History of Earthquakes
• Earthquakes have been recorded as early as 1177 B.C. in
China. Of course earthquakes have been a part of myth and
legend since the dawn of man. In Greek Mythology,
Posseidon (Neptune in the Roman pantheon) was "God of the
Sea". Yet one of his powers was thought to be that of "earth
shaker". As a tsunami is often the result of an earthquake,
this was an appropriate power for a sea god.
• In European history, the earliest recorded earthquake
occurred in 580 B.C. In North America the great earthquakes
of 1811-1812 occurred near New Madrid, Missouri. The
magnitude of the quakes are not known, but they
are estimated to have been about 8 on the Richter scale.
There were actually three large quakes with
aftershocks between and for months after. The quake was so
wide-spread it was felt as far away as Boston.
The most destructive quake in U.S.
history occurred in San Francisco in
1906, it caused the deaths of over 700
people. The great Alaskan earthquake
of 1964 was twice as powerful, but less
destructive due to the low population
density of the area struck. The Chilean
quake of 1960 was the biggest quake
ever recorded. It came in at 9.5 on the
The study of earthquakes is called seismology. The
earliest seismologists were the Chinese who worked
hard to record their quakes in detail. They even
developed a means to predict earthquakes by filling a
ceramic jar to the brim with water and leaving it set.
If the water overflowed the jar, then an earthquake
was imminent. Of course, this means of prediction
was unreliable and uncertain.
It is thought that some animals may feel vibrations
from a quake before humans, and that even minutes
before a quake dogs may howl and birds fly
erratically. However, evidence for such sensitivity by
animals is purely anecdotal
Aristotle was one of the first Europeans to create a theory
about the origin of Earthquakes. He thought that they were
the result of heavy winds. Not much more study was
concentrated on earthquakes until the mid-1700s
when London was hit by a devastating quake and
a tsunami struck Lisbon, Portugal shortly after. John Mitchell
in England and Elie Bertrand in Switzerland began a
comprehensive study of the timing and severity of
Soon scientists from several countries were exchanging
observations and theories on earthquakes. In the 1820's Chile
became an area of interest to seismologists. After an
earthquake there, it was noticed that the elevation of the
coastline had changed. This
was substantiated by the Captain of the H.M.S.
Beagle, Robert Fitzroy. (The ship also carried Charles
Darwin who was studying the flora and fauna of the
In the 1850s Robert Mallet, figured out a means to
measure the velocity of seismic waves. Meanwhile, in
Italy, Luigi Palmieri invented an
electromagnetic seismograph, one of which
was installed near Mount Vesuvius and another at the
University of Naples. These seismographs were the
first seismic instruments capable of routinely
detecting earthquakes imperceptible to human
In 1872 a U.S. scientist named Grove Gilbert figured
out that earthquakes usually center around a fault
line. It was after the 1906 earthquake in San Francisco
that Harry Reid hypothesized that earthquakes were
likely the result of a build-up of pressure along these
It was about 1910 that Alfred Wegener published his
theory of plate tectonics to explain volcanic and
Since then, seismologists have continued to work at a
furious pace, building better instruments, computer
models, theories and forecast to study the causes and
effects of earthquakes.
What is the elastic rebound theory?
• The elastic rebound theory was developed by Harry
Fielding Reid, an American geophysicist who was studying
the 1906 San Francisco earthquake.
He observed that points on the Earth's surface distant from
the San Andreas fault had gradually moved prior to the
earthquake whereas points on the surface directly on and
around the fault had not.
During the earthquake the points next to the fault zone
which had originally been static had suddenly shifted to
match up with the points at a greater distance from the
locked fault zone.
He concluded that this was due to the accumulation
of elastic strain within the Earth's crust around the
fault zone and that when the stress that caused this
strain exceeded the strength of the rock mass or fault
zone in the crust it suddenly ruptured. This caused the
stored energy (termed elastic potential energy) to be
released in one instant, causing an earthquake, and
also meant that the rock mass around the fault zone
that had originally been locked in position, snapped
or rebounded to match the position of the rock mass
at a greater distance from the fault. As such he coined
the term "elastic rebound" to describe this
Following the great 1906 San Francisco earthquake, Harry Fielding Reid
examined the displacement of the ground surface around the
San Andreas Fault.
From his observations he concluded that the
earthquake must have been the result of the elastic rebound of
previously stored elastic strain energy in the rocks on either side of the
fault. In an interseismic period, the Earth's plates (see plate tectonics)
move relative to each other except at most plate boundaries where
they are locked. Thus, if a road is built across the fault as in the figure
panel Time 1, it is perpendicular to the fault trace at the point E, where
the fault is locked
The far field plate motions (large arrows) cause the rocks in the region
of the locked fault to accrue elastic deformation, figure panel Time 2.
The deformation builds at the rate of a few centimeters per year, over a
time period of many years. When the accumulated strain is great
enough to overcome the strength of the rocks, an earthquake occurs.
During the earthquake, the portions of the rock around the fault that
were locked and had not moved 'spring' back, relieving the
displacement in a few seconds that the plates moved over the entire
interseismic period (D1 and D2 in Time 3). The time period between
Time 1 and Time 2 could be months to hundreds of years, while the
change from Time 2 to Time 3 is seconds. Like an elastic band, the more
the rocks are strained the more elastic energy is stored and the greater
potential for an event. The stored energy is released during the rupture
partly as heat, partly in damaging the rock, and partly as elastic waves.
Modern measurements using GPS largely support Reid’s theory as the
basis of seismic movement, though actual events are often more
What Is Seismology?
• Seismology is the study of
earthquakes and seismic waves that
move through and around the earth.
A seismologist is a scientist who
studies earthquakes and seismic
What Are Seismic Waves?
• Seismic waves are the waves of
energy caused by the sudden
breaking of rock within the earth or
an explosion. They are the energy
that travels through the earth and is
recorded on seismographs.
Types of Seismic Waves
• There are several different kinds of seismic waves,
and they all move in different ways. The two main
types of waves arebody waves and surface waves.
Body waves can travel through the earth's inner
layers, but surface waves can only move along the
surface of the planet like ripples on water.
Earthquakes radiate seismic energy as both body
and surface waves.
• Travelling through the interior of the
earth, body waves arrive before the
surface waves emitted by an
earthquake. These waves are of a
higher frequency than surface waves
• The first kind of body wave is the P wave or primary wave.
This is the fastest kind of seismic wave, and, consequently,
the first to 'arrive' at a seismic station. The P wave can
move through solid rock and fluids, like water or the liquid
layers of the earth. It pushes and pulls the rock it moves
through just like sound waves push and pull the air. Have
you ever heard a big clap of thunder and heard the
windows rattle at the same time? The windows rattle
because the sound waves were pushing and pulling on the
window glass much like P waves push and pull on rock.
Sometimes animals can hear the P waves of an earthquake.
Dogs, for instance, commonly begin barking hysterically
before an earthquake 'hits' (or more specifically, before the surface
waves arrive). Usually people can only feel the bump and rattle of these
P waves are also known as compressional waves, because of the
pushing and pulling they do. Subjected to a P wave, particles move in
the same direction that the the wave is moving in, which is the direction
that the energy is traveling in, and is sometimes called the 'direction of
The second type of body wave is the S wave or secondary wave, which is the second
wave you feel in an earthquake. An S wave is slower than a P wave and can only move
through solid rock, not through any liquid medium. It is this property of S waves that
led seismologists to conclude that the Earth's outer core is a liquid. S waves move rock
particles up and down, or side-to-side--perpindicular to the direction that the wave is
travelling in (the direction of wave propagation).
• Travelling only through the crust, surface
waves are of a lower frequency than body
waves, and are easily distinguished on a
seismogram as a result. Though they arrive
after body waves, it is surface waves that are
almost enitrely responsible for the damage
and destruction associated with
earthquakes. This damage and the strength
of the surface waves are reduced in deeper
The first kind of surface wave is called a Love wave, named
after A.E.H. Love, a British mathematician who worked out
the mathematical model for this kind of wave in 1911. It's the
fastest surface wave and moves the ground from side-to-side.
Confined to the surface of the crust, Love waves produce
entirely horizontal motion.
The other kind of surface wave is the Rayleigh wave, named for John
William Strutt, Lord Rayleigh, who mathematically predicted the
existence of this kind of wave in 1885. A Rayleigh wave rolls along the
ground just like a wave rolls across a lake or an ocean. Because it rolls, it
moves the ground up and down, and side-to-side in the same direction
that the wave is moving. Most of the shaking felt from an earthquake is
due to the Rayleigh wave, which can be much larger than the other
What is an Earthquake
Focus and Epicenter?
Where is the earthquake focus? The focus of an
earthquake is the point where the rocks start to
fracture. It is the origin of the earthquake. The
epicenter is the point on land directly above the
Focus of an earthquake
• The focus is also called the hypocenter of
an earthquake. The vibrating waves
travel away from the focus of the
earthquake in all directions. The waves
can be so powerful they will reach all
parts of the Earth and cause it to vibrate
like a turning fork.
Epicenter of an earthquake
• Directly above the focus on the Earth's surface
is the earthquake epicenter. Earthquake
waves start at he focus and travel outward in
all directions. Earthquake waves do not
originate at the epicenter.
• Most news stories on earthquakes will list the
epicenter of an earthquake and then tell how
deep the earthquake was from the epicenter.
• Shallow-focus earthquakes occur between 0 and 40
miles deep. Shallow-focus earthquakes are much
more common than deep-focus earthquakes.
Crustal plates moving against each other produce
most of the shallow-focus earthquakes here on
• Shallow-focus earthquakes are much more
dangerous than deep-focus earthquakes. They
release 75% of all the energy produced by
earthquakes each year.
• Deep-focus earthquakes occur 180
miles or more below the Earth's
surface. These earthquakes occur in
island arc or deep ocean trenches
where one plate is slipping over
another in subduction zones.
To Do List - Before An Earthquake!
Develop a plan of action in the event of an earthquake.
Agree with friends and family on a contact point that is outside of the quake zone. With one
contact point, you will avoid tying up phone lines, and friends and family can go to one safe
source for news about you and others in the zone.
Locate the safe and the dangerous spots around your home and office so that you can act
quickly should the need arise.
Check your house to make sure it is up to earthquake codes. Is the foundation bolted to the
house? Is your hot water heater bolted down?
Know how to shut down utilities at their source to ensure quick closing of gas and water
leaks and to secure electrical line
Protect fragile valuables with latched cupboards, cushioning, etc.
Make sure you have a kit that includes a jackknife, a flashlight, a GPS device, a waterfilter,
and a solar/windup radio.
Make sure there is always plenty of gas in your vehicle.
• Store flammable liquids outside of the house.
• Prepare a package of emergency suplies that includes food, water, a first-aid kit, and cash. It
is best to have this cache be capable of lasting the entire family for at least three days.
To Do List - During An Earthquake!
Do not panic. Take care of the basics first, get under a large, stable piece of
furniture like a table and hold on to it to keep it from moving away from you. If
outside, get your feet on the ground and duck to avoid flying debris.
If you are inside do not try to rush out of the building. Shattering glass and falling
bricks can be a great hazard for anyone leaving a building. Do not try to take
elevators or stairs during the quake.
Avoid windows and glass doors, planters, bookcases, furniture on wheels,
chimneys, kitchens and shopping mall walkways.
If you are outdoors when the quake strikes, stay away from buildings, dams, gas
and water mains, power lines, trees, fuel tanks, vehicles, or anything else that
could fall on you or roll over you.
If you are driving, pull off the road and stay clear of bridges, overpasses
and parking garages
Avoid smoking or using an open flame in case of a gas main leak.
Prepare for aftershocks
If you are on a waterfront head for high ground immediately; earthquakes often
create huge waves capable of incredible damage.
To Do List - After An Earthquake!
Make sure you get your shoes on, there may be considerable glass shards and
other object that could injure your feet. Medical attention may not be immediately
available, so this is a bad time to hurt yourself through negligence.
Do not move anyone who is seriously injured unless other danger is imminent. If
possible, let trained medical personnel make this judgement.
If you join rescue work, tread warilly as building foundations will be weak and
there is the possibility of an aftershock.
If you think that there may be damage to utilities then shut them off.
Use your radio to find out information about the quake, what other dangers might
be lurking and what to do about them.
Avoid tying up communication lines. Unhung phones can cause systems to shut
down, so be sure to hang up all phones.
Be judicious about leaving your present location to search for loved ones or to
travel home. Moving about just after an earthquake can bring you up against
unexpected hazards like broken gas mains, unpassable roads and downed power
Remember to preserve your water
supplies (toilet tanks, hot water heaters). Don't
trust unknown water sources, make sure you boil
the water or chlorinate.
Watch out for looters, some may be armed and
Police and military personnel may be sent into the
area. Be sure you do not appear to be a looter to
Finally, do not panic. There will be plenty to do and
to worry about. Take care that children are
reassured. Remember, help is on the way.
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