زمین لرزه-Earthquake
Earthquake
The sudden movement of the Earth caused by
the abrupt release of accumulated strain along a fault in the interior. The
released energy passes through the Earth as seismic waves (low-frequency sound
waves), which cause the shaking. Seismic waves continue to travel through the
Earth after the fault motion has stopped. Recordings of earthquakes, called
seismograms, illustrate that such motion is recorded all over the Earth for
hours, and even days, after an earthquake.
Characteristics
Earthquakes vary immensely in size, from
tiny events that can be detected only with the most sensitive seismographs, to
great earthquakes that can cause extensive damage over widespread areas.
Although thousands of earthquakes occur every day, and have for billions of
years, a truly great earthquake occurs somewhere in the world only once every
year. When a great earthquake occurs near a highly populated region, tremendous
destruction can occur within a few seconds. In 1976, 600,000 people were killed
in
Cause
The locations of earthquakes which occurred
between 1957 and 2000 are shown on the map in Fig. 1. The map shows that
earthquakes are not distributed randomly over the globe but tend to occur in
narrow, continuous belts of activity. Approximately 90% of all earthquakes occur
in these belts, which define the boundaries of the Earth's plates. The plates
are in continuous motion with respect to one another at rates on the order of
centimeters per year; this plate motion is responsible for most geological
activity.
Fig. 1 Seismicity of the Earth from 1957 to
2000; depths to 700 km (435 mi). Earthquakes are plotted as circles; the plate
boundaries are black. (After E. R. Engdahl et al., Global teleseismic earthquake
relocation with improved travel times and procedures for depth determination,
Bull. Seis. Soc. Amer., 88:722–743, 1998; and A. Villaseñor et al., Toward a
comprehensive catalog of global historical seismicity, Eos, 78:581, 583, 588,
American Geophysical Union, 1997)
Plate motion occurs because the outer cold,
hard skin of the Earth, the lithosphere, overlies a hotter, soft layer known as
the asthenosphere. Heat from decay of radioactive minerals in the Earth's
interior sets the asthenosphere into thermal convection. This convection has
broken the lithosphere into plates which move about in response to the
convective motion in a manner shown schematically in Fig. 2. The plates move
apart at oceanic ridges. Magma wells up in the void created by this motion and
solidifies to form new sea floor. This process, in which new sea floor is
continually created at oceanic ridges, is called sea-floor spreading. Since new
lithosphere is continually being created at the oceanic ridges by sea-floor
spreading, a like amount of lithosphere must be destroyed somewhere. This occurs
at the oceanic trenches, where plates converge and the oceanic lithosphere is
thrust back down into the asthenosphere and remelted. The melting of the
lithosphere in this way supplies the magma for the volcanic arcs which occur
behind the trenches. Where two continents collide, however, the greater bouyancy
of the less dense continental material prevents the lithosphere from being
underthrust, and the lithosphere buckles under the force of the collision,
forming great mountain ranges such as the Alps and
Fig. 2 Movement of the lithosphere over the
more fluid asthenosphere. In the center, the lithosphere spreads away from the
oceanic ridges. At the edges of the diagram, it descends again into the
asthenosphere at the trenches. (After B. Isacks, Oliver, and L. R. Sykes,
Seismology and the new global tectonics, J. Geophys. Res., 73:5855–5899,
American Geophysical Union, 1968)
According to the theory of plate tectonics,
the motion of the plates is very similar to the movement of ice floes in arctic
waters. Where floes diverge, leads form and water wells up, freezing to the
floes and producing new floe ice. The formation of pressure ridges where floes
converge is analogous to the development of mountain ranges where plates
converge. See also: Orogeny; Plate
tectonics
Stick-slip friction and elastic
rebound
As the plates move past each other, little
of the motion at their boundaries occurs by continuous slippage; most of the
motion occurs in a series of rapid jerks. Each jerk is an earthquake. This
happens because, under the pressure and temperature conditions of the shallow
part of the Earth's lithosphere, the frictional sliding of rock exhibits a
property known as stick-slip, in which frictional sliding occurs in a series of
jerky movements, interspersed with periods of no motion—or sticking. In the
geologic time frame, then, the lithospheric plates chatter at their boundaries,
and at any one place the time between chatters may be hundreds of years.
