The initial automatic detection of the event was made on 6 January 2016 at 01:36 UTC by 27 primary seismic stations stations, the closest being primary seismic (PS) station PS31, Wonju, Korea and PS37, Ussuriysk, Russia. One of the furthest was PS06, La Paz, Bolivia.
The initial estimate of the event's magnitude was 4.9, which was revised to 4.85 in the course of further analysis.
The data was made available to Member States immediately, as were the results of the first automatic analysis that followed around one hour later. More refined automatic processing was made available within two and six hours of the event, which is currently being studied by the CTBTO's analysts.
We can say now that the event of 6 January 1:30 UTC is indeed a man-made explosion.Executive Secretary Lassina Zerbo
In the process, data from additional seismic stations including auxiliary seismic stations is being used. The International Monitoring System (IMS) consists of 50 primary and 120 auxiliary seismic stations, of which 42 and 107 respectively are certified, see interactive map.
The preliminary analysis showed that the event occurred in the vicinity of the nuclear test site of the Democratic People's Republic of Korea (DPRK). The characteristics of the waveforms were similar to the event detected on 12 February 2013, when the DPRK last declared to have conducted a nuclear test. That event was detected by a total of 94 seismic and 2 infrasound stations.
The International Monitoring System did exactly what it was supposed to do this morning.Randy Bell, Director, International Data Centre
Result of seismic analysis: a man-made explosion
In the course of 7 January, the Revised Event Bulletin (REB) was issued, which is the result of human analysis. The REB clearly identified the event as a man-made explosion. This explosion can only be classified as nuclear once corresponding airborne radioactivity is detected.
The Reviewed Event Bulletin used data from 77 seismic station to study the event, of which 74 were used so far for the location. The error ellipse has shrunken to +/- 9.1 km or 214 km2, so well within Treaty requirements for an on-site inspection.
Potential for Radionuclide Detections
Should traces of radioactivity have been released from the event, typically in the form of the radioactive noble gas xenon, they would need to be transported through the atmosphere to one of the radionuclide stations in the region, detected, sampled and analyzed. Results, if any, can be expected within days or weeks. After the 2013 announced nuclear test, xenon was detected around 55 days after the event.
Under the Comprehensive Nuclear-Test-Ban Treaty (CTBT), the determination of an event's nature - nuclear explosion or not - lies with the Member States and their National Data Centres.
The atmospheric transport simulation below is based on the hypothesis that a release of radionuclides occurred immediately and continuously from the location of the seismic detection.
Update 11 March 2016
Lacking the ability to conduct an on-site inspection, which can only be requested after entry into force, conclusive evidence of the nature of the event – whether it was nuclear or not – is dependent on the detection of relevant radionuclides that can be correlated to the seismic event. CTBTO analysts have been closely monitoring data from IMS stations since the event occurred, and it is confirmed that the system is working perfectly. Several recent IMS radionuclide measurements could be consistent with either a release from a test or with other possible sources, and are therefore not conclusive. Due to a number of factors such as the isotopes’ half-life and atmospheric conditions, the probability of making such a detection continues to diminish over time. Nevertheless, the organization remains on full alert in case of any new evidence being detected.