Major earthquakes are known to cause buildings to collapse, dishevel roads, shear pipe lines, disrupt communication, set fires, and inflict injuries which sometimes result in death. While significant advances have been made to reinforce structures against earthquakes, little has been done to reduce secondary hazards. Toppled furniture, objects thrown off shelves and out of cabinets continue to pose a hazard to people and property during a major earthquake. Injuries aside, significant property is lost due to major earthquakes as a result of objects thrown out of cabinets and broken on the floor.
In this regard, a reliable apparatus to detect major earthquakes and activate safety devices would be of benefit. One which initiates safety measures before the onset of the earthquake's destructive ground motions, even more so. A forewaming of a few seconds would be most beneficial by latching cabinets closed, securing objects in place, shutting off gas supplies, deactivating electrical equipment, activating automated safety measures and warning people to take cover so they can safely ride out the earthquake.
Seismologists know well that the P wave of an earthquake precedes all others. Next the S wave, followed by the most destructive L and R surface waves. This sequence is due to the speed and manner by which the various shock waves propagate through and on the earth. The P wave (Primary, compression, push pull) is the fastest, followed by the S wave (Secondary, shear, side to side) both of which propagate directly through the earth. When these body waves reach the surface they give rise to the more destructive L (Love, long undulations) and R (Rayleigh, Rolling) surface waves. Because the P wave propagates faster, directly through the earth, it typically arrives seconds before the S wave, which in turn also arrives seconds before more destructive L and R surface waves.
Conveniently, the amplitude of the P wave provides a direct measure of an earthquake's potential destruction. Combined with the observation that the P wave motion in the vicinity of the epicenter is predominantly up and down results in a distinct precursory measure of an earthquake's potential destruction.
As it turns out, the sequence of shock waves provides a second chance to detect a major earthquake and activate safety devices before the onset of its destructive ground motions. The S wave which follows closely after the P wave also provides a forewarning of the more destructive L and R surface waves. The S wave has both horizontal and vertical components and, like the P wave, the amplitude of the vertical component of the S wave provides a direct measure of the potential destruction of an earthquake.
In either case, it is the vertical component of the P and S waves which provides a direct measure of an earthquake's local magnitude, seconds before the onset of more intense ground motion. No other source on earth produces the unique vertical threshold accelerations and displacements characteristic of the P and S waves of a major earthquake. Given this knowledge, all that is needed is a device which isolates and responds to the initial vertical shocks of a major earthquake and thereby activate safety devices before the onset of more destructive ground motions.