NTN - Chapter 3 - The Change in Technical Requirements

Chapter 3: Developing a Monitoring System

In broad terms, a seismological monitoring system has three basic elements: 1) a network of stations supplying continuously recorded data, 2) a trigger system that initiates the collection of data from the network for specific events, and 3) analysis of the triggered events. These three elements are common both to earthquake monitoring - for seismic risk and scientific understanding - and to the monitoring of underground nuclear explosions. The basic elements needed for monitoring a Comprehensive Test Ban Treaty (CTBT) are available today. Many of these elements, however, exist only in prototype form, and others will need to be modified and/or expanded.

This chapter begins with a review of the new technical requirements for monitoring a CTBT and a discussion of the various types of seismic stations that can be applied to the task. We then describe earthquake monitoring, as an analogy for the open retrieval of global seismic data.

The Change in Technical Requirements

As discussed in the previous chapter, there has been a major shift in priorities for seismic monitoring. In the bilateral environment, the emphasis was on specific test sites. Accumulated knowledge of testing practices minimized concern about discriminating between nuclear explosions, industrial explosions, and earthquakes. The U.S. placed special emphasis on recording sites along the southern rim of the U.S.S.R. and on special arrays in Scandinavia for monitoring the Soviet test sites in Kazakhstan and Novaya Zemlya. Likewise, the Soviet monitoring program used sites in Central Asia especially sensitive to signals from the U.S. test site in Nevada, the French site in the Pacific, and the Chinese test site at Lop Nor.

Sensing equipment and analysis were tuned to detecting explosions in these specific geographical areas and to estimating their yield. In particular, U.S. practices for monitoring the major test sites emphasized the use of arrays, with greatest sensitivity to seismic signals which travel distances 1000's of kilometers through the Earth's interior.

Today the focus is no longer on yield estimation at specific test sites, but rather on the detection, accurate location, and identification of low-magnitude seismic events. Accordingly, we need to change our monitoring focus from teleseismic waves (those waves that travel distances greater than around 1500 kilometers) to regional waves (those waves that generally travel less than 1500 kilometers).

Teleseismic waves travel primarily through the Earth´s deep interior, where physical properties vary gradually over large distances. Regional waves are generally trapped within the Earth's uppermost layers, where physical variations are large and frequent. Regional waves, therefore, are complex and easily influenced by the characteristics of how and where the seismic event occurred, and the material and structure through which the seismic wave travels.

Unfortunately for monitoring, the properties of the Earth's crust in most parts of the world are significantly different from those found near the major test sites.¹ As a result, experience gained from monitoring these test sites will not necessarily transfer to sites of proliferation concern. Regional wave progagation needs to be understood, or at least calibrated, for many parts of the world. The understanding of regional wave propagation is an area where university and government researchers working in the related field of earthquake seismology and Earth structure can make, for example, significant contributions to the nuclear monitoring effort.

To a large degree, the monitoring task is assisted by advances in telecommunications and computing. With new developments in seismometer design, low power electronics, computers, and communication technology, there have been revolutionary improvements in the monitoring of seismic events. In contrast to the past, when seismic measurements were severely restricted by analog recording systems, ground motion can now be recorded over the broad range of frequencies and amplitudes useful to seismology. The new digital broadband seismic stations are able to record a wide range of seismic signals covering ground vibrations that are as rapid as tens of times per second (high frequency) to ones that last as long as hundreds or thousands of seconds for one oscillation (long period). As illustrated in the box below ("The 1992 Chinese Test") global communications now allow the rapid transfer of seismic data from remote sites to centralized locations for processing, analysis, and archiving.

The 1992 Chinese Test

On May 21, 1992 the Chinese detonated a high-yeild (magnitude 6.6) nuclear explosion at their test site in Lop Nor. University seismologists in the United States accessed data from the IRIS open seismic station in Obninsk (outside of Moscow) via a satellite telemetry link and sent the data across the United States on the Internet system. The figure below, comparing the record from the explosion to an earthquake of similar magnitude and distance, was made within a day of the explosion through the collaborative effort of research scientists in Russia, California, and Colorado. Data was retrieved in a similar fashion for the smaller (magnitude 5.8) Chinese nuclear explosion on October 5, 1993.

**The 1992 Chinese Test**
On May 21, 1992 the Chinese detonated a high-yeild (magnitude 6.6) nuclear explosion at their test site in Lop Nor. University seismologists in the United States accessed data from the IRIS open seismic station in Obninsk (outside of Moscow) via a satellite telemetry link and sent the data across the United States on the Internet system. The figure below, comparing the record from the explosion to an earthquake of similar magnitude and distance, was made within a day of the explosion through the collaborative effort of research scientists in Russia, California, and Colorado. Data was retrieved in a similar fashion for the smaller (magnitude 5.8) Chinese nuclear explosion on October 5, 1993.

Nuclear Testing and Nonproliferation

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