The periods between major earthquakes is
thus one during which strain slowly builds up near the plate boundary in
response to the continuous movement of the plates. The strain is ultimately
released by an earthquake when the frictional strength of the plate boundary is
exceeded. This pattern of strain buildup and release was discovered by H. F.
Reid in his study of the 1906 San Francisco earthquake. During that earthquake,
a 250-mi-long (400-km) portion of the San Andreas fault, from Cape Mendocino to
the town of Gilroy, south of San Francisco, slipped an average of 12 ft (3.6 m).
Subsequently, the triangulation network in the San Francisco Bay area was
resurveyed; it was found that the west side of the fault had moved northward
with respect to the east side, but that these motions died out at distances of
20 mi (32 km) or more from the fault. Reid had noticed, however, that
measurements made about 40 years prior to the 1906 earthquake had shown that
points far to the west of the fault were moving northward at a slow rate. From
these clues, he deduced his theory of elastic rebound, illustrated schematically
in Fig. 3. The figure is a map view, the vertical line representing the fault
separating two moving plates. The unstrained rocks in Fig. 3a are distorted by
the slow movement of the plates in Fig. 3b. Slippage in an earthquake, returning
the rocks to an unstrained state, occurs as in Fig. 3c. See also: Fault and fault structures
Fig. 3 Schematic of elastic rebound theory. (a)
Unstrained rocks (b) are distorted by relative movement between the two plates,
causing strains within the fault zone that finally become so great that (c) the
rocks break and rebound to a new unstrained position. (After C. R. Allen, The
San Andreas Fault, Eng. Sci. Mag., Calif. Inst. Technol., pp. 1–5, May
1957)
Classification
Most great earthquakes occur on the
boundaries between lithospheric plates and arise directly from the motions
between the plates. Although these may be called plate boundary earthquakes,
there are many earthquakes, sometimes of substantial size, that cannot be
related so simply to the movements of the plates.
Near many plate boundaries, earthquakes are
not restricted to the plate boundary itself, but occur over a broad zone—often
several hundred miles wide—adjacent to the plate boundary. These earthquakes,
which may be called plate boundary–related earthquakes, do not reflect the plate
motions directly, but are secondarily caused by the stresses set up at the plate
boundary. In Japan, for example, the plate boundaries are in the deep ocean
trenches offshore of the Japanese islands, and that is where the great plate
boundary earthquakes occur. Many smaller events occur scattered throughout the
Japanese islands, caused by the overall compression of the whole region.
Although these small events are energetically minor when compared to the great
offshore earthquakes, they are often more destructive, owing to their greater
proximity to population centers.
Although most earthquakes occur on or near
plate boundaries, some also occur, although infrequently, within plates. These
earthquakes, which are not related to plate boundaries, are called intraplate
earthquakes, and can sometimes be quite destructive. The immediate cause of
intraplate earthquakes is not understood. Some of them can be quite large. Three
of the largest earthquakes known to have occurred in the United States were part
of a sequence of intraplate earthquakes which took place in the Mississippi
Valley, near New Madrid, Missouri, in 1811 and 1812. Another intraplate
earthquake, in 1886, caused moderate damage to Charleston, South Carolina.
In addition to the tectonic types of
earthquakes described above, some earthquakes are directly associated with
volcanic activity. These volcanic earthquakes result from the motion of
undergound magma that leads to volcanic eruptions.
