The tragic March 11, 2011 earthquake in Japan gives an opportunity to explain how timing technologies play an important role in earthquake detection, monitoring and prediction. News reports tell us about the physical location of the earthquake, including its depth below the crust of the earth. In addition, reports indicate that parts of Japan may have shifted up to 2,5 meters and the axis of the earth has been offset by about 10 cm. These measurements are made possible by GPS-based precision timing.
Vibrations from earthquakes are measured using seismometers. These extremely sensitive instruments not only detect the vibrations but are also coupled to GPS receivers which make it possible to time-stamp the vibration data with high precision. Since the GPS time stamp is accurate to better than 100 ns on any location on the surface of the earth, it is possible to determine the location of the earthquake by using the relative delay of the reception of the signal from many different seismograms around the world. This relative delay is used to “triangulate” to the location of the seismic event. This computation is more than a simple geometrical exercise because the speed of sound (or vibration) in the earth varies with diffraction and refraction effects between the different layers of the earth, including its liquid core. In addition, an earthquake often does not occur at a single geometrical point, but can occur along several hundred km long fault lines. For earthquake detection, the accuracy of 100 ns is not directly necessary due to the relatively slow propagation of the waves. However, the GPS synchronization is still vital because it ensures that the clocks in seismometers never drift relative to each other, which would result in increasing cumulative errors.
GPS technology is also used to determine the shift of continents or islands. Here again the precision timing available with differential dual-frequency GPS signals makes it possible to determine shifts as small as 5 mm. (It should be noted that this precision is not possible with the low-cost receivers in current consumer navigation and mobile phones). Measurement of these shifts is also made on an on-going basis, to monitor the gradual sliding of the continental plates and the associated build-up of pressure that may trigger an earthquake.