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 Kôbe, 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