DEFINITION OF ORIGINS OF EARTHQUAKES

Earthquakes can originate from sudden motion along a fault,

from a volcanic eruption, or from bomb blasts. Not every

fault is associated with active earthquakes; in fact, most

faults are no longer active but were active at some time in the

geologic past. Of the faults that are active, only some are

characterized as being particularly prone to earthquakes.

Some faults are slippery, and the two blocks on either side

just slide by each other passively without producing major

earthquakes. In other cases, however, the blocks stick to each

other and deform like a rubber band until they reach a certain

point where they suddenly snap, releasing energy in an

earthquake event.

Rocks and materials are said to behave in a brittle way

when they respond to built-up tectonic pressures by cracking,

breaking, or fracturing. Earthquakes represent a sudden brittle

response to built-up stress and are almost universally activated

in the upper few kilometers of the Earth. Deeper than

this, the pressure and temperature are so high that the rocks

simply deform like silly putty, do not snap, and are said to

behave in a ductile manner.

An earthquake originates in one place and then spreads

out. The focus is the point in the Earth where the earthquake

energy is first released. The epicenter is the point on the

Earth’s surface that lies vertically above the focus.

When big earthquakes occur, the surface of the Earth

actually forms into waves that move across the surface, just

as in the ocean. These waves can be pretty spectacular and

also extremely destructive. When an earthquake occurs, these

seismic waves move out in all directions, just like sound

waves, or ripples that move across water after a stone is

thrown in a still pond. After the seismic waves have passed

through the ground, the ground returns to its original shape,

although buildings and other human constructions are commonly

destroyed.

During an earthquake, these waves can either radiate

underground from the focus—called body waves—or aboveground

from the epicenter—called surface waves. The body

waves travel through the whole body of the Earth and move

faster than surface waves, though surface waves cause most

of the destruction associated with earthquakes because they

briefly change the shape of the surface of the Earth when they

pass. There are two types of body waves: P, or compressional

waves, and S, or secondary waves. P-waves deform material

through a change in volume and density, and these can pass

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