Page 249 - Encyclopedia Of World History

earthquakes 599

We learn geology the morning after the earthquake. • Ralph Waldo Emerson (1803–1882)

by human activities, such as construction of dams, min- Strategy against
ing, and nuclear explosions. For example, a reservoir fill- Earthquakes
ing in Koyna, India, induced a catastrophic earthquake in The increasing density of population boosts the potential
December 1967 that caused 177 deaths. effects of earthquakes, especially in urban areas with high
As mentioned earlier, the majority of earthquakes are seismic activity—for example, San Francisco, which was
caused by the movement of tectonic plates, as explained struck by a huge earthquake on 18 April 1906. For this
by the continental drift theory of Wegener.Tectonic plates reason, anti-seismic building codes are important.Today,
are large segments of the earth’s lithosphere (the outer, proper planning and regulation of new buildings and
rigid shell of the earth that contains the crust, continents, seismic upgrading of existing buildings can safeguard
and plates). The earth’s surface consists of nine major most types of buildings against earthquake shocks. One
plates: six continental plates (the North American, South obstacle to adhering to anti-seismic building codes is the
American, Eurasian,African, Indo-Australian, and Antarc- high cost; this is true particularly in cities of the Third
tic plates) and three oceanic plates (the Pacific, Nazca, World and of countries at the stage of economic takeoff.
and Cocos plates). Tectonic plates move in relation to The 19 September 1985 Mexico City earthquake, for
each other and along faults over the deeper interior. example, occurred 200 kilometers from Mexico City, but
Faults are fractures or zones of fractures in rock along the shaking of loose sediments in the city was much
which the two sides have been displaced relative to each stronger than at the epicenter. Nearly 10,000 people
other parallel to the fractures, for example, the well- died, and the city was heavily damaged as poorly con-
known San Andreas fault in California, which separates structed buildings collapsed.The earthquake destroyed as
the Pacific plate (on which lie San Francisco and Los many as 100,000 housing units and countless public
Angeles) from the North American plate. buildings.
When lava is upwelling at the midoceanic (mid-Pacific,
mid-Atlantic) ridges, rock moves slowly on either side of Earthquake Catastrophes—
the ridges across the surface of the earth. New plates are Examples
constantly being created, and other plates must also be The following earthquake catastrophes from different
absorbed at the so-called subduction zones (where the regions in different time periods provide opportunities to
edge of one plate descends below the edge of another). examine human-environment interaction and the influ-
Earthquakes are divided into shallow earthquakes (depth ence of each on the other over time and across places:
of focus <65 km), intermediate depth earthquakes (65–
300 km), and deep earthquakes (>300 km).The subduc- 1. The 18 April 1906 San Francisco, California, earth-
tion zones are where most deep-focus earthquakes occur. quake
Earthquakes, volcanoes, mountain building, and sub- 2. The 17 January 1995 Hanshin-Awaji, Kobe, Japan,
duction zones are therefore generally explained as con- earthquake
sequences of steady, large, horizontal surface motions. 3. The 17 August Izmit, and 12 November 1999 Ducze,
Most tectonic plates contain both dry land and ocean Turkey, earthquakes
floor. At present, those plates containing Africa, Antarc- 4. The 26 December 2003 Bam, Iran, earthquake
tica, and North and South America are growing, whereas
the Pacific plate is getting smaller. When plates collide 1906 San Francisco,
(for example, the African and the Eurasian or the Indo- California, Earthquake
Australian and the Eurasian) big mountain chains, such The 18 April 1906 San Francisco earthquake with a mag-
as the Alps and the Himalayas, arise.This phenomenon nitude of 7.8 remains one of the most damaging in the
is accompanied by persistent earthquake activity. history of California.The magnitude of an earthquake is