Sequences
Earthquakes often occur in well-defined
sequences in time. Tectonic earthquakes are often preceded, by a few days to
weeks, by several smaller shocks (foreshocks), and are nearly always followed by
large numbers of aftershocks. Foreshocks and aftershocks are usually much
smaller than the main shock. Volcanic earthquakes often occur in flurries of
activity, with no discernible main shock. This type of sequence is called a
swarm.
Size
Earthquakes range enormously in size, from
tremors in which slippage of a few tenths of an inch occurs on a few feet of
fault, to the greatest events, which may involve a rupture many hundreds of
miles long, with tens of feet of slip. Accelerations exceeding 1 g (acceleration
due to gravity) can occur during an earthquake. The velocity at which the two
sides of the fault move during an earthquake is only 1–10 mi/h (10–100 cm/s),
but the rupture front spreads along the fault at a velocity of nearly 5000 mi/h
(3 km/s). The earthquake's primary damage is due to the generated seismic waves,
or sound waves which travel through the Earth, excited by the rapid movement of
the earthquake. The energy radiated as seismic waves during a large earthquake
can be as great as 1012 cal (4.19 × 1012 joules), and the power emitted during
the few hundred seconds of movement as great as a billion megawatts.
The size of an earthquake is given by its
moment: average slip times the fault area that slipped times the elastic
constant of the Earth. The units of seismic moment are dyne-centimeters. An
older measure of earthquake size is magnitude, which is proportional to the
logarithm of moment. Magnitude 2.0 is about the smallest tremor that can be
felt. Most destructive earthquakes are greater than magnitude 6; the largest
shock known was the 1960 Chile earthquake, with a moment of 1030
dyne-centimeters (1023 newton-meters) or magnitude 9.5. It involved a fault 600
mi (1000 km) long slipping 30 ft (10 m). The magnitude scale is logarithmic, so
that a magnitude 7 shock is about 30 times more energetic than one of magnitude
6, and 30 × 30, or 900 times, more energetic than one of magnitude 5. Because of
this great increase in size with magnitude, only the largest events (greater
than magnitude 8) significantly contribute to plate movements. The smaller
events occur much more often but are almost incidental to the process.
The intensity of an earthquake is a measure
of the severity of shaking and its attendant damage at a point on the surface of
the Earth. The same earthquake may therefore have different intensities at
different places. The intensity usually decreases away from the epicenter (the
point on the surface directly above the onset of the earthquake), but its value
depends on many factors and generally increases with moment. Intensity is
usually higher in areas with thick alluvial cover or landfill than in areas of
shallow soil or bare rock. Poor building construction leads to high intensity
ratings because the damage to structures is high. Intensity is therefore more a
measure of the earthquake's effect on humans than an innate property of the
earthquake.
Effects
Many different effects are produced by
earthquake shaking. Although the fault motion that produced the earthquake is
sometimes observed at the surface, often other earth movements, such as
landslides, are triggered by earthquakes. On rare occasions the ground has been
observed to undulate in a wavelike manner, and cracks and fissures often form in
soil. The flow of springs and rivers may be altered, and the compression of
aquifers sometimes causes water to spout from the ground in fountains. Undersea
earthquakes often generate very long-wavelength water waves, which are sometimes
called tidal waves but are more properly called seismic sea waves, or tsunami.
These waves, almost imperceptible in the open ocean, increase in height as they
approach a coast and often inflict great damage to coastal cities and
ports. See also: Landslide; Tsunami
Prediction
Earthquake prediction research has been
going on for nearly a century. A successful prediction, specifying the time,
location, and magnitude of an earthquake, would save lives and billions of
dollars in housing and infrastructure costs. Unfortunately, successful
earthquake predictions are extremely rare. There are two basic categories of
earthquake predictions: forecasts (months to years in advance) and short-term
predictions (hours or days in advance). Forecasts are based a variety of
research, including the history of earthquakes in a specific region, the
identification of fault characteristics (including length, depth, and
segmentation), and the identification of strain accumulation. Data from these
studies are used to provide rough estimates of earthquake sizes and recurrence
intervals.
