Advances in Conception about Earthquakes - Earthquake Engineering for Structural Design - page 112

FUTURE WORLD
AFTER250MA
Figure 4.8 After Pangaea - future world (modified after Bokor, 2007)
4.1.6. Revolution in Seismology: Global Positioning System(GPS)
For Seismology, since 1990, there are several factors contributing to advances in
development of knowledge. First, the creation around the world networks of
seismic stations to detect ground motions provides a large amount of seismic data
throughout the world. Second, the development of computer technologies allows
seismologists to analyze the massive quantities of new data and to model the
tectonic plate movements. But the real revolution after 1990 is due to the
development of space-based geodetic investigation, the Global Positioning System
(GPS) . This system is a constellation of 24 satellites (Fig. 4.9a), which is used for
navigation and precise geodetic position measurements. These satellites are placed
in orbits at about 3.75 times the Earth radius (NASA, 2005). Daily position
estimates are determined from satellite signals, which are recorded by almost 1000
GPS receivers on the ground. Horizontal velocities due to the motion of the Earth's
tectonic plates and deformations in plate boundary zones are represented in maps
(Fig. 4.9b). The GPS system has greatly contributed to the knowledge of regional
tectonics and fault movements through its ability to measure, at sub-centimeter
precision, the relative positions of points on the Earth's surface (Jackson, 2001).
Re-measurement at a later date then gives the changes in relative positions, values
which can be converted into the relative direction of tectonic plate movement, the
velocity of this movement and the produced stresses. So, today, due to GPS, one