INTRODUCTION TO EARTHQUAKES
An earthquake is the vibration of the earth produced by a release of energy
as a sudden movement of rock mass occurs along faults.
Earthquakes are very common geologic phenomena:
-over 800,000 earthquakes occur every year
-nearly all of these are very minor, too small to be felt by people
-but about 20 earthquakes of M=7.0 (the size of the 1989 Loma Prieta
earthquake) or larger occur around the world annually.
Large earthquakes are among the most destructive of natural phenomena.
-during historic times, earthquakes in populated areas have caused
50,000 or more deaths at least 16 times.
The most destructive historic earthquake occurred in China in 1556
-it is estimated that 830,000 people died
-one reason for the huge death toll was that people in the region lived
in cave dwellings excavated from bluffs of soft sediment, which
collapsed during the earthquake.
More recently, another large earthquake in China in 1976 killed at least
250,000 people
-some estimates as high as 700,000.
Because of their destructive power, scientists have studied earthquakes very
intently, with the hope of understanding their exact causes and
mechanisms of occurrence
-and to use this knowledge to find ways of predicting the location and
time of occurrence of large earthquakes.
Earthquakes have also provided earth scientists with a powerful tool for
studying the properties and structure of the earth's interior
-vibrations set up by large earthquakes propagate throughout the earth
-studies of the travel times and travel paths of these vibrations allow us
to determine the density and other physical properties of rocks
below the surface
-and how these properties vary with depth
-revealed the layered structure of the earth, with divisions into core,
mantle, and crust.
ORIGIN OF EARTHQUAKES: ELASTIC REBOUND
The way in which earthquake vibrations are generated was not understood
by geologists when the large 1906 earthquake struck San Francisco and
northern California.
During this earthquake, much of northern part of the San Andreas fault
ruptured: from Hollister northward to Cape Mendocino.
-the shaking lasted in San Francisco for 48 seconds.
-during this time, the crust west of the fault shifted northwestward
-horizontal offset at the fault reached as much as 6 m (20 ft) in Marin
County.
After the earthquake, studies revealed that strain had been building up in the
region surrounding the fault for a long time
- the evidence was found in land surveys made during the 50 years
preceding the quake
-the surveys showed that distant points on either side of the fault were
slowly being displaced during the decades prior to the quake
-the displacement was horizontal, parallel to the fault, as much as 3 m
(10 ft)
-but no movement occurred on the San Andreas fault itself during this
time: the fault was locked by friction
-displacement resulted from gradual deformation spread through the
rocks on either side of the fault, across a zone tens of km wide.
Survey lines crossing the fault were being bent by the build-up of strain in
the rocks surrounding the fault:
On the basis of this evidence, the Elastic Rebound mechanism of
earthquake generation was proposed.
This theory is based on the fact that when rocks are subjected to small,
temporary stress, they behave like elastic materials:
-they change shape temporarily
-but when stress is removed, they spring back to their original shape
-in effect, when elastic materials deform, they store up elastic energy,
which is released when stress is removed, and enables them to
return to their original shape.
The bending of rocks adjacent to a locked fault stores up elastic energy and
increases the stress along the fault
-eventually, stresses acting parallel to the fault exceed the friction that
prevents movement
-the fault suddenly slips, with movements as rapid as several meters in
a few seconds
-as the fault slips, the rocks adjacent to the fault suddenly "unbend",
and spring back to their original shape, releasing the stored elastic
energy
-the sudden motion of the unbending rocks generates the vibrations
that we feel as an earthquake.
The potential for earthquakes is greatest along active faults that are
temporarily locked
-the size of the earthquake is related to how long the fault has been
locked, how much elastic strain has built up, and how large a
portion of the fault slips.
The slip that occurs during a major earthquake may not relieve all of the
built-up stress:
-after a major quake, additional smaller offsets commonly occur along
faults
-these generate smaller earthquakes called aftershocks
-despite their smaller magnitude, aftershocks can do significant
damage because many structures may be badly weakened by the
main quake.
Major earthquakes may also be preceded by smaller quakes called
foreshocks
-monitoring faults for foreshocks is one strategy for earthquake
prediction.
SEISMIC WAVES
The vibrations produced by an earthquake radiate outward from the
earthquake focus in all directions
-the study of earthquake vibrations is called seismology
-and the instruments used to measure and record seismic vibrations are
called seismographs.
-the record of these vibrations is a seismogram.
