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 Richter scale.

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

earthquakes.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

coast.)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 beings.

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

faults.It was about 1910 that Alfred Wegener published his

theory of plate tectonics to explain volcanic and seismic activity.

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 phenomenon.

Following the great 1906 San Francisco earthquake, Harry Fielding Reid examined the displacement of the ground surface around the

San Andreas Fault.[1] From his observations he concluded that theearthquake 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

complicated.

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 waves.

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.

BODY 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

.

P 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 just

before an earthquake 'hits' (or more specifically, before the surface waves arrive). Usually people can only feel the bump and rattle of these

waves.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

wave propagation

S WAVES 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).

SURFACE WAVES• 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 earthquakes.

LOVE WAVESThe 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.

RAYLEIGH WAVES

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 waves.

What is an EarthquakeFocus 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.

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

• 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 Earth.

• 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

• 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.

WHERE DO EARTHQUAKES OCCUR, AND HOW OFTEN?

HOW EARTHQUAKES WORK?

JAPAN EARTHQUAKES AERIAL VIEW

WHERE DO EARTHQAUKES COMMONLY OCCUR?

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 lines.

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 dangerous.

Police and military personnel may be sent into the area. Be sure you do not appear to be a looter to them.

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.