Special Report - Nonproliferation in an evolving world
Seismic detectives go underground
In the era of nuclear nonproliferation, the ability to monitor seismic activity is critical. Seismology stations around the world are active every day recording disturbances that include tens of thousands of earthquakes a year, possible nuclear explosions, mining explosions and even construction blasts.
Researchers at Pacific Northwest National Laboratory are using data gathered by seismology stations to develop a mathematical framework for identifying and locating seismological events around the world. Their work, part of the National Nuclear Security Administration's Ground Based Nuclear Explosion Monitoring Research and Engineering Program, will be used by the U.S. government to monitor explosions and weapons tests.
To build a framework, researchers use various measurements or discriminants from the seismologic data. One discriminant is energy wave pattern. Compressional waves include energy that is pushed outward in all directions, as in an explosion. Shear waves include energy that moves up and down as in an earthquake.
Using compressional and shear wave data from known events, PNNL researchers have built statistical models that describe what energy waves look like for earthquakes and for explosions. "When we have a new event coming down the line and we don't know what it is, we can ask if its energy waves most closely match the earthquake model or the explosion model," said PNNL's Dale Anderson, principal investigator on this project.
In addition to creating statistical models for each discriminant, PNNL researchers are adding a new twist by mathematically combining the discriminants into a model to more accurately identify a seismic event.
These models account for uncertainty in measuring individual seismic discriminants. "The better we can estimate a discriminant, such as depth, the less uncertainty we will have in the final decision about the type of event," said Debbie Carlson, a PNNL mathematician working with Anderson.
Because people can dig only so far into the earth to plant explosives, researchers can use this approach to translate depth measurements into precise statements about the event.
"It's not only the depth estimate, but it's also how confident you are of the depth," Anderson said. "If it is 100 kilometers plus or minus 90, we may not be very confident of an identification decision, but if it is 100 kilometers plus or minus 10, we know it's not an explosion."
Anderson and Carlson's seismology work addresses two types of problems: teleseismic waves—those that travel all the way through the earth, and regional waves—those that remain mostly in the earth's crust.
They have just completed developing a mathematical framework for teleseismic waves and are starting to work with regional waves. "The physical properties of regional events are different than those of teleseismic events," said Anderson. "We use a probability model to mathematically marry the physics of both."
Joining this information will allow researchers access to more information when they are determining whether an event is an earthquake or an explosion.
Seismograms show the amplitude of shear energy (Lg) is larger for an earthquake than for an explosion. Scientists at PNNL are using compressional (represented by Pg) and shear wave data from seismic events to build statistical models that will ultimately help distinguish earthquakes from explosions.