Geology Reference
In-Depth Information
Tangshan, China, completely leveled the city because hardly
any structures were built to resist seismic forces. In fact, most
had unreinforced brick walls, which have no fl exibility, and
consequently they collapsed during the shaking.
The magnitude-6.4 earthquake that struck India in 1993
killed approximately 30,000 people, whereas the magnitude-
6.7 Northridge, California, earthquake one year later resulted
in only 61 deaths. What is the reason for such a difference in
the death toll? Both earthquakes occurred in densely popu-
lated regions, but in India, the brick and stone buildings
could not withstand ground shaking; most collapsed and en-
tombed their occupants.
generated when the sea floor undergoes sudden, vertical
movements. Many result from submarine earthquakes, but
volcanoes at sea or submarine landslides can also cause them.
For example, the 1883 eruption of Krakatau between Java
and Sumatra generated a large sea wave that killed 36,000 on
nearby islands.
Once a tsunami is generated, it can travel across an en-
tire ocean and cause devastation far from its source. In the
open sea, tsunami travel at several hundred kilometers per
hour and commonly go unnoticed as they pass beneath ships
because they are usually less than 1 m high and the distance
between wave crests is typically hundreds of kilometers.
When they enter shallow water, however, the wave slows
down and water piles up to heights anywhere from a meter
or two to many meters high. The 1946 tsunami that struck
Hilo, Hawaii, was 16.5 m high! In any case, the tremendous
energy possessed by a tsunami is concentrated on a shoreline
when it hits either as a large breaking wave or, in some cases,
as what appears to be a very rapidly rising tide.
A common popular belief is that a tsunami is a sin-
gle large wave that crashes onto a shoreline. Any tsu-
nami consists of a series of waves that pour onshore for
as long as 30 minutes followed by an equal time during
which water rushes back to sea. Furthermore, after the
fi rst wave hits, more waves follow at 20- to 60-minute in-
tervals. Approximately 80 minutes after the 1755 Lisbon,
Portugal, earthquake, the fi rst of three tsunami, the larg-
est more than 12 m high, destroyed the waterfront area
and killed thousands of people. Following the arrival of
a 2-m-high tsunami in Crescent City, California, in 1964,
curious people went to the waterfront to inspect the dam-
age. Unfortunately, 10 were killed by a following 4-m-high
wave!
One of nature's warning signs of an approaching tsu-
nami is a sudden withdrawal of the sea from a coastal region.
In fact, the sea might withdraw so far that it cannot be seen
and the seafl oor is laid bare over a huge area. On more than
one occasion, people have rushed out to inspect exposed
reefs or to collect fi sh and shells only to be swept away when
the tsunami arrived.
During the December 2004 tsunami, however, a 10-year-
old British girl saved numerous lives because she recognized
the warning signs that she had learned in a school lesson on
tsunami only two weeks before. While vacationing with her
mother on the island of Phuket, Thailand, a popular resort
area, the girl noticed the water quickly receding from the
beach. She immediately told her mother that she thought a
tsunami was coming, and her mother, along with the resort
staff, quickly warned everyone standing around watching
the water recede to clear the beach area. Their quick action
resulted in many lives being saved.
Following the tragic 1946 tsunami that hit Hilo, Hawaii,
the U.S. Coast and Geodetic Survey established a Pacific
Tsunami Early Warning System in Ewa Beach, Hawaii. This
system combines seismographs and instruments that detect
earthquake-generated sea waves. Whenever a strong earth-
quake takes place anywhere within the Pacifi c basin, its location
Fire
In many earthquakes, particularly in urban areas, fire is a
major hazard. Nearly 90% of the damage done in the 1906
San Francisco earthquake was caused by fi re. The shaking
severed many of the electrical and gas lines, which touched
off fl ames and started fi res throughout the city. Because the
earthquake ruptured water mains, there was no effective
way to fi ght the fi res that raged out of control for three days,
destroying much of the city.
Eighty-three years later, during the 1989 Loma Prieta
earthquake, a fire broke out in the Marina district of San
Francisco. This time, however, the fi re was contained within
a small area because San Francisco had a system of valves
throughout its water and gas pipeline system so that lines
could be isolated from breaks (see Geo-inSight on pages 204
and 205).
During the September 1, 1923, earthquake in Japan,
fi res destroyed 71% of the houses in Tokyo and practically all
of the houses in Yokohama. In all, 576,262 houses were de-
stroyed by fi re, and 143,000 people died, many as a result of
fi re. A horrible example occurred in Tokyo where thousands
of people gathered along the banks of the Sumida River to
escape the raging fi res. Suddenly, a fi restorm swept over the
area, killing more than 38,000 people. The fires from this
earthquake were particularly devastating because most of
the buildings were constructed of wood, and were fanned by
20-km/hr winds.
Tsunami: Killer Waves
On December 26, 2004, a magnitude-9.0 earthquake
struck 160 km off the west coast of northern Sumatra,
Indonesia, generating the deadliest tsunami in history
(
Figure 8.16a). Within hours, walls of water as high as
10.5 m pounded the coasts of Indonesia, Sri Lanka, India,
Thailand, Somalia, Myanmar, Malaysia, and the Maldives,
killing more than 220,000 people and causing billions of
dollars in damage.
This earthquake generated what is popularly called a
“tidal wave,” but is more correctly termed a seismic sea wave
or tsunami , a Japanese term meaning “harbor wave.” The
term tidal wave nevertheless persists in popular literature and
some news accounts, but these waves are not caused by or
related to tides. Indeed, tsunami are destructive sea waves
 
 
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