While an average of 20,000 quakes above magnitude 4.0 are reported annually, seismologists figure that at magnitude 2.5, that number is 120,000-a formidable string of ambiguous rumblings to sift through. In addition, another hundred thousand chemical explosions are set off by mining operations. While these are often less than magnitude 3.0, determined testers can hide nuclear explosions by synchronizing the blasts. Researchers Scott Phillips and Craig Pearson of the Los Alamos National Laboratories are compiling more detail of the seismic signatures of all possible events into a library. Similar work with underwater blasts is also underway.
Underwater explosions create an enormous expanding bubble filled with scorching gases.
The growth eventually loses momentum, and the water pressure squeezes the gassy globule back down. The internal pressure rebuilds and the gas and water pressure continue this tango, cyclically expanding and contracting the walls of the gas pocket as it rises to the surface. Hydrophones, as they are called, consist of a pressure sensitive floating two-foot long cylindrical bump of on a hose that can feel sound waves. Even small chemical explosions are extremely loud. An underwater nuclear explosion is likely to bust everyone's eardrums.