A sensitive globe-spanning instrument array is monitoring the Earth. It was set up under the Comprehensive Nuclear-Test-Ban Treaty (CTBT), first signed by more than 100 nations in 1996.
From earthquakes in Nepal to volcanic eruptions in Chile, from meteors crashing to Earth to the songs of migrating whales in the Indian Ocean, The Global Ear hears all.
… On a typical day, the centre analyses over 30,000 seismic signals to identify about 130 seismic events that meet stringent criteria.
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Since the first stations in this network officially began reporting data back to the headquarters of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) in Vienna, Austria, in 2000, the network has steadily expanded.
Today, about 85 percent of the envisioned network of 337 seismic, hydroacoustic, infrasound and radionuclide detecting stations worldwide have been installed, tested and certified — enough for the network to fulfill its main function. A series of nuclear tests conducted by North Korea in the past decade demonstrated the network’s ability to detect signatures and confirm the location of blasts.
“Our mission is to contribute, as we can, to make this world safe and secure,” says CTBTO executive secretary Lassina Zerbo. “We’ve put together a deterrent that makes sure today that no nuclear test explosion relevant to the development of a nuclear weapon would go undetected.”
And beyond showing would-be weapons testers that they can’t hide their activities, the wealth of global monitoring data produced is being put to work elsewhere — from monitoring meteorites and volcanic eruptions to nuclear reactor accidents and sea life.
The verification regime consists of three main branches: the International Monitoring System (IMS), the International Data Center (IDC) and an on-site inspection regime ….
Underwater cable is installed to connect hydroacoustic sensors to the recording station on land in the Juan Fernández Islands off Chile in 2003.
Four global instrument networks, designed to continuously monitor for hints of a nuclear explosion, make up the IMS, which serves as the front line of the CTBTO’s verification regime. The largest of these networks is currently made up of about 150 seismic stations (out of 170 planned) with an ear to the ground to listen for vibrations potentially generated by a blast. Like other seismometers, these instruments record the intensity, frequency and incoming direction of seismic waves traveling through Earth’s interior and across its surface. Because of the high speeds of seismic waves, these stations offer the IMS’s most rapid detection method, capable of detecting events within seconds to tens of minutes depending on an event’s size and proximity.
In addition to its seismic network, the IMS employs two additional “waveform” methods, which detect sound waves traveling underwater and in the atmosphere. Just 10 (of a planned 11) stations — located on and offshore of islands in the Atlantic, Indian and Pacific oceans — currently constitute the IMS’s hydroacoustic network, making it the system’s smallest. But because sound travels through water very efficiently, particularly through a layer of seawater about 1 kilometer below the surface that is specifically monitored by hydrophones, this group of stations still offers worldwide coverage of the oceans.
The radionuclide station on the South Atlantic island of Tristan da Cunha is equipped to measure both radioactive particulates and noble gases.
Meanwhile, 48 stations (of a planned 60) scattered across the oceans and continents monitor the skies for low-frequency infrasound produced by volcanoes, earthquakes, aurorae and other natural phenomena, as well as by aircraft, rockets and explosions of all manner. Like hydroacoustic waves, infrasound — which has frequencies below 20 hertz, roughly the minimum audible to humans — travels efficiently through the atmosphere and can circle the globe in about 40 hours. The IMS specifically monitors infrasound with frequencies between 0.02 and 4 hertz. Each IMS station hosts an array of four to 15 microbarometers, often spread over several square kilometers, which pick up the tiny variations in air pressure associated with infrasound waves.
Together, the seismic, hydroacoustic and infrasound networks offer ample coverage of the planet to detect the sound and shaking that would accompany a nuclear explosion. In many cases, an event is likely to be picked up by at least two of the networks, says CTBTO seismo-acoustic officer and infrasound specialist Pierrick Mialle, but not always. In the case of an aboveground test, “infrasound might be the only waveform technology to pick up” a clear signal, he notes, so having all three offers valuable redundancy.
These three networks are further complemented by another, capable of sensing the radioactive fallout from a nuclear explosion. Detecting fallout is a comparatively slow and low-resolution method for determining when and where an explosion may have occurred. However, it offers unique evidence — in the form of radioactive isotopes, or radionuclides — not available from the other technologies that an event was indeed nuclear in nature. Known proportions of the radionuclides barium-140 and cesium-137, among others, are produced by nuclear fission of uranium or plutonium atoms during a nuclear explosion, explains Ted Bowyer, a nuclear physicist at Pacific Northwest National Laboratory in Washington state and chair of the radionuclide experts panel that works with CTBTO.
When released to the atmosphere, Bowyer says, most of these radionuclides latch onto dust particles and are then wafted around the atmosphere at the whim of global wind patterns. To monitor for these radioactive particles, the IMS currently has 63 stations worldwide (of a planned 80 stations) routinely collecting air samples and searching for the telltale gamma-rays they emit. Eleven separate radionuclide laboratories (with five additional labs awaiting certification) also periodically analyze samples to provide quality assurance for the 63 stations, 22 of which are further equipped to detect radioactive noble gases produced in explosions, chiefly xenon.
