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Erosion
by increased volcanic activity;
and because this is also the most active volcanic region of the world, this zone is called the Pacific Ring of Fire.
A rupture sets off many kinds
of vibrations, called seismic waves. Some travel through the Earth
and are called body waves, while others travel near the surface
and are called surface waves. Body waves are used to pinpoint the starting point (also called
the hypocenter or focus) of the earthquake, while surface waves are the ones that cause damage
due to ground shaking. (See also: Acceleration; Amplification; Epicenter; Shock wave.)
Earthquakes are measured and located using instruments called seismographs. For this, readings from three stations are needed. The waves are analyzed to see how long it took between the first waves (P waves) and the second waves
(S waves) to arrive. From this data the distance to the earthquake focus can be calculated. Multiplying the time in seconds between the arrival of the two waves by eight gives
the distance in kilometers. Three such measurements locate the focus accurately by a method called triangulation.
Earthquakes and their effects are measured using two scales. The amount of energy in the earthquake is measured on the Richter
scale, while the damage that the earthquake does is measured on the Modified Mercalli scale. (For more on the effects of earthquakes see: Ground failure; Liquefaction; Seiche; Seismic deformation; Tsunami.)
(See also: Aftershock; Mainshock; Seismic gap; Seismic zone; Shadow zone; Shallow earthquake.)
 Earthquake—Ground movement can have very severe effects on buildings. This one has tilted as a result of ground liquefaction.
Ejecta
A general term for anything thrown into the air from a volcano during an eruption. Ejecta is also another word for tephra.
Epicenter
The point on the Earth’s surface directly above the focus (hypocenter) of an earthquake.
 Epicenter—The epicenter is the point on the surface directly above the focus of the earthquake.
Erosion
The twin processes of breaking down a rock (called weathering) and then removing the debris (called transporting). Erosion cuts into the cones of volcanoes. It also takes away soft rocks faster than hard rocks, so that hard volcanic rocks stand out in the landscape (see: Igneous rock). Dikes and sills, as well as volcanic plugs,
are all striking landscape features that developed from the effects of erosion carrying away the soft rocks around them.
Epicenter
Focal depth
Direction of rupture
Fault plane
Earthquake focus or hypocenter
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