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A Closer Look

Earthquake Safety
DURING THE SHAKING it. The car's springs will absorb some shorting, close the switch at the main
1. Don't panic. of the shaking, and it will offer you meter box.
2. If you are indoors, stay there. Seek protection. 2. Turn on radio or television (if
protection under a table or desk or in possible) for emergency bulletins.
a doorway. Stay away from glass. Don't AFTER THE SHAKING 3. Stay off the telephone except to report
use matches, candles, or any open 1. Check but do not turn on utilities. If an emergency.
flame; douse all fires. 4. Stay out of severely damaged
you smell gas, open windows, shut
3. If you are outside, move away from off the main valve, and leave the buildings that could collapse in
buildings and power lines and stay in building. Report the leak to the utility aftershocks.
the open. Don't run through or near company, and don't reenter the build- 5. Don't go sightseeing; you will only
buildings. ing until it has been checked out. If hamper the efforts of emergency
4. If you are in a moving car, bring it to a water mains are damaged, shut off personnel and repair crews.
stop as quickly as possible,* but stay in the main valve. If electrical wiring is

*Not near buildings, an overpass, or any other structure.
Source: Data from NASA.

at speeds of 90 km/h (55 mi/h), and produce a wave height of 0.6
EXAMPLE 19.3 to 3 m (2 to 10 ft) when they break on the shore. A tsunami, on
Compare the energy released by an earthquake of 7.2 magnitude to an the other hand, is produced by some strong disturbance in the
earthquake with a magnitude of 5.2. seafloor, travels at speeds of 725 km/h (450 mi/h), and produces a
wave height of over 8 m (about 25 ft) when it breaks on the shore.
SOLUTION A tsunami may have a very long wavelength of 200 km (120 mi)
compared to the wavelength of ordinary wind- generated waves of
Determine the number of magnitude differences, and multiply the en-
400 m (1,300 ft). Because of its great wavelength, a tsunami does
ergy change factor by itself for each change in magnitude.
not just break on the shore, then withdraw. Depending on the
richter magnitude 1 = 7.2
seafloor topography, the water from a tsunami may continue to
richter magnitude 2 = 5.2 rise for 5 to 10 minutes, flooding the coastal region before the
energy difference = ? wave withdraws. A gently sloping seafloor and a funnel-shaped
magnitude bay can force tsunamis to great heights as they break on the shore.
difference = richter magnitude 2 − richter magnitude 1 The current record height was established in 1971 when a tsu-
= 5.2 − 7.2 nami broke at 85 m (278 ft) on an island south of Japan. Waves
= −2 magnitudes of such great height and long wavelength can be very destructive,
energy difference = (30)(30) sweeping away trees, lighthouses, and buildings up to 30 m (100 ft)
above sea level. The size of a particular tsunami depends on the
= 900
nature of how the seafloor was disturbed. Generally, a “big” earth-
The negative magnitude difference means the first earthquake released
quake that causes the seafloor suddenly to rise or fall favors the
approximately 900 times more energy than the second earthquake.
generation of a large tsunami.
On December 26, 2004, an earthquake measuring 9.0 on the
EXAMPLE 19.4 Richter scale occurred west of the Indonesian island of Sumatra.
Compare the amount of movement of the ground (surface wave ampli- This was the largest quake worldwide in four decades. The
tude) associated with the two earthquakes in Example 19.2. (Answer: focus was about 10 km (about 6 mi) beneath the ocean floor at
The second earthquake had 100 times less ground motion.) the interface of the India and Burma Plates and was caused by
the release of stresses that develop as the India Plate subducts
beneath the overriding Burma Plate. A 100 km (62 mi) wide rup-
Tsunami is a Japanese term now used to describe the very ture occurred in the ocean floor, about 1,200 km (746 mi) long
large ocean waves that can be generated by an earthquake, land- and parallel to the Sunda trench. The ocean floor was suddenly
slide, or volcanic explosion. Such large waves were formerly uplifted more than 2 m (about 7 ft), with a movement about
called “tidal waves.” Since the large, fast waves were not associ- 10 m (about 33 ft) to the west-southwest. This displacement acted
ated with tides or tidal forces in any way, the term tsunami or as a huge paddle at the bottom of the ocean, vertically displac-
seismic sea wave is preferred. ing billions of tons of water and triggering a tsunami. The tsunami
A tsunami, like other ocean waves, is produced by a distur- created a path of destruction across the 4,500 km (about 2,800 mi)
bance. Most common ocean waves are produced by winds, travel wide Indian Ocean over the next 7 hours. Series of very large

488 CHAPTER 19 Building Earth’s Surface 19-12