Some earthquake energy sets up a wave-like disturbance of the earth's
surface: surface waves
-like the ripples on the surface of water
But most earthquake energy moves as body waves:
-waves that move through the rocks in the interior of the earth.
There are two types of seismic body waves:
-Primary or P-waves, also called compressional waves
-Secondary or S-waves, also called transverse waves.
P-waves
As P-waves travel through rock, the rocks oscillate back and forth in the
direction the wave is travelling
-rocks are alternately pushed and pulled in the direction of wave
motion
-or rocks are alternately compressed and expanded.
A P-wave can be simulated with a large spring
-if the spring is stretched, and you bunch the coils at one end
-when released, the zone of compression travels longitudinally down
the spring, followed by a zone of expansion.
Sound traveling through the air moves in this same way, as alternate
compressions and expansions of the air
-because solids, liquids, and gasses can all be compressed elastically,
P-waves can travel through all three phase states of matter.
S-waves
S-waves are what physicists call transverse waves:
-rocks oscillate back and forth at right angles to the direction of wave
travel
S-waves can be illustrated by tying one end of an elastic rope to something
and shaking the other end
-an S-wave travels down the rope and reflects back from the tied end
S-waves produce temporary changes in shape of the rocks which are
recovered as the waves pass
-S-waves propagate readily through rocks, but cannot be transmitted
through a liquid or gas (unlike P-waves)
-if you try to push a liquid sideways, it simply flows
-it does not spring back when the stress is removed
This difference between P-waves and S-waves allows us to determine which
parts of the earth's interior are solid and which parts are liquid.
Surface waves
The motion of surface waves is similar to that of S-waves, but the motion
dies out downward below the surface
-the ground surface oscillates back and forth at right angles to the
direction of wave travel
-both up-and-down and horizontal side-to-side motions occur in
different types of surface waves.
-when large shallow earthquakes occur, observers near the epicenter
can sometimes see the ground surface undulating up and down by
a meter or more as the surface waves pass.
Seismic Waves and Seismograms
How can we recognize different seismic waves on a seismogram?
-for earthquakes some distance away from a recording station, this
turns out to be fairly easy
-the different waves arrive at the seismograph at different times
-because the different wave types travel at different velocities.
The first wave to arrive is the P-wave: which is the source of the name.
-the P-wave is the fastest type of seismic wave
-the exact velocity depends upon rock type, rock density, and depth
-for granite at shallow depths in the crust, Vp = 6 km/sec
The S-wave is the second wave to arrive:
-it is significantly slower than the P-wave
-for granite, Vs = 3.5 km/sec
-for any rock type, the P-wave travels 1.7 times as fast as the S-wave.
The S-waves also usually have a higher amplitude than the P-waves
-the wiggles on the seismograph trace are more pronounced.
Surface waves are slightly slower than S-waves:
-travel at 0.9 times the velocity of S-waves
-so they are the last to arrive
-usually have much higher amplitudes than the body waves.
LOCATING EARTHQUAKES
The source of an earthquake is the point where a fault first begins to rupture
-this nearly always occurs some distance below the surface
-for many deeper earthquakes, the zone of rupture never reaches the
surface, and there may be no surface offset.
This point of origin of the earthquake waves is called the earthquake focus.
-the epicenter of an earthquake is the point on the earth's surface
directly above the focus.
The difference in velocity and arrival times of P-waves and S-waves
provides the data that allow earthquake epicenters to be located.
When an earthquake is recorded at a seismograph station, we don't initially
know the exact time at which the earthquake began
So we can't use the arrival time of the P-wave alone, for example, to
determine how far away the epicenter is.
But for any seismic station more than a few miles from the earthquake
epicenter, there will be a time lag between the arrival of the P and S
waves.
-as the waves radiate away from the epicenter, the slower S-wave lags
farther and farther behind the P-wave
-so the difference in arrival times increases with distance.
If both P and S-waves are recorded at our station, we can easily measure the
time lag between their first arrivals
-that time difference will correspond to a specific distance from the
epicenter.
Unfortunately, with a single seismograph station, we don't know the
direction from which the seismic waves were traveling.
But the epicenter must lie on a circle centered on the seismograph station,
with a radius equal to the calculated distance
In order to find the exact location of the epicenter, we must have distance
measurements from at least three seismograph stations
-the circles drawn around each station should all intersect in a single
common point: the epicenter.
