A Multidisciplinary Study of the DPRK Nuclear Tests

The Democratic People Republic of Korea announced
two underground nuclear tests carried out in their territory respectively
on October 9th, 2006 and May 25th, 2009. The scarce
information on the precise location and the size of those explosions
has stimulated various kinds of studies,mostly based on seismological
observations, by several national agencies concerned with theNuclear
Test Ban Treaty verification.Weanalysed the available seismological
data collected through a global high-quality network for the two tests.
After picking up the arrival times at the various stations, a standard
location program has been applied to the observed data. If we use all
the available data for each single event, due to the different magnitude
and different number of available stations, the locations appear quite
different. On the contrary, if we use only the common stations, they
happen to be only few km apart from each other and within their
respective error ellipses. A more accurate relative location has been
carried out by the application of algorithms such as double difference
joint hypocenter determination (DDJHD) and waveform alignment.
The epicentral distance between the two events obtained by these
methods is 2 km, with the 2006 event shifted to the ESE with respect
to that of 2009. We then used a dataset of VHR TerraSAR-X satellite
images to detect possible surface effects of the underground tests. This
is the first ever case where these highly performing SAR data have
been used to such aim. We applied InSAR processing technique to
fully exploit the capabilities of SAR data to measure very short displacements
over large areas. Two interferograms have been
computed, one co-event and one post-event, to remove possible
residual topographic signals. A clear displacement pattern has been
highlighted over a mountainous area within the investigated region,
measuring a maximum displacement of about 45 mm overall the
relief. Hypothesizing that the 2009 nuclear test had been carried out
close to the area where the displacement has been observed through
the DInSAR technique, its relation with the epicenter location
obtained through seismological processing has been discussed as a
possible alternative hypothesis with respect to the preferred solutions
reported by the nuclear explosion database (NEDB). The distance of
about 10 km between the two places can be considered acceptable in
light of the possible systematic location shifts commonly observed in
the seismological practice over a global scale. The difference between
the mb magnitudes of the two tests could reflect differences in geological
conditions of the two test sites, even if the yield of the two
explosions had been the same.

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