WHAT IS AN EARTHQUAKE?
Where Do Earthquakes Happen?
Why Do Earthquakes Happen?
How Are Earthquakes Studied?
How To Locate The Earthquake's Epicenter?
SCALES FOR EARTHQUAKE MEASUREMENT
What Are Earthquake Hazards?
• WHAT IS AN EARTHQUAKE?
• Where Do Earthquakes Happen?
• Why Do Earthquakes Happen?
• How Are Earthquakes Studied?
• How To Locate The Earthquake's Epicenter?
• SCALES FOR EARTHQUAKE MEASUREMENT
• What Are Earthquake Hazards?
WHAT IS AN EARTHQUAKE?
Vibration of the earth caused by the sudden
release of energy usually as a result of
displacement of rock blocks along plate
boundaries and faults.
WHAT CAUSES AN EARTHQUAKE?
Removal & addition of fluids to wells
Artificial water reservoirs/Dam
4. Where Do Earthquakes Happen?
• Earthquakes occur all the time all over the world, both along plate
edges and along faults.
• Along Plate Edges:
• Most earthquakes (around 90%) occur along the edge of the oceanic
and continental plates. The earth's lithosphere (the outer layer of the
planet) is made up of several pieces, called plates. The plates under the
oceans are called oceanic plates and the rest are continental plates.
The plates are moved around by the motion of a deeper part of the
earth (the mantle) that lies underneath the crust. These plates are
always bumping into each other, pulling away from each other, or past
each other. The plates usually move at about the same speed that our
fingernails grow. Earthquakes usually occur where two plates are
running into each other or sliding past each other.
6. Along Faults
• Earthquakes can also occur far from the edges of plates,
along faults. Faults are cracks in the earth where sections
of a plate (or two plates) are moving in different directions.
Faults are caused by all that bumping and sliding the plates
do. They are more common near the edges of the plates.
• Types of Faults
• Normal faults are the cracks where one block of rock is
sliding downward and away from another block of rock.
These faults usually occur in areas where a plate is very
slowly splitting apart or where two plates are pulling away
from each other. A normal fault is defined by the hanging
wall moving down relative to the footwall, which is moving
7. Types of Faults
• Normal faults are the cracks where one block
of rock is sliding downward and away from
another block of rock. These faults usually
occur in areas where a plate is very slowly
splitting apart or where two plates are pulling
away from each other. A normal fault is
defined by the hanging wall moving down
relative to the footwall, which is moving up.
9. Reverse Fault
• Reverse faults are cracks formed where one plate is
pushing into another plate. They also occur where a
plate is folding up because it's being compressed by
another plate pushing against it. At these faults, one
block of rock is sliding underneath another block or
one block is being pushed up over the other. A
reverse fault is defined by the hanging wall moving
up relative to the footwall, which is moving down.
11. Strike-Slip Fault
• Strike-slip faults are the cracks between two
plates that are sliding past each other. You
can find these kinds of faults in California. The
San Andreas fault is a strike-slip fault. It's the
most famous California fault and has caused a
lot of powerful earthquakes.
13. Why Do Earthquakes Happen?
• Earthquakes are usually caused when rock underground suddenly
breaks along a fault. This sudden release of energy causes the
seismic waves that make the ground shake. When two blocks of
rock or two plates are rubbing against each other, they stick a little.
They don't just slide smoothly; the rocks catch on each other. The
rocks are still pushing against each other, but not moving.
• After a while, the rocks break because of all the pressure that's
built up. When the rocks break, the earthquake occurs. During the
earthquake and afterward, the plates or blocks of rock start
moving, and they continue to move until they get stuck again.
• The spot underground where the rock breaks is called the focus of
the earthquake. The place right above the focus (on top of the
ground) is called the epicenter of the earthquake.
14. How Are Earthquakes Studied?
• Seismologists study earthquakes by going out
and looking at the damage caused by the
earthquakes and by using seismographs.
• A seismograph is an instrument that records the
shaking of the earth's surface caused by seismic
• The term seismometer is also used to refer to
the same device, and the two terms are often
15. EARTHQUAKE TERMINOLOGY
• Focus: Location within the crust where the
rocks failed under stress. Also called
• Epicentre: Point on the earth’s surface
directly above the focus.