For seismograph stations close to the epicenter, the depth of the earthquake
focus also affects the travel time of seismic waves
-seismic waves from a deep earthquake will take longer to reach a
nearby station than waves from a shallower earthquake
-travel-time data from a number of nearby seismic stations can be used
to establish the focal depth.
Earthquake foci vary in depth from 5 km to 700 km below the surface.
-seismologists have divided earthquakes into three depth ranges (which
are somewhat arbitrary)
-shallow earthquakes are less than 70 km deep.
-intermediate depth earthquakes are between 70 and 300 km deep.
-deep earthquakes are deeper than 300 km.
About 90% of all earthquakes are shallow-focus earthquakes.
-intermediate and deep earthquakes are exceptional, and played an
important role in working out the plate-tectonic theory.
EARTHQUAKE MAGNITUDE AND INTENSITY
Earthquakes vary greatly in strength and in the effects they produce at the
surface. Two different ways to compare the relative strength of
earthquakes are the magnitude and intensity.
Magnitude
The magnitude of an earthquake is a fairly objective measure of the strength
of an earthquake.
-is determined from the ground motions measured by seismic
instruments.
The basic assumption in magnitude measurement is that the amplitude of
seismic waves recorded on seismograms is related to the amount of
energy released in the earthquake
-the larger the earthquake, the greater the amplitude of the vibrations.
The technique for determining earthquake magnitude was developed by
Charles Richter and associates at Cal Tech in 1935
-the magnitude scale is now called the Richter Scale
Magnitude is determined by measuring the amplitude of the largest wave
recorded by a standard type of seismograph: this is usually the surface
wave.
-but seismic waves spread out and weaken as they move away from the
source
-the wave amplitude diminishes at more distant seismic stations
-so observed amplitudes are corrected for these variations due to
distance from the source
Because the size of earthquakes varies enormously, the amplitudes of ground
motions measured by seismographs differs by factors of thousands from
earthquake to earthquake.
-so to obtain a scale which expresses variations in amplitude in small
numbers, the Richter Scale is logarithmic
This means that an increase in Richter magnitude of 1.0 between two
earthquakes is equal to a ten-fold increase in wave amplitude
-a magnitude 5.0 earthquake has a wave amplitude 10 times that of a
magnitude 4.0 quake
-a magnitude 6.0 quake has an amplitude 100 times higher
-a magnitude 7.0 quake has an amplitude 1,000 times higher.
The smallest earthquakes that can be felt by people under ideal
circumstances are about magnitude 1.5
-damage to structures may occur at magnitudes greater than 4.5.
The Richter scale has no inherent upper limit
-but the largest earthquake magnitude that has been measured this
century is 8.9, in two earthquakes.
-this appears to represent a natural upper limit, related to the strength
of rocks in the crust.
Seismic wave amplitude correlates in a crude way with the amount of energy
released by an earthquake
-an increase in magnitude of 1.0 increases the amount of seismic
energy released by a factor of 30, not just 10.
-so a magnitude 6.0 earthquake releases 900 times more energy than a
magnitude 4.0 quake.
The largest earthquakes (over Magnitude 8) release millions of times more
energy than the smallest earthquakes felt by people.
-as a result, they are much rarer:
Richter Magnitude
Number per year
7.5) occurred
within seismic gaps that had been previously identified by
seismologists.
Recognizing seismic gaps on active faults helps to predict where we can
expect to experience major earthquakes
-but much more detailed studies are required to try to predict when we
can expect such a quake to occur.
Recurrence Intervals
Some indications of the timing of quakes can be revealed by the past
behavior of faults
-for many faults, there is evidence that earthquakes of similar
magnitude repeatedly break the same segment of the fault, and
produce similar offsets.
-the time between these repeated quakes is called the recurrence
interval.
One of the best examples of such regular behavior is part of the San Andreas
fault near Parkfield in central California
-Parkfield is the southern termination of the "creeping" section of the
fault
-a few miles NW of town, the fault takes a slight bend, which serves to
isolate the Parkfield section from the creeping section.
Earthquakes have occurred on the Parkfield section in 1857, 1881, 1901,
1922, 1934, and 1966
-the average recurrence interval has been about 22 years +-2, with the
1934 quake coming 10 years early.
The 1857 Parkfield quake occurred several hours before the Ft. Tejon quake
-seismologists speculate that the displacement on the Parkfield
segment may have triggered the larger offset on the segment of the
fault to the south, which must have been already strained to the
breaking point.