Unlike most radionuclides, which tend to react chemically with their environments, the noble gases are inert. “So, if you have an underground detonation … the most likely things to escape are those elements that are not reactive, and xenon is one of those,” Bowyer says, which makes it very useful in nuclear monitoring.
… On the receiving end of the 10 to 15 gigabytes of data amassed each day is the IDC, located at CTBTO headquarters in Vienna. Here, a staff of about 100 works to analyze data; improve methods and advance scientific understanding of the geophysical and geochemical phenomena relevant to monitoring; liaise with member states; maintain the IDC’s communications and operate its computer networks, as well as the more than 280 IMS stations, each of which is owned and staffed by the member states.
“What we’re really aiming for is continuous high sensitivity of the entire network,” so no corner of the globe ever goes unmonitored, says IDC director Randy Bell. However, he says, “integrating the data from the four different detection methods is a challenge.” Thus, there are several stages of data processing and analysis. Computer algorithms first identify signals of seismic or acoustic origin — in the case of the waveform technologies — in individual station data, and then correlate signals from multiple stations to refine estimated timings, locations and sizes of detected events. Analysts review these automated event lists, correcting and amending them if signals have been misdiagnosed (or are simply missing) to produce daily “reviewed event bulletins.” Similarly, daily radiation reports from radionuclide stations are first reviewed automatically, then by radionuclide specialists.
In any given day, Bell says, IDC analysts review roughly 100 to 300 new events, most of seismic origin, although some days the tally tops 1,000.
Making sense of all of these events is complicated by the fact that the different types of signals travel at different speeds and are detected at different times. “While a seismic signal might propagate around the Earth and be recorded here and processed all within [hours],” he says, “we then have to integrate data that’s late arriving, such as the radionuclide data,” which may come in days or even weeks after the event that produced the corresponding seismic signals.
Beyond reviewing events, the IDC also screens out those events likely to have been caused by natural phenomena from potentially human-induced events based on distinctive features in the detected signals. The character of seismic waves produced by an underground event offers clues to its origin, for example, as does its depth (seismic waves emanating from great depths can’t be anthropogenic given the limits of drilling technology). Similarly, certain kinds of hydroacoustic signals are thought to be produced only by earthquakes, thus ruling out a human origin.
But separating the natural from the potentially anthropogenic is as far as the IDC goes. “We don’t decide whether an event is nuclear or non-nuclear in nature,” Bell says. “The job of the CTBTO … is to operate the network, and receive, gather, store, process and disseminate the data,” he says. “Determination of whether a violation of the treaty has occurred is a decision left to the member states.”
… Since the treaty was first signed, three nuclear weapons tests — in 2006, 2009 and 2013 — have occurred, all conducted underground by North Korea. Because the CTBT was not in force, these weren’t treaty violations strictly speaking, and thus, technically, no determinations were made based on IMS data. But that doesn’t mean the explosions went unnoticed.
Seismicity originating from each of the tests was picked up at dozens of the network’s seismic stations. The most recent explosion, on Feb. 12, 2013, was also heard by regional infrasound stations. The first IMS data following the 2013 event were made available to member states within hours, according to a CTBTO press release. Further confirmation came weeks later when an IMS radionuclide station detected radioactive xenon in the air over Takasaki, Japan. Atmospheric transport modeling suggested that the timing and location of the gas’s emission likely coincided with the test.
These results have “absolutely” demonstrated the CTBTO’s ability to fulfill its intended purpose, says Zerbo, who was a research geophysicist in the mining industry prior to joining the CTBTO in 2004. And other recent events have further shown its promise, he adds. The 2011 Tohoku earthquake off Japan’s coast, for example, put all four of the IMS’s monitoring technologies to use: The earthquake and ensuing tsunami “were well-detected by our seismic and hydroacoustic stations,” Zerbo says. “Then the infrasound stations detected the explosion of the [Fukushima Daiichi] nuclear power plant itself, and our radionuclide technology followed the dispersion of radioisotopes around the globe.”
In fact, the IMS detects many other natural and human-induced activities on a regular basis, from numerous earthquakes to whale sounds to relatively small, nonnuclear explosions. Radionuclide stations have detected spikes in atmospheric cesium-137 linked to forest fires burning trees that had taken up the element following the 1986 Chernobyl reactor meltdown. The infrasound network, meanwhile, can hear volcanoes erupting thousands of kilometers away, Mialle says, and on Feb. 15, 2013 (coincidentally just three days after North Korea’s nuclear test), it picked up its largest event to date when a stunning meteor tore across the sky over Chelyabinsk, Russia.
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Can anyone access the daily reports? Not yet, it seems.
