Tuesday, September 1, 2009
Lawrence Livermore National Laboratory is leading a joint
project with Los Alamos and Sandia national
laboratories, as well as the Air Force Technical Applications Center (AFTAC) and
Quantum Technology Sciences, Inc., to improve the accuracy of regional seismic
travel time (RSTT) predictions to detect and locate low-yield nuclear
explosions.
While seismic research is most often associated with
earthquakes, seismic technology is also the primary means to detect, locate and
identify underground nuclear explosions.
Energetic seismic waves propogate over long distances in the earth's crust and mantle.
Underground nuclear testing plays a pivotal role in the
persistent and well-documented efforts by states to develop and improve
explosive nuclear devices.
Nuclear tests are no longer frequent. However, there are 30-40
earthquakes of magnitude 4 and greater every day—about 10,000 per year. A
magnitude 4 earthquake releases energy on the order of a one-kiloton nuclear
explosion. Identification and location of the rare, and possibly covert nuclear
test, within the cacophony of natural and man-made background seismic activity,
is a major national security scientific challenge that NNSA and its labs are in
a unique position to meet.
Scientists study the seismic traces (waveform records of the
surface ground motion as a function of time, acquired by digital equipment)
from networks of seismometers all over the world.
The long-term effort to improve seismic event location
accuracy significantly increases the accuracy of RSTT predictions. The newly
developed RSTT model embodies three-dimensional variations in seismic wave
speed in the earth's crust as well as lateral variability in seismic-wave speed
in the earth's upper mantle.
The RSTT model increases the location accuracy of small
events, previously undetectable at great distance. Tests across Eurasia show that the RSTT model improves median location
accuracy by 46 percent (from 17.3 km using a standard one-dimensional model to
9.3 km using the RSTT model).
NNSA efforts have reduced regional location error for small
yield events to a level that, until recently, was only achieved for large,
globally recorded events.
This NNSA-funded effort has resulted in a significant
improvement in regional seismic event location accuracy and further improvement
can be expected as complementary research projects mature, thus improving our
ability to detect lower yield events.
Original
article
SOURCE: Lawrence Livermore National Lab