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EARTH QUAKE REFERENCE FILES

EARTHQUAKE REFERENCE FILES Earthquake, shaking of the earth?s surface caused by

rapid movement of the earth?s rocky outer layer. Earthquakes occur when energy stored within

the earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to

the surface of the earth by earthquake waves. The study of earthquakes and the waves they create

is called seismology. Scientists who study earthquakes are called seismologists. (Webster?s

p.423) The destruction an earthquake causes, depends on its magnitude or the amount of shaking

that occurs. The size varies from small imperceptible shaking, to large shocks felt miles around.

Earthquakes can tear up the ground, make buildings and other structures collapse, and create

tsunamis (large sea waves). Many Lives can be lost because of this destruction. (The Road to

Jaramillo p.211) Several hundred earthquakes, or seismic tremors, occur per day around the

world. A worldwide network of seismographs detect about one million small earthquakes per

year. Very large earthquakes, such as the 1964 Alaskan earthquake, which measured 8.6 on the

Richter scale and caused millions of dollars in damage, occur worldwide once every few years.

Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California (magnitude 7.0), and

the 1995 tremor in K?be, Japan (magnitude 6.8), occur about 20 times a year. Moderate

earthquakes also cause millions of dollars in damage and can harm many people. (The Road to

Jaramillo p.213-215) In the last 500 years, several million people have been killed by

earthquakes around the world, including over 240,000 in the 1976 T?ang-Shan, China,

earthquake. Worldwide, earthquakes have also caused severe property and structural damage.

Good precautions, such as education, emergency planning, and constructing stronger, more

flexible structures, can limit the loss of life and decrease the damage caused by earthquakes.

(The Road to Jaramillo p.213-215,263) AN EARTHQUAKES ANATOMY Seismologists

examine the parts of an earthquake, like what happens to the earth?s surface during an

earthquake, how the energy of an earthquake moves from inside the earth to the surface, and

how this energy causes damage. By studying the different parts and actions of earthquakes,

seismologists learn more about their effects and how to predict ground shaking in order to

reduce damage. (On Shifting Ground p.109-110) Focus and Epicenter The point within the earth

along the rupturing geological fault where an earthquake originates is called the focus, or

hypocenter. The point on the earth?s surface directly above the focus is called the epicenter.

Earthquake waves begin to radiate out from the focus and follow along the fault rupture. If the

focus is near the surface between 0 and 70 km (0 and 40 mi.) deep shallow focus earthquakes are

produced. If it is deep below the crust between 70 and 700 km (40 and 400 mi.) deep a deep

focus earthquake will occur. Shallow-focus earthquakes tend to be larger, and therefore more

damaging, earthquakes. This is because they are closer to the surface where the rocks are

stronger and build up more strain. (The Ocean of Truth p.76 & The road to Jaramillo p.94-97)

Seismologists know from observations that most earthquakes originate as shallow-focus

earthquakes and most of them occur near plate boundaries areas where the earth?s crustal plates

move against each other. Other earthquakes, including deep-focus earthquakes, can originate in

subduction zones, where one tectonic plate subducts, or moves under another plate. (The Ocean

of Truth p.54-56) I Faults Stress in the earth?s crust creates faults places where rocks have

moved and can slip, resulting in earthquakes. The properties of an earthquake depend strongly

on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists

categorize faults according to the direction of the fault slip. The surface between the two sides of

a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an

angle into the earth. When the rock hanging over the dipping fault plane slips downward into the

ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the

bottom wall, the fault is called a reverse fault or a thrust fault. Both normal and reverse faults

produce vertical displacements, or the upward movement of one side of the fault above the other

side, that appear at the surface as fault scarps. Strike slip faults are another type of fault that

produce horizontal displacements, or the side by side sliding movement of the fault, such as seen

along the San Andreas fault in California. Strike-slip faults are usually found along boundaries

between two plates that are sliding past each other. (Plate Tectonics p.49-53) II Waves The

sudden movement of rocks along a fault causes vibrations that transmit energy through the earth

in the form of waves. Waves that travel in the rocks below the surface of the earth are called

body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S,

waves. The S waves, also known as shearing waves, cause the most damage during earthquake

shaking, as they move the ground back and forth. (Plate tectonics p.133) Earthquakes also

contain surface waves that travel out from the epicenter along the surface of the earth. Two types

of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and

Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage

to structures, as they shake the ground underneath the foundations of buildings and other

structures. Body waves, or P and S waves, radiate out from the rupturing fault starting at the

focus of the earthquake. P waves are compression waves because the rocky material in their path

moves back and forth in the same direction as the wave travels alternately compressing and

expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6

to 7 km (4 mi.) per second. P waves are followed by S waves, which shear, or twist, rather than

compress the rock they travel through. S waves travel at about 3.5 km (2 mi.) per second. S

waves cause rocky material to move either side to side or up and down perpendicular to the

direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists

to locate the focus and epicenter of an earthquake. As P and S waves move through the interior

of the earth, they are reflected and refracted, or bent, just as light waves are reflected and bent by

glass. Seismologists examine this bending to determine where the earthquake originated.

(Encarta 98) On the surface of the earth, Rayleigh waves cause rock particles to move forward,

up, backward, and down in a path that contains the direction of the wave travel. This circular

movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular

path onto a beach. The second type of surface wave, the Love wave, causes rock to move

horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical

displacements. Rayleigh and Love waves always travel slower than P waves and usually travel

slower than S waves. (The Floor of the Sea p.76-78, 112-115) III CAUSES Most earthquakes are

caused by the sudden slip along geologic faults. The faults slip because of movement of the

earth?s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic

plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each

other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when

pressure within the crust increases slowly over hundreds of years and finally exceeds the strength

of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs,

increase stress in the earth?s crust. (Encarta 98) ELASTIC REBOUND THEORY In 1911

American seismologist Harry Fielding Reid studied the effects of the April 1906 California

earthquake. He proposed the elastic rebound theory to explain the generation of earthquakes that

occur in tectonic areas, usually near plate boundaries. This theory states that during an

earthquake, the rocks under strain suddenly break, creating a fracture along a fault. When a fault

slips, movement in the crustal rock causes vibrations. The slip changes the local strain out into

the surrounding rock. The change in strain leads to aftershocks, which are produced by further

slips of the main fault or adjacent faults in the strained region. The slip begins at the focus and

travels along the plane of the fault, radiating waves out along the rupture surface. On each side

of the fault, the rock shifts in opposite directions. The fault rupture travels in irregular steps

along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations

that propagate as seismic waves. After the earthquake, strain begins to build again until it is

greater than the forces holding the rocks together, then the fault snaps again and causes another

earthquake. (Plate tectonics p.56-59) DISTRIBUTION Seismologists have been monitoring the

frequency and locations of earthquakes for most of the 20th century. They have found that the

majority of earthquakes occur along plate tectonic boundaries, while there are relatively few

intraplate earthquakes, that occur within a tectonic plate. The categorization of earthquakes is

related to where they occur, as seismologists generally classify naturally occurring earthquakes

into one of two categories: interplate and intraplate. Interplate earthquakes are the most

common; they occur primarily along plate boundaries. Intraplate earthquakes occur within the

plates at places where the crust is fracturing internally. Both interplate and intraplate earthquakes

may be caused by tectonic or volcanic forces. (Naked Earth p.134-135) I Tectonic Earthquakes

Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a

fault. The released energy is produced by the strain on the rocks due to movement within the

earth, called tectonic deformation. The effect is like the sudden breaking and snapping back of a

stretched elastic band. (The Ocean of truth p.122) II Volcanic Earthquakes Volcanic earthquakes

occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes.

Volcanic earthquakes are caused by the upward movement of magma under the volcano, which

strains the rock locally, and leads to an earthquake. As the fluid magma rises to the surface of the

volcano, it moves and fractures rock masses and causes continuous tremors that can last up to

several hours or days. Volcanic earthquakes occur in areas that are associated with volcanic

eruptions, such as in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and

at isolated hot spots such as Hawaii. (Plate tectonics p.74) LOCATIONS Seismologists use

global networks of seismographic stations to accurately map the focuses of earthquakes around

the world. After studying the worldwide distribution of earthquakes, the pattern of earthquake

types, and the movement of the earth?s rocky crust, scientists proposed that plate tectonics, or the

shifting of the plates as they move over another weaker rocky layer, was the main underlying

cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories

and discoveries. Scientists now use the plate tectonics theory to describe the movement of the

earth’s plates and how this movement causes earthquakes. They also use the knowledge of plate

tectonics to explain the locations of earthquakes, mountain formation, deep ocean trenches, and

predict which areas will be damaged the most by earthquakes. It is clear that major earthquakes

occur most frequently in areas with features that are found at plate boundaries: high mountain

ranges and deep ocean trenches. Earthquakes within plates, or intraplate tremors, are rare

compared with the thousands of earthquakes that occur at plate boundaries each year, but they

can be very large and damaging. (On shifting ground p.17-19) Earthquakes that occur in the area

surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of

80 percent of the energy released in earthquakes worldwide. Japan is shaken by more than 1000

tremors greater than 3.5 in magnitude each year. The western coasts of North and South America

are very also active earthquake zones, with several thousand small to moderate earthquakes each

year. (U.S.G.S.) Intraplate earthquakes are less frequent than plate boundary earthquakes, but

they are still caused by the internal fracturing of rock masses. The New Madrid, Missouri,

earthquakes of 1811 and 1812 were extreme examples of intraplate seismic events. Scientists

estimate that the three main earthquakes of this series were about magnitude 8.0 and that there

were at least 1500 aftershocks. (The ocean of truth p.67-69) EFFECTS Ground shaking leads to

landslides and other soil movement. These are the main damage causing events that occur during

an earthquake. Primary effects that can accompany an earthquake include property damage, loss

of lives, fire, and tsunami waves. Secondary effects, such as economic loss, disease, and lack of

food and clean water, also occur after a large earthquake. (On shifting ground p.47) Ground

Shaking and Landslides Earthquake waves make the ground move, shaking buildings and

structures and causing poorly designed or weak structures partially or totally collapse. The

ground shaking weakens soils and foundation materials under structures and causes dramatic

changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like

liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil

beneath structures and buildings weakens. Shaking may also dislodge large earth and rock

masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of

lives or further property damage. (The road to Jaramillo p.43-45) REDUCING DAMAGE

Earthquakes cannot be prevented, but the damage they cause can be greatly reduced with

communication strategies, proper structural design, emergency preparedness planning,

education, and safer building standards. In response to the tragic loss of life and great cost of

rebuilding after past earthquakes, many countries have established earthquake safety and

regulatory agencies. These agencies require codes for engineers to use in order to regulate

development and construction. Buildings built according to these codes survive earthquakes

better and ensure that earthquake risk is reduced. (On shifting ground p.56) Tsunami

early-warning systems can prevent some damage because tsunami waves travel at a very slow

speed. Seismologists immediately send out a warning when evidence of a large undersea

earthquake appears on seismographs. Tsunami waves travel slower than seismic P and S waves

in the open ocean, they move about ten times slower than the speed of seismic waves in the

rocks below. This gives seismologists time to issue tsunami alerts so that people at risk can



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