An example of an earthquake forecast is the
identification of seismic gaps, portions of the plate boundaries that have not
ruptured in a major earthquake for a long time. These regions are most likely to
experience great earthquakes in the future. Figure 4 shows seismic gaps for the
circum-Pacific region and indicates which gaps were most likely to experience a
large or great earthquake in 1989–1999. Large earthquakes did occur in several
of the likeliest gaps, but many large earthquakes occurred in less likely gaps
as well. Earthquake probability estimates are another example of forecasting.
Geologic, geodetic, and seismic information are combined to estimate the
frequencies of damaging earthquakes in a specific region. Recent regional
earthquake probability estimate studies have resulted in forecasts of an 80–90%
probability of a magnitude 7 or larger earthquake in the southern California
region before 2024, and a 70% probability of a magnitude 6.7 or larger
earthquake in the San Francisco Bay region before 2030.
Fig. 4 Major seismic gaps, western Pacific. The
plate boundaries are shown in black. Black squares mark the locations of seismic
gaps with a 50% or greater chance of being filled by a large earthquake between
1989 and 1999. Black dots are magnitude 7.0 or larger earthquakes that occurred
between 1989 and 1999. (After S. P. Nishenko, Pure Appl. Geophys., 135:169–259,
1991)
Short-term earthquake prediction is still
entirely in the realm of ongoing research, and no method is known to be
reliable. Evidence is emerging that short-term prediction may be inherently
impossible due to the complex and chaotic nature of the earthquake process.
Christopher H. Scholz
Kaye M. Shedlock
Deep
Earthquakes
Most earthquakes occur at depths shallower
than about 50 km (30 mi) [Fig. 5a] and are usually found near plate boundaries.
A few percent of all shocks occur at depths of 300–700 km (183–427 mi), depths
that correspond to earth pressures of 100,000–250,00 atm (1–2.5 × 1010 Pa; Fig.
5b and Fig. 6). That the mantle can suddenly rupture rather than flow
plastically at such conditions has elicited wonder since deep earthquakes were
first discovered in the 1920s. Modern insight into these phenomena has come from
scientific advances of plate tectonics, seismic tomography, and the mineral
physics of the deep mantle based on very high pressure experiments on mantle
minerals.
Fig. 5 Map showing the global distribution of
shallow and deep earthquakes. (a) Earthquakes shallower than 40 km (25 mi). (b)
Earthquakes deeper than 300 km (186 mi; open circles with dates indicate the
locations of the 13 largest recorded deep shocks, including the 1994 Bolivian
event).
Fig. 6 Depth histogram of earthquakes compared
to the mineralogical structure of the mantle. (a) Depth histogram of
well-located earthquakes (1964–1991) in relationship to seismic-wave speed
discontinuities caused by mineralogical changes in normal mantle and in cold
slabs. (b) Hypothetical mineralogical structure of very cold slabs and normal
mantle, emphasizing the phase changes associated with the olivine component
[(Mg,Fe)2SiO4] of the mantle [α (olivine), β (modified spinel) and γ (spinel)];
Mw + Pv are magnesiowüstite and perovskite, the higher-pressure minerals
dominating the lower mantle. Transformational faulting is a shear instability
that can occur in metastable olivine under stress and can produce deep
earthquakes.
Most, if not all, deep earthquakes occur in
inclined belts inside slabs, the cold, dense, and strong lithospheric plates
that dive deeply into the Earth's mantle in places where plates are converging.
Seismic waves have been used to image variations in the seismic wave speeds in
the Earth. These anomalies in seismic tomographic images reflect differences in
temperature, mineralogy, or composition. As expected, deep earthquakes occur in
the high-wave-speed anomalies that mark cold slabs, anomalies that have been
traced to depths as great as 2000 0km (1220 mi) or more (Fig. 7).