The last three Parkfield quakes have been recorded by seismic instruments
-all had magnitudes of 5.6, with a M=5.1 foreshock occurring 17
minutes before the main quake.
-each ruptured the same 20-mile segment of the fault, with the rupture
beginning at the north end.
-seismograms of these three earthquakes recorded at distant stations
are nearly identical in character and amplitude, suggesting similar
amount of energy was released.
Based on the average recurrence interval of 22 years, the next Parkfield
quake was expected between 1988 and 1993, and is now overdue
-just as the 1934 quake came early, the expected quake appears to be
late
-even for this most well-behaved of fault segments, earthquakes are
not as predictably periodic as we would like.
Unfortunately, the U.S.G.S. bet heavily on the predicted arrival time
-they hoped to catch an earthquake in action
-installed hundreds of monitoring devices to record changes in the
crust around the fault before and after the earthquake
-hoping to get data that would help define the mechanism of triggering
of the earthquake, and reveal possible precursor phenomena that
might be used to help provide short-term earthquake predictions.
For larger-magnitude earthquakes, the recurrence interval is considerably
longer than 22 years
-may be over 200 years
-too long to accurately determine it with our short recorded history in
California.
However, if we know the amount of offset that occurs in each major quake,
and the long-term rate at which strain accumulates along the fault, we
can calculate the expected average interval between quakes.
For example, assume that we determine the long-term rate of movement on a
fault averages out to 5 cm/year
-but this occurs in abrupt offsets of 5 m (500 cm) during large
earthquakes
-the earthquake offset divided by the long-term rate gives the interval
between earthquakes
-in this case the recurrence interval is 100 years.
This is the time it takes for elastic strain to build up along the locked fault to
the level at which friction along the fault is finally overcome, and fault
slip can occur.
Both the characteristic offsets during major earthquakes and the long-term
strain rate must be determined by painstaking geologic studies of active
faults
-and recent techniques allow direct estimates of recurrence intervals of
major earthquakes.
-these studies of ancient earthquake effects are sometimes called
paleoseismology.
PALEOSEISMOLOGY
Paleoseismology involves studies of offset landforms such as stream
channels
-and studies of young sediment layers that accumulate in stream
channels, ponds, and marshes along faults.
Stream channels that cross a strike-slip fault like the San Andreas fault have
their downstream portion progressively offset from the upstream portion
-for a while, the stream will flow along the fault, connecting upper and
lower portions
-eventually the distance becomes too great
-a new lower valley is cut straight across the fault, but it will be offset
in turn.
One such offset stream is Wallace Creek, along the San Andreas fault north
of Los Angeles
-the young, active channel has been offset about 130 m by the fault
-an older, abandoned channel is also preserved, offset an even greater
distance.
Studies of the abandoned channel showed that it is filled up by sediment
deposited by a single debris flow
-a flowing mixture of water, mud, and gravel triggered by heavy rains,
that had flowed down the channel and plugged it.
-fragments of fossil wood in the channel fill deposit were dated by the
Carbon-14 technique: 3,700 years old.
This plugging of the old channel was the event that triggered the cutting of
the present downstream channel segment
-this segment therefore began to form 3,700 years ago
-has been offset about 130 m in that time
-dividing the total offset by the time yields a long-term slip rate of 3.4
cm/year for the fault.
The Wallace Creek area is part of the San Andreas fault that was last broken
by the 1857 Ft. Tejon earthquake
-amount of offset in this region was about 9 meters.
-some small streams in the area have several abandoned and offset
downstream channels
-the regular spacing between the offset channels suggests that 9 meters
is the typical offset that occurs during major earthquakes in this
area.
If the 130 meters of offset of Wallace Creek is due to successive 9-m offsets
during large quakes
-this would require about 14 such quakes (130/9)
If the 14 earthquakes occurred at regular intervals during the 3700 years
-the average interval between earthquakes would be 3700/14, or about
265 years.
-since the last major quake was in 1857, the most likely time for the
next great earthquake on this part of the fault would be in 2120.
Other studies of the San Andreas fault have looked in more detail at
sediment layers accumulating in ponds and marshes along the fault
-during the long intervals between earthquakes, sediment gradually
accumulates at the surface, burying the last fault break
-but during the next earthquake, this layer may be broken by the fault
movement
-after the earthquake, sediment will again accumulate to form a new,
younger layer that covers up the break.