• Fault Plane: Surface of the fault along which
the rock blocks are slipped during the
• The first seismograph was invented in 132 A.D. by the
Chinese astronomer and mathematician Chang Heng. He
called it an "earthquake weathercock”.
• In 136 A.D. a Chinese scientist named Choke updated this
meter and called it a "seismoscope”. Columns of a viscous
liquid were used in place of metal balls.
• The height to which the liquid was washed up the side of the
vessel indicated the intensity and a line joining the points of
maximum motion also denoted the direction of the tremor.
20. How To Read a Seismogram?
• Seismogram is the record of a seismograph either on a paper or
on a magnetic tape.
• When you look at a seismogram, there will be wiggly lines all
across it. These are all the seismic waves that the seismograph
has recorded. Most of these waves were so small that nobody felt
them. These tiny microseisms can be caused by heavy traffic near
the seismograph, waves hitting a beach, the wind, and any
number of other ordinary things that cause some shaking of the
seismograph. There may also be some little dots or marks evenly
spaced along the paper. These are marks for every minute that
the drum of the seismograph has been turning. How far apart
these minute marks are will depend on what kind of seismograph
22. SEISMIC WAVES
SEISMIC WAVE- DEFINITION
TYPES OF SEISMIC WAVES
SPEED OF SEISMIC WAVES
PROPAGATION OF SEISMIC WAVES
PROPAGATION OF P & S WAVES THROUGH MANTLE AND CORE
EARTH'S INTERNAL STRUCTURE
23. The propagation velocity of the waves
depends on density and elasticity of the
medium. Velocity tends to increase with
depth, and ranges from approximately 2
to 8 km/s in the Earth's crust up to
13 km/s in the deep mantle.
There are many types of seismic waves,
body wave, surface waves.
Body waves travel through the interior of
the Earth. They create raypaths refracted
by the varying density and modulus
(stiffness) of the Earth's interior.
Seismic waves are waves of energy that travel through the Earth's layers, and are a
result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic
energy. Many other natural and anthropogenic sources create low amplitude waves
commonly referred to as ambient vibrations. Seismic waves are studied by
geophysicists called seismologists. Seismic wave fields are recorded by a
seismometer, hydrophone (in water), or accelerometer.
The density and modulus, in turn, vary according to temperature, composition, and phase. This
effect is similar to the refraction of light waves.
24. Compressional or P-Waves
P-waves are the first waves to arrive on a complete record of ground shaking
because they travel the fastest (their name derives from this fact - P is an
abbreviation for primary, first wave to arrive). They typically travel at speeds
between ~1 and ~14 km/sec. The slower values corresponds to a P-wave traveling in
water, the higher number represents the P-wave speed near the base of Earth's
The velocity α of a P wave depends on the elastic properties and density of a
material. If we let k represent the bulk modulus of a material, µ the shear-modulus,
and ρ the density, then the P-wave velocity, which we represent by a, is defined by:
Secondary , or S waves, travel slower than P waves and are also called "shear" waves
because they don't change the volume of the material through which they propagate,
they shear it. S-waves are transverse waves because they vibrate the ground in the
direction "transverse", or perpendicular, to the direction that the wave is traveling.
The S-wave speed, call it β, depends on the shear modulus and the density
An important distinguishing characteristic of an S-wave is its inability to propagate
through a fluid or a gas because a fluids and gasses cannot transmit a shear stress and
S-waves are waves that shear the material.
Secondary , or S waves
25. P Compressional, Primary Wave, Body wave, Longitudinal
The deformation propagates. Particle motion consists of alternating compression and
dilation (extension). Particle motion is parallel to the direction of propagation
(longitudinal). Material returns to its original shape after the wave passes.
26. S Shear, Secondary Wave , Body wave, Transverse
The deformation propagates. Particle motion consists of alternating transverse motion.
Particle motion is perpendicular to the direction of propagation (transverse). The
transverse particle motion shown here is vertical but can be in any direction; however the
Earth’s approximately horizontal layers tend to cause mostly SV (in the vertical plane) or
horizontal (SH) shear motions. Material returns to its original shape after the wave
27. Love Wave
Love waves are transverse waves that vibrate
the ground in the horizontal direction
perpendicular to the direction that the waves
are traveling. They are formed by the interaction
of S waves with Earth's surface and shallow
structure and are dispersive waves. The speed at
which a dispersive wave travels depends on the
wave's period. In general, earthquakes generate
Love waves over a range of periods from 1000 to
a fraction of a second, and each period travels at
a different velocity but the typical range of
velocities is between 2 and 6 km/second.