Fig. 7 Tomographic image of the Australian
plate subducting beneath Indonesia and associated slab earthquakes. The broken
lines mark the mantle transition zone at depths of 410–660 km (250–403 mi); the
upper and lower mantles are above and below this depth interval. Note that the
compressional wave-speed anomaly of the slab extends deeply into the lower
mantle, whereas slab earthquakes cease near 660 km (410 mi). (Image courtesy of
Dr. Wim Spakman, University of Utrecht)
Curiously, earthquakes occur no deeper than
650–700 km (397–427 mi), far shallower than the maximum depths to which slabs
descend. This abrupt shutoff and the gradual onset of the deep earthquake
population at 300–350 km (183–214 mi) bracket approximately the transition zone
of the mantle where seismic wave speeds abruptly increase (Fig. 6a).
High-pressure experiments indicate that the mineralogy of the mantle changes at
those depths and pressures from upper-mantle mineralogy (dominantly olivine and
pyroxenes) to the minerals spinel, ilmenite, and majorite in the transition zone
and, in turn, to the lower-mantle perovskite and oxide minerals. Slab mantle
penetrating through the transition zone is expected to transform to these denser
minerals.
Most deep earthquakes occur in the depth
interval of the transition zone where upper-mantle slab minerals are
reconstructed to their denser structural forms. Attention has therefore been
drawn to the possibility that deep earthquakes are somehow caused by the
mineralogical transformation of slabs as they descend into and through this
region. Early speculation was that deep earthquakes represent rapid implosions
that might occur when slab minerals transform suddenly to their denser,
high-pressure forms. The patterns of seismic waves that radiate from deep
earthquake sources indicate, however, that such disturbances represent slip on a
fault, as do shallow earthquakes. If a connection exists between deep
earthquakes and mantle phase changes, the underlying process must facilitate
failure by faulting.
A clue to the nature of this possible
connection comes from the observation that deep earthquakes do not occur in all
slabs, only in those that are very cold because they are descending at very fast
rates. Low slab temperatures are important because such conditions favor the
metastable persistence of upper-mantle minerals in the coldest interiors of
slabs as they descend into the transition zone (Fig. 6b). Laboratory studies
show that some minerals deformed under metastable conditions will rupture by an
unusual shear instability in which the mineral is transformed to denser minerals
in the shear zone. This shear instability, called transformational faulting, is
not inhibited by high pressures and hence is an attractive candidate for the
faulting mechanism of deep earthquakes. According to this theory, deep
earthquakes do not occur in the lower mantle because low-density upper-mantle
slab rocks are too buoyant to sink into the lower mantle.
An extraordinary demonstration of the
potential scale of deep seismic faulting was demonstrated by the great deep
earthquake that occurred at a depth of about 640 km (390 mi) beneath the
Amazonian rainforest of
Kaye M. Shedlock
Bibliography
G. L. Berlin, Earthquakes and the Urban
Environment, 3 vols., 1980
B. Bolt, Earthquakes and Geological
Discovery, 1993
Earthquake Information Bulletin,
Facing Geological Hazards,
C. Lomnitz, Fundamentals of Earthquake
Prediction, 1994
T. Rikitaki, Earthquake Forecasting,
1982
S. Kirby et al., Metastable mantle phase
transformations and deep earthquakes in subducting oceanic lithosphere, Rev.
Geophys., 34:261–306, 1996
T. Lay, Structure and Fate of Subducting
Slabs, Academic Press,
C. Frohlich, Deep earthquakes, Sci. Amer.,
260(1):48–55, 1989
E. R. Engdahl, R. van der Hilst, and R.
Buland, Global teleseismic earthquake relocation with improved travel times and
procedures for depth determination, Bull. Seis. Soc. Amer., 88:722–743,
1998
A. Villaseñor et al., Toward a comprehensive
catalog of global historical seismicity, Eos Trans. Amer. Geophys.
K. M. Shedlock and L. C. Pakiser,
Earthquakes,
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Surfing the Internet for Earthquake Data
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