Earthquakes
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 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 evacuate the coastal
area as a preventative measure
to reduce related injuries or deaths.
Scientists radio or telephone the
information to the Tsunami Warning Center
in Honolulu and other stations.(The
floor of the sea p.59) Engineers minimize
earthquake damage to buildings by
using flexible, reinforced materials that
can withstand shaking in buildings.
Since the 1960s, scientists and
engineers have greatly improved earthquake
resistant designs for buildings
that are compatible with modern architecture and
building materials. They use
computer models to predict the response of the
building to ground shaking
patterns and compare these patterns to actual seismic
events, such as in the
1994 Northridge, California, earthquake and the 1995 Kôbe,
Japan,
earthquake. They also analyze computer models of the motions of buildings
in
the most hazardous earthquake zones to predict possible damage and to
suggest
what reinforcement is needed. (Martin Alfred p.62) Structural Design
Geologists
and engineers use risk assessment maps, such as geologic hazard
and seismic
hazard zoning maps, to understand where faults are located and
how to build near
them safely. Engineers use geologic hazard maps to predict
the average ground
motions in a particular area and apply these predicted
motions during
engineering design phases of major construction projects.
Engineers also use
risk assessment maps to avoid building on major faults or
to make sure that
proper earthquake bracing is added to buildings constructed
in zones that are
prone to strong tremors. They can also use risk assessment
maps to aid in the
retrofit, or reinforcement, of older structures. (The
ocean of truth p.23) In
urban areas of the world, the seismic risk is greater
in non-reinforced
buildings made of brick, stone, or concrete blocks because
they cannot resist
the horizontal forces produced by large seismic waves.
Fortunately,
single-family timber-frame homes built under modern construction
codes resist
strong earthquake shaking very well. Such houses have laterally
braced frames
bolted to their foundations to prevent separation. Although
they may suffer some
damage, they are unlikely to collapse because the
strength of the strongly
jointed timber-frame can easily support the light
loads of the roof and the
upper stories even in the event of strong vertical
and horizontal ground
motions.(On shifting groung p.73) Emergency
Preparedness Plans Earthquake
education and preparedness plans can help
significantly reduce death and injury
caused by earthquakes. People can take
several preventative measures within
their homes and at the office to reduce
risk. Supports and bracing for shelves
reduce the likelihood of items falling
and potentially causing harm. Maintaining
an earthquake survival kit in the
home and at the office is also an important
part of being prepared. (On
shifting ground p.97) In the home, earthquake
preparedness includes
maintaining an earthquake kit and making sure that the
house is structurally
stable. The local chapter of the American Red Cross is a
good source of
information for how to assemble an earthquake kit. During an
earthquake,
people indoors should protect themselves from falling objects and
flying
glass by taking refuge under a heavy table. After an earthquake,
people
should move outside of buildings, assemble in open spaces, and
prepare
themselves for aftershocks. They should also listen for emergency
bulletins on
the radio, stay out of severely damaged buildings, and avoid
coastal areas in
the event of a tsunami. (The floor of the sea p.46) In many
countries,
government emergency agencies have developed extensive earthquake
response
plans. In some earthquake hazardous regions, such as California,
Japan, and
Mexico City, modern strong motion seismographs in urban areas
are now linked to
a central office. Within a few minutes of an earthquake,
the magnitude can be
determined, the epicenter mapped, and intensity of
shaking information can be
distributed via radio to aid in response
efforts.(The floor of the sea p.18)
STUDYING EARTHQUAKES Seismologists
measure earthquakes to learn more about them
and to use them for geological
discovery. They measure the pattern of an
earthquake with a machine called a
seismograph. Using multiple seismographs
around the world, they can
accurately locate the epicenter of the earthquake, as
well as determine its
magnitude, or size, and fault slip properties. (Alfred
Wegener &
encarta 98) I Measuring Earthquakes An analog seismograph consists
of a base
that is anchored into the ground so that it moves with the ground
during an
earthquake, and a spring or wire that suspends a weight, which
remains
stationary during an earthquake. In older models, the base includes a
rotating
roll of paper, and the stationary weight is attached to a stylus, or
writing
utensil, that rests on the roll of paper. During the passage of a
seismic wave,
the stationary weight and stylus record the motion of the
jostling base and
attached roll of paper. The stylus records the information
of the shaking
seismograph onto the paper as a seismogram. Scientists also
use digital
seismographs, computerized seismic monitoring systems that record
seismic
events. Digital seismographs use re-writeable, or multiple-use, disks
to record
data. They usually incorporate a clock to accurately record seismic
arrival
times, a printer to print out digital seismograms of the information
recorded,
and a power supply. Some digital seismographs are portable;
seismologists can
transport these devices with them to study aftershocks of a
catastrophic
earthquake when the networks upon which seismic monitoring
stations depend have
been damaged. (Plate Tectonics p.56-58, 64) There are
more than 1000 seismograph
stations in the world. One way that seismologists
measure the size of an
earthquake is by measuring the earthquake’s seismic
magnitude, or the
amplitude of ground shaking that occurs. Seismologists
compare the measurements
taken at various stations to identify the
earthquake’s epicenter and determine
the magnitude of the earthquake. This
information is important in order to
determine whether the earthquake
occurred on land or in the ocean. It also helps
people prepare for resulting
damage or hazards such as tsunamis. When readings
from a number of
observatories around the world are available, the integrated
system allows
for rapid location of the epicenter. At least three stations are
required in
order to triangulate, or calculate, the epicenter. Seismologists
find the
epicenter by comparing the arrival times of seismic waves at the
stations,
thus determining the distance the waves have traveled. Seismologists
then
apply travel-time charts to determine the epicenter. With the present
number
of worldwide seismographic stations, many now providing digital signals
by
satellite, distant earthquakes can be located within about 10 km (6 mi.)
of
the epicenter and about 10 to 20 km (6 to 12 mi.) in focal depth.
Special
regional networks of seismographs can locate the local epicenters
within a few
kilometers. (the Ocean of truth) . All magnitude scales give
relative numbers
that have no physical units. The first widely used seismic
magnitude scale was
developed by the American seismologist Charles Richter in
1935. The Richter
scale measures the amplitude, or height, of seismic surface
waves. The scale is
logarithmic, so that each successive unit of magnitude
measure represents a
tenfold increase in amplitude of the seismogram
patterns. This is because ground
displacement of earthquake waves can range
from less than a millimeter to many
meters. Richter adjusted for this huge
range in measurements by taking the
logarithm of the recorded wave heights.
So, a magnitude 5 Richter measurement is
ten times greater than a magnitude
4; while it is 10 x 10, or 100 times greater
than a magnitude 3 measurement.
(The floor of the sea p.89-91) Today,
seismologists prefer to use a different
kind of magnitude scale, called the
moment magnitude scale, to measure
earthquakes. Seismologists calculate moment
magnitude by measuring the
seismic moment of an earthquake, or the
earthquake’s strength based on a
calculation of the area and the amount of
displacement in the slip. The
moment magnitude is obtained by multiplying these
two measurements. It is
more reliable for earthquakes that measure above
magnitude 7 on other scales
that refer only to part of the seismic waves,
whereas the moment magnitude
scale measures the total size. The moment magnitude
of the 1906 San Francisco
earthquake was 7.6; the Alaskan earthquake of 1964,
about 9.0; and the 1995
Kôbe, Japan, earthquake was a 7.0 moment magnitude; in
comparison, the
Richter magnitudes were 8.3, 8.6, and 6.8, respectively for
these tremors.
(U.S.G.S.) Earthquake size can be measured by seismic intensity
as well, a
measure of the effects of an earthquake. Before the advent of
seismographs,
people could only judge the size of an earthquake by its effects
on humans or
on geological or human-made structures. Such observations are the
basis of
earthquake intensity scales first set up in 1873 by Italian
seismologist M.
S. Rossi and Swiss scientist F. A. Forel. These scales were
later superseded
by the Mercalli scale, created in 1902 by Italian seismologist
Guiseppe
Mercalli. In 1931 American seismologists H. O. Wood and Frank Neumann
adapted
the standards set up by Guiseppe Mercalli to California conditions
and
created the Modified Mercalli scale. Many seismologists around the world
still
use the Modified Mercalli scale to measure the size of an earthquake
based on
its effects. The Modified Mercalli scale rates the ground shaking by
a general
description of human reactions to the shaking and of structural
damage that
occur during a tremor. This information is gathered from local
reports, damage
to specific structures, landslides, and peoples’ descriptions
of the damage.
(The road to Jaramillo p.122) II Predicting Earthquakes
Seismologists try to
predict how likely it is that an earthquake will occur,
with a specified time,
place, and size. Earthquake prediction also includes
calculating how a strong
ground motion will affect a certain area if an
earthquake does occur. Scientists
can use the growing catalogue of recorded
earthquakes to estimate when and where
strong seismic motions may occur. They
map past earthquakes to help determine
expected rates of repetition.
Seismologists can also measure movement along
major faults using global
positioning satellites (GPS) to track the relative
movement of the rocky
crust of a few centimeters each year along faults. This
information may help
predict earthquakes. Even with precise instrumental
measurement of past
earthquakes, however, conclusions about future tremors
always involve
uncertainty. This means that any useful earthquake prediction
must estimate
the likelihood of the earthquake occurring in a particular area in
a specific
time interval compared with its occurrence as a chance event. (The
ocean of
truth p.29) The elastic rebound theory gives a generalized way of
predicting
earthquakes because it states that a large earthquake cannot occur
until the
strain along a fault exceeds the strength holding the rock masses
together.
Seismologists can calculate an estimated time when the strain along
the fault
would be great enough to cause an earthquake. As an example, after
the
1906 San Francisco earthquake, the measurements showed that in the 50
years
prior to 1906, the San Andreas fault accumulated about 3.2 meters (10
feet) of
displacement, or movement, at points across the fault. The maximum
1906 fault
slip was 6.5 meters (21 feet), so it was suggested that 50 years x
6.5
meters/3.2 meters, about 100 years, would elapse before enough energy
would
again accumulate to produce a comparable earthquake. (Plate
Tectonics)
Scientists have measured other changes along active faults to
try and predict
future activity. These measurements have included changes in
the ability of
rocks to conduct electricity, changes in ground water levels,
and changes in
variations in the speed at which seismic waves pass through
the region of
interest. None of these methods, however, has been successful
in predicting
earthquakes to date. (U.S.G.S) Seismologists have also
developed field methods
to date the years in which past earthquakes occurred.
In addition to information
from recorded earthquakes, scientists look into
geologic history for information
about earthquakes that occurred before
people had instruments to measure them.
This research field is called
paleoseismology. Seismologists can determine when
ancient earthquakes
occurred. (The floor of the sea p.118) Seismology,
basically, the science of
earthquakes, involving observations of natural ground
vibrations and
artificially generated seismic signals, with many theoretical and
practical
ramifications. A branch of geophysics, seismology has made
vital
contributions to understanding the structure of the earth’s
interior.
(Webster’s) SEISMIC PHENOMENA Different kinds of seismic waves are
produced by
the deformation of rock materials. A sudden slip along a fault,
for example,
produces both longitudinal push-pull (P) and transverse shear
(S) waves.
Compressional trains of P waves, set up by an quick push or
pull in the
direction of wave propagation, cause surface formations to shake
back and forth.
Sudden shear displacements move through materials with
slower S-wave velocity as
vertical planes shake up and down. When P and S
waves encounter a boundary such
as Mohorovièiæ discontinuity (Moho), which
lies between the crust and the
mantle, they are partly reflected, refracted,
and transmitted, breaking up into
several other types of waves as they pass
through the earth. Travel times depend
on compressional and S-wave velocity
changes as they pass through materials with
different elastic properties.
Crustal granitic rocks typically show P-wave
velocities of 6 km/sec, where as
underlying mafic and ultramafic rocks show
velocities of 7 and 8 km/sec. In
addition to P and S waves—body-wave
types—two surface seismic waves are the
Love waves, named for the British
geophysicist Augustus E. H. Love, and
Rayleigh waves, named after the British
physicist John Rayleigh. These waves
travel fast and are guided in their
propagation by the earth’s surface.
(Plate Tectonics p.142) INTRAMENTS OF
STUDY Longitudinal, transverse, and
surface seismic waves cause vibrations at
points where they reach the earth’s
surface. Seismic instruments have been
designed to detect these movements
through electromagnetic or optical methods.
The main instruments, called
seismographs, were perfected following the
development by the German
scientist Emil Wiechert of a horizontal seismograph
about the turn of the
century. (Naked Earth p.36-42) Some instruments, such as
the electromagnetic
pendulum seismometer, employ electromagnetic recording; that
is, induced
tension passes through an electric amplifier to a galvanometer.
A
photographic recorder scans a rapidly moving film, making
sensitive
time-movement registrations. Refraction and reflection waves are
usually
recorded on magnetic tapes, which are readily adapted to computer
analysis.
Strain seismographs, employing electronic measurement of the
change in distance
between two concrete pylons about 30 m (100 ft.) apart,
can detect compressional
and extensional responses in the ground during
seismic vibrations. The Benioff
linear strain seismograph detects strains
related to tectonic processes, those
associated with propagating seismic
waves, and tidal yielding of the solid
earth. Still more recent inventions
used in seismology include rotation
seismographs; tiltmeters; wide frequency
band, long-period seismographs; and
ocean bottom seismographs. (Alferd
Wegener p.118-120) Similar seismographs are
deployed at stations around the
world to record signals from earthquakes and
underground nuclear explosions.
The World Wide Standard Seismograph Network (WWSSN)
incorporates some 125
stations. (U.S.G.S.) Richter Who? Richter, Charles
(1900-1985), American
seismologist who wrote fundamental seismology texts, and
who established an
earthquake magnitude scale with German-American
seismologist
BenoGutenberg. (Encarta 98) Richter was born in Ohio but
moved to Los Angeles as
a child. He attended Stanford University and received
his undergraduate degree
in 1920. In 1928 he began work on his Ph.D. in
theoretical physics from the
California Institute of Technology
(Caltech), but before he finished it, he was
offered a position at the
Carnegie Institute of Washington. At this point, he
became fascinated with
seismology. After he worked at the new Seismological
Laboratory in
Pasadena, under the direction of Beno Gutenberg. In 1932 Richter
and
Gutenberg developed a standard scale to measure the relative sizes
of
earthquake sources, called the Richter scale. In 1937 he returned to
Caltech,
where he spent the rest of his career, eventually becoming professor
of
seismology in 1952. Richter and Gutenberg also worked to locate and
catalog
major earthquakes and used them to study the deep interior of the
earth.
Together they wrote a very influential textbook, published in
1954, called
Seismicity of the Earth. In 1958 Richter published the
textbook, Elementary
Seismology, which many consider his greatest
contribution to the field. Richter
visited Japan on a Fulbright Fellowship in
1959-1960. (Encarta 98) Richter was
also involved in public awareness and
safety issues surrounding earthquakes,
taking a sensible stance rather than
using scare tactics. He was devoted to his
work in science and learned
several languages in order to read the global
earthquake literature. Richter
was so interested in earthquakes, he even
installed a seismograph in his
living room of his Los Angeles home. He
influenced Los Angeles building codes
that city officials credited with saving
many lives in the 1971 earthquake in
San Fernando, California. After retirement
he continued to work on earthquake
safety design. (Encarta 98) (PUT MONTH)
EARTHQUAKE FINDINGS During the
month of march we charted all of the bigger
earthquakes that occurred . We
charted the earthquakes measuring from 4 to 7 on
the Richter scale. We
plotted this data to see where most of the earthquakes
would occur. Also to
see how high most of the quakes would be on the scale.
According to our
analyses most of the earthquakes occurred around the plate
boundaries.
Especially in South America along the South American plate and
Mexico
along the North American plate. Yet, to our surprise there weren’t
many
earthquakes whatsoever, along the boundary between the Eurasian plate
and the
African plate. We also found Seismic activity in some unusual
areas like the
arctic region above Europe and the Antarctic region. Most of
the quakes we
recorded were not generally large either. Most of them were
recorded at 4 on the
Richter scale. There were not many large earthquakes
in the month of March. The
largest quake we recorded was 6.8 in Xizang-India
border region. We also found
that there were an unusually high number of
earthquakes in the month of March.
From the data that we collected we
noticed that earthquakes can also occur in
the middle of the ocean. In
conclusion from the data we have constructed we came
to find out that large
earthquakes are rare and far in between. We have come to
realize how
devastating earthquakes can really be to people and
their
surroundings.
Bibliography
Kidd, J.S. & Kidd, R. A.
(1997). On shifting ground "the story of
continental drift". New York: Facts
on File, Inc. Erickson, J. (1992). Plate
Tectonics. New York: Facts on
File, Inc. Glen, W. (1982). The Road to Jaramillo.
Stanford, California:
The Stanford University Press. Menarld, H.W. (1986). The
Ocean of Truth.
Princeton, N.J.: The Princeton University Press Suhwartzbach, M.
(1986).
Alfred Wegener. Madison, Wisconsin.: Science Teck Inc. Vogel, S.
(1992).
Naked Earth. New York: Dutton Books. Wertenbacher, W. (1974). The
Floor of the
Sea. Boston Massachusetts.: Little Brown and Co. Internet.
(1999).
wwwneic.cr.usgs.gov/neis/bulletin.html. Computer source.: Internet
explorer.
Apsell, P. S. (Producer). (1990). Nova Earthquake. [Video
Tape]. Western Video