FORENSIC SEISMOLOGY SUPPORTS CTBTO FOR
Atomic warfare started with two bombs in the summer of 1945. In the decades since then, thousands of nuclear explosives have been tested. And for millions of citizens, thousands of activists and hundreds of disarmament negotiators, every new explosion during those years was just as much a threat to peace as those two over Hiroshima and Nagasaki.
Awed by the destructive power of nuclear weapons, scientists and others began discussing banning further weapons tests shortly after Trinity, the first test of a nuclear explosive in 1945. Since then, a succession of treaties has slowly narrowed the lawful testing environments. For example, the Limited Test Ban Treaty, ratified in 1963, banned nuclear explosions in the air, oceans, and space, while the Threshold Test Ban Treaty, ratified in 1988, limited underground nuclear weapon tests to 150 kilotons. Other articles of the treaty describe the international monitoring system, on-site inspections, confidence-building measures, organization of the treaty's executive council and the technical secretariat, and measures toredressviolations.
Seismic networks have always been instrumental in learning exactly how deep explosions differ from ordinary earthquakes, . Programs have been consolidated, and a large international seismic network has been set up. This nuclear "neighborhood watch," part of the International Monitoring System (IMS), will be able to hear an explosion of one kiloton, and pinpoint it to within about 40 kilometers, anywhere on Earth.
Each day, these stations will transmit enormous amounts of data via satellite to the International Data Center in Vienna, which in turn distributes it to national data centers around the world. Computers at the international center will process the raw data, associate segments of the data stream with specific events, and estimate the location of those events. Analysts will then review the processed data and send a daily bulletin to all parties to the treaty.
In turn, national data centers will have the responsibility to make judgments about the true nature of any suspect events. These national centers will have access to all raw data available at the international center. They will also have the right to use their own computer analyses, informational databases, and data gathered by their own technical resources. Most importantly, each nation will apply its own criteria for distinguishing between compliance and noncompliance
The IMS of the CTBTO have some 170 seismic stations (50 primary and 120 auxiliary), and will coexist with three other networks: 80 air-sampling stations that will detect radioactive byproducts of any nuclear test, 11 sets of ocean-mounted hydrophones, and 60 atmospheric stations to pick up the infrasound made by explosions. All of the data?and this is where the envelope gets pushed?will be sent in real time to a central facility, open to anyone in the world. Primary seismic stations of the CTBT monitoring network. The CTBT forbids all nuclear tests, including those intended for peaceful purposes, and creates an international monitoring network to search for evidence of clandestine nuclear explosions .
. Under the terms of the Comprehensive Test Ban Treaty, a nation suspecting another of conducting a nuclear test may request that the treaty's 51-member Executive Council conduct an on-site inspection to determine the nature of the suspect event. The requesting nation may introduce evidence acquired on its own to strengthen its case to the organization. On-site inspections must be approved within 96 hours of receiving an inspection request because of the need to observe short-lived nuclear phenomena that are produced by a nuclear tes under the current Threshold Test Ban Treaty (banning explosions exceeding
150 kilotons), determining accurate explosive yield is the critical issue.
Most nuclear tests near the threshold treaty's limit generate seismic magnitudes of about 6 or greater on the Richter scale. Seismic signals from these tests travel thousands of miles through Earth's relatively homogeneous core and mantle and are readily picked up by far-away seismic stations for relatively straightforward characterization .
Accurately locating and characterizing signals at these so-called regional distances pose a significant challenge,. "It's a much harder job because we can't use global models of Earth. We have to calibrate region by region, seismic station by seismic station." Successfully meeting the regional distance challenge, has been the most difficult aspect of the effort over the past several years.
that complicating the task is the huge number of events that, at first cut, can resemble a small nuclear detonation. Stations will be recording a constant stream of background noise that includes earthquakes, lightning, meteors, sonic booms, navy armament testing, mining explosions, construction activities and other industrial operations, nuclear reactor operations and accidents, natural radioactivity, and even strong wind and ocean waves.
"As we consider the possibility of smaller and smaller clandestine tests, the number of background events, both natural and human made, becomes immense.
For example, more than 200,000 earthquakes similar in seismic magnitude to a small nuclear explosion occur in the world every year. Many of these background events can be disregarded because of their depth or similarity to other events known to be nonnuclear. However, many will not be identified so readily. As a result Data Centers will require a set of tools, largely data-processing software, modeling capability, and reference databases, to perform "forensic seismology" to separate a weak potential nuclear test from background noise.
One essential tool will be a comprehensive database that includes seismic patterns and the location of mines and seismically active regions. This database must also include information on how Earth's crust and mantle affect the travel time and amplitude of seismic signals as they make their way to international stations. "We want to be sure that data relayed by individual stations are interpreted in light of their regional settings so that the location and nature of an event are properly determined," potential treaty violators might be tempted to detonate a nuclear device in the center of a large underground cavity, a technique called decoupling. Under the CTBT, however, the critical issues will be to determine that a nuclear explosion--no matter its size--took place and to pinpoint its location accurately.
A nation attempting to conceal a test could attempt to minimize the seismic signals. Such signals from a small nuclear test could be well below magnitude 4, with resulting measurable signals traveling 1,000 miles or less. What's more, the signals would likely be confined to Earth's upper mantle and crust, an extremely heterogeneous environment that distorts, and even blocks, parts of the signals
The seismic signal from such a test is reduced by a factor of up to 70 through a muffling effect that reduces the amplitude of the signal. A 1-kiloton nuclear explosion, for example, would produce a magnitude in the range of approximately 2.5 to 3 on the Richter scale when tested in a large underground cavity. Seismic signals of the lower magnitude are produced frequently in a large number of mine explosions worldwide, and many thousands of earthquakes are in this range. scientists have investigated the signal effects possible with blasts conducted in cavities formed from different rock types.
Researchers have also attempted to gain a more complete understanding of the seismic signals caused by routine mining operations. They have joined with colleagues from the U.S. Geological Survey and Russian scientists to calibrate seismic wave propagation in regions of the former Soviet Union. Livermore scientists have also monitored different types of seismic signals from operations in mines.
A potential nuclear explosion in key areas of interest can be detected and identified down to much smaller magnitudes. In other words, , the world will soon have strong international monitoring and analysis capabilities to help determine international compliance with the Comprehensive Test Ban Treaty.
While seismic network research is progressing along many fronts, specialists have devoted their energies to advancing hydroacoustic monitoring technology. They have combined fundamental research on detecting the propagation of underwater sound waves with contributions to the Knowledge Base's storehouse of underwater signals from earthquakes, volcanoes, shipping activity, and chemical explosions from military testing. "A lot of background underwater events have to be taken into account," although they are not as pervasive as land activities such as mining.
They also provided analyses showing the economic advantages of fixed hydroacoustic stations (connected by cable to recording sites on land) over unmoored, floating buoys. On the basis of this work, a network of six hydrophones and five island seismometers was chosen as the international system to detect and locate underwater explosions and, in some cases, explosions in the low atmosphere.
The network takes advantage of the fact that underwater explosions generate acoustic waves (in the frequency range of 1 to 100 hertz) that can travel completely across an ocean basin--in some cases, more than 10,000 miles. The acoustic waves travel along the SOFAR (sound fixing and ranging) channel, described as "a wave guide for ocean acoustic energy that depends on temperature, density, and depth." However, waves traveling in this channel can be blocked or weakened by land masses and regions of shallow or cold water. Livermore modeling of the properties of this channel during CTBT negotiations was important in determining the global distribution of hydroacoustic stations.