Earth Quake Reference Files

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 KÐ"Ò'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