Block 7


The Effect of Earthquakes and Why They Happen

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The Ring of Fire is a major area in
the basin of the Pacific Ocean where a high percentage of earthquakes and
volcanic eruptions occur.  Within a 40,000-km
horseshoe shape that is casted upon the perimeter of the Pacific plate, it is
associated with a nearly continuous series of oceanic trenches, volcanic arcs,
volcanic belts, and plate movements (Wikipedia, 2017). Four-hundred-fifty-two
volcanoes are in this mountain chain that rests atop the basin of the Pacific (What is the
“Ring of Fire?”, 2017), and the reason why
a large majority of volcanoes are a part of this volcanic mountain chain is due
to the constant subduction of the tectonic plates.  In addition, many earthquakes are the result
of the constant subduction.

is a geological occurrence that happens when one plate of oceanic lithosphere
(the term lithosphere is used to describe the rigid outer part of the earth, which consists of
the crust and upper mantle) is forced under another plate (What is the
“Ring of Fire?”, 2017).   Three types of earthquakes are produced depending
upon the interaction of the tectonic plates, divergent plate boundary,
convergent plate boundary, and the transform plate boundary.  A divergent boundary occurs
when two tectonic plates move away from each other.  Along these boundaries, lava spews from long
fissures creating new crust.  As the
magma solidifies it transforms into a dark, dense rock called Basalt (NOAA, 2013).  Convergent plate boundaries happen when two
plates collide together.  When the plates
strike, one plate rises upward while the other down.  Typically, when this happens a mountain ridge
is formed parallel to the boundary.  Also, a typical volcano can be formed this
way.  Powerful earthquakes shake a wide
area on both sides of the boundary (NOAA, 2013).  The phenomenon when two plates slide against
one another is known as a transform plate boundary.  As the plates alternately press in a
propagational direction against each other, earthquakes are produced through a
wide boundary zone.  In contrast to
convergent and divergent boundaries, no magma is formed. Thus, crust is cracked
and broken at transform margins, but is not created or destroyed (NOAA, 2013).

When one of the three plates
create tension, the focal point of an earthquake is determined, also known as
the epicenter.  The focal point refers to
the depth at which an earthquake is initiated. 
Earthquakes happen at three depths. 
Shallow earthquakes occur in depths less than 70-km.  When the depth is between 70-km and 300-km it
commonly is classified as mid-focus or intermediate-depth earthquakes.  But in subduction zones, where older and
colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths in the mantle, ranging from 300-km up
to 700-km.  Generally, the shallower the earthquake the more damaging they
are (William Spence, 1989).

the formation of an earthquake’s hypocenter or focus point, fault lines form
from the direct result of sliding plates. 
There are three variants of fault lines: Strike-Slip Faults, Normal Faults,
and Reverse Faults (Oskin, 2017).  Strike-Slip Faults indicate the horizontal
movement of rocks but have little to no vertical movement.  Normal Faults are likewise to divergent plate
boundaries, because the earth’s crust is pulled away manipulating the landscape.  Reverse Faults, also called thrust faults, slide one block of crust on top
of another. These faults are commonly found in collisions zones, where tectonic
plates push up mountain ranges (Oskin, 2017).

an earthquake ensues, there are
two main categories of seismic waves, body waves and surface waves.  Body waves are of two types: compressional or primary
waves and shear or secondary waves. P-waves and S- waves are called “body
waves” because they can travel through the interior of a body such as the
Earth’s inner layers, from the focus of an earthquake to
distant points on the surface (BRAILE, 2007).  P-waves are the fastest and strongest seismic
wave being able to travel through solid rock and liquid.  They travel by vibrating in a fixed direction
of propagation called a longitudinal wave (BRAILE, 2007).  S-waves are the second waves recorded by the
seismograph.  They travel in a series of
undulated motions called transverse waves.  S-waves also possess a weaker frequency with
it only capable of traveling through solid rock rather than a liquid medium and
are the aftershock of the P-waves (BRAILE, 2007).  A
surface wave is a seismic wave that is trapped near the surface of
the Earth. Though it is the slowest of the waves, it causes great
destruction when it breaches the surface. When
these waves are recorded they are categorized into different levels of

magnitude of an earthquake is used to describe the intensity and relative size
of an earthquake with each level being ten times stronger than the previous
level.  Earthquakes with a magnitude of
2.5 or less on the Richter Scale is considered minor and cannot be felt on the
surface.  When the magnitude reaches 2.5
to 5.4 minor shakes can be felt.  As an
earthquake’s magnitude increases from then on, from 5.5 to 6.0 minor damage is
inflicted upon buildings with greater magnitudes from 6.1 to 8.0 or greater,
major damage is induced on structural foundations and land (MichiganTech,

There are other ways an earthquake can be produced other than from the
movement of the tectonic plates. 
Earthquakes can be of the direct result of human activity.  These man-made tremors are called induced seismicity.  Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity that
alters the stresses and strains on the Earth’s crust.  Most induced seismicity is of a low magnitude. Examples of induced
earthquakes are waste water disposal wells. 
It is a common misconception that fracking is the major influence of
induced earthquakes, but wastewater disposal wells typically operate for
longer durations and inject much more fluid than hydraulic fracturing, making
them more likely to induce earthquakes (USGS, n.d.).  For example, in Oklahoma, which has the most
induced earthquakes in United States, only 1-2% of the earthquakes can be
linked to hydraulic fracturing operations. The remaining earthquakes are
induced by wastewater disposal.  But
most injection wells are not associated with felt earthquakes. A combination of
many factors is necessary for injection to induce felt earthquakes. These
include: the injection rate and total volume injected; the presence of faults
that are large enough to produce earthquakes that are felt; stresses that are
large enough to produce earthquakes; and the presence of pathways for the fluid
pressure to travel from the injection point to faults (USGS,
n.d.).  Other examples that result in induced
earthquakes are artificial lakes, mining, and geothermal energy.  Another way in which an earthquake can be
produced is through volcanic activity.  Earthquakes
related to volcanic activity may produce hazards which include ground cracks,
ground deformation, and damage to manmade structures. There are two general
categories of earthquakes that can occur at a volcano: volcano-tectonic
earthquakes and long period earthquakes.

Earthquakes produced by stress changes in
solid rock due to the injection or withdrawal of magma are called volcano-tectonic
earthquakes (Chouet, 1993). These earthquakes can cause land to
subside and can produce large ground cracks. These earthquakes can occur as
rock is moving to fill in spaces where magma is no longer present.
Volcano-tectonic earthquakes don’t indicate that the volcano will be erupting
but can occur at any time.

The second category of volcanic earthquakes are long period earthquakes which
are produced by the injection of magma into surrounding rock. These earthquakes
are a result of pressure changes during the unsteady transport of the magma.
When magma injection is sustained a lot of earthquakes are produced (Chouet,
1993). This type of activity indicates that a volcano is about to erupt.
Scientists use seismographs to record the signal from these earthquakes. This
signal is known as volcanic



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