Geology Reference
In-Depth Information
with a rough approximation of the location of the earthquake,
the kind and extent of the damage done, and the effects of
local geology on different types of building construction
(
This means that it would take approximately 30 earthquakes
of magnitude 6 to equal the energy released in one earth-
quake of magnitude 7.
The 1964 Alaska earthquake with a magnitude of 8.6
released nearly 900 times more energy than the 1994 North-
ridge, California, earthquake of magnitude 6.7! And the
Alaska earthquake released more than 27,000 times as much
energy as an earthquake with a magnitude of 5.6 would have.
We mentioned that more than 900,000 earthquakes are re-
corded around the world each year. This fi gure can be placed
in better perspective by reference to Table 8.3, which shows that
the vast majority of earthquakes have a Richter magnitude of
less than 2.5 and that great earthquakes (those with a magnitude
greater than 8.0) occur, on average, only once every fi ve years.
The Richter Magnitude Scale was devised to measure
earthquake waves on a particular seismograph and at a specifi c
distance from an earthquake. One of its limitations is that it
underestimates the energy of very large earthquakes because it
measures the highest peak on a seismogram, which represents
only an instant during an earthquake. For large earthquakes,
though, the energy might be released over several minutes and
along hundreds of kilometers of a fault. For example, during
the 1857 Fort Tejon, California, earthquake, the ground shook
for longer than 2 minutes and energy was released for 360 km
along the fault. Despite their shortcomings, however, Richter
magnitudes still usually appear in news releases.
Seismologists now commonly use a somewhat different
scale to measure magnitude. Known as the s
eismic-moment
magnitude scale
, this scale takes into account the strength
of the rocks, the area of a fault along which rupture occurs,
and the amount of movement of rocks adjacent to the fault.
Because larger earthquakes rupture more rocks than smaller
earthquakes and rupture usually occurs along a longer
segment of a fault and therefore for a longer duration, these
very large earthquakes release more energy. For example, the
December 26, 2004, Sumatra, Indonesia, earthquake that
generated the devastating tsunami created the longest fault
rupture and had the longest duration ever recorded.
Thus, magnitude is now frequently given in terms of
both Richter magnitude and seismic-moment magnitude.
For example, the 1964 Alaska earthquake is given a Richter
magnitude of 8.6 and a seismic-moment magnitude of 9.2.
Because the Richter Magnitude Scale is most commonly used
in the news, we will use that scale here.
Figure 8.11). Because intensity is a measure of the kind of
damage done by an earthquake, insurance companies still
classify earthquakes on the basis of intensity.
Generally, a large earthquake will produce higher inten-
sity values than a small earthquake, but many other factors
besides the amount of energy released by an earthquake also
affect its intensity. These include distance from the epicenter,
focal depth of the earthquake, population density and geol-
ogy of the area, type of building construction employed, and
duration of shaking.
A comparison of the intensity map for the 1906 San
Francisco earthquake and a geologic map of the area shows a
strong correlation between the amount of damage done and
the underlying rock and soil conditions (Figure 8.11). Dam-
age was greatest in those areas underlain by poorly consoli-
dated material or artifi cial fi ll because the effects of shaking
are amplifi ed in these materials, whereas damage was less in
areas of solid bedrock. The correlation between the geology
and the amount of damge done by an earthquake was fur-
ther reinforced by the 1989 Loma Prieta earthquake, when
many of the same areas that were extensively damaged in the
1906 earthquake were once again heavily damaged.
◗
If earthquakes are to be compared quantitatively, we must
use a scale that measures the amount of energy released and
is independent of intensity. Charles F. Richter, a seismolo-
gist at the California Institute of Technology, developed
such a scale in 1935. The
Richter Magnitude Scale
mea-
sures earthquake
magnitude
, which is the total amount
of energy released by an earthquake at its source. It is an
open-ended scale with values beginning at zero. The largest
magnitude recorded was a magnitude-9.5 earthquake in
Chile on May 22, 1960 (Table 8.1).
Scientists determine the magnitude of an earthquake
by measuring the amplitude of the largest seismic wave as
recorded on a seismogram (
Figure 8.12). To avoid large
numbers, Richter used a conventional base-10 logarithmic
scale to convert the amplitude of the largest recorded seismic
wave to a numerical magnitude value. Therefore, each whole-
number increase in magnitude represents a 10-fold increase
in wave amplitude. For example, the amplitude of the largest
seismic wave for an earthquake of magnitude 6 is 10 times
that produced by an earthquake of magnitude 5, 100 times
as large as a magnitude-4 earthquake, and 1000 times that of
an earthquake of magnitude 3 (10 × 10 × 10 = 1000).
A common misconception about the size of earthquakes
is that an increase of one unit on the Richter Magnitude
Scale—a 7 versus a 6, for instance—means a 10-fold increase
in size. It is true that each whole-number increase in mag-
nitude represents a 10-fold increase in the wave amplitude,
but each magnitude increase of one unit corresponds to a
roughly 30-fold increase in the amount of energy released
(actually, it is 31.5, but 30 is close enough for our purposes).
◗
EFFECTS OF EARTHQUAKES?
Certainly, earthquakes are one of nature's most destructive
phenomena. Little or no warning precedes earthquakes, and
once they begin, little or nothing can be done to minimize
their destructive effects, although planning before an earth-
quake can help.
The number of deaths and injuries, as well as the amount
of property damage, depends on several factors. Generally
speaking, earthquakes that occur during working hours and
school hours in densely populated urban areas are the most
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