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Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, location, and magnitude of an earthquake can be determined from the data recorded by seismograph stations. The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.
The
effect of an earthquake on the Earth's surface is called the intensity.
The intensity scale consists of a series of certain key responses such
as people awakening, movement of furniture, damage to chimneys, and
finally--total destruction. Although numerous intensity scales have
been developed over the last several hundred years to evaluate the
effects of earthquakes, the one currently used in the United States is
the Modified Mercalli (MM) Intensity Scale. It was developed in 1931 by
the American seismologists Harry Wood and Frank Neumann. This scale,
composed of 12 increasing levels of intensity that range from
imperceptible shaking to catastrophic destruction, is designated by
Roman numerals. It does not have a mathematical basis; instead it is an
arbitrary ranking based on observed effects.