Table 7.1: (continued)Number Description
IX Damage considerable in specially designed
structures; well-designed frame structures
thrown out of plumb. Damage great in
substantial buildings, with partial collapse.
Buildings shifted off foundations.
X Some well-built wooden structures de-
stroyed; most masonry and frame structures
destroyed with foundations. Rails bent.
XI Few if any structures still standing. Bridges
destroyed. Rails bent greatly.
XII Damage total. Lines of sight and level dis-
torted. Objects thrown into air.ThereweremanyproblemswiththeMercalliscale. Whatpeoplefeelandseeinanearthquake
is affected by how far they are from the earthquake’s focus, the type of rock that lies beneath
them, the construction type of the nearby buildings, and many other factors. Different
observers will also perceive the experience differently. For example, one might exaggerate
while the other downplays the damage done. With the Mercalli scale, comparisons between
earthquakes are difficult to make.
To address these problems, in 1935 Charles Richter developed hisRichter magnitude
scale. The Richter scale measures the magnitude of the largest jolt of energy released in
an earthquake. Because Richter’s scale is logarithmic, the amplitude of the largest wave
increases 10 times from one integer to the next. For example, the amplitude of the largest
seismic wave of a magnitude 5 quake is 10 times that of a magnitude 4 quake and 100 times
that of a magnitude 3 quake. One integer increase in magnitude roughly correlates with a
30-fold increase in the amount of energy released. A difference of two integers on the Richter
scale equals a 1,000-fold increase in released energy.
Seismologists recognize that the Richter scale has limitations, since it measures the height
of the greatest earthquake wave. A single sharp jolt will measure higher on the Richter scale
than a very long intense earthquake that releases more energy. In other words, earthquakes
that release more energy are likely to do more damage than those that are short, but have a
larger single jolt. Using the Richter scale, a high magnitude may not necessarily reflect the
amount of damage caused.
Themomentmagnitudescaleisthecurrentmethodofmeasuringearthquakemagnitudes.
This method measures the total energy released by an earthquake and so more accurately
reflects its magnitude. Moment magnitude is calculated from the area of the fault that
is ruptured and the distance the earth moved along the fault. Like the Richter scale, the