Crustal plates do not glide smoothly past one another. Instead, tremendous friction exists along plate boundaries. The stress (a force) of plate motion builds strain (a deformation) in the rocks until friction is overcome and the sides along plate boundaries or fault lines suddenly break loose. The two sides of the fault plane then lurch into new positions, moving from centimeters to several meters, and release enormous amounts of seismic energy into the surrounding crust. This energy radiates throughout the planet, diminishing with distance, but sufficient enough to register on instruments worldwide.
In 2007, southern Sumatra, Indonesia, was hit with M 8.4 and M 7.8 quakes, near the same plate boundary where four other quakes greater than M 7.9 had occurred since 1998. At this boundary, the Australian plate moves northeast, subducting beneath the Sundaplata on which Sumatra rides, at a relative speed of 6.6 cm/year (2.6 in./year). This is where the M 9.1 Sumatra-Andaman earthquake struck December 26, 2004, triggering the devastating Indian Ocean tsunami (more on this in Chapter 16); the quake and seismic wave took 228,000 lives, go to http://earthquake.usgs.gov/earthquakes/.
What are the mechanisms that produce such dramatic tectonic events? Why do most quakes strike in total surprise, such as in Haiti and Chile in 2010?
Crustal plates do not glide smoothly past one another. Instead, tremendous friction exists along plate boundaries. The stress (a force) of plate motion builds strain (a deformation) in the rocks until friction is overcome and the sides along plate boundaries or fault lines suddenly break loose. The two sides of the fault plane then lurch into new positions, moving from centimeters to several meters, and release enormous amounts of seismic energy into the surrounding crust. This energy radiates throughout the planet, diminishing with distance, but sufficient enough to register on instruments worldwide.In 2007, southern Sumatra, Indonesia, was hit with M 8.4 and M 7.8 quakes, near the same plate boundary where four other quakes greater than M 7.9 had occurred since 1998. At this boundary, the Australian plate moves northeast, subducting beneath the Sundaplata on which Sumatra rides, at a relative speed of 6.6 cm/year (2.6 in./year). This is where the M 9.1 Sumatra-Andaman earthquake struck December 26, 2004, triggering the devastating Indian Ocean tsunami (more on this in Chapter 16); the quake and seismic wave took 228,000 lives, go to http://earthquake.usgs.gov/earthquakes/.What are the mechanisms that produce such dramatic tectonic events? Why do most quakes strike in total surprise, such as in Haiti and Chile in 2010?
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