1.2.1 Seismic hazards and risks
One of the primary motivations for studying earthquakes and
seismology is the destruction caused by large earthquakes. In
many parts of the world, seismic risks are significant, whether
they are popularly recognized (as in Japan, where schools conduct earthquake drills) or not. Much of the challenge in assessing and addressing seismic hazards is that in any given area
large earthquakes are relatively rare on human time scales, but
can cause great destruction when they occur.
Earthquakes primarily occur at the boundaries where the
100 km-thick tectonic plates converge, diverge, or slide past
each other. Although the plates move steadily, their boundaries
are often “locked,” and do not move most of the time. However, on time scales of a few hundred years, the boundary slips
suddenly, and the accumulated motion is released in an earthquake. Figure 1.2-1 shows the locations of m b ≥ 4 earthquakes
between 1963 and 1995. The earthquakes nicely define the
plate boundaries, although some earthquakes also occur in
intraplate regions, away from plate boundaries.
The energy released by large earthquakes is striking (Fig. 1.22). For example, the 1906 San Francisco earthquake involved
about 4 m of slip on a 450 km-long fault, releasing about
3 × 10 16 Joules 2 of elastic energy. This energy is equivalent to
a 7 megaton nuclear explosion, much larger than the 0.012
megaton bomb dropped on Hiroshima. The largest recorded
earthquake, the 1960 Chilean event in which about 21 m of
slip occurred on a fault 800 km long and 200 km across,
released about 10 19 J of elastic energy, more than a 2000 Mt
bomb. This earthquake released more energy than all the
nuclear bombs ever exploded, the largest of which was 58 Mt.
For comparison, the total global human annual energy consumption is about 3 × 10
20 J.
Fortunately, the largest earthquakes are infrequent, because
the energy released accumulates slowly over a long time. The
San Francisco earthquake occurred on the San Andreas fault
in northern California, part of the boundary along which the
Pacific plate moves northward relative to the North American
plate. Studies using the Global Positioning System satellites
show that away from the plate boundary the two plates move
by each other at a speed of about 45 mm/yr. Most parts of
the San Andreas fault are “locked” most of the time, but slip
several meters in a large earthquake every few hundred years.
A simple calculation suggests that such earthquakes should occur on average about every 4000 mm/(45 mm/yr) or 90 years.
The real interval is not uniform, for reasons that are unclear,
and is longer, because some of the motion occurs on other
faults.
Because plate boundaries extend for more than 150,000 km,
and some earthquakes occur in plate interiors, earthquakes
occur frequently somewhere on earth. As shown in Table 1.2-1,
Table 1.2-1 Numbers of earthquakes per year.
Earthquake
Number
Energy released
magnitude (M s )
per year
(10
15 J/yr)
≥8.0
0–1
0–1,000
7–7.9
12
100
6–6.9
110
30
5–5.9
1,400
5
4–4.9
13,500
1
3–3.9
>100,000
0.2
Based upon data from the US Geological Survey National Earthquake
Information Center. Energy estimates are based upon an empirical
formula of Gutenberg and Richter (Gutenberg, 1959), and the magnitude
scaling relations of Geller (1976), and are very approximate.
3 As part of his incorrect prediction of a magnitude 7 earthquake in the Midwest in
1990, I. Browning claimed that he had successfully predicted the 1989 Loma Prieta
earthquake. In fact, he had said that near the date in question there would be an earthquake somewhere in the world with magnitude 6, a prediction virtually guaranteed to
be true.
2 The SI unit of energy is 1 Joule (J) = 1 Newton meter (N-m) = 10 7 ergs = 10 7 dyncm. Nuclear explosions are often described in megatons (Mt), equivalent to
1,000,000 tons of TNT or 4.2 × 10 15 J.
an earthquake of magnitude 7 occurs approximately monthly,
and an earthquake of magnitude 6 or greater occurs on average
every three days. 3 Earthquakes of a given magnitude occur
about ten times less frequently than those one magnitude
smaller. Because the magnitude is proportional to the logarithm
of the energy released, most of the energy released seismically is
in the largest earthquakes. A magnitude 8.5 event releases more
energy than all the other earthquakes in a given year combined.
Hence the hazard from earthquakes is due primarily to large
(typically magnitude greater than 6.5) earthquakes.
In assessing the potential danger posed by earthquakes or
other natural disasters, it is useful to distinguish between hazards and risks. The hazard is the intrinsic natural occurrence of
earthquakes and the resulting ground motion and other effects.
The risk is the danger the hazard poses to life and property.
Hence, although the hazard is an unavoidable geological fact,
the risk is affected by human actions. Areas of high hazard can
have low risk because few people live there, and areas of
modest hazard can have high risk due to large populations and
poor construction. Earthquake risks can be reduced by human
actions, whereas hazards cannot (hence the US government’s
National Earthquake Hazards Reduction Program is, strictly
speaking, misnamed).
These ideas are illustrated by Table 1.2-2, which lists some
significant earthquakes and their societal consequences. As
shown, some very large earthquakes caused no fatalities
because of their remote location or deep focal depth. In general,
the most destructive earthquakes occur where large populations live near plate boundaries. The highest property losses
occur in developed nations where more property is at risk,
whereas fatalities are highest in developing nations. Although
the statistics are often imprecise, the impact of major earthquakes can be enormous. Estimates are that the 1990 Northern
Iran shock killed 40,000 people, and that the 1988 Spitak
1.2 Seismology and society 11
One of the primary motivations for studying earthquakes and
seismology is the destruction caused by large earthquakes. In
many parts of the world, seismic risks are significant, whether
they are popularly recognized (as in Japan, where schools conduct earthquake drills) or not. Much of the challenge in assessing and addressing seismic hazards is that in any given area
large earthquakes are relatively rare on human time scales, but
can cause great destruction when they occur.
Earthquakes primarily occur at the boundaries where the
100 km-thick tectonic plates converge, diverge, or slide past
each other. Although the plates move steadily, their boundaries
are often “locked,” and do not move most of the time. However, on time scales of a few hundred years, the boundary slips
suddenly, and the accumulated motion is released in an earthquake. Figure 1.2-1 shows the locations of m b ≥ 4 earthquakes
between 1963 and 1995. The earthquakes nicely define the
plate boundaries, although some earthquakes also occur in
intraplate regions, away from plate boundaries.
The energy released by large earthquakes is striking (Fig. 1.22). For example, the 1906 San Francisco earthquake involved
about 4 m of slip on a 450 km-long fault, releasing about
3 × 10 16 Joules 2 of elastic energy. This energy is equivalent to
a 7 megaton nuclear explosion, much larger than the 0.012
megaton bomb dropped on Hiroshima. The largest recorded
earthquake, the 1960 Chilean event in which about 21 m of
slip occurred on a fault 800 km long and 200 km across,
released about 10 19 J of elastic energy, more than a 2000 Mt
bomb. This earthquake released more energy than all the
nuclear bombs ever exploded, the largest of which was 58 Mt.
For comparison, the total global human annual energy consumption is about 3 × 10
20 J.
Fortunately, the largest earthquakes are infrequent, because
the energy released accumulates slowly over a long time. The
San Francisco earthquake occurred on the San Andreas fault
in northern California, part of the boundary along which the
Pacific plate moves northward relative to the North American
plate. Studies using the Global Positioning System satellites
show that away from the plate boundary the two plates move
by each other at a speed of about 45 mm/yr. Most parts of
the San Andreas fault are “locked” most of the time, but slip
several meters in a large earthquake every few hundred years.
A simple calculation suggests that such earthquakes should occur on average about every 4000 mm/(45 mm/yr) or 90 years.
The real interval is not uniform, for reasons that are unclear,
and is longer, because some of the motion occurs on other
faults.
Because plate boundaries extend for more than 150,000 km,
and some earthquakes occur in plate interiors, earthquakes
occur frequently somewhere on earth. As shown in Table 1.2-1,
Table 1.2-1 Numbers of earthquakes per year.
Earthquake
Number
Energy released
magnitude (M s )
per year
(10
15 J/yr)
≥8.0
0–1
0–1,000
7–7.9
12
100
6–6.9
110
30
5–5.9
1,400
5
4–4.9
13,500
1
3–3.9
>100,000
0.2
Based upon data from the US Geological Survey National Earthquake
Information Center. Energy estimates are based upon an empirical
formula of Gutenberg and Richter (Gutenberg, 1959), and the magnitude
scaling relations of Geller (1976), and are very approximate.
3 As part of his incorrect prediction of a magnitude 7 earthquake in the Midwest in
1990, I. Browning claimed that he had successfully predicted the 1989 Loma Prieta
earthquake. In fact, he had said that near the date in question there would be an earthquake somewhere in the world with magnitude 6, a prediction virtually guaranteed to
be true.
2 The SI unit of energy is 1 Joule (J) = 1 Newton meter (N-m) = 10 7 ergs = 10 7 dyncm. Nuclear explosions are often described in megatons (Mt), equivalent to
1,000,000 tons of TNT or 4.2 × 10 15 J.
an earthquake of magnitude 7 occurs approximately monthly,
and an earthquake of magnitude 6 or greater occurs on average
every three days. 3 Earthquakes of a given magnitude occur
about ten times less frequently than those one magnitude
smaller. Because the magnitude is proportional to the logarithm
of the energy released, most of the energy released seismically is
in the largest earthquakes. A magnitude 8.5 event releases more
energy than all the other earthquakes in a given year combined.
Hence the hazard from earthquakes is due primarily to large
(typically magnitude greater than 6.5) earthquakes.
In assessing the potential danger posed by earthquakes or
other natural disasters, it is useful to distinguish between hazards and risks. The hazard is the intrinsic natural occurrence of
earthquakes and the resulting ground motion and other effects.
The risk is the danger the hazard poses to life and property.
Hence, although the hazard is an unavoidable geological fact,
the risk is affected by human actions. Areas of high hazard can
have low risk because few people live there, and areas of
modest hazard can have high risk due to large populations and
poor construction. Earthquake risks can be reduced by human
actions, whereas hazards cannot (hence the US government’s
National Earthquake Hazards Reduction Program is, strictly
speaking, misnamed).
These ideas are illustrated by Table 1.2-2, which lists some
significant earthquakes and their societal consequences. As
shown, some very large earthquakes caused no fatalities
because of their remote location or deep focal depth. In general,
the most destructive earthquakes occur where large populations live near plate boundaries. The highest property losses
occur in developed nations where more property is at risk,
whereas fatalities are highest in developing nations. Although
the statistics are often imprecise, the impact of major earthquakes can be enormous. Estimates are that the 1990 Northern
Iran shock killed 40,000 people, and that the 1988 Spitak
1.2 Seismology and society 11