The CTBTO has seen a marked increase in its ability to locate and analyze nuclear test explosions since North Korea's first nuclear test in 2006. This is largely a result of an increase in the number of completed monitoring stations in the CTBTO's International Monitoring System (IMS), combined with increasingly larger nuclear explosions by North Korea. The IMS uses a combination of seismic, hydroacoustic, infrasound, and radionuclide stations to detect nuclear test explosions.
In October 2006, the IMS was approximately 60% complete, with 99 seismic stations in operation. The organization was able to detect the 4.1 magnitude North Korean test with 22 seismic stations around the world and one radionuclide station located in Yellowknife, Canada. The test explosion was estimated within a location ellipse of 880 square kilometers.
In May 2009, the IMS was over 70% complete, with 130 operational seismic stations. As a result of the increase in the IMS' sensor network, the organization was able to detect the 4.52 magnitude event with 61 seismic stations, although no radioactive noble gases were detected from the North Korean test tunnel. The explosion was estimated to be within a location ellipse of 265 square kilometers.
At the time of the February 2013 test, the IMS network was over 85% complete with 160 seismic stations reporting to the IMS. The network was able to detect the 4.9 magnitude nuclear test within a location ellipse of 181 square kilometers and with 94 seismic stations. In addition, two infrasound stations were able to register the event for the first time. Again, no radioactive noble gases were detected.
The CTBTO has reduced its location estimates by nearly 700 square kilometers and is now able to detect test explosions with over four times as many seismic stations compared to October 2006. This high degree of accuracy ensures that states will not be able to confidently test nuclear weapons clandestinely.
When completed, the IMS will consist of 337 facilities in 89 countries. This will include 170 seismic, 11 hydroacoustic, 60 infrasound, and 80 radionuclide stations. In addition, the network will also have 16 laboratories to analyze radionuclide samples.
The CTBTO currently has 183 signatories and has been ratified by 159 states. Ratification of the treaty by eight remaining Annex 2 states, one of which is North Korea, is necessary for the treaty to enter into force. Once the remaining Annex 2 countries ratify the treaty, a state party can request on-site inspections in the case of a suspected nuclear test. This will allow inspectors from the CTBTO to inspect an area of 1,000 square kilometers for telltale signs of a recent nuclear test explosion.