Rayleigh waves are the slowest of all the
seismic wave types and in some ways the most
complicated. Like Love waves they are
dispersive so the particular speed at which
they travel depends on the wave period and
the near-surface geologic structure, and they
also decrease in amplitude with depth. Typical
speeds for Rayleigh waves are on the order of
1 to 5 km/s.
28. Love Wave Motion, Surface Wave, Long waves
The deformation propagates. Particle motion consists of alternating transverse motions.
Particle motion is horizontal and perpendicular to the direction of propagation
(transverse). To best view the horizontal particle motion, focus on the Y axis (red line)
as the wave propagates through it. Amplitude decreases with depth. Material returns to
its original shape after the wave passes.
29. R Rayleigh Wave Motion, Surface Wave, Long waves
The deformation propagates. Particle motion consists of elliptical motions (generally
retrograde elliptical as shown in the figure) in the vertical plane and parallel to the
direction of propagation. Amplitude decreases with depth. Material returns to its original
shape after the wave passes.
30. Seismic waves travel fast, on the order of kilometers per second (km/s). The
precise speed that a seismic wave travels depends on several factors, most
important is the composition of the rock. We are fortunate that the speed
depends on the rock type because it allows us to use observations recorded on
seismograms to infer the composition or range of compositions of the planet. But
the process isn't always simple, because sometimes different rock types have the
same seismic-wave velocity, and other factors also affect the speed, particularly
temperature and pressure. Temperature tends to lower the speed of seismic
waves and pressure tends to increase the speed. Pressure increases with depth in
Earth because the weight of the rocks above gets larger with increasing depth.
Usually, the effect of pressure is the larger and in regions of uniform composition,
the velocity generally increases with depth, despite the fact that the increase of
temperature with depth works to lower the wave velocity.
Seismic Wave Speed
A Stoneley wave is a type of large amplitude Rayleigh wave that propagates along a
solid-fluid boundary. They can be generated along the walls of a fluid-filled borehole
, being an important source of coherent noise in VSPs and making up the low
frequency component of the source in sonic logging. The equation for Stoneley
waves was first given by Dr. Robert Stoneley (1894 - 1976), Emeritus Professor of
31. Characteristics of Different seismic waves
• The P wave will be the first wiggle that is bigger than the
rest of the little ones.
• S waves are usually bigger than the P waves. If there aren't
any S waves marked on seismogram, it probably means
the earthquake happened on the other side of the planet.
• The surface waves (Love and Rayleigh waves) are, often
larger, waves marked on the seismogram. Surface waves
travel a little slower than S waves so they tend to arrive at
the seismograph just after the S waves. Surface waves
may be the only waves recorded a long distance from
32. Table 1: Seismic Waves Characteristics
Particle Motion Typical Velocity Other Characteristics
and dilations (“pulls”)
which are directed in the
same direction as the
wave is propagating
(along the ray path); and
to the wavefront.
VP ~ 5 – 7 km/s in
crust; >~ 8 km/s in
and core; ~1.5
km/s in water;
~0.3 km/s in air.
P motion travels fastest in
materials, so the P-wave is the
first-arriving energy on a
seismogram. Generally smaller
and higher frequency than the
S and Surface-waves. P waves
in a liquid or gas are pressure
waves, including sound waves.
to the direction of
propagation, and the ray
such that particle motion
is in vertical or
VS ~ 3 – 4 km/s in
crust; >~ 4.5 km/s
in Earth’s mantle;
~ 2.5-3.0 km/s in
(solid) inner core.
S-waves do not travel through
fluids, so do not exist in Earth’s
outer core (inferred to be
primarily liquid iron) or in air or
water or molten rock
(magma). S waves travel
slower than P waves in a solid
and, therefore, arrive after the
perpendicular to the
generally parallel to
the Earth’s surface.
VL ~ 2.0 - 4.4 km/s in
the Earth depending
on frequency of the
and therefore the
penetration of the
waves. In general,
the Love waves
travel slightly faster
than the Rayleigh
Love waves exist because of the
Earth’s surface. They are largest at
the surface and decrease in
amplitude with depth. Love waves
are dispersive, that is, the wave
velocity is dependent on frequency,
generally with low frequencies
propagating at higher velocity.
