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other structures. In addition, when a quake occurs in a populated
area, power and gas lines are often ruptured, causing numerous
fires. The actual mechanism of earthquake generation eluded
geologists until H. F. Reid of Johns Hopkins University conducted
a study following the great 1906 San Francisco earthquake. The
earthquake was accompanied by horizontal surface displacements
of several meters along the northern portion of the San Andreas
Fault. Field studies determined that during this single earthquake,
the Pacific plate lurched as much as 4.7 m northward past the
adjacent North American plate. What Reid concluded from his
investigations is this. Tectonic stresses acting over tens to
hundreds of years slowly deform the crustal rocks on both sides of
a fault. When deformed by differential stress, rocks bend and store
elastic energy, much like a wooden stick does if bent. Eventually,
the frictional resistance holding the rocks in place is overcome.
Slippage allows the deformed (strained) rock to “snap back” to its
original, stress-free shape. The “springing back” was termed
elastic rebound by Reid because the rock behaves elastically,
much like a stretched rubber band does when it is released. The
vibrations we know as an earthquake are generated by the rock
elastically returning to its original shape.

Task 3. Answer the following questions, using the
vocabulary from Task 1.
0. What are the violent shaking and destruction caused by
earthquakes the result of?
0. What is the name of the point at the surface directly above
the focus?
0. What is the name of a massive amount of energy released
during large earthquakes?
0. Can seismic energy be detected and recorded by sensitive
instruments located around the world?
0. Why are large earthquakes triggered near a major population
centre very dangerous?

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