Sediment sequences such as this are studied by digging trenches across the
active fault trace
-for the San Andreas fault, the fault motion is dominantly horizontal
-but when the horizontal offset is several meters or more, there is
usually at least a little vertical offset that shows up in the trench
walls.
In looking at such a sequence in a trench wall, we use principles of relative
age dating to determine the sequence of events
-the principle of cross-cutting relationships tells us that the fault is
younger than any layer that it cuts
-the principle of superposition tells us that the overlying, unbroken
layer must be younger than the fault motion.
In ponds and marshes, the sediments often contain fragments of charcoal
form fires, or pieces of wood, leaves, or marsh vegetation
-new laboratory techniques allow even small fragments to be dated
using the Carbon-14 technique
-if many layers can be dated, then the age of each faulting event
(earthquake) can be bracketed by these absolute ages.
One of the first studies of this kind was done at Pallett Creek northeast of
Los Angeles
-the southernmost part of the fault that ruptured in 1857
At Pallett Creek, swamp deposits about 6 meters thick were studied
-contained about 80 recognizable sediment layers, deposited over the
last 2,000 years.
-the sequence showed the evidence of a number of large earthquakes,
whose ages could be determined with a possible error of a few
decades: 1720, 1550, 1350, 1080
-in fact, there was evidence for 12 major quakes in the past 1700 years.
The average recurrence interval for major earthquakes in the Pallett Creek
area was calculated to be about 150 years
-but the interval varied from a high of 300 years to a low of 55 years.
Paleoseismic studies such as this have been undertaken along many parts of
the San Andreas fault system
-mostly sponsored by the U.S.G.S.
-they suggest that most parts of the San Andreas fault system are less
regular than the Parkfield segment
-recurrence intervals are at best only a guide to enable geologists to
focus attention on faults most likely to rupture in the near future.
EARTHQUAKE PRECURSORS
The main hope for development of short-term earthquake predictions in the
1970's lay in studies of various types of physical phenomena that have
been seen to occur in the months or days prior to some major
earthquakes
-these short-term phenomena can be called earthquake precursors
Changes in earthquake frequency
Large earthquakes generally occur in seismic gaps
-for many large historic quakes, small earthquakes have begun
occurring in the seismic gap in the months prior to the major
earthquake
-these sometimes increase in magnitude, and culminate in strong
foreshocks in the final days before the main quake.
So monitoring of lock faults for signs of increased earthquake frequency is
one major component of efforts toward predicting earthquakes
-although a sudden increase in activity suggests an increased
likelihood of a major earthquake on a locked fault, it does not tell
us when such an earthquake will occur.
Ground uplift and tilting
Locallized changes in the level of the ground surface sometimes occur in the
area of an impending earthquake
-these can be detected by repeated ground surveys
-and by sensitive instruments called tiltmeters which can detect
minute changes in the tilt of the land surface
-imagine that we could pick up the United States by one edge like a
rigid dinner plate
-if we picked up the eastern U.S. and raised it by 2 inches, tiltmeters
elsewhere in the country could detect the very slight resulting tilt.
A major earthquake occurred on the west coast of Japan in 1964 (M = 7.5)
-this is the quake that caused the tilting of apartment buildings due to
liquefaction.
Much of the coastal area around the epicenter underwent slow uplift in the
early part of the century
-but this uplift accelerated about 10 years prior to the quake, then
leveled off
-and the ground began to subside again several years before the quake
-unfortunately, this pattern in the survey data was not discovered until
after the earthquake
-but if it occurred again in the future it might provide some warning.
Water levels in wells
The level of underground water has been seen to rise and fall in wells before
large earthquakes
-presumably this occurs because of deformation of the rocks
surrounding the fault, which changes the pressure on the water
contained in fractures and pores in the rock.
Electrical resistivity of the earth
If electrodes are placed in the ground several kilometers apart, a weak
electrical current will flow between them
-this current flows mostly through the water that saturates the rock
-the resistance to current flow can be measured, and is the electrical
resistivity
-the electrical resistivity of rocks along active faults has been
monitored in some areas in the U.S. and the Soviet Union
-some changes in resistivity have been seen before earthquakes
-related to changes in volume of cracks and pores that contain water.
Data from Soviet studies suggest that resistivity first increases, then
decreases just before an earthquake.
