Seismic signature helps reveal nuclear tests

Seismologists around the world have their ears to the ground, listening for secret nuclear explosions among other seismic activities such as earthquakes. Now, researchers say they have found an easier way to identify these nuclear tests from seismic data.

An array of seismic monitoring stations called the International Monitoring System (IMS) is positioned around the globe to detect the rumblings of covert nuclear weapons tests. The system supports the Comprehensive Nuclear Test Ban Treaty, which prohibits member countries from testing nuclear weapons (see Geotimes, March 2007). An ongoing challenge for the IMS is to accurately differentiate between different types of seismic events, including nuclear explosions, volcanic eruptions, mine blasts and, of course, earthquakes. But a new way to discern a certain type of mine blast from the rest of these seismic events is making seismic monitoring of nuclear activity more accurate, researchers reported in June in the Bulletin of the Seismological Society of America.

Mining companies use several types of explosions. One type is the delay-fired mine blast, in which multiple explosive-laden boreholes are drilled in a grid pattern and then detonated in sequence. Because of their large size, delay-fired mine blasts can be confused with nuclear explosions in the seismic record, says Stephen Arrowsmith of the Los Alamos National Laboratory in New Mexico and lead author of the study.

By scrutinizing the seismic data with the help of an enhanced algorithm, however, the two events can be distinguished. "Different types of events are rich in different frequencies," Arrowsmith explains. Examining common frequency patterns for a given event can yield a "seismic signature" as unique to the event as a written signature is to a human, he says.

Using seismic data from an area of Wyoming considered ideal for its abundance of mine blasts and minor earthquakes, Arrowsmith and his co-authors analyzed 76 seismic events that occurred between January and May 2004. Each event was classified as either a mine blast or an earthquake.

The authors compared their experimental, seismographic classifications with actual blast records from one of the largest mines in the area and found that they had successfully identified 74 out of 76 seismic events, or 97 percent. That represents an improvement over the 89 percent of events correctly identified by a similar method used by the same authors in a previous study. In the more recent study, the authors used data from multiple seismometers in a single location instead of only one seismometer. This method allowed them to eliminate seismic background noise, which gave a clearer depiction of event-identifying frequency patterns. Delay-fired mine blasts appear uniquely "scalloped" on spectral frequency graphs, according to the study, and this distinctive shape can be used to successfully identify delay-fired mine blasts in the seismic record.

This finding represents a strong contribution to the monitoring of nuclear weapons testing, the researchers say, though human chicanery should not be underestimated in the realm of international power politics. "There is [still] some concern that nations may attempt to mask nuclear weapons tests by performing delay-fired mine blasts simultaneously with nuclear blasts," Arrowsmith says.

Ari Hartmann
Geotimes contributing writer

Links:
"Put to the Nuclear Test: Seismology and the International Monitoring System," Geotimes, March 2007 
"Small nuclear war could pose large climate consequences," Geotimes online, Web Extra, Dec. 12, 2006


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