Nuclear explosion in North Korea, February 12, 2013
Observations, magnitudes and location estimates
At 04:08:39 Norwegian time on February 12, 2013, the seismological observatory NORSAR recorded seismic signals generated by the announced underground nuclear explosion in North Korea. This test was the third of its kind, the first two having occurred on October 9, 2006, and May 25, 2009.
The signals recorded on the NORSAR array indicate a seismic magnitude of 5.0. By comparison the reported nuclear tests in 2009 and 2006 had magnitudes 4.7 and 4.2 respectively.
The event has been located at 41.28 N, 129.07 E, a distance of 7356 km from NORSAR’s seismic array in Hedmark, southern Norway. With the seismic signals taking approximately 11 minutes to propagate over this distance, the observations indicate that the explosion took place at 03:57:51 Norwegian time.
The plot displays, to a common scale, the recordings on the NORSAR
seismic array in Hedmark of the 2006, 2009, and 2013 nuclear tests in
North Korea.
The magnitudes (mb) are based upon the NORSAR seismic
signals only and the estimate of the yield is provided in terms of
kiloton TNT.
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Relative location of the North Korean Nuclear Tests
When two seismic events occur very close to each other, there may be sufficient similarity between the signals generated at any given station that very accurate relative times may be calculated by cross-correlating the one waveform against the other. The accuracy in these relative arrival times can exceed greatly the accuracy with which the arrival of a seismic phase can be read on a seismogram and, by measuring these very small time differences at many seismic stations observing the event from different directions, we may be able to locate the two events relative to each other with a high level of confidence. This principle forms the basis of the double difference location algorithms (e.g. Richards et al., 2006; Waldhauser and Ellsworth, 2000) which have been used to create earthquake catalogs of unprecedented detail.
When North Korea carried out their second nuclear test in 2009, it was evident that the signals generated by this explosion were very similar to those generated by the test in 2006. Several independent studies (Murphy et al., 2010; Selby, 2010; Wen and Long, 2010) examined closely the time differences between stations, both regional and global, and there is general agreement that the location of the explosion in 2009 is approximately 2 km to the West and slightly to the North of the 2006 test. These high precision relative location estimates have also been examined in relation to high-resolution satellite imagery of the test-site region (see also Schlittenhardt et al., 2010).
The figure below shows the locations that Wen and Long (2010) provide for the 2006 and 2009 tests based on both precision seismology and analysis of satellite imagery. The symbol to the right is the location of the 2006 explosion.
The signals generated by the test on February 12, 2013, are remarkably similar to those generated by the test in 2009. Using correlation-based travel-time differences from selected IMS seismic array stations, and applying the grid-search method of Selby (2010), we fixed the location of the 2009 test at 41.2939 N and 129.0817 E (marked by the symbol to the left in the satellite image) and calculated a residual or misfit for a grid of trial epicenters for the location of the 2013 explosion. The location with the smallest misfit value is located within 500 meters of the assumed site of the 2009 explosion and somewhat to the South West. The geometrical distribution of observing stations is very good and the location of this optimal fit appears to be quite stable to the removal of selected stations from the inverse problem.
A grid of trial epicenters scanned for goodness-of-fit between the
predicted and observed traveltime differences using the method of
(Selby, 2010). The white stars indicate the locations of the 2009 and
2006 explosions as provided by (Wen and Long, 2010). The location of the
May 25, 2009, DPRK explosion is fixed at 41.2939 N and 129.0817 E and
the colours indicate the size of the misfit. The minimum residual is
associated with a location very close to the assumed site of the 2009
explosion: within 500 meters and to the South West. The depth was held
constant for all trial hypocenters. | |  |
References
Murphy, J.R., Kohl, B.C., Stevens, J.L., Bennett, T.J., Israelsson, H.G., 2010. Exploitation of the IMS and other data for a comprehensive, advanced analysis of the North Korean nuclear tests, in: Proceedings of the 2010 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies. Report LA-UR-10-05578. Los Alamos National Laboratory, pp. 456–465.
Richards, P., Waldhauser, F., Schaff, D., Kim, W.-Y., 2006. The Applicability of Modern Methods of Earthquake Location. Pure and Applied Geophysics 163, 351–372.
Schlittenhardt, J., Canty, M., Grünberg, I., 2010. Satellite Earth Observations Support CTBT Monitoring: A Case Study of the Nuclear Test in North Korea of Oct. 9, 2006 and Comparison with Seismic Results. Pure and Applied Geophysics 167, 601–618.
Selby, N.D., 2010. Relative locations of the October 2006 and May 2009DPRK announced nuclear tests using International Monitoring System seismometer arrays. Bulletin of the Seismological Society of America. Volume 100 (4), pp. 1779-1784.
Waldhauser, F., Ellsworth, W.L., 2000. A Double-Difference Earthquake Location Algorithm: Method and Application to the Northern Hayward Fault, California. Bulletin of the Seismological Society of America 90, 1353–1368.
Wen, L., Long, H., 2010. High-precision Location of North Korea’s 2009 Nuclear Test. Seismological Research Letters 81, 26–29.
