Geography Reference
In-Depth Information
When a major earthquake occurs on land, devastation is limited to a few tens or perhaps a hundred
miles of the point where the quake occurred. The tsunami, however, may travel for hundreds or even
thousands of miles with little loss of energy. By means of tsunamis, therefore, earthquakes can wreak
havoc far away in places where the earthquake itself is never felt.
Fortunately, global earthquake monitoring now makes it possible to warn tsunami-prone cities of an
approaching tidal wave, hopefully in time to evacuate people from low-lying areas. This know-how
has been unevenly applied, however. Generally, affluent countries have been able to put in place good
civil defense systems while poor countries have not. As is the case with earthquakes, therefore, the
geography of wealth and poverty has much to do with the resulting human toll.
A matter of magnitude
The amount of destruction that results from an earthquake depends on a couple of factors, one of
which is its strength. Earthquakes vary remarkably in their power. Some can barely be felt. Others
can knock you off your feet or buildings off their foundations. Two methods are used to measure the
power of earthquakes. Because both make use of a series of numbers, they are referred to as
scales
.
The Richter Scale
Probably the more famous of the two scales is the
Richter Scale
, which was formulated in
1935 by a
seismologist
(one who studies earthquakes) named Charles F. Richter. This scale
indicates ground motion in an earthquake. It ranges from 0 to 9, but theoretically can go high-
er. The numbers are
logarithmic
. That means each whole number is 10 times greater than the
preceding whole number. Thus, an earthquake that measures 7.0 on the Richter Scale releases
10 times more energy than one that measures 6.0, and 100 times as much as one that measures
5.0. Of course, the ground motion of earthquakes is not limited to multiples of the number 10.
Thus when earthquakes are reported in Richter terms, you may see numbers such as 6.3 or 4.8.
The Mercalli Scale
Less famous is the
Mercalli Scale
. This was devised in 1902 by a gentleman whose first name was
Giuseppe and whose last name you can guess. This measures the intensity or violence of an earth-
quake, particularly in terms of damage caused to human-built structures. Expressed by a series of
Roman numerals, I to XII, the higher numbers reflect increasing damage. Table 6-1 tells you what the