Radon content of ground water
Radon is a gas produced by the radioactive decay of uranium in rocks
-it diffuses out of the rocks and dissolves in groundwaters
But radon itself is radioactive
-has a half-life of only 3.8 days, so it does not remain in groundwater
for very long
-any change in radon content in the water must result from current
changes in the rate of addition of radon to the water from the
surrounding rock.
Increases in the radon content of groundwater have been measured before
some major earthquakes
-these increases might result from fracturing of the rock, that would
enable radon to escape from the rock into the water more rapidly.
Animal Behavior
For centuries, unusual animal behavior has been reported to have occurred
before earthquakes
-birds becoming agitated and flying away
-dogs running outside of houses
-horses and cows running around wildly
-snakes and frogs emerging from hibernation in the middle of winter
It is certainly possible that some animals can sense phenomena that humans
cannot
-such as variations in electrical fields or magnetic fields
-related to the physical changes going on in the rocks before a quake
For example, some biologists believe the homing sense in birds such as
geese may lie in their ability to sense the earth's magnetic field.
On the other hand, most reports of unusual animal behavior have been
gathered second and third-hand, and after the earthquake
-it is human nature to see cause-and-effect connections between
unusual events happening close together in time
-similar odd behavior occurring at other times may be ignored.
The U.S.G.S. has funded several experiments and surveys to test animal
behavior during earthquakes
-one was a survey of 50 households in Willits CA after a M = 4.7
quake in 1977
-17 out of the 50 households reported unusual behavior
-for a larger quake in Montana in 1978, only 1 person out of 35
interviewed reported any unusual behavior
-from this the researchers concluded that some individual animals
within some species may be able to sense an impending quake.
Another 4-year project in the Bay Area set up a phone hotline that people
could call anytime they saw a pet or other animal behaving in strange
ways.
-over this time, 13 tremors of M 4 or higher occurred in the area
-by looking at phone records of the place and time of the calls,
researchers were able to see if there had been an increased
incidence of unusual behavior before the quakes
-about half of the quakes showed significant increases in the number of
calls in the preceding month
-but the other half showed nothing unusual.
As with other precursors, it appears that unusual animal behavior is also a
very hit-or-miss thing
-cannot be relied upon in earthquake prediction efforts
EXAMPLES OF EARTHQUAKE PREDICTIONS
The Chinese have achieved the only success with short-term earthquake
predictions in a populated area
-the most famous one for an earthquake which occurred in 1975 near
the city of Haicheng in northeast China, inhabited by 100,00
people.
As early as 1970, the State Seismological Bureau had identified the
Haicheng area as the likely location of future large earthquakes
-in 1974, surveys revealed an increase in the rate of uplift of the
surface
-the authorities expanded their network of instruments in the area, and
enlisted thousands of amateur observers to assist the seismologists
By the end of 1974, farmers had reported that water levels were changing in
wells, and scientists detected increases in radon levels.
On December 20, residents were warned that a large quake was imminent
-the people slept outside in the snow for two nights, but no quake
occurred.
On February 1, 1975, a tiny quake occurred near Haicheng (M < 1),
followed the next day by several more, in an area where no quakes had
occurred previously
-by this time 70% of the water wells being monitored showed changes
in water levels, and a number were spouting water at the surface
During February 3, the local earthquakes began to get larger and to occur
more rapidly
-increased to M = 4 by the next morning, then dropped off
At 10:00 AM on Feb. 4, the seismologists issued a prediction that a major
quake would strike in the next day or two
-in Haicheng, the people were evacuated from buildings into tents and
other temporary shelters
-disaster relief facilities were mobilized
-shops and businesses closed
-foreshocks continued throughout the day, and at 7:36 in the evening
the main quake struck, with M = 7.6.
90% of the buildings in Haicheng were severely damaged or destroyed
-but only a few people were killed, because everyone was outside and
well away from the buildings.
During the following year, 1976, three other damaging earthquakes were
successfully predicted in China.
But in July of 1976, a M = 8.0 earthquake struck Tangshan, a city of 1
million people east of Beijing,
-the quake struck entirely without warning
-there had been no foreshocks, and no other precursor phenomena
observed
-the death toll was 240,000 people, making this one of the most tragic
natural disasters in history.
The following month, in August 1976, the Chinese authorities issued an
earthquake warning for parts of southeastern China
-many people slept outdoors in tents for almost 2 months
-but no earthquake occurred.
Unfortunately, events such as these indicate that precursor phenomena do
not always occur before large earthquakes
-or they may occur without being followed by a quake.
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