Depth of penetration of the Love
waves is also dependent on
frequency, with lower frequencies
penetrating to greater depth.
Motion is both in the
perpendicular (in a
vertical plane), and
“phased” so that the
motion is generally
elliptical – either
VR ~ 2.0 - 4.2 km/s in
the Earth depending
on frequency of the
and therefore the
penetration of the
Rayleigh waves are also dispersive
and the amplitudes generally
decrease with depth in the Earth.
Appearance and particle motion are
similar to water waves. Depth of
penetration of the Rayleigh waves is
also dependent on frequency, with
lower frequencies penetrating to
35. How To Locate The Earthquake's Epicenter?
• To figure out just where that earthquake happened, one need to
look at the seismogram and to know at least two other
seismographs recorded for the same earthquake. One will also
need a map of the world, a ruler, a pencil, and a compass for
drawing circles on the map.
• One minute intervals are marked by the small lines printed just
above the squiggles made by the seismic waves (the time may be
marked differently on some seismographs). The distance
between the beginning of the first P wave and the first S wave
tells you how many seconds the waves are apart. This number
will be used to tell you how far your seismograph is from the
epicenter of the earthquake.
38. • Measure the distance between the first P wave and the first S
wave. In this case, the first P and S waves are 24 seconds apart.
• Find the point for 24 seconds on the left side of the chart below
and mark that point. According to the chart, this earthquake's
epicenter was 215 kilometers away.
• Measure the amplitude of the strongest wave. The amplitude is
the height (on paper) of the strongest wave. On this
seismogram, the amplitude is 23 millimeters. Find 23 millimeters
on the right side of the chart and mark that point.
• Place a ruler (or straight edge) on the chart between the points
you marked for the distance to the epicenter and the amplitude.
The point where your ruler crosses the middle line on the chart
marks the magnitude (strength) of the earthquake. This
earthquake had a magnitude of 5.0.
39. Finding the Epicenter
• Check the scale on your map. It should look something like a piece
of a ruler. All maps are different. On your map, one centimeter could
be equal to 100 kilometers or something like that.
• Figure out how long the distance to the epicenter (in centimeters) is
on your map. For example, say your map has a scale where one
centimeter is equal to 100 kilometers. If the epicenter of the
earthquake is 215 kilometers away, that equals 2.15 centimeters on
• Using your compass, draw a circle with a radius equal to the number
you came up with in Step #2 (the radius is the distance from the
center of a circle to its edge). The center of the circle will be the
location of your seismograph. The epicenter of the earthquake is
somewhere on the edge of that circle.
41. SCALES FOR EARTHQUAKE MEASUREMENT
• Two scales are used for the measurement of
1) Richter Magnitude scale and
2) Modified Mercalli Intensity scale.
•How Are Earthquake Magnitudes Measured?
•The magnitude of most earthquakes is measured on the
Richter scale, invented by Charles F. Richter in 1934. The
Richter magnitude is calculated from the amplitude of
the largest seismic wave recorded for the earthquake,
no matter what type of wave was the strongest. The
Richter magnitudes are based on a logarithmic scale (base
42. The Modified Mercalli Intensity Scale (MMI)
• Besides Magnitude scale another way to measure the strength
of an earthquake is to use the Mercalli scale.
• Invented by Giuseppe Mercalli in 1902, this scale uses the
observations of the people who experienced the earthquake to
estimate its intensity.
• The Mercalli scale isn't considered as scientific as the Richter
scale. The amount of damage caused by the earthquake may
not accurately record how strong it was either.
SCALES FOR EARTHQUAKE MEASUREMENT
43. The Mercalli intensity scale is a seismic scale used for measuring the intensity of an
earthquake. It measures the effects of an earthquake, and is distinct from the moment
magnitude usually reported for an earthquake (sometimes misreported as the Richter
magnitude), which is a measure of the energy released. The intensity of an earthquake is
not totally determined by its magnitude.
The scale quantifies the effects of an earthquake on the Earth's surface, humans,
objects of nature, and man-made structures on a scale from I (not felt) to XII (total
destruction). Values depend upon the distance to the earthquake, with the highest
intensities being around the epicentral area.
Data gathered from people who have experienced the quake are used to determine an
intensity value for their location. The Mercalli (Intensity) scale originated with the widely
used simple ten-degree Rossi-Forel scale which was revised by Italian
volcanologist, Giuseppe Mercalliin 1884 and 1906.
In 1902 the ten-degree Mercalli scale was expanded to twelve degrees by Italian
physicist Adolfo Cancani. It was later completely re-written by the German geophysicist
August Heinrich Sieberg and became known as the Mercalli-Cancani-Sieberg (MCS)
The Mercalli-Cancani-Sieberg scale was later modified and published in English by Harry
O.Wood and Frank Neumann in 1931 as the Mercalli-Wood-Neumann (MWN) scale. It
was later improved by Charles Richter, the father of the Richter magnitude scale.
The scale is known today as the Modified Mercalli scale (MM) or Modified Mercalli
Intensity scale (MMI)
45. • Intensities typically increase close to an earthquake's
epicenter, allowing seismologists to interpret maps such
as this for the general location of historical earthquakes.
• Note the locations of unusually high intensities (up to IX)
far north of the earthquake's epicenter, near San
Francisco Bay. During this earthquake, soft and water-
saturated soils near the Bay amplified the effects of the
shaking. The amplified shaking, together with soil
liquefaction effects, caused some well-built structures to
collapse and yielded the intensity IX rating at those
47. What Are Earthquake Hazards?
• The first main earthquake hazard (danger) is the effect of
ground shaking. Buildings can be damaged by the
shaking itself or by the ground beneath them settling to
a different level than it was before the earthquake
• Buildings can even sink into the ground if soil liquefaction
• Liquefaction is the mixing of sand or soil and groundwater
(water underground) during the shaking of a moderate or strong
earthquake. When the water and soil are mixed, the ground
becomes very soft and acts similar to quicksand. If liquefaction
occurs under a building, it may start to lean, tip over, or sink
• The ground firms up again after the earthquake has past and
the water has settled back down to its usual place deeper in the
ground. Liquefaction is a hazard in areas that have
groundwater near the surface and sandy soil.
49. Why does liquefaction occur?
To understand liquefaction, it is important to recognize
the conditions that exist in a soil deposit before an
earthquake. A soil deposit consists of an assemblage
of individual soil particles. If we look closely at these
particles, we can see that each particle is in contact
with a number of neighboring particles. The weight of
the overlying soil particles produce contact forces
between the particles - these forces hold individual
particles in place and give the soil its strength.
51. STRONG SURFACE WAVES
• Buildings can also be damaged by strong surface waves
making the ground heave and lurch. Any buildings in the
path of these surface waves can lean or tip over from all the
movement. The ground shaking may also cause landslides,
mudslides, and avalanches on steeper hills or mountains, all
of which can damage buildings and hurt people. The second
main earthquake hazard is ground displacement (ground
movement) along a fault. If a structure (a building, road,
etc.) is built across a fault, the ground displacement during
an earthquake could seriously damage or rip apart that
52. GROUND DISPLACEMENT
• The second main earthquake hazard is
ground displacement (ground movement)
along a fault.
• If a structure (a building, road, etc.) is built
across a fault, the ground displacement
during an earthquake could seriously
damage or rip apart that structure.
• The third main hazard is flooding.
• An earthquake can rupture (break) dams
or levees along a river. The water from the
river or the reservoir would then flood the
area, damaging buildings and maybe
sweeping away or drowning people.
• Tsunamis and seiches can also cause a great deal of
• A tsunami is what most people call a tidal wave, but it
has nothing to do with the tides on the ocean. It is a
huge wave caused by an earthquake under the ocean.
Tsunamis can be tens of feet high when they hit the
shore and can do enormous damage to the coastline.
• Seiches are like small tsunamis. They occur on lakes
that are shaken by the earthquake and are usually only
a few feet high, but they can still flood or knock down
houses, and tip over trees.
55. TSUNAMI CHARACTERISTICS
• While everyday wind waves have a wavelength (from crest to crest) of about
100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the
deep ocean has a wavelength of about 200 kilometres (120 mi). Such a
wave travels at well over 800 kilometres per hour (500 mph), but due to the
enormous wavelength the wave oscillation at any given point takes 20 or 30
minutes to complete a cycle and has an amplitude of only about 1 metre (3.3
ft).This makes tsunamis difficult to detect over deep water. Ships rarely
notice their passage.
• As the tsunami approaches the coast and the waters become shallow, wave
shoaling compresses the wave and its velocity slows below 80 kilometres
per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12
mi) and its amplitude grows enormously, producing a distinctly visible wave.
Since the wave still has such a long wavelength, the tsunami may take
minutes to reach full height.
56. When the wave enters shallow water, it slows
down and its amplitude (height) increases.
• The fourth main earthquake hazard is fire. These fires can be
started by broken gas lines and power lines, or tipped over
wood or coal stoves. They can be a serious problem, especially
if the water lines that feed the fire hydrants are broken, too. For
example, after the Great San Francisco Earthquake in 1906,
the city burned for three days. Most of the city was destroyed
and 250,000 people were left homeless.
• Most of the hazards to people come from man-made structures
themselves and the shaking they receive from the earthquake.
The real dangers to people are being crushed in a collapsing
building, drowning in a flood caused by a broken dam or levee,
getting buried under a landslide, or being burned in a fire.
60. Earthquake Prediction
• Earthquake prediction is a popular pastime for psychics and pseudo-scientists, and
extravagant claims of past success are common. Predictions claimed as "successes"
may rely on a restatement of well-understood long-term geologic earthquake
hazards, or be so broad and vague that they are fulfilled by typical background
seismic activity. Neither tidal forces nor unusual animal behavior have been useful for
predicting earthquakes. If an unscientific prediction is made, scientists can not state
that the predicted earthquake will not occur, because an event could possibly occur
by chance on the predicted date, though there is no reason to think that the predicted
date is more likely than any other day.
• Scientific earthquake predictions should state where, when, how big, and how
probable the predicted event is, and why the prediction is made. The National
Earthquake Prediction Evaluation Council reviews such predictions, but no generally
useful method of predicting earthquakes has yet been found.
• Because of their devastating potential, there is great interest in predicting the location
and time of large earthquakes. Although a great deal is known about where
earthquakes are likely, there is currently no reliable way to predict the days or months
when an event will occur in any specific location.
61. • Although we are not able to predict individual earthquakes, the world's largest
earthquakes do have a clear spatial pattern, and "forecasts" of the locations and
magnitudes of some future large earthquakes can be made. Most large earthquakes
occur on long fault zones around the margin of the Pacific Ocean.
• It may never be possible to predict the exact time when a damaging earthquake will occur,
because when enough strain has built up, a fault may become inherently unstable, and
any small background earthquake may or may not continue rupturing and turn into a large
earthquake. While it may eventually be possible to accurately diagnose the strain state of
faults, the precise timing of large events may continue to elude us. In the Pacific
Northwest, earthquake hazards are well known and future earthquake damage can be
greatly reduced by identifying and improving or removing our most vulnerable and
• This is because the Atlantic Ocean is growing a few inches wider each year, and the
Pacific is shrinking as ocean floor is pushed beneath Pacific Rim continents. Geologically,
earthquakes around the Pacific Rim are normal and expected. The long fault zones that
ring the Pacific are subdivided by geologic irregularities into smaller fault segments which
62. • Earthquake magnitude and timing are controlled by the size of a fault segment, the
stiffness of the rocks, and the amount of accumulated stress. Where faults and plate
motions are well known, the fault segments most likely to break can be identified. If a
fault segment is known to have broken in a past large earthquake, recurrence time
and probable magnitude can be estimated based on fault segment size, rupture
history, and strain accumulation. This forecasting technique can only be used for
well-understood faults, such as the San Andreas. No such forecasts can be made for
poorly-understood faults, such as those that caused the 1994 Northridge, CA and
1995 Kobe, Japan quakes.
• One well-known successful earthquake prediction was for the Haicheng, China
earthquake of 1975, when an evacuation warning was issued the day before a M 7.3
earthquake. In the preceding months changes in land elevation and in ground water
levels, widespread reports of peculiar animal behavior, and many foreshocks had led
to a lower-level warning. An increase in foreshock activity triggered the evacuation
warning. Unfortunately, most earthquakes do not have such obvious precursors. In
spite of their success in 1975, there was no warning of the 1976 Tangshan
earthquake, magnitude 7.6, which caused an estimated 250,000 fatalities.