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A: Earthquakes can strike any
location at any time. But history shows they occur in the
same general patterns year after year, principally in three
large zones of the earth. The world's greatest earthquake
belt, the circum-Pacific seismic belt, is found along
the rim of the Pacific Ocean, where about 81 percent of
the world's largest earthquakes occur. The belt extends
from Chile, northward along the South American coast through
Central America, Mexico, the West Coast of the United States,
and the southern part of Alaska, through the Aleutian Islands
to Japan, the Philippine Islands, New Guinea, the island
groups of the Southwest Pacific, and to New Zealand. This
earthquake belt was responsible for 70,000 deaths in Peru
in May 1970, and 65 deaths and a billion dollars' damage
in California in February 1971.  Why
do so many earthquakes originate in this belt? This is a
region of young, growing mountains and deep ocean trenches
which invariably parallel mountain chains. Earthquakes necessarily
accompany elevation changes in mountains, the higher part
of the earth's crust, and changes in the ocean trenches,
the lower part. The second important belt, the Alpide,
extends from Java to Sumatra through the Himalayas, the
Mediterranean, and out into the Atlantic. This belt accounts
for about 17 percent of the world's largest earthquakes,
including some of the most destructive, such as the Iran
shock that took 11,000 lives in August 1968, and the Turkey
tremors in March 1970 and May 1971 that each killed over
1,000. All were near magnitude 7 on the Richter scale. The
third prominent belt follows the submerged mid-Atlantic
Ridge.
The remaining shocks are scattered
in various areas of the world. Earthquakes in these prominent
seismic zones are taken for granted, but damaging shocks
occur occasionally outside these areas. Examples in the
United States are New Madrid, Missouri, and Charleston,
South Carolina. Many years, however, usually elapse between
such destructive shocks.
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