Book Report on Earthquakes

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Earthquakes


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 Kobe, 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
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