In May 2009, not long after 2am GMT, 120 states received an automatic notification alerting them to the possibility that a nuclear test had just been conducted. The notification contained estimates of the place - the northwest corner of North Korea in approximately the same place as a previous test in 2006 - and also estimates of the time - 9:54am local time.
It also included the magnitude: the test had measured 4.52 on the Richter scale, bigger than the 4.1 nuclear test in 2006, which at the time had been internationally condemned. After two and four hours, the states received updated notifications with even more accurate information.
Not long after the first notification was received, the North Korean government also issued a statement saying that they had conducted a nuclear test.
The notifications used information from the International Monitoring System (IMS), a global sensor network run by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). The decision to set up the sensor network, in Geneva, Switzerland, in 1996, was made possible due to the end of the Cold War.
"Cold War hostilities had ceased following the peaceful dissolution of the Soviet Union. Nuclear weapon possessor states had also gathered ample amounts of data from the nearly 2,050 nuclear tests conducted until then," said Annika Thunborg, the head of communications for the organization.
Today, there are 182 countries signed up to the treaty and 154 of these have also ratified it. However, there are still nine outstanding nuclear-technology capable countries that have to fully adhere or ratify in order to make the treaty enter into force.
A network of sensors
The sensor network is composed of seismic, infrasound, hydroacoustic and radionuclide technologies. They monitor and track any large movement of energy generated by large explosions, which create acoustic or seismic waves that spread in the atmosphere, ground, or oceans, and they also sniff the air for radioactivity. At completion, the network will contain 321 seismic monitoring stations and 16 radionuclide laboratories, positioned all over the world, and currently over 280 such facilities are fully operational.
With this network, "all militarily significant nuclear explosions can be detected," said physicist Richard Garwin at the organization's Science & Technology 2011 Conference held in June 2011 in Vienna, Austria.
"The number of AutoDRM [automatic email-based message system] and subscription requests from member states served each month was around 396,000 on average for 2010," said Thunborg.
Since the network contains seismic sensors, almost all of these were other seismic events, such as earthquakes and volcanic eruptions, including the Japanese Tōhoku earthquake in March 2011.
Collecting data in Vienna
"The stations of the International Monitoring System send all their data to the International Data Centre of the CTBTO [in Vienna, Austria] where all computing is carried out. Data from at least three stations needs to be available to enable identification of an event as a possible nuclear explosion and its source location. The Data Centre receives around six gigabytes of data daily," said Thunborg.
In a year, then, more than 3.5 TBs of data can be collected. "The CTBTO makes use of internal computer resources only, a number of separate clusters of Linux computers. No supercomputers, or external grids or clouds are employed. While a computer grid is not currently used, it cannot be excluded. In the future the need for one might arise," she said.
"We want to reach out to new scientific communities, timeframes and look for new applications. We want to turn an area of weapons of mass destruction into areas of mass collaboration for verification," said Tibor Tóth, CTBTO Executive Secretary at the organization's conference.
One of the five major topics at the conference was "Advances in computing, processing, and visualization for verification applications".
Dealing with the data
Computer algorithms analyze the datasets in near-real time to pick up signatures of large explosions. This information is processed and organized into three bulletins called Standard Event Lists (SELs) that are sent to all member states of the CTBTO who request it, just like with the North Korean test of 2009.
The first bulletin contains seismic and hydroacoustic data, the second summarizes statistics from auxiliary seismic stations, including additional infrasound data. And the third bulletin accounts for any last minute figures from monitoring stations.
The results of this automatic processing are then reviewed by human analysts, who discard events which are not real, add signals which have not been associated to an event, and correct and improve the location estimates. The confirmed and corrected events are distributed via the Reviewed Event Bulletin (REB), in which the radionuclide data is also included.
The science of our planet
"This kind of activity [CTBTO] is really, really fundamental in driving not just the questions of nuclear detection, but it also drives the understanding of the science of our planet," said David Strangway, a geophysicist who works with the CTBTO on developing better earthquake monitoring and prediction.
"The network already contributed to speedy tsunami warning alerts. And during the Fukushima accident, the data from our radionuclide sensors helped trace the levels and dispersion of radioactivity worldwide," Thunborg said.
"There is also great scientific interest in using CTBTO data for climate change research and infrasound data could help improve aviation safety following volcanic eruptions and dispersion of a volcanic ash cloud."