releases (yield Y in kt TNT) of
 tamped (no decoupling) underground
 nuclear explosions have been calibrated
 against their measured seismic magnitudes
 using some NTS tests:
           mb = log Y + 4.0
           Ms = log Y + 2.0
 So a 1 kt explosion is equivalent to an
 earthquake of mb 4.0.

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 Detect Underground Explosions
 Underground   nuclear explosions are
  detected and located the same way as
  earthquakes.
 With the International Monitoring System
  (IMS), global detection magnitude
  thresholds are 3 to 3.5 (20 to 100 t TNT)




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Detection Magnitude Threshold




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IMS Detection Threshold




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IMS Detection Threshold




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    Differences between nuclear
    explosions and earthquakes
 Explosions have much smaller source dimension.
       Log R (in km) = (1/3) log Y - 2
    It’s ~1 km for a 1 Mt device. An earthquake of
  this size (Ms 8) usually have a dimension of 100
  km.
 Explosions have shorter time duration
    A 1 Mt device has a duration of ~1 sec, as
  compared to 30 sec for a similar sized earthquake.
 So explosions generate more high-frequency
  signals than earthquakes.

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          Mb:Ms Discrimination
   A simple and effective way to quantify the amount of low
    frequency vs. high frequency energy is by comparing two
    different magnitudes for the event: the body wave
    magnitude (mb) and the surface wave magnitude (Ms)
   Since surface waves are lower frequency than body waves,
    their amplitude depends on the amount of low frequency
    energy created by the source.
   Since body waves are higher frequency than surface
    waves, their amplitude depends on the amount of high
    frequency energy created by the source.



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Differences between nuclear
explosions and earthquakes
                                                  Explosions
                                                   produce strong
                                                   compressional
                                                   wave (P-wave).
                                                   Seismic signals
                                                   are depleted in
                                                   S-wave
                                                   Earthquakes
                                                   produce strong
                                                   S-wave.




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 Difference in Radiation Pattern
 When   a seismic event occurs the seismic
  waves that are generated are not necessarily
  the same in every direction
 In general the amplitude of a seismic wave
  depends not only on the size of the source
  and the source-receiver distance, but also on
  the direction the receiver is from the source
 This direction dependence is called the
  radiation pattern of the source.


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 Difference in Radiation Pattern
 Earthquakes   have much more complicated
  radiation patterns than explosions
 Explosions tend to have nearly isotropic
  radiation patterns (the same in all
  directions) while earthquakes have many
  “lobes” in their patterns
 Therefore the observed radiation pattern of
  a source can be used as a discriminant


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Earthquake Radiation Pattern




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Earthquake First Motions




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General Focal Mechanisms




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             Difference in depth
 Underground explosions are close to surface (<2
  km)
 Earthquakes are much deeper.(>2km)
 But it is hard to resolve the depth just using first P
  wave arrivals?
    – Because the depth “trades-off” with the origin time.
    – In other words, you can get the same fit to teleseismic
      arrival times by making your potential location a little
      deeper but also a little later.
    – Alternatively, making your prospective location a little
      shallower and a little earlier gives the same fit.

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        Depth Determination
 Accurate determination of depth requires
 the use of surface reflected waves (labeled
 as pP and sP) in addition to first P.




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Depth Determination

        pP
                                                seismometer


earthquake       P



    Arrival time of pP minus
   arrival time of P gives depth!




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              Evasion Methods
 The so-called evasion methods are ways to either
  reduce the seismic signals from underground tests
  below the detection threshold of monitoring
  network, or to make the explosion to be identified
  as an earthquake.
 Three groups of methods have been suggested:
    – Decoupling
    – Multiple explosions
    – Hide-in-earthquake


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                 Decoupling
 To reduce seismic signal strength by firing the
  explosion in low-coupling medium (such as dry
  alluvium) or in a cavity.
 The decoupling factor is defined as the amplitude
  ratio of the seismic signals from a fully contained
  explosion in hard rock to from a decoupled
  explosion.
 A decoupling factor up to 10 can be achieved for
  dry alluvium.
 A problem is to avoid leakage of radioactive
  material and surface deformation. This demands
  thick alluvial deposit (~600 m for a 1 kt shot).
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Decoupling




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                 Decoupling
 Another way of decoupling is to fire the explosion
  in a large cavity.
 A factor of 120 can be achieved in theory if the
  cavity is large enough so that the shock-wave
  deformation is elastic (fully decoupled).
 A factor of 70 was achieved in one US test in
  which a 0.38 kt explosion was detonated in the
  cavity produced by a 5.3-kt explosion (see movie)



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Decoupling
                         The main difficulty
                          is to obtain large,
                          standing cavities. To
                          fully decouple a 5 kt
                          explosion, a
                          spherical cavity of
                          radius 100 m at a
                          depth of 800 m is
                          needed.



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           Multiple explosions
 To utilize several nuclear explosions, placed a a
  specific geometrical pattern and fired a a certain
  time sequence, to produce seismic signals similar
  to those generated by earthquakes.
 It is aimed to reduce the mb and increase Ms to
  evade the mb:Ms criterion.
 The effect of the technique is difficult to control
  with reasonable confidence
 It is also difficult to escape the radiation pattern
  identification.

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    Hide-in-earthquake Method
 To hide the seismic signals from the explosion in
  signals from a large earthquake.
 Either a large nearby earthquake of very large
  earthquakes in any part of the world can be used.
 Sometimes partial hiding of surface waves is
  sought to evade the mb:Ms identification.
 The decision to fire must be made as soon as the
  earthquake parameters have been estimated and
  found to be appropriate. This is not easy and the
  opportunities are few. The risk of being caught by
  a global seismic network is also high.
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Hide-in-earthquake Method




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    The 1998 Indian Nuclear Tests
   India announced that it detonated three underground
    nuclear tests on May 11, 1998, the first since the 1974
    “peaceful nuclear explosion”.
Mr. Vajpayee, Indian PM, “Today at 1545 hrs, India conducted three
                                 underground nuclear tests in the Pokhran
                                 range. The tests were conducted with a fission
                                 device, a low yield device and a thermonuclear
                                 device. The measured yields are in line with
                                 expected values. Measurements have also
                                 confirmed that there was no release of
                                 radioactivity into the atmosphere. These were
                                 contained explosions like the experiment
                                 conducted in may 1974. I warmly congratulate
                                 the scientists and engineers who have carried
                                 out these successful tests.”

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    The 1998 Indian Nuclear Tests
   Two days later, India announced that two more
    tests were conducted near the previous test site.
                                                  According to info released by
                                                  India, the first blast had a
                                                  yield of 12 kt, the second
                                                  was a thermonuclear device
                                                  with a yield of 43 kt. The
                                                  remaining three were all low
                                                  yield devices (0.2 to 0.6 kt).



 "The tests... have provided critical data for the validation of our capability in
the design of nuclear weapons of different yields for different applications and
different delivery systems,"
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Seismic Observation of the Tests
   The May 11 tests were detected by global seismic network.




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Seismic Observation of the Tests




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Seismic Observation of the Tests




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Seismic Observation of the Tests
   The May 11 tests were reported in the USGS/PDE as one
    event of mb 5.2 and Ms 3.5.
   This put the total yield of the first three devices as 12-25
    kt, much less than the 12+43=55 kt claimed by India.
   There is no sign of multiple explosions more than 0.5 s
    apart (see waveform recordings).
   Teleseismic waveforms are similar to those from the
    previous 1974 test which was claimed to be 12 kt but was
    estimated as 4 to 6 kt explosion.
   This led people question the claimed success of India’s 45
    kt thermonuclear device.


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             The May 13 Tests?
   No event was detected for the May 13 tests. The
    absence of seismic signal at the closest station
    NIL put the total yield of the tests to be < 25 t.




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