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About 85 percent of all earthquakes are of a shallow
focus, occurring in the top 70 km (about 45 mi) of the sur- Myths, Mistakes, & Misunderstandings
face. Only about 3 percent are earthquakes of a deep focus.
Most earthquakes are shallow ones because (1) the nature of Do Earthquakes Swallow People?
the rocks near the surface (brittle) differs from that of those It is a myth that the ground opens up, swallows people, then
under great pressure and higher temperatures (plastic) and closes during an earthquake. There might be shallow crevasses
(2) the mechanism that results in earthquakes (plate tecton- that form during an earthquake, but faults do not open up and
ics) has the greatest resistance to movement near the surface close during an earthquake. Movement occurs along a fault, not
perpendicular to it. If faults opened up, no earthquake would
between the plates.
occur because there would be no friction to lock the sides of the
fault together.
MEASURING EARTHQUAKE STRENGTH
The intensities of earthquakes vary widely, from the many
that are barely detectable to the few that cause widespread The size of an earthquake can be measured in terms of
destruction. Destruction is caused by the seismic waves, which vibrations, in terms of displacement, or in terms of the amount
cause the land and buildings to vibrate. Vibrations during small of energy released at the site of the earthquake. The larger the
quakes can crack windows and walls, but vibrations in strong quake, the larger the waves recorded on a seismometer. From
quakes can topple buildings. For example, the January 2010 these recorded waves, scientists assign a number called the
strong Haiti earthquake collapsed more than 300,000 buildings, magnitude. Magnitude is a measure of the energy released dur-
and the February 2010 stronger Chile earthquake collapsed ing an earthquake. Earthquake magnitude is often reported by
more than 500,000 buildings. Injuries and death are usually the media using the Richter scale (Table 19.1). The Richter
the result of falling debris from crumbling buildings or falling scale was developed by Charles Richter, a seismologist at the
bridges. Fire from broken gas pipes was a problem in the 1906 California Institute of Technology in the early 1930s. The scale
and 1989 earthquakes in San Francisco and the 1994 earthquake was based on the widest swing in the back-and-forth line traces
in Los Angeles. Broken water mains made it difficult to fight the of a seismograph recording. The higher the magnitude of an
1906 San Francisco fires, but in 1989 fireboats and fire hoses us- earthquake, as measured by the Richter scale, the greater the
ing water pumped from the bay were able to extinguish the fires. (1) severity of the ground-shaking vibrations and (2) energy
Other effects of earthquakes include landslides and dis- released by the earthquake. An increase of 1 on the Richter
placement of the land surface. Vertical, horizontal, or both scale means that the amount of movement of the ground
vertical and horizontal displacement of the land can occur increased by a factor of 10 and the amount of energy released
during a quake. People sometimes confuse cause and effect increased by a factor of 30. An earthquake mea suring below 3
when they see a land displacement, saying such things as, on the scale is usually not felt by people near the epicenter. The
“Look what the earthquake did!” The fact is that the move- largest earthquake measured so far had a magnitude over 9, but
ment of the land probably produced the seismic waves (the there is actually no upper limit to the scale. Today, professional
earthquake). The seismic waves did not produce the land dis- seismologists rate the size of earthquakes in dif ferent ways,
placement. Such displacements from a single earthquake can depending on what they are comparing and why, but each way
be up to 10 to 15 m (about 30 to 50 ft), but such displacements results in logarithmic scales similar to the Richter scale.
rarely happen.
The effect of an earthquake on people and buildings can
be used to determine the relative intensity of the earthquake.
The modified Mercalli scale expresses such intensities with TABLE 19.1
Roman numerals that range from I to XII. In general, intensi- Effects of earthquakes of various magnitudes
ties of I through VI are concerned with increasing levels of
awareness by people. Intensity level I is “not felt,” and level VI Richter Magnitudes Description
is “felt by all.” Intensity levels V through VII are concerned 0–2 Smallest detectable earthquake
with increasing levels of damage to plaster, dishes, furniture, 2–3 Detected and measured but not generally
and so forth. Level V, for example, describes “some dishes bro- felt
ken.” Levels VIII through X are concerned with increasing lev- 3–4 Felt as small earthquake but no damage
els of damage to structures. Finally, an XI on the Mercalli scale occurs
means that few, if any, buildings remain standing. Level XII 4–5 Minor earthquake with local damage
means total destruction with visible waves moving across the 5–6 Moderate earthquake with structural
ground surface. As you can see, the Mercalli measure of rela- damage
tive earthquake intensity has its advantages since it requires 6–7 Strong earthquake with destruction
no instruments. It could also be misleading since the intensity 7–8 Major earthquake with extensive damage
could vary with the type of construction, the type of material and destruction
(clay or sand, for example) under the buildings, the distance 8–9 Great earthquake with total destruction
from the epicenter, and various combinations of these factors.

19-11 CHAPTER 19 Building Earth’s Surface 487