Geology Reference
In-Depth Information
be surprising that a comparatively small earthquake can have
disastrous effects, whereas a much larger one might go largely
unnoticed, except perhaps by seismologists.
The destructive effects of earthquakes include ground
shaking, fire, seismic sea waves, and landslides, as well as
panic, disruption of vital services, and psychological shock.
In some cases, rescue attempts are hampered by inadequate
resources or planning, conditions of civil unrest, or simply
the magnitude of the disaster.
P
S
Amplitude
= 23 mm
0
P - S = 24 seconds
10
20
500
400
50
40
100
50
20
10
5
2
1
0.5
Ground shaking, the most obvious and immediate effect of an
earthquake, varies depending on the earthquake's magnitude,
distance from the epicenter, and type of underlying materials in
the area—unconsolidated sediment or fi ll versus bedrock, for
instance. Certainly, ground shaking is terrifying, and it may be
violent enough for fi ssures to open in the ground. Nevertheless,
contrary to popular myth, fi ssures do not swallow up people
and buildings and then close on them. And although California
will no doubt have big earthquakes in the future, rocks cannot
store enough energy to displace a landmass as large as Califor-
nia into the Pacifi c Ocean, as some alarmists claim.
The effects of ground shaking, such as collapsing build-
ings, falling building facades and window glass, and toppling
monuments and statues, cause more damage and result in
more loss of life and injuries than any other earthquake haz-
ard. Structures built on solid bedrock generally suffer less
damage than those built on poorly consolidated material
such as water-saturated sediments or artifi cial fi ll.
Structures built on poorly consolidated or water-saturated
material are subjected to ground shaking of longer duration
and greater S-wave amplitude than structures built on bedrock
(
6
300
30
5
200
20
4
100
60
40
10
8
6
4
3
2
0.2
0.1
Amplitude
(mm)
1
20
5
2
0
Magnitude
0
Distance
(km)
P - S
(seconds)
◗
Figure 8.12
Richter Magnitude Scale The Richter Magnitude Scale
measures the total amount of energy released by an earthquake at
its source. The magnitude is determined by measuring the maximum
amplitude of the largest seismic wave and marking it on the right-hand
scale. The difference between the arrival times of the P- and S-waves
(recorded in seconds) is marked on the left-hand scale. When a line is
drawn between the two points, the magnitude of the earthquake is the
point at which the line crosses the center scale.
Figure 8.13). In addition, fill and water-saturated sedi-
ments tend to liquefy, or behave as a fl uid, a process known
as
liquefaction
. When shaken, the individual grains lose cohe-
sion and the ground fl ows. Two dramatic examples of damage
◗
Average Number of Earthquakes of Various
Magnitudes per Year Worldwide
T ABLE 8.3
Average Number
per Year
Magnitude
Effects
<2.5
Typically not felt but recorded
900,000
2.5-6.0
Usually felt; minor to
moderate damage to
structures
31,000
Water
6.1-6.9
Potentially destructive,
especially in populated
areas
100
7.0-7.9
Major earthquakes; serious
damage results
20
Seismic
wave
>8.0
Great earthquakes; usually
result in total destruction
1 every 5 years
Source:
Modifi ed from
Earthquake Information Bulletin,
and B. Gutenberg
and C. F. Richter,
Seismicity of the Earth and Associated Phenomena
(Princeton, NJ: Princeton University Press, 1949).
Bedrock
Well-
consolidated
sediments
Poorly
consolidated
sediments
Bay mud
(water-
saturated)
◗
Figure 8.13
Relationship between Seismic Wave Amplitude
and Underlying Geology The amplitude and duration of seismic
waves generally increase as the waves pass from bedrock to poorly
consolidated or water-saturated material. Thus, structures built
on weaker material typically suffer greater damage than similar
structures built on bedrock because the shaking lasts longer.
destructive and cause the most fatalities and injuries. However,
magnitude, duration of shaking, distance from the epicenter,
geology of the affected region, and type of structures are also
important considerations. Given these variables, it should not
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