figure, refers to the future possibility of bilateral monitoring agreements, distinctly separate from the CTBT. The magnitude estimates listed on this figure are from the IDC and other public sources. SOURCE: Seismology Subcommittee

As depicted in Figure D-1 (and Figure 2-7), over the past decade the capabilities of the U.S. and other countries to detect small tests (and, hence, to detect attempts at evasive testing) have improved greatly due to the implementation of regional monitoring methods, increases in data availability and quality and the application of new discriminants. Fundamental improvements in our ability to monitor underground nuclear testing have occurred in the following three areas:

1.  Use of regional seismic waves
The routine use of “regional” waves—seismic waves that travel at higher frequencies within the earth’s crust and that travel distances up to 1,600 km (1,000 miles) have fundamentally changed the strategies used for nuclear monitoring. Regional seismic waves travel through the earth’s crust and uppermost mantle (the top few tens of kilometers of the earth’s interior) at high frequencies. Particularly important is the availability today of data from seismic stations throughout the Middle East, North Africa, Russia, Kazakhstan, Mongolia, China and South Korea. Access to these regions permits countries of special concern to the United States to be monitored at much closer distances and thus down to events of much smaller size. While research into regional seismic methods was underway when the CTBT text was finalized more than a decade ago, it was during the past decade that many of these new research products began to be implemented into the routine operational systems that continue today.

2.  Increased data coverage, quality, and availability
Over the last decade, the amount of seismological data that is available to detect nuclear weapons tests (including evasively-conducted tests) has increased approximately 10 fold. Improvements in global digital communication networks have increased the capacity to transmit these large amounts of data from around the world in real or near-real time by approximately 100-fold. Computer power, data storage and retrieval increased approximately 10-fold. Furthermore, much of this newly available data is coming from nations and areas that were previously inaccessible to the United States.

3.  Improvements in our ability to distinguish the seismic signals of a small nuclear explosion from those of naturally occurring earthquakes
The use of digital data now provides for ground motion to be recorded continuously with high sampling rates over broad frequency ranges. This has led to new methodologies that use the characteristics of the ground motion that are recorded at various frequencies to distinguish small explosions from naturally occurring earthquakes. Access to high-frequency seismic data has allowed for applications of new discriminants that have improved by about a factor of 10 or more our capability to distinguish explosions from naturally occurring earthquakes. The number of problem events that occur (fewer than about one per year) is now small enough that on-site inspections are feasible under the verification provisions of the treaty.

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