All earthquakes produce a series of different types of shock waves,
which travel at different speeds and carry different amounts of energy.
The fastest shock waves, and the first to arrive, are called P waves. These
travel about twice as fast as the next fastest shock waves and rarely cause
any damage. The next shock waves to arrive are called Shear waves (or S
waves), followed by Love waves and Rayleigh waves, which cause most of
the damage by earthquakes.
Since the P waves arrive first, they can provide an advance notice
of the heavy shaking to follow. The warning time is about 1 second for
every 5 miles of distance to the Hypocenter of the earthquake. This amount
of warning can give automated systems plenty of time to respond and enough
time for people to react.
The use of P waves to produce an early warning system has been hampered
in the past by two technical problems: how to eliminate false positives
(triggering on man-made vibrations); and how to distinguish between small
earthquakes (which should be ignored) from large ones. SWS has solved both
of these problems.
The QuakeGuard seismic warning system relies on the early detection
of non-destructive, fast moving waves emanating from the origin of an earthquake.
An earthquake produces four types of waves that emanate from its Hypocenter;
P or primary waves, S or shear waves, L or Love waves, and R or Rayleigh
waves. A P-wave, non-destructive and imperceptible to humans, is
a vertical motion wave that travels faster and can be sensed much earlier,
depending upon distance from epicenter, than destructive S- and R-waves.
The QuakeGuard technology detects the non-destructive P-waves while
filtering other sources of vibrations that can lead to false alarms.
The elimination of false warnings is a result of QuakeGuard's patented
DSP algorithms that filter detected vibrations to isolate the signature
waveforms of a seismic event that has just occurred. Depending on
the geological composition of the terrain and the distance from the epicenter
of the seismic event, a warning of 10 to 60 seconds is possible.
2008 United States National Seismic Hazard Maps
SWS has received multiple patents for discriminating between conventional
earth tremors and those that are precursors to an earthquake, and the use
of multiple sensors to discriminate between man-made vibrations and earthquakes.
These patents cover technology to eliminate false positives, which is a
key differentiator, advanced sensor design and several signal analysis
techniques. SWS has corresponding international patent applications pending.
"The results of the tests showed, in general, that QuakeGuardTM
is sensitive enough to detect small vertical ground motions, thereby providing
a warning time before the arrival of more damaging horizontal motions produced
by earthquakes. Warning time will allow users enough time to take precautionary
action in the event of an upcoming seismic shaking. The warning time will
depend, of course, on the earthquake type, the epicentral and focal distances,
and the geological conditions of the site where the device is to be installed."
Dr. Carlos E. Ventura,
Earthquake Research Laboratories,
University